Patent Application
20170236954
The present disclosure relates in general to the fields of photovoltaics and semiconductor microelectronics. More particularly, the present disclosure relates to the methods, architectures, and apparatus related to high-efficiency back-contact crystalline silicon photovoltaic solar cells.
Currently, crystalline silicon (both multi-crystalline and mono-crystalline silicon) has the largest market share in the photovoltaics (PV) industry, currently accounting for about 85% of the overall global PV market share. Although moving to thinner crystalline silicon solar cells is long understood to be one of the most potent and effective methods for PV cost reduction (because of the relatively high material cost of crystalline silicon wafers used in solar cells as a fraction of the total PV module cost), utilizing thinner crystalline wafers is hampered by the problem of thin wafers being extremely fragile, mechanical breakage during wafer handling and cell processing, and the resulting production yield losses caused by thin and fragile silicon wafers. Other problems include inadequate light trapping in the thin cell structure because silicon is an indirect bandgap semiconductor material and absorption of longer wavelength red and infrared photons (particularly those in the wavelength range of about 900 nm to 1150 nm) requires relatively long optical path lengths—often much larger than the wafer thickness itself. Further, using known designs and manufacturing technologies it is often difficult to balance the requirement of high mechanical yield and reduced wafer breakage rate with high manufacturing yields in PV factories in a cost effective manner.
Relating to substrate (semiconductor absorber) thickness, for current crystalline silicon wafer solar cells, moving even slightly thinner than the current thickness range of 140 μm to 200 μam starts to severely compromise mechanical yield during cell and module manufacturing. This is particularly a big challenge for larger cell sizes such as 156 mm×156 mm and 210 mm×210 mm cells (compared to the smaller 125 mm×125 mm cells). Thus, manufacturable solutions directed to process very thin solar cell structures, such as with cell semiconductor absorbers thinner than about 100 μm down to micron-size-scale and submicron thickness, often must utilize a cell process during which the cell is fully supported by a either a temporary and/or a permanent host carrier throughout the process flow, or a cell process which utilizes a novel self-supporting, stand-alone, substrate with an accompanying structural innovation. This structural innovation must allow the cell substrate to be extremely robust against breakage in high throughput solar cell and module factories. Examples of the latter are the novel 3-Dimensional honeycomb and pyramidal structures formed with crystalline silicon thin films.
On the cell architecture side, back-junction/back contacted monocrystalline semiconductor (such as monocrystalline silicon) solar cells are conducive to very high efficiency. This is primarily because there is no metal shading associated losses on the front side as well as no emitter on the front which helps result in a high blue response. Moreover, the use of n-type base enables much higher minority carrier lifetime compared to p-type base, as well as no Light-Induced Degradation (LID). In addition, the back-contact/back-junction cell with n-type base may use well-established silicon nitride frontside passivation and anti-reflection coating layer with positive fixed charges in the passivation layer (or layer stack) comprising silicon nitride providing for improved frontside surface passivation with reduced Frontside Surface Recombination Velocity (FSRV) enabled by field-assisted passivation. Further, backside metal may be made thicker and with a higher area coverage (e.g., well over 90%) to ensure very low series resistance (or very high metal interconnect electrical conductivity) without worrying about the trade-off with shading that is often a consideration for front contacted cells. Back contacted/back junctions cells are, in particular, highly conducive to being combined with very thin (e.g., solar cell substrates for at least two distinct reasons. Firstly, high-efficiency back contacted/back junction cells have a stringent requirement of having minority carrier diffusion length (known as Leff) at least 5× (by a factor of at least approximately 5) the thickness of the substrate (or the active crystalline semiconductor absorber). A very thin (e.g., with crystalline semiconductor layer thickness less than about 80 microns and more preferably less than about 50 microns) solar cell substrate enables this requirement without demanding a very high bulk substrate lifetime or a very high quality material, thus, can be done in practice on a cheaper starting material having eliminated the most stringent substrate quality requirements. This indirectly gives a further cost advantage: the quality of the material can be relaxed in addition to it being thinner. A second reason is related to the process flow which enables fabrication of the back contact/back junction cells (will be discussed further in the following sections). Because back contacted cell architecture and related process flow may be catered to have all high temperature process steps (i.e., any cell process steps with process temperatures in the range of approximately 400° C. to ˜1150° C.) on one side of the cell, the requirement for a carrier of the thin substrate when it is going through processing on the other sides are considerably eased. Thus, using a very thin substrate (e.g., with crystalline semiconductor layer thickness less than about 80 microns and more preferably less than about 50 microns) in conjunction with a back-contacted/back-junction architecture may represent an ideal solar cell combination.
In the past, there have been attempts in solar PV R&D to use carriers such as glass for thin substrates; however, these carriers have suffered from serious limitations including relatively low maximum processing temperatures in the case of soda lime glass (or most other non-silicon foreign materials), with the processing temperatures being limited to well below approximately 400° C.—which potentially may compromise the solar cell efficiency. There have also been attempts to make small area (for example, cell areas well below 10 cm2) thin cells which do not have serious breakage concerns (while they still suffer from the thermal processing limitations, including limitation of process temperatures to well below approximately 400° C.); however, large cell areas (areas well above 100 cm2) are often required for commercial viability through cost-effective manufacturing.
Therefore, a need has arisen for fabrication methods and designs relating to metallization for solar cells. In accordance with the disclosed subject matter, methods, structures, and apparatus for multi-level metallization of solar cells are provided. These innovations substantially reduce or eliminate disadvantages and problems associated with previously developed solar cells.
According to one aspect of the disclosed subject matter, fabrication methods and structures relating to multi-level metallization of solar cells are described. In one embodiment, a back contact solar cell comprises a substrate having a light receiving frontside surface and a backside surface for forming patterned emitter and base regions. A first electrically conductive metallization layer is patterned on the backside base and emitter regions. An electrically insulating layer is formed on the first electrically conductive metallization layer and a second electrically conductive metallization layer is formed on the electrically insulating layer. The second electrically conductive metallization layer is connected to the first electrically conductive metallization layer through conductive via plugs formed in the electrically insulating layer.
These and other advantages of the disclosed subject matter, as well as additional novel features, will be apparent from the description provided herein. The intent of this summary is not to be a comprehensive description of the subject matter, but rather to provide a short overview of some of the subject matter's functionality. Other systems, methods, features and advantages here provided will become apparent to one with skill in the art upon examination of the following FIGURES and detailed description. It is intended that all such additional systems, methods, features and advantages included within this description be within the scope of the claims.
The features, nature, and advantages of the disclosed subject matter may become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference numerals indicate like features and wherein:
The following description is not to be taken in a limiting sense, but is made for the purpose of describing the general principles of the present disclosure. The scope of the present disclosure should be determined with reference to the claims. Exemplary embodiments of the present disclosure are illustrated in the drawings, like numbers being used to refer to like and corresponding parts of the various drawings.
And although the present disclosure is described with reference to specific embodiments, such as crystalline silicon and other fabrication materials, one skilled in the art could apply the principles discussed herein to other materials, technical areas, and/or embodiments without undue experimentation.
The disclosed subject matter provides various structures and manufacturing methods for high-efficiency back-junction/back contacted solar cells specifically using thin crystalline semiconductor absorbers such as monocrystalline silicon with the cell absorber layer (or substrate), preferably ranging in thickness from about less than one micron (1 μm) up to about one hundred microns (100 μm), and even more particularly ranging in thickness from about one micron (1 μm) to about fifty microns (50 μm). The cell structures and manufacturing methods provided also apply to thicker crystalline semiconductor substrates or absorbers, ranging in thickness from about 100 μm to about 200 μm (which also includes the thickness range for more conventional CZ or FZ wafer thicknesses). The crystalline solar cell substrates may be formed either using chemical-vapor-deposition (CVD) methods including epitaxial growth (such as atmospheric-pressure epitaxy) or other crystalline silicon material formation techniques (including but not limited to the the so-called kerfless slicing or exfoliation methods utilization proton implantation, metal-stress-induced exfoliation, or laser). Various embodiments of manufacturing methods as it pertains to all aspects of processing very thin crystalline semiconductor solar cell substrates may be extended to other types of materials and to wafer based approaches, including kerfless cleavage methods such as the implantation-assisted wafer cleavage methods. Key attributes of various cell embodiments provided include substantially reduced semiconductor (e.g., silicon) material consumption, very low manufacturing cost, high cell efficiency, and relatively high energy yield, thus, improved solar photovoltaic module performance. Specifically, this stems from the combinations of the unique cell design architectures and manufacturing methods of this invention, which entail manufacturing back junction/back contacted solar cells using thin crystalline semiconductor layers, yielding very high conversion efficiency on thin crystalline semiconductor substrates, yielding very low cost. While the variety of disclosed embodiments may be applied to various crystalline semiconductor materials (such as silicon, gallium arsenide, germanium, etc.), preferred embodiments for monocrystalline silicon are provided (which also apply to the other monocrystalline semiconductors including gallium arsenide, germanium, gallium nitride, etc.).
The disclosed subject matter provides innovations particularly pertaining to very thin crystalline solar cells (from about 1 micron up to 150 microns, and more preferably cell absorbers in the thickness range of about 1 micron to about 50 microns) with the back junction/back contact architecture. First, novel very thin (thickness range of 1 micron to 150 microns) back contacted/back junction crystalline silicon cell structures are provided. Secondly, methods for manufacturing back contacted/back junction crystalline silicon cell structures are provided. Thirdly, methods for supporting thin substrates (using Carriers) while they have being processed through the line and while they are deployed in the field are provided. Various combinations of these three categories create a myriad set of structures, process flows, and thin cell support carriers.
The final structures obtained using these unique combinations are back-contact solar cells. Importantly, although this disclosure provides many unique sets of set of structures, process flows, and thin cell support carriers, it is understood that not all sets of possible process flows based are explicitly covered by this document, and the ones which are not covered are implied based on the cell design and process flow architectures disclosed herein. Several process flows and alternative embodiments are provided with detail herein allowing one with skill in art to combine various disclosed aspects.
This disclosure provides various host carrier methods and structures used for supporting the thin semiconductor (such as thin monocrystalline silicon) cell. We start by first addressing the category pertaining to handling and supporting thin film silicon substrate (henceforth, TFSS) through its manufacturing and permanently reinforcing it—this is shown as Thin Carrier 1 and Thin Film carrier 2 in
High manufacturing yield is a pre-requisite for commercially viable thin silicon solar technology. Very thin solar cells (from about 1 micron up to 150 microns, and more preferably cell absorbers in the thickness range of about 5 microns to about 60 microns) discussed in this document are fully and continually supported throughout the cell handling and processing to maintain high manufacturing yield and for commercial viability. This means that thin cells are never processed or handled without either temporary or permanent support attachments (also called substrate carriers). These thin semiconductor cells are also permanently supported (and reinforced) once assembled in the photovoltaic modules for installation and operation in the field to maintain mechanical resilience, reliability, and high yield during module lamination/packaging, field installation, and field operation. Because both sides of the solar cell need to be accessed and processed (to complete the cell backside and sunnyside), in general, two carriers are required for TFSS (in order to always support the thin semiconductor substrate throughout handling, processing, and final module packaging): one for processing each face of the solar cell. The carriers must satisfy several important criteria: firstly, they should be cost-effective (i.e., very low cost per cell or very low cost per peak watt). Their combined amortized cost should be less than the cost of the silicon in the thin cell that they save (compared to the traditional wafer-based solar cells). Secondly, at least one of the carriers should be able to withstand relatively high temperature processing (particularly at temperatures in the range of approximately 300° C. up to as much as 1150° C.) required in manufacturing of high-efficiency solar cells, without any complications due to mismatch in coefficient of thermal expansion (CTE) and/or due to undesirable impurities being introduced into the cell. In addition, if only one of the carriers is able to support high temperature cell processing (i.e., high-temperature processing to form the cell substrate itself using CVD epitaxy as well as to complete the cell backside device structure as required), the process flow should be such that all the necessary high temperature processing steps are on this high-temperature-capable carrier (which will serve as temporary reusable carrier). As mentioned before these particular criteria are highly favorable to back contacted/back junction cell, hence, truly enabling high efficiency back-contact, back-junction thin cells. Thirdly, at least one of the carriers should preferably be able to withstand wet processing and final cell metallization required for manufacturing solar cells. An example of a key wet processing step includes silicon front surface random pyramid texturing etch in diluted and heated alkaline (comprising KOH and/or NaOH and/or TMAH) solutions. Fourthly, once the first side (preferably the cell backside for the back-contact/back-junction cell processing) is partially or fully processed, the carrier (which serves as temporary reusable carrier) should be such that the thin cell (Thin Film Semiconductor Substrate: TFSS) may be easily detached or lifted off from the carrier on demand with high yield and with the TFSS layer transferred to the other carrier in conjunction with the lift-off detachment process (attached on the side which was processed first, preferably the cell backside for back-contact cell) for processing of the second side. Subsequently, in the case, where the first side (preferably the cell backside) was only partially processed, the remaining process steps (for instance, such as completion of the final cell metallization) can be completed using, for example, various embodiments detailed below. Preferably within the embodiments of this invention, the high-temperature-capable temporary carrier and the high-temperature processing steps precede the permanent carrier as well as the wet processing and final cell metallization steps. Moreover, starting with the formation of the thin-silicon substrate using CVD epitaxy till the pre-lift-off attachment of the permanent carrier to the TFSS layer, all the process steps performed on the TFSS while on the temporary carrier are preferably dry processing steps (no wet processing on the temporary carrier other than a wet porous silicon process step prior to the formation of the TFSS layer by CVD epitaxy). Furthermore, the cell contact metallization is preferably performed after formation of the cell contacts and prior to the attachment of the permanent carrier and prior to the lift-off separation of the TFSS layer from the temporary reusable carrier or template.
Supporting Carrier #1 (i.e., reusable template) for TFSS. For the combination of TFSS with back contact/back junction architecture two choices for the first carrier are disclosed, henceforth carrier 1. These options are shown in
When the porous silicon/epi technique on the host carrier is compared with the thick wafer/ingot and implant induced separation technique several trade-offs can be identified. The wafer/ingot with implant has the advantage of not needing porous silicon and epitaxial growth and the accompanying reactors (however, it has dependency on polysilicon feedstock and ingot growth). On the other hand it needs rather expensive MeV proton implantation capital equipment and high energy consumption to operate the implanters. The quality of silicon can be high depending on the cost of the ingot and it can potentially also allow wet processing. A downside is that because the ingot may have <111> orientation in order to eliminate the need for excessively high proton implanation doses, the wafer may rely on more-expensive and damage-producing dry texturing as opposed to the standard wet texturing. The porous silicon/epi combination has the advantage that it is compatible with standard alkaline wet texturing and the substrate doping can be modulated/graded to whatever is conducive for high efficiency requirements. Also, very thin silicon cell substrates (down to about one micron) are possible using a porous silicon/epi manufacturing method and the doping profile can be engineered and adjusted during the epitaxial growth process (not possible for the thin silicon layers produced by proton implantation).
Supporting Carrier #2 for TFSS: Backplane. The second carrier, in the specific context of back contacted/ back junction cells should preferably satisfy several criteria. An obvious one is that it must support the TFSS through the remaining process steps. Secondly, it should protect the prior processing on the side where it is attached (backside for our specific architecture), while the other side (frontside) is being processed. This requires that the second carrier preferably be relatively immune or resistant to the wet chemistry that is used during processing of the frontside (particularly and primarily the wet chemistry used to clean and texture the TFSS sunnyside). Thirdly, it may or may not have high-conductivity metallization layer (preferably comprising aluminum and/or copper) as its integral part. For the case where it does have metallization, in addition to being a carrier (preferably a very low cost permanently attached carrier), it provides metallization which seamlessly attaches to the metal on the cell with low resistance. Finally, though not as high a priority, it should have thermal processing capability sufficient for achieving excellent frontside passivation (therefore, preferably at least up to a temperature of about 180° C. and more preferably at least up to a temperature of about 250° C. or even 300° C.) without producing cracks in TFSS due to any CTE mismatch with silicon and without degradation of the carrier material. This second carrier attached to the solar cell backside henceforth, will be identified as the solar cell “Backplane.”
Several backplanes embodiments are disclosed herein and outlined in
Carrier 2 (the backplane) may be divided into two broad classes (
One of the driving forces behind the partial processing paradigm on the non-sunny side (i.e., cell backside) is to ensure that if potentially deleterious materials (including lifetime degrading materials), such as copper, are part of the backside processing, they do not contaminate carrier 1, which can be reused for carrying other TFSS (hence, preventing the risk of metallic cross contamination in the production line). This prevents cross contamination in the manufacturing line and the resulting efficiency degradation (hence enabling high-yield template reuse without the risk of cross contamination to the cells). Thus, an idea behind partial processing on the non-sunny side is to introduce the potentially lifetime-degrading materials and processes (such as high-conductivity copper plating metalliation) after the TFSS is detached and released from carrier 1, hence, eliminating the risk of cross contamination.
Three sub-categories of backplanes within the partial processing paradigm are shown in
The second and third subcategories, “Backplanes without Metallization” and “Backplanes with Metallization,” of backplanes with partial backside processing are characterized by backplanes which are permanent (in contrast with aforementioned FSR). A difference between these two categories is that the “Backplanes without Metallization” do not have thick metallization integrated or embedded in their structure; rather, this metallization is put on toward the back-end after the frontside (sunny-side) is processed. Whereas, “Backplanes with Metallization” have a thick second level of metallization (for instance, a patterned metal foil) integrated into the backplane. The thick metallization layer on the backplane connects to the thin metallization layer on the TFSS, forming the second layer of interconnects, and may also contain busbars. This thick high-electrical-conductivity metallization layer (preferably made of aluminum and/or copper) decreases resistance for back-contacted cells.
This disclosure details three particular embodiments within the “Backplanes without Metallization” subcategory of backplanes. Importantly, this should not be construed as limitation of this paradigm to these three embodiments. The first case is called Back Surface Reinforcement or “BSR.” In this process flow, the TFSS is released from the template (the first carrier) using a permanent backside reinforcement. The permanent backside reinforcement only partially covers the backside, thus allowing processing on the backside through the open areas, after front side processes are completed also with the BSR support. A structural example of this is a backplane made in a grid pattern with a substantially large open area between the grids providing access to the backside for last several processing steps on the non-sunny backside.
The second embodiment of a permanent “Backplanes without Metallization” is a design known as acronym “PLUTO.” In this process flow, a simple and cheap backplane material (e.g., a relatively low-CTE Pre-preg material comprising a mixture of resin and fibers) is attached to the TFSS, while it is attached to the first carrier. The backplane attachment may be a direct bonding/lamination (if material has adhesive in it) or use an intermediate adhesive layer, for example a dielectric adhesive (DA) which may be printed using means such as screen printing (or applied using a spray coater or a roller coater). The pre-preg assembly/material choices should be such that they meet the following criteria:
After all frontside processing is completed using the PLUTO backplane, access holes (100's to 1000's of holes) are drilled, preferably using a high-productivity laser drilling tool, through the backplane (such as a pre-preg material) and remaining cell metallization is finished, preferably either by plating or using a combination of screen printing of a patterned electrically conductive seed paste and attachment of a pre-patterned metal foil layer (comprising aluminum and/or copper). These holes provide access to the underlying on-cell patterned metal which was formed while the TFSS was on the template (specific examples will be illustrated during the subsequent discussion on process flows). The drilling of holes may be accomplished using a myriad of laser and mechanical methods, in a specific example this can be accomplished using a fast throughput CO2 laser. Requirements for the drilling technique include fast throughput, no damage to the TFSS or the underlying metal on the TFSS, a reliable way to clean the laser opened contacts (if necessary) to have low resistance electrical access to the underlying metal on the TFSS, and proper alignment of the holes to the underlying metal. Subsequent to the laser drilling the rest of the metallization (comprising a second level of metal) may be finished using several method including plating (both electroless and/or electroplating), direct thick metal write techniques such as flame spray, attaching a cheap bread-board with metallization to the backplane, attaching metal foil fingers after screen printing of a patterned conductive seed paste, or having the metallization as part of the module assembly in approaches such as monolithic module assembly (MMA). A slight modification process includes an embodiment in which the pre-preg has pre-drilled holes prior to its attachment/lamination to the TFSS (to eliminate the risk of laser-drilling-induced damage to the TFSS) and is protected by another easily removable cheap thin material layer or sheet (such as a thin Mylar sheet or another suitable material). In this embodiment, the removable protective sheet will be preferably removed after completion of the sunnyside cell processing (including at the wet texture and PECVD passivation processes), and prior to completion of the final cell metallization (or prior to module assembly in the case of MMA).
The third embodiment, “Cu Plugs,” of a permanent “Backplanes without Metallization” of
Another embodiment uses a single sided or a dry frontside texturing process so that the need to protect the partially processed backside is obviated and all access points can be pre-opened (using laser drilling or mechanical drilling or punching) either before attachment of the backplane or before processing the frontside.
The “Backplanes with Metallization” subcategory of backplanes, as shown in
The OASIS backplane embodiment has several components. First, it consists of a metallic backplate which may or may not also serve as the metallization layer. This metallization layer, which in a particular embodiment is patterned into inter-digitated fingers with busbars, may be made, for example, from Al foils or solderable aluminum foils. The Al foils may be pre-coated or pre-plated with nickel and Sn (or a Sn solder alloy) to provide better adhesion of the conductive vias which connect the second level of interconnects to the first level of interconnects on the TFSS. The backplate may be protected from chemical attack on the top by a suitable protective layer such as Z68, EVA or prepreg or another suitable polymeric/plastic cover sheet. These layers are ultimately opened up to provide access for the testing and module connections from the top. During lamination of the patterned metal to the EVA or Z68 like material, substantial planarity must be accomplished by exploiting the flow of the attachment material such that the final assembly should be substantially planar from both top and bottom. At the planar bottom of this assembly, the connection of the Al foil metal to the underlying TFSS metal is made using selective conductive posts or vias in the dielectric layer which has gaps to accommodate the electrically conductive vias. The conductive vias (henceforth conductive epoxy or CE) and the dielectric material (henceforth dielectric epoxy or DE) in the preferred embodiment are screen printed on either the TFSS or on the backplane. CE material requirements include cost effectiveness, highly conductive, it may be screen printable in a preferred embodiment, and that it attaches with low contact resistance to both the overlying backplane metal and the underlying TFSS metal. DE material requirements include cost effectiveness, that it is a non-electrically-conductive dielectric, it may be screen printable in a preferred embodiment, and it adheres well to both the overlying backplane material (both metal and the EVA or Z68 dielectric encapsulant) and the underlying TFSS materials consisting of both TFSS metal and dielectric. For example, the OASIS backplane may have a myriad of variations based on choices in the following categories:
Specific examples in which these backplanes may be fit into process flows for forming back contact solar cells are outlined in the manufacturing methods below.
General Structures and Methods for TFSS-based Back Junction/Back Contacted Solar Cells
The above discussion pertained to the choices and combinations pertaining to first and second (backplane) carriers for ensuring high processing/manufacturable yield for TFSS back junction/back contacted solar cells. The following section deals with manufacturing methods and process flows for an entire TFSS based solar cell with these carriers. While depicting process flows, in several cases, the backplane is abstracted. This abstraction may be replaced by any of the several backplane options that were discussed in the above sections. In addition, the combination of the backplane with specific flows may either be used with the template/Porous Silicon (PS) based carrier 1 or the Ingot (or Thick Wafer)/implant based carrier 1. Specific flows pertaining to these two cases will be shown.
Ex-situ vs. In-situ Emitter. The process flows shown
General Structural and Manufacturing Method attributes shared by the process flows. Specific examples of a class of final back junction/back contacted solar cell structure and methods for manufacturing are detailed below. Note, that the structures and methods are not limited to these specific examples. A wider range of examples may be derived using the aforementioned general carrier methods by those skilled in the related art. For those specific structure and methods detailed herein, identified common attributes include:
Flow Option 1: This process flow uses APCVD PSG to make the base doping. PSG layer is deposited and phosphorus is driven in either using a batch furnace anneal or using pulsed nanosecond laser hot ablation of the PSG layer (in the latter case, to dope the underlying TFSS and to ablate the PSG layer for base contact opening).
Flow Option 2: This process uses screen printed (or stencil printed) silicon nanoparticle phosphorous or silicon nanoparticle phosphorous ink applied by inkjet (or aerosol jet) printing. This will be followed by a thermal anneal.
Flow Option 3: This option uses screen printing of the phosphorous paste or phosphorous ink applied by inkjet printing. This is followed by thermal anneal in a batch furnace equipment to drive in the dopant.
Flow Option 4: This process uses Phosphorous Oxychloride POCl3 as the starting phosphorous dopant material (process is preferably performed in a POCl3 tube furnace). It requires post diffusion phosphorus glass wet etch or HF vapor etch.
Sub-categories of the four flow option categories of
Flow Option 1: APCVD PSG based base doping. There are two sub-categories in this class: a) Hot ablation using pulsed ns laser processing to drive base and emitter contact using laser (and to concurrently open the base and emitter contact holes), b) Cold ablation (preferably using pulsed ps laser processing) where furnace anneal makes the base contact diffusion regions.
As shown in
Laser hot ablation is followed in one embodiment for metal 1 by physical vapor deposition (PVD) based deposition of a thin aluminum or Al—Si layer (such as with plasma sputtering or evaporation). This aluminum (Al) layer serves the function of both back surface reflector (BSR) in conjunction with the backside passivation dielectric stack as well as is instrumental in making a good electrical contact to the device base and emitter regions. The contact resistance of Al (or Al doped with Si) PVD to both the doped emitter and the base contact regions is critical. The PVD process may also be done either as a hot PVD (depositing the Al layer while the cell substrate is heated to a temperature in the approximate range of 150° C. to 450° C., or a post PVD anneal may be performed between 150° C. to 450° C. if needed. This is to ensure better contact resistance (hence, higher fill factor) as well as take advantage of a better passivation in presence of Aluminum (Al anneal) and H2 from the APCVD layers (effectively performing a forming gas anneal to improve the backside passivation and to improve the cell Voc). Subsequently, other PVD metal layers may be deposited depending on the needs of the adhesion, reflectivity requirements and laser metal isolation requirements. In one rendition, combination of NiV (or Ni) and Sn can also be sputtered as second and third layers on top of Al using PVD and in situ after sputter deposition of Al. The function of this metal stack with a top layer of Sn will be to ensure that the adhesion of the backplane metal or M2 is not compromised (hence, improving the cell fill factor and long-term reliability). In a variation of this stack, the Al/NiV/Sn stack can be annealed below the melting point of Sn to give a solder like anneal between Sn and NiV. Subsequently, pulsed picoseconds laser is used to isolate and pattern both base and emitter metal areas. A typical design is interdigitated finger design. In a preferred embodiment, no busbars and only interdigitated fingers are defined on the cell for Ml. This minimizes electrical shading under the busbar and increases cell efficiency. However, other embodiments with busbars as well as with other designs such as mini-cells can be defined with the metal ablation laser process. In general, the specific dimensions including pitch of emitter/base lines is dictated by several device design considerations including, but not limited to, base and emitter diffusion resistance. PVD could entail vacuum sputtering, vacuum evaporation, ion-beam deposition (IBD), atmospheric arc spray and other thermal physical vapor coating methods. In a different and less preferred embodiment, screen printing of resist, followed by etch can also be used to isolate base and emitter patterns. However, there is a risk in this approach due to performing wet processing on template (for metal etching and resist stripping wet steps).
In another variation of the metallization process, instead of using PVD metal (which includes vacuum techniques such as sputtering, evaporation etc), prevalent metal screen printed approach can be used. This approach has the advantage of not using vacuum process which tends to be both expensive as well as present the danger of dislodging the epi substrate prematurely from the mother template due to the delamination pressure from the porous silicon interface while the cells are in vacuum. In the metal screen printed embodiment, in general, a base and an emitter metal is screen printed (this may be a single screen print process using a single aluminum paste material) and fired to make metallization contact to the emitter and base diffusion regions, where the base and emitter diffusions are created using several possible techniques, one of which is described above and several others will be detailed subsequently. The rest of the process flow remains similar. The screen printed metal or metals can be either co-fired or sequentially fired, and may be the same for base and emitter or be dissimilar. Further, the screen printed metals may be fritted, lightly fritted, or frit-less (such as a suitable fritless aluminum paste). Specific examples of this process may entail screen printing a frit-less Al metal paste on both emitter and base and co-firing using the same process step. The M1 metal pattern will depend on the underlying cell design; however, in general, it can include segmented metal lines to reduce wafer level stress and to reduce the risk of microcrack formation in TFSS. Another example of this process could entail screen printing and firing Ag for the phosphorous contact, while screen printing and firing Al for the base contact. These screen printed lines or fingers can be continuous or segmented. In this rendition, if it is segmented on the base, the PSG may be deposited selectively in the base area, thus, creating pockets of base contact doping. Subsequently, the Ag metal may be fired through the doping sources (PSG in this case) to create contacts within the base pockets. This approach can have efficiency advantage by ensuring much smaller base contact minority carrier recombination, hence improving both Voc and Jsc of the solar cell. It also obviates the need for opening base contact using laser process. This segmented metal design is possible only because of the versatility of the backplane. The backplane allows a vertical draw of current while summing it at the backplane layer. In case there is difficulty in providing backplane level connection at the same tight pitch as the Ag metal segmentations (which may be dictated by other device constraints), a continuous metal can be screen printed (for example Al) on top of Ag segments at the same time that the emitter metal is printed. Care has to be taken to ensure that this metal (Al) does not penetrate through the PSG oxide which may be avoided using the right choice of the metal paste.
Although, not explicitly mentioned in the following sections dealing with other variations of the process flow, it is understood that the direct write metal screen printed option can be utilized in lieu of the PVD metal option for the subsequently discussed process flows as well.
In the specific embodiment shown in
And although several types of backplanes were discussed in the earlier section, the two embodiments are detailed as follows:
The current still needs to be drawn out to the top of the backplane from the face down side. Following are two general schemes for this: firstly, to wrap the Al foil around the edge of the backplane on to the other side (henceforth, Wrap-around Busbars). Risks with this scheme include difficulty in protecting the wrapped foil during some of the subsequent steps. In the second scheme, a few through holes are drilled and current is accessed at these locations from the underlying foil. Several ways to create these holes are disclosed herein.
Although, the process flow remains similar with either of the backplane embodiments discussed above, the Al foil configuration is detailed in the remaining process flows. Backplane assembly is attached to the cell/template (
After release, the template is cleaned and send back for reuse for porous silicon and epi for the next Reuse. The TFSS attached to the backplane assembly (which is the second and the permanent carrier) is now cleaned on the QMS (or porous silicon) side and textured. In one specific embodiment, this may be performed in one shot using hot KOH based chemistries such as KOH/SCD or KOH/IPA combination (wherein KOH may be replaced with NaOH). This is followed by post texture clean which in one case can be done using HF/HCl combination. Subsequently, the TFSS is taken for its final process step on the sunny-side which is deposition of the (hydrogenated) SiNx ARC and passivation layer. Because of the presence of backplane assembly, the maximum temperature of this process is limited to a low value which may be in the range of 150° C. to 300° C. depending on the choice of the backplane material. A method by which a satisfactory passivation may be achieved at low temperature for back contacted cells is discussed in an earlier passivation. It suffices to mention that this will involve excellent cleaning post texture and deposition of a thin (e.g., 3 nm to 10 nm) amorphous silicon (a-Si) or amorphous silicon oxide layer before SiN. The SiN preferably has to be rich in positive charges to repel the positively charged minority carrier holes away from the surface and reduce surface recombination
The final step in the process flow of
Option 2C (
Note that, in all the options with silicon nanoparticles (flow option 2 in
Flow Option 3 of
In option 3A (
Option 3B (
Option 3C (
Flow Option 4: POCl3 based Base doping.
In option 4A (
In option 4B (
In option 4C (
Minimum cell process flow. This section describes a variation of the process flows described as option 1 above (using PSG for making base contact). In this variation, several steps are combined and the CE print step is eliminated to use a reduced number of tools for creating the high-efficiency, back contact thin cell. A defining attribute of these minimum steps flows is that the screen printing of conductive epoxy is eliminated by using a low-temperature solder alloy (e.g., 58%Bi-42%Sn with 138° C. solder melting point, or Bi-45%Sn-0.33%Ag with a melting point of 140-145° C.), formed both as an overlayer on top of the cell Al metal/mirror as well as on the backplane metal fingers with a pre-formed pin grid array on the metal fingers. Once the backplane is aligned and placed on the cell, the backplane pin grid array is solder attached to the cell during the thermal lamination process.
Non Epi Bulk thin substrate process flows. Previously, two types of carrier 1 examples were disclosed. The first type of carrier 1 uses a template and the second type of carrier 1 is a thicker wafer or ingot from which thin CZ or FZ slices are cleaved or exfoliated using a myriad of available techniques, including hydrogen ion implantation. The following section describes the cell level process flows utilizing backplane innovation in conjunction with the wafer cleaving approach to obtain a thin silicon substrate. Proton implant based cleaving produces <111> textured substrates, which would preferably require dry texturing. The embodiments show proton implant cleavage/slicing of ultrathin substrates (e.g., about 1 μm to 80 μm thick substrates separated/cleaved from much thicker reusable wafers, e.g., wafers or bricks which are several mm or several cm thick).
In
Specific Process flows for Bulk CZ and FZ wafers using backplane technology. In this category of flows representative back contacted/back junction process flows for bulk CZ (Czochralski) and FZ (Float Zone) wafers using backplane technology are detailed. Among distinguishing factors includes insertion of backplane and also the extensive use of pico-second laser processes for direct pattern definition. Although, not explicitly mentioned, if desired, the backplane technology may be used on the bulk FZ and CZ wafers to thin them down by etching to form much thinner cell absorbers, which may be useful when cheap bulk wafers are desired that may not render very high lifetimes. These cheaper, relatively lower lifetime wafers may also be of p-type bulk doping. Although, all the process flows depicted are examples of wafers with the preferred doping which is n-type base (bulk) doping.
Five categories of flows are detailed below—each category having two subcategories. Subcategories are distinguished by the method which is used to deposit and pattern metal on the cell. In the first sub-category, analogous to the previously described flows in this document, PVD along with laser based metal isolation processes are used to obtain patterned base and emitter metals. In the second sub-category, a direct patterned metal write technique is used in lieu of the PVD/laser isolation steps. Full process flows of the five main categories are detailed in the figures and descriptions; however, the categories may be defined according to the following characteristics:
In addition to the flow family 1B outlined in
As seen in
After epitaxial silicon layer deposition, a first BSG layer is deposited with a rather low concentration of boron to later provide a lightly doped emitter in the bulk of the back surface region. This process is followed by laser ablation (preferably picosecond laser) of the area where the emitter contact is to be formed. This and the subsequent structuring processes can advantageously contain parallel lines across the structure. Areas of emitter contacts and base contacts are aligned in an alternating interdigitated pattern. In certain zones, namely in the zones where later in the process the busbars are located on the metal 2 layer (second metal deposition), it may be advantageous to deviate from the linear, parallel, interdigitated base and emitter contact pattern. This deviation is employed to drastically reduce the electrical shading that is otherwise experienced underneath each busbar. Next the second BSG layer is deposited with a comparatively higher concentration of boron, such as to provide the highly doped emitter contact region (e.g., with p++ doping). Subsequently, the area for the base contact is laser ablated, preferably using a picosecond laser. Next, a PSG layer is deposited, to serve as the precursor for the phosphorus doped base contact. Subsequently, the dopants are driven in during a multi-functional high temperature process step which optionally can contain a neutral ambient such as nitrogen, optionally followed by an oxidizing ambient such as oxygen or water steam (and furthermore, optionally comprising backend lower temperature gettering and finally forming gas anneal). The junctions are now driven in. Contact can subsequently be made by laser ablation in the contact area, preferably using picosecond laser. Next, the metal 1 (first metal deposited and metal positioned closest to cell) is deposited and structured, either using PVD of for instance a stack of Al, Ni or NiV and Sn, followed by patterning using for instance, picosecond laser ablation, or by screen printing, aerosol printing, ink jet or otherwise printing one or more layers of aluminum containing paste. The aluminum paste may be selected to contain in a first layer some silicon to reduce spiking into the junctions upon subsequent annealing, or other spike reducing agents. In a second layer, also attributed to the structure of metal 1, the paste or ink can be selected to contain a suitable grain structure to harmonize well with the later via access hole drilling which is employed to make contact between metal 1 and metal 2. Other selection criteria are the optimized conductivity to have a low line resistance within metal 1 (M1). Especially for the lower ink or paste it is also crucial to select the correct paste for a low contact resistance to both the base and the emitter. Where desired, different pastes or inks, even containing different metals, may be employed to make the contact to the baser contact diffusion versus to the emitter contact diffusion. For example, the initial metal 1 layer can be a thin layer of ink such as nickel ink which can be deposited very locally in the contact regions and then turned into a silicide by heating, preferably in a self limiting process. However, the subsequent layer of metal 1 be treated at low enough temperatures to provide the lowest resistivity phase of the respective silicide formed. It is to be noted that in order to facilitate a good process window for later via access hole laser drilling while at the same time keeping metal 1 consumption (thickness) and cost per cell in check, it may be advisable to locally print thicker aluminum metal paste pads underneath the designated via holes areas, while printing a much thinner aluminum paste elsewhere on the cell to form the continuous or segmented fingers. This design may be formed, for example, by printing additional metal paste material in the area of the via hole (hence, double screen printing of the metal paste) or also by increasing the line width in the area of the via hole for better alignment tolerance or by combinations of the former and the latter.
It is to be noted that all flows and structures described in this disclosure may, in alternative embodiments, use printing processes for metal 1 paste such as ink jet, aerosol or screen printing, although PVD followed by picosecond (or fs) laser ablation patterning is explicitly mentioned as the method for metal 1 deposition. Subsequent to paste or ink printing, the pastes or inks can be suitably baked and annealed. Next, the backplane can be attached, for instance, but not limited to lamination of a suitable low-CTE prepreg material, or by first screen printing and heat or radiation treatment, such as by UV radiation, of another adhesive filler, optionally between the metal 1 spaces in order to planarize the surface prior to the backplane lamination. If such an additional adhesive is used, the backplane material, such as prepreg, can be laminated to the relatively planarized surface structure afterwards.
The lamination material, such as prepreg, may be smaller in extent than the template side dimension for instance a few millimeters on each side. For instance, for a standardized 156 mm×156 mm final cell it may be advantageous to have a lamination material just larger, for instance about 158 mm ×158 mm, and a template just larger than that, for instance about 165 mm ×165 mm.
After the lamination, in the area just outside the lamination region an ablated trench of silicon can be cut partially or fully through the epitaxial film with a laser, preferably a nanosecond UV laser, or alternatively employing thermal laser separation, a process where locally an area is heated using a travelling laser beam and subsequently cooled, using a trailing jet of mist, water or other coolant such as helium, thereby creating a cleaving front which can be terminated in the region of the release layer, formed by the porous silicon and thereby at the interface between the epitaxial layer and the template.
Following such preparation, the laminated reinforced thin film solar substrate (TFSS) may be released from the template, preferably by a pulling process, a peeling process, a pull-peeling process or through the support of sonication, such by either immersing the TFSS and template stack in an ultrasonic bath or by adding ultrasonic energy to a dry release station with the capability of applying vacuum to both sides of the stack, or by vacuum oscillation or by a combination of the above. After release of the TFSS the remaining template undergoes a process in which the area outside of the active released area is stripped of remaining epitaxial material, by grinding it off, by the use of water or other liquid pressure, by chemical removal or by a combination of the above. Subsequently, the template is cleaned and inspected and then fed back into circulation, for another round of porous silicon formation, epitaxial film deposition and so on.
The released TFSS is then trimmed to size, preferably using one or a combination of several lasers, for instance a UV or green nanosecond laser. Such trimming to size can also contain a partial ablation trench just inside of the edge boundary, to make the structure less prone to propagation of microcracks from the outside of the device. After trimming the TFSS is then textured, for instance using an alkaline texture chemistry, such as KOH with a suitable additive, followed by a post texture clean, for instance using HF and HCl, and finishing with a hydrophobic surface (for instance, using an HF-last cleaning step). Next, the TFSS receives the front side passivation, for instance by deposition of a-Si or a-SiOx, followed by ARC layer deposition, such as Silicon nitride (SiN), all preferably performed using PECVD.
The silicon nitride also contributes to the front side passivation by providing hydrogen as well as a positive charge to repel the base minority carriers. Either during the deposition or at a later step such as end of the line, the passivation layer and interface may be annealed, for instance, using a forming gas or a neutral ambient or in vacuum, to improve passivation. Such anneal may be performed at a temperature in the range of about 200° C. up to the maximum temperature allowable by the backplane material, as well as ensuring no crystallization of amorphous silicon (or silicon oxide) and ensuring formation of no microcracks. The maximum allowable temperature may be as high as about 300° C. to 350° C.
Subsequently, the backside of the wafer receives via holes, preferably drilled using a CO2 laser and stopping on or just inside the metal 1 layer. Next, the metal 2 deposition is employed, which may be arranged orthogonally to metal 1. An exception is the bus bar area if it is desired to be part of metal 2. As stated before, underneath the bus bars, metal 1 fingers as well as emitter and base regions are preferably arranged differently in order to minimize overall electrical shading from the bus bar area.
Prior to metal 2 deposition, a surface clean of the contact may be employed, such as with a sub-atmospheric or atmospheric plasma etch or cleanup to remove native oxide. For the metal 2 application, various techniques, such as those described above, may be employed, including a PVD seed which is later patterned using resist print, plating of Cu and Sn, resist strip and local seed layer etch or a pattern or unpatterned printed seed layer, such as printed nickel ink or paste (or a copper ink or paste), followed by suitable baking and subsequent copper plating. Alternatively, the metal 2 layer may also be applied using thermal spraying such as flame spraying of Al, Al with Zn, or Cu or Cu followed by Sn. The thermal spraying may be performed in lines or through a patterned mask that is periodically cleaned.
Dimensions for the metal 2 layer may be relaxed as the region access is mainly achieved by the smaller dimension metal 1 layer and metal 2 layer being arranged orthogonally to metal 1. The laminated backplane serves among other functions (such as permanent support and reinforcement) also that of the isolating dielectric between metal 1 and metal 2 layers and to provide the matrix for the via holes which provide access between the two layers (M1 and M2). Exemplary thickness dimensions for the cell of
If the bus bar is not part of the cell but rather part of the module, then the geometries in the cell may be simplified and it is possible to have both metal 1 and metal 2 both completely contain parallel interdigitated fingers only, arranged orthogonally between metal 1 and metal 2.
However, another advantage of having structures in metal 1 not being completely linear is that this design allows for a recess or exclusion of the area of metal 2 coverage within the TFSS area and thereby a sealing of the edges of the TFSS during the plating process. Such sealing prevents contamination of the active absorber area with potentially detrimental metal plating solutions containing for instance Cu.
It also may be advantageous to have the interdigitated metal lines of the metal 1 layer segmented, especially in cases such as a rather thick printed metal paste. Segmentation is to be arranged such that the contact to metal 2 is still made as well so that the series resistance throughout the line does not noticeably deteriorate. When these requirements are met, for example for line segments between about 0.5 and 5 centimeter in length, then the segmentation may prevent generation of microcracks as well as excess bow and stress initiated by the shrinkage of the metal 1 lines during a paste anneal or during process steps subsequent to the metal deposition or metal paste anneal.
Importantly, alternative dielectrics may be formed and used on the backside of the cell. For a p-type emitter, such as a boron doped emitter, it may be advantageous to have a passivation dielectric in contact with the emitter region which provides a negative charge. Therefore, in a variation of all previously described structures and flows, be they on absorber layers generated from epitaxially deposited films, from CZ wafer or from otherwise processed absorber layers such as those cleaved using high energy such as MeV implantation and splitting, it is also possible to have material such as a thin (e.g., thickness in the range of about 5 nm to 50 nm) aluminum oxide (preferably formed by APCVD or ALD) as the first layer contacting the backside (and therefore the top of the epitaxial layer).
Exemplary thickness dimensions for the cell of
As an alternative to the above deposition sequences, it is also possible to apply the aluminum oxide at a later point in time—as shown by the flow in
For instance, after diffusion of the junctions using one of the above schemes which utilize BSG, PSG and USG, it is possible to strip these APCVD oxide layers, for instance using an HF dip or preferably an HF vapor etch, followed by suitable residue removal by gas stream. Then the aluminum oxide is deposited directly onto the silicon, which in turn already contains the suitable emitter and base contact diffusions. Optionally the aluminum oxide can be thick enough or capped with other deposited oxide, such as USG, in order to prevent pin hole shunting of the subsequent metal 1 deposition. Further processing proceeds as described above for all other embodiments.
The metal 1 layer provides, in addition to electrical contact, a mirror for the photons that pass through the thin absorber layer. Therefore, a very effective minor is advantageous for harvesting and converting a higher amount of photons by reflecting the infrared photons for improved photon trapping and energy harvesting. The area coverage of metal, as well as its specific reflectivity, play important roles for this function. In order to increase the area of coverage, deposit a thin, PVD based metal, the PVD layer on a previously patterned structure—as shown in
Additionally, the geometrical structure of the on-template processes may be optimized. In addition to the structures mentioned above which enable harvesting electrical current underneath the bus bar regions, there are other geometrical structures, especially for metal 1, that may be employed advantageously and which fan out underneath the bus bars which are located on metal layer 2. However, for simplicity most of the lines of emitter and base regions as well as contacts are parallel interdigitated lines—simplified structures depicted in
However, it is also possible to have both the base diffusion regions and the base contact opening regions laid out in the form of islands in a sea of emitter area—geometries shown in
When base islands are employed, the mean path of holes to travel to the emitter for current collection may be reduced, thereby increasing the hole collection efficiency.
The same concepts hold true for the case of the above described selective emitter formation using two boron dopant sources, for example two different BSG layers, as described above—
Similarly, in most of the presented disclosures metal 1 has been generated using PVD and subsequent laser ablation. However, all structures and methods are fully compatible and applicable as well to any direct write metal 1 application methods, such as screen printing, ink jet or aerosol jet printing and thermal or flame spraying.
Also, in most of disclosed embodiments, the annealing of the passivation has been employed in in-situ anneal methods. However, all processes and structures are also fully applicable to such conditions where the passivation anneal is carried out ex-situ at a suitable point after the passivation material deposition. Advantages to ex-situ annealing include the following: The ex-situ anneal reduces the stringency of matching the thermal expansion coefficients between all the materials involved, mainly the active TFSS absorber material, such as silicon, the backplane material, as well as the metal 1 paste material and the optional additional adhesive that is employed at least between the metal 1 lines and between the active absorber material such as silicon and the backplane laminate. When the passivation itself is done at a sufficiently low temperature, say at or below 220° C. in a sophisticated deposition tool, such as a PECVD machine, then having a subsequent anneal at a higher temperature, such as 300° C. d, can be done in a very simple tool, such as an oven, and in a simple, potentially coin stacked fashion with optional interleaves between TFSS. This sequence of processing alleviates handling concerns caused by residual CTE mismatching between the materials involved.
Heterojunctions. Most silicon based solar cells on the market today are based on homojunctions. Heterojunctions, especially those with a wider bandgap emitter, benefit from the potential of a higher Voc and thus higher efficiency capability. Several cost effective ways for providing heterojunctions in conjunction with thin silicon cells are provided. The heterojunction is achieved mainly by the introduction of hydrogenated amorphous silicon (a-Si) in the emitter, which provides a wider bandgap when compared to crystalline silicon. One main task when processing such cells with amorphous silicon is to retain the effective process temperature after the amorphous silicon deposition below the crystallization temperature of silicon, typically below 400 deg C. In practice, deposition of amorphous Si (or silicon oxide) is done using PECVD at a temperature in the range of about 150° C. to 200° C.
The processes are applicable to thin silicon, such as silicon generated using epitaxial deposition on top of a porous silicon layer, as well as implant/cleave based thin silicon architectures, as well as CZ wafer- and thinned CZ wafer-based cells.
The following description provides processing methods and designs utilizing a permanent support structure (“backplane”) providing a permanent reinforcement that will not be removed after it is applied to the thin Si wafers and may be used in solar module panels together with front or back contacted thin Si solar cells. Additionally, the disclosed backplanes provide for the extraction of electrical current and power from thin solar cells with suitably low loss. The disclosed permanent support structures enable the handling and support of thin solar cells through necessary process steps including, but not limited to, edge definition or trimming, texturization and clean, as well as passivation and anti reflective coating (ARC) deposition and optional follow-on anneals, by means of thermal, microwave, or radiation such as laser energy. Additionally, the permanent support structures further support contacting schemes, such as via openings and various metallization and dielectric material application schemes including, but not limited to, deposition, printing, plating, laminating metal or metal containing or in general conductive films as well as dielectrics, including intra-cell, cell to cell and cell to module contacting.
The disclosed subject matter details novel methods and structures for reinforcing very thin Silicon (Si) solar wafers and cells to reduce breakages and to furnish contacts to emitter and base during the manufacturing process. These methods and structures are motivated by the solar cell industry's movement from standard Si solar cell thicknesses of 180 to 250 μm towards thinner cells in order to reduce Si usage and thus material costs—Si wafer production technology has advanced rapidly in reducing the wafer thickness. The fabrication of Si wafers with thicknesses less than 30 μm has been demonstrated through various methods such as layer transfer and epitaxial Si deposition. However, the industry is generally unable to manufacture Si solar cells of thickness less than about 140 μm because of the significant increase in cell breakages and lower manufacturing yields. The disclosed subject matter provides for the handling of much thinner silicon through the solar cell line with high yield, with thicknesses down to tens of microns or even less, thus reducing costs associated with breakages. Currently, the industry standard substrate thickness is greater than 180 μm. And while solar cell manufacturers have begun using Si wafers as thin as 140 μm, Si wafers less than 140 μm thick are often too fragile for usage in high volume manufacturing processes. It is anticipated that aggressive cost savings may be achieved with solar cell material less than about 50 μm thick without significant detrimental impact to cell performance as less silicon allows for a cheaper solar cell (silicon material cost constitutes a substantial fraction of total solar cell cost).
As previously noted solar cell substrates may be shaped in a variety forms including, but not limited to, standard pseudo squares, squares and hexagons. The size and the area of the substrate also varies, for example 125 mm×125 mm or ×156 mm or even much larger cells including but not limited to 210 mm×210 mm. Further, substrate material may be either mono-, poly- or multi-crystalline silicon. The disclosed subject matter is applicable to various types of substrates as distinguished by the source and the shape of the substrates. For instance, it is applicable to at least two categories:
A) Flat wafers from ingots obtained using either Czochralski (CZ) or float zone (FZ) techniques (textured or untextured) or multi-crystalline cast ingots which are obtained using techniques such as wire-sawing, polishing, lapping, etching, or ion implantation (Hydrogen or Helium) based slicing of bulk ingots.
B) Epitaxially or polycrystalline grown substrates which are produced directly using any of the precursors used for depositing silicon such as silicon tetrachloride (STC), trichlorosilane (TCS), dichlorosilane (DCS), or Silane. These substrates may or may not have the dopant diffusions as is customary in a finished solar cell such as back surface field (BSF), bulk doping, front surface field (FSF), and emitter, as part of the epitaxial growth process. The method is widely and equally applicable to any of the several combinations of doping which form a solar cell. For example: (1) n-type bulk doping using phosphorous with a boron-doped p-type emitter and (2) p-type bulk doping using boron with a phosphorous-based n-type emitter. A preference has been noted in the use of n-type doped base layers with p-type emitters as these n-type based solar cells tend to exhibit the light induced degradation effects often seen in silicon solar cell material with a boron doped p-type base.
Several embodiments of fabricating the epitaxial substrate are possible. In one embodiment the epitaxial substrate is grown on top of a sacrificial layer on a mother template and is subsequently dislodged. The mother template is then reused (for instance by residue removal, optional reconditioning by e.g. bevel or area lapping or grinding, cleaning and re-formation of the sacrificial layer) several times to grow more epitaxial substrates. The sacrificial layer has to transfer information about the crystalline structure in the mother template to the epitaxial layer and is removed selectively with respect to the substrate and the mother template. One specific embodiment of the sacrificial layer is porous silicon, whose porosity may be modulated to achieve both the aforementioned critical functions.
Within the epitaxial substrate embodiment, several possibilities distinguished by the underlying, starting, mother template are possible. Although, not limited to these, a few of these possibilities are described as examples below.
In the above description, the release layer is comprised of porous silicon and the epitaxial layer is silicon as well. However, the disclosed subject matter is also applicable to the use of other release layer methods such as those generated by the implantation of hydrogen to form a cleave release region or the use of a laser that is focused inside the silicon to form a release or cleave region. In addition, the disclosed subject matter is also applicable to active absorber material other than silicon, including hetero-epitaxial combinations such as silicon with germanium, carbon or mixtures thereof, as well as materials from the III-V family such as gallium arsenide (GaAs), which can, for instance, be grown on top of germanium or a graded silicon germanium region which in turn is grown onto a porous silicon layer and which is designated to allow for lattice matching between GaAs and underlying silicon in order to grow good quality GaAs on essentially a silicon substrate with a release layer.
For the aforementioned substrates (flat wafers from ingots and epitaxial substrates), if the thickness of the deposited silicon is substantially thin or if the processing conditions are not compatible with the materials used for permanent reinforcement, it may be necessary to introduce a carrier to temporarily support the solar cell during processing until it is suitable for permanent reinforcement. Possibilities for temporary reinforcement include (but are not limited to) mobile carriers utilizing electrostatic, vacuum, or combination of electrostatic and vacuum methods etc. These structures will substantially strengthen and reinforce the thin substrate thus ensuring a high manufacturing yield. However, the disclosed subject matter provides a permanent reinforcement for use in solar module panels together with the front or back contacted thin Si solar cells.
Further, in the case of an epitaxial substrate formed on a template with a release layer, the disclosed subject matter provides for continuous thin substrate support during the manufacturing process. For example, in the early, preferably dry and potentially high temperature process stages using the template as reinforcement and in the later, preferably low temperature and potentially wet process stages, using backplane reinforcement structures and methods. Thus, the disclosed subject matter entails materials, designs, structures and methods to fabricate permanent support structures that enable the manufacturing of solar cells with thin active absorber layers (“thin solar cells”) and the structures of the resulting solar cells. Further, the disclosed subject matter provides for the integration of permanent support structures within various embodiments of cell manufacturing flows—the backplane structures, materials and methods disclosed may be employed for fabrication of photovoltaic solar cells that utilize high efficiency thin film solar cell substrates.
An advantageous design for the disclosed thin film solar cell structures are back junction, back contacted cells where the reinforcement is applied onto the side that contains the back junctions and back contacts. However, cell designs with at least one polarity of the contact on the front side may also be supported using the disclosed subject matter in combination with low temperature processing, typically below 250° to 350° C., is used to manufacture the front side contact if the front side contact is manufactured after the attachment of the reinforcement. An effectively low temperature process may utilize laser annealing that only heats the front surface while keeping the back surface cold enough for the backplane material to sustain the process. Methods for front side contacting include, for example, the formation of front side lines of Al or other metals with subsequent laser annealing for contacting and optional emitter junction formation, and contacts at the front side or patterned implantation followed by laser or other substantially low temperature annealing for forming the junctions, followed by a suitable metallization scheme such as Aluminum deposition, either patterned using deposition, printing or spraying, or unpatterned with subsequent patterning.
The intent of this disclosure is to enable the reinforcement and thus high yield manufacturing of many types of thin film structures while the focus of the disclosed embodiments is on presenting solutions for the often more challenging process of manufacturing back-contacted cells. Examples for viable structures and methods for manufacturing the thin film solar substrate (TFSS) up to the point of metallization are described generally in the process flows of
The process flow starts with a cleaned re-usable semiconductor wafer, called a template. A release layer, such as porous semiconductor material, is then deposited on the surface of the template. In the case of silicon wafers, this may be porous silicon. The porous silicon layer may comprise at least two zones of different porosities, where the top layer is preferably of a lower porosity than the bottom layer. The bottom layer serves as a designated weak layer while the top layer is reflowed in a subsequent bake step in an epi reactor prior to the silicon layer deposition and the reflow reconstructs the surface to provide a seed surface to enable epitaxial deposition. In the ensuing epitaxial deposition, which may be carried out at high temperature using at least one silicon containing gas such as trichlorosilane (TCS) mixed in hydrogen (H2), a thin layer of semiconductor, for instance silicon, is deposited on top of the porous layer on top of the template. This layer may serve as the thin active absorber layer, or light capturing layer, for the solar cell. Shown, the active absorber base layer is an n-type layer, formed by, for example, the addition of phosphine (PH3) during the deposition step. The PH3 may be optionally diluted in hydrogen. Graduation of PH3 flow during the deposition may be employed to achieve doping gradients in the film where desired.
After epitaxial deposition, further steps comprise the formation and structuring of the emitter layer, e.g. by atmospheric pressure chemical vapor deposition (APCVD) of borosilicate glass (BSG), and laser ablation of BSG where desired in order to make openings for base contacts. A subsequent optional step includes deposition of undoped silicate glass (USG), followed by laser ablation in order to later generate a separation zone between base contact and emitter. Then, phosphorus-silicate glass (PSG) may be deposited as a precursor to later form a highly n-doped base contact. Undoped layers may be used for separation of each layer where needed. A subsequent thermal drive-in step, optionally with oxidation in at least one step to form a good interface with the semiconductor (such as silicon), may be used to drive-in the doped diffusion profiles. A laser may then be used to ablate the dielectric in desired contact areas, which enables contacting with a subsequent metallization step. Suitable lasers for above ablation processes include picoseconds lasers and especially picoseconds UV lasers which may cause little or no subsurface damage to the underlying semiconductor.
It is to be noted that after the backplane attachment and structuring processes disclosed herein, the template may be re-used after the release of the backplane reinforced structured thin film solar substrate (TFSS) from said template. This re-use requires clean-up steps to render the template ready again for the next round of porous layer formation and epitaxial deposition.
Generally, the disclosed backplane structures utilize orthogonal current extraction. In back contacted solar cells, current typically needs to travel along long distances as both contacts are on the same side—thus a large area planar electrical contact may not be easily realized. To reduce electrical shading metal finger pitch typically needs to be kept small while finger height needs to be substantial, which often results in a costly and high stress process for metal finger formation on back contacted solar cells. Such high stresses can even prevent the move to larger substrate sizes for conventional back contact cells.
The disclosed subject matter provides a solution to high cost and high stress processes associated with back contact metal finger formation through the use of orthogonal current extraction. Metal fingers on the thin solar cell are kept thin, the current is then guided up through contacting dots, which can be comprised of conductive adhesives such as, but not limited to, silver epoxy or of solder, or of deposited or printed next level metal. The remainder of the area or most of the remainder around the contacting dots is covered by a printed dielectric adhesive or by a dielectric adhesive sheet, providing electrical isolation against the backplanes. Such dielectric sheets can for instance consist of prepreg that is laminated to the thin film solar substrate (TFSS) and later has vias drilled into it in areas where contact between metal layer 1 and metal layer 2 is to be established.
Current is then extracted orthogonally where large emitter and base fingers in the backplane structure contact the respective small emitter and base fingers on the thin film solar cell substrate (TFSS). By the use of this orthogonal transfer, the individual distance that the current has to travel within the thin metal layer on the cell is miminized or kept relatively small thereby strongly reducing the electrical series resistance experienced in the structure and in turn enabling thin metal fingers on the thin solar cell.
While the first and second layer metal lines are typically orthogonal to each other, some modifications may be utilized. In the case where a bus bar is to be implemented on the cell as part of the second layer metal, in a normal completely orthogonal arrangement the area under the bus bar would suffer from substantial electrical shading, as in the region of the respective bus bar via drilling to contact the opposing first metal layer line is precluded by the presence of the bus bar and opposing carriers would not get collected or would have to travel far within the active absorber area (e.g. silicon) to be collected by the closest finger of their respective second layer metal. Here, it may be advantageous to have, underneath the bus bar, a pattern of interwoven first metal lines that connect either directly to the bus bar (the metal line having the same polarity as the bus bar) or for the other polarity to the closest fingers of the second metal layer. Using this architecture, electrical shading is reduced greatly and only the series resistance of the first metal layer contributes to additional losses compared to the situation in the bulk of the cell where the first metal layer lines and second metal layer lines are arranged orthogonally and in alternating polarity, respectively.
An explanation in broad and general terms the variations of embodiments of the various backplane flows follows disclosing typical layers, structures, materials, functions and unit processes that are associated with the backplane reinforcement flow. Importantly, not every embodiment of the backplane or processing methods requires all described layers and functionalities.
Several cell layers and structures may be associated with the disclosed backplane flows and with layers that immediately influence the backplane structures and methods. In the following, such layers and structures are listed and described in general in an order starting with the layers and structures closest to the thin film solar cell (TFSS) and ending with the layers on the backside of the cell (closest to the layers that are in contact with the module).
On the TFSS are suitably patterned dielectric layer or layers on top of the thin film solar substrate that are deposited or grown on the thin film, for example while the thin film is on the template. Underneath the dielectric or dielectrics, are zones of emitters and bases (emitter and base regions) and base contacts of the thin film substrate. One of the functions of such layers is to provide dielectric isolation between terminals and of metal lines from active areas of the thin film solar substrate, and secondly to be used as dopant sources for forming emitter and/or base contacts. Methods and embodiments providing the dielectric layers include grown or deposited dielectric layers such as undoped or doped glasses with optional subsequent dopant drive-ins, thermal anneals, and/or thermal oxidations.
The contact opening of at least one of the zones of emitter and/or base (emitter and base regions) using suitable patterning methods, such as laser ablation, etch paste, lithography, and etch is employed to provide local access to the doped zones with suitable contact areas. Contact areas need to be optimized with the parameters of best contact and shunt resistance, as well as with providing a minimized area with high recombination rate for carriers. Depending on the process flow, such contact opening may be performed later in the cell process flow, but generally is performed prior to forming first layer metallization.
Metal contacts (herein also referred to as the first metallization layer or first electrically conductive interconnect layer) are deposited on the TFSS to at least one or both of the emitter and base areas. The first metallization layer (or layers) may be patterned as metal fingers, such as interdigitated metal electrodes, on the TFSS (while attached to the template if template processing is used to form the substrate) which may deposited using PVD or other methods such as printing of patterned metal layer or layers. The base and emitter metal contact layers forming the first metallization layer are suitably isolated from each other and can be patterned using laser ablation, printing, lithography and etching, etching pastes, or other methods. A function of the first metallization layer is to provide contact to at least one of emitter and base areas of the cell and to route current from the cell terminals (emitter and base) to the next backplane layer/level; and second to provide surfaces that can give low contact resistance, such as aluminum which gives low contact resistance to both p- and highly doped n-type material in silicon whereas optional materials on top of aluminum may provide good contact resistance to the next layer/level. Third, the first metallization layer may provide a surface that can later be plated, such as a Sn or Ni or NiV or Ta coated surface, if the next level of metal is applied using plating. Fourth, the first metallization layer may provide a good stopping layer in the case that a dielectric layer deposited, such as by lamination, on top of the first metallization layer is to be drilled, using a laser drilling for example. Example methods of depositing a first metallization layer are PVD, evaporation, screen printing, ink jet printing, and aerosol jet printing. Example materials and embodiments are PVD layers or stacks such as Al itself or AlSi1%, Al with Ni or NiV, and optionally Sn or SnAg, Al with Ta or Pd or Ag. Thick Al or AlSi1%, such as layer greater than 0.5 micron thick, may serve as particularly suitable reflectors in the far infrared and thus act as stopping layers for CO2 laser based drilling of subsequent via holes in further cell processing. Other examples are PVD stacks with additional locally printed pads for better contacting with the next layer, to provide better margins to stop laser drilling, and provide mechanical locks to prevent aligned pre-drilled dielectric from shifting during lamination. Such pads may consist of pastes containing Al or Ag such as conductive epoxy. Alternatively, printed metal or metals may be used, such as printed Al or printed Al with small amounts of Si (AlSi) or combinations thereof, optionally also with local caps of Ag for better contact and better reflectance for ensuing laser drilling process. For such printed layers, the metal may be printed in fingers, interrupted fingers, or dots that are aligned to subsequent metal vias. Refractory metals such as Ti, Co or Ni that may be printed, for instance using ink jet or screen printing and which can locally form silicides when suitably heated, may also be used as first metallization layer or part of a first metallization layer. Such silicides may be optionally used underneath other metals, for example underneath printed Al or AlSi.
The next level dielectric layer (herein referred to as the second dielectric layer) acts as an adhesion layer for the TFSS and additional components of the backplane. The second dielectric layer also serves as an isolating dielectric which enables the orthogonal arrangement between metal fingers on the TFSS (the first electrically conductive interconnect layer) and the large metal fingers on top of the dielectric or within the backplane (the second electrically conductive interconnect layer). The second dielectric may also provide protection from chemical attack to the backside of the TFSS along with the first electrically conductive interconnect layer and first dielectric layer in processing embodiments where the second dielectric acts as the outermost layer of the structure at the time of wet processing, such as texturing and post-texture cleaning. The second dielectric also serves to provide mechanical stability to the backplane reinforcement for the attached active absorber layer, comprised of the thin film silicon solar cell substrate. Deposition methods for the second dielectric layer include, a pre-drilled dielectric sheet which is attached using a lamination process, a post-drilled sheet which is attached using a lamination process and is undrilled at the time of lamination and of subsequent wet processing and drilled after said wet processing steps, and a patterned dielectric adhesive which may be, for example, printed either onto the TFSS surface or on the backplane side of the TFSS-backplane structure. Example materials for the second dielectrics layer include first dielectric sheets such as prepreg, EVA, Z68 PE sheet and others which are patterned via pre-lamination or post-lamination drilling (in the case of prepreg preferably using a laser such as for example a CO2 laser). Alternatively, punching or stamping processes may be used for perforation of such sheets. Printed dielectric adhesives, such as thermo-plastic or B-stageable materials, may also be used as a second dielectric. Other example second dielectric layers include a sandwich structure of sheets of dielectrics, such as prepreg, EVA, Z68, or others, covered with a protective material such a Tedlar, Mylar, Teonex such as Q83 or other PEN or PET materials, where at least one of the layers is continuous to secure protection and at least one or all of the other layers are either continuous (in the case of post lamination drilling) or perforated in the case of pre-lamination drilling. The latter allowing for easy low contact resistance access to the underlying metal fingers. Another example second dielectric layer includes a randomly or regularly but unaligned perforated sheet, such as in the case of the immersion contact bonding structure.
It is to be noted that in the case of performing subsequent wet processing steps in an embodiment where there is no wet chemistry contact to the back side of the backplane-reinforced TFSS, a protective sheet may not be necessary during the wet processing and, also, drilling of access via holes may then be performed even at any point prior to the wet processing.
The via hole (also referred to as contact openings) in the attached dielectric provides access between the underlying first level metal fingers on the TFSS (the first electrically conductive interconnect layer) and the next level metal on the backplane (the second electrically conductive interconnect layer. Drilling the via holes after lamination or keeping the via holes covered with a protective sheet provides, as in the case of the Pluto structure described below, protection of the underlying metal on the TFSS during the texture, clean, and front surface passivation steps and thus enables immersion of the reinforced structure into wet chemical baths. Via holes (contact openings) in the dielectric may be formed by drilling, preferably using a laser as described above, or in the case of a printed dielectric adhesive leaving areas unprinted where a via hole is desired.
The next level metal routes the current through the vias and either onto the next level of metal on the backplane or directly to cell-to-cell or to module connectors depending on the backplane structure and process embodiment. Typical materials and embodiments of via filling materials are conductive epoxy or more generally conductive adhesives which may be either stencil or screen printed into the vias or applied prior to applying a pre-drilled dielectric sheet. Typical materials also include solder or solder pastes, such as those containing Ag, Cu,Sn, Bi or mixes thereof, including SnBi mix which may be particularly advantageous due to its low solder apply temperature of approximately 140° C. which is in the same range as or even lower than attractive backplane dielectric processing temperatures.
Subsequently to at least partial via filling or even omitting via filling, the next level metal deposited serves to provide large width metal fingers on top of the dielectric (herein referred to as the second electrically conductive interconnect layer or second metallization layer). For the more desirable case where no additional via filling metal is used inbetween, said second level metal directly is used to make contact to the underlying first level metal in the drilled vias. Such large metal fingers may consist of plated metal, optionally with a prior blanket PVD seed which is then covered by a patterned and later removed dielectric print for emitter metal from base metal separation. The latter print is later removed and an etchback process may be carried out to remove the blanket seed metal. For plated fingers, optionally the seed may also be printed or deposited using a shadow mask, such that it is pre-patterned. Depending on the existence of a bus bar structure, a larger amount of contacting points during the plating of the finger structure may be employed. Rather than being deposited or built up by methods such as printing, spraying or plating, the large metal fingers (the second electrically conductive interconnect layer) may also consist of preformed fingers made of for instance solderable aluminum, i.e. Al with a thin coat of Ni, NiV and optionally Sn. For structural strength, such finger lines may be interlocked or may be tiles which may also be optionally interlocked. Another example for depositing the orthogonal fingers includes sprayed metal, such as the use of flame or thermal spraying. Yet another option is a flexible printed foil which can locally be attached to the underlying vias by the solder or conductive adhesive points—such printed foils are not unlike those used for flex circuitry or flex connectors.
The metal fingers embodiments may optionally include a bus bar design. If not, then subsequent contacting through soldering or printing of conductive adhesive may connect the backplane and with it the cell to the module. It is to be noted that for some embodiments, the printing of conductive material into the holes is not required and that rather, after the optional clean-up of the drilled via holes, optionally together with a removal of native oxide on metals, the seed layer for the next level metal (the second electrically conductive interconnect layer) can be applied directly into the opened via holes.
Optional additional layers, particularly as applied to the oasis and hybrid structures described herein, include:
Also, the structures, materials and methods covered in this disclosure allow for a multitude of potential implementation variations which cannot all be explicitly described. It is the intention of this disclosure to cover all such implementations, if at least one part of the presented embodiment is implemented and utilized in a comparable fashion. In addition to final structures, specific method or process flows along with some variations to achieve the final structure may be shown for each of the cases. The process flows and structures below assume a very thin silicon needing carrier support as this case is more general. Thicker silicon not requiring carrier support is a specific case of the more general case presented here.
For descriptive purposes, the present application provides several backplane and process flow embodiments, including: the Pluto structure, the Oasis structure, the Hybrid structure, and the immersion contact bond structure. However, the backplane structural and processing elements disclosed may be used in any number of combinations and variations by one skilled in the art.
A multitude of similar structures may be envisioned with this scheme in mind, for instance structures that consist of more than one metal to form the contact to the on-TFSS metal fingers. Common to the structures shown is a two-layer metal design where the outer, second layer metal (metal 2) is arranged essentially orthogonal to the inner, first layer metal (metal 1). Further, the dimensions of the second layer metal are much larger and easily manufactured. A second group of structural embodiments, hereinafter referred to as oasis, is defined by the following two concepts. First, at least at some point in time the structure relies on orthogonal or quasi-orthogonal current transfer, a concept that is described in the following structure attributes: Orthogonal finger design for orthogonal current extraction including: 1) interlocked fingers to provide structural integrity and keep the cell-backplane arrangement from bowing or warping, and 2) stress relief cuts in fingers; and Tile design for orthogonal current extraction including: 1) segmented fingers (tiles) to reduce the CTE mismatch related stress between the thin solar cells and the backplane material in the direction of the fingers, and 2) interlocked tiles to provide structural integrity and keep the cell-backplane arrangement from bowing or warping. The second oasis characteristic is that at the time of the texture and passivation processes in the solar cell manufacturing process, at least one additional layer of metal next to the metal layer that makes the contact to the base and emitter in the semiconductor is already integrated into the backplane. Thus the oasis backplane is an integrated structure with two metal layers, metal 1 and metal 2.
An important structural differentiation may be drawn between a single backplane lamination process where all components are laid up together and laminated at the same time, and a process where the second layer metal is laminated into a flat backplane and is embedded into the surrounding dielectric prior to a second lamination to the TFSS which, at that point, may be supported by the template by means of a release layer of suitable strength. In the latter case, the a backplane may be manufactured, stored and staged separately from the TFSS with potential benefits to cost and logistics. Also, in this case, there is an option: either one or both the dielectric adhesives that provide adhesion between the TFSS and the backplane and the conductive material that is used for the contact between the on-TFSS metal fingers and the large metal fingers that are part of the backplane may be applied either to the backplane side or to the TFSS side prior to lamination.
An embodiment of a third group of structural embodiments, hereinafter referred to as a hybrid structure, are depicted in the top and side views of
The hybrid structure of
An embodiment of a fourth group structural embodiments, hereinafter referred to as an immersion contact bonding structure, are depicted in the cross-sectional diagrams of
The front end of the process may begin with a wet clean of the re-used or fresh template, followed by the formation of the release layer, for example a bilayer of porous silicon with low porosity on top of high porosity. Subsequently, the active absorber cell area is deposited, for example using epitaxial deposition of silicon using trichlorosilane (TCS) gas and a dopant, for example phosphine (PH3) to generate an n-type base, in hydrogen. Optionally such a deposition may be arranged to have more than one distinct doping concentration region as a function of depth. Subsequently layers of doped glass are deposited, for example using atmospheric pressure chemical vapor deposition (APCVD), followed by patterning processes using a picoseconds laser.
In one embodiment, the first glass layer contains a lighter amount of emitter dopant (boron in boroslicate glass—BSG), optionally capped with a layer of undoped silicate glass (USG), in order to form a less heavily doped emitter, followed by ablation of the borosilicate glass in areas where a more heavily doped emitter is to be generated which in turn serves to provide a low resistance contact to the emitter metal 1. After this, a more heavily doped BSG layer (BSG2) is deposited in the region of the metal 1 contact to the emitter, optionally with a USG cap layer. Then the area for the base contact is ablated, preferably using a picosecond laser. Subsequently, the phosphosilicate glass (PSG) layer is deposited which serves as the dopant source for phosphorus which in turn is to generate the heavily N+-type doped base contact region to form a low resistance contact to the base. In a subsequent step, the profile is thermally annealed, thereby driving in the junctions. Optionally the annealing ambient may be chosen between a neutral and an oxidizing ambient, the latter to serve to form a high quality interface at the backside to enable low back surface recombination velocity. As a next step, the contact areas to emitter and base doped junctions are opened to enable contacting of the subsequently applied metal 1 layer, where metal 1 may be for example a printed layer or sequence of printed layers, for example consisting of aluminum (Al) or AlSi to form a low resistance contact of metal 1 to the junctions while avoiding spiking through the junctions. The printed metal layer or layers may optionally be thermally annealed prior to the next steps. At this point, the front end of the device may be considered completed, and backplane related steps may begin.
The next steps may comprise either single step lamination or backplane preparation, followed by lamination to thin film solar substrate on template. Such lamination is preferably carried out in a vacuum and at elevated temperatures to cure the laminate. A pressure is applied to secure uniform and reliable adhesion. Pressure can be variable throughout the thermal and vacuum cycle that the structure undergoes. Various embodiments with respect to the lamination process and tool are possible, including stacking multiple templates with laminates, separated by release sheet and pressure distributing buffer layers or having multiple templates laminated side by side in a large tray arrangement. Such large tray arrangements may themselves be stacked into commercial laminators with multiple slots (daylights) which are all heated, typically from above and below or from one side only. Hydraulic press elements may be used to apply the pressure. Sufficiently chosen thick sheets of cellulose or rubber or other suitably compliant sheets may be used to overcome pressure differentials due to local stack height variations or due to different template heights, which respectively may be caused by templates of differing age or re-use count being laminated at the same time. It is to be noted that prior to lamination of the backplane material (e.g. prepreg) it may be advantageous to apply an additional adhesive, as described earlier in the same disclosure.
The next steps involve post-lamination release of thin film solar substrate (TFSS), laminated to backplane, from the template. Either prior to the lamination or prior to the release of the TFSS it may be advised to outline the shape of the TFSS with a laser cut either through the epitaxial film outside of the backplane or through backplane and the epitaxial fillm. Care is to be taken to minimize template damage from cutting past the epitaxial layer and into the template. Laser based technology called thermal laser separation may be used in this cutting process in which a heating laser beam is immediately followed and traced by a cooling spot, provided by a jet of cold liquid or mist such as water or a cold gas such as helium for example. By doing so, a cleave may be initiated through the silicon which in turn terminates at the interface between the TFSS and the template, in the region of the release layer. Next are edge preparation steps including trimming (cutting) the edges, and optionally decoupling the fragile thin film from the edge of the reinforced thin substrate. The outer edges of the device may be cut to size by mechanical trimming, such as shearing or stamping, or by laser trimming. The corners of the device may be chosen to be cut using a chamfer or otherwise suitable shape to dull the corners and make them less prone to handling damage throughout subsequent process steps.
Next are wet (or optionally dry) texturing steps followed by a post texture clean and drying. Texturing may be preceded by one or more surface preparation steps, for instance by a mechanical roughening step, such as grit blasting, to aide in the later formation of proper pyramids, or by surface treatment such as organic residue removal or forming a thin chemical oxide to aide texturing.
Next are passivation steps at low temperatures with an optional dry bake with or without vacuum assistance prior to passivation layer deposition. Example viable passivation layers for low temperature being amorphous silicon (a-Si) or silicon oxide or sub-stoichiometric silicon oxide, silicon oxy-nitride or silicon nitride. Alternatively, the passivation layer such as a chemical oxide or oxynitride may be deposited in a wet process tank.
Next are anti-reflective coating steps using materials such as silicon nitride, Al2O3, or other suitable dielectric, preferably with very low absorption in the wavelength range capable of generating carriers in silicon and with suitable built-in charge to repel the respective minority carriers. Optionally, forming gas or other thermal anneal may be used to improve the front surface passivation. Optionally, a laser anneal from the front to improve the front surface passivation and also optionally bulk quality and back surface passivation depending on the laser processing parameters and penetration depth of the chosen laser wavelength or wavelengths. The next step consists of opening contacts to the next buried layer of the cell terminals. Depending on the chosen backplane structure, the next buried layer may be, for example: the patterned metal layer on the cell that was deposited onto the cell and prior to lamination; contact pads that were deposited on said patterned metal layer only in areas where contact access is needed, or; a buried next level routing of metal, optionally essentially orthogonally arranged with respect to the original metal connectors. This contacting process may be performed using laser or mechanical hole or slit drilling into the protective/dielectric layer. Optionally, prior to this step the surface is protected by a sheet or material that prevents plating or contamination of the front side during the later plating process
Subsequently, the underlying metal is contacted through the contact opening by one of several optional means, for example: an optional surface preparation step to promote adhesion and /or platability of a seed metal; deposition of a seed metal by PVD, plating, printing including screen printing, ink jetting, aerosol jetting, stencil printing, or spraying such as flame or thermal spraying; in the case of a non-patterned deposition, a patterning step, such as printed resist, or; plating in non-resist covered areas, followed by resist removal and seed layer etchback (all of which are processes common in plating technology) Typical metallization materials include, for example, a starting layer of nickel, followed by copper and ending with tin or other solderable capping layer, and printed layers may contain suitable metals, including silver and alloys, nickel, copper, aluminum and tin. In the case of a PVD seed layer, choices include but are not limited to Sn, Ni, NiV, Al, Pd, Ta, Cu, Ag or alloys.
After optional testing and binning, contact to the solar module may be readily achieved using solderable stringer ribbons for example. The stringer ribbons may be for instance straight or dog-bone shaped and may optionally contain black or blackened areas, for instance in areas visible to the module customer, in order to retain an all-black appearance as well as to optionally serve as electrical isolation where needed. A final encapsulation is performed, for example, using common solar backside encapsulants.
The following disclosure relates to exemplary structures and process flows presented for descriptive purposes. A main difference between the pluto and the oasis structure is that at the time of wet processing or other form of texturization of the front surface of the epitaxial thin film, the pluto reinforcement structure does not contain any other metal structure except for on-cell metal emitter and base contact fingers, henceforth called first layer metal, whereas the oasis structure contains at least part of a second layer metallization.
The on-cell metal may be deposited using either a blanket deposition technique such as physical vapor deposition (PVD) or evaporation (e.g. via electron beam or thermal evaporation) with subsequent patterning using for instance laser ablation or direct patterned deposition of the metal or metal precursors using screen printing, typically with a subsequent thermal step for baking, sintering or drive-in. Importantly, the description below holds similarly for a PVD and for an evaporation based process. In the following, wherever not otherwise noted, PVD is used to represent all other large area blanket deposition type processes. Such blanket films may be deposited over the whole epitaxial cell structure on the template or a shadow mask can be implemented during the deposition to avoid deposition where not desired, for instance at the very edge of the template or outside of the active structure. Shadow masking may also be used to define the active or metal contacted area.
A schematic representation of examples for different embodiments of process flows of the pluto and oasis structures as well as pluto and oasis hybrid structures are depicted in
For all the presented backplane embodiments, a viable processing flow and structure prior to the backplane part of the process has been disclosed herein. For example, in one starting substrate embodiment an epitaxial cell structure which is supported by the template has the contacts opened to the semiconductor areas of emitter and base. The contact to the base may have a highly doped contact area for low contact resistance, while the emitter is optionally a selective emitter with a highly doped area around the contact to the primary metal. These contacts may be opened using various techniques, as shown in the example embodiment of
Subsequently the first layer metal is formed. This layer is referred to herein as the first layer metal even if it consists of several metals or several structures within. In one embodiment, the first metal structure is preferably aluminum or aluminum with a small amount of silicon to reduce spiking to ensure an ohmic contact to both p-type and n-type regions. If PVD is used to deposit the material, then the choice is typically that of a single material such as aluminum since the deposition is commonly performed for the whole cell area and structured later. The blanket deposited material is later patterned. Several options for patterning exist, in one example embodiment the metal is structured using laser ablation. Several options of laser ablations are possible, such as using picosecond laser ablation. The metal is preferably patterned such that alternating lines of emitter and base contact metal is formed, on top of the alternating lines of emitter and base contact openings.
If instead of PVD, a printing process is used for the first metal, such as screen printing or aerosol printing with—depending on the material—subsequent thermal processing, then aluminum or aluminum with a small amount of silicon to reduce spiking for both contacts may be used or aluminum for the p-type region contact and another metal, such as silver or others for the n-type region contact. The material of choice will also depend on its performance as a minor. Good mirror performance (specular or Lambertian) can improve the overall light to electricity conversion, especially for the longer wavelengths which is important for cells using thin silicon. Alternatively, silicide forming refractory metals may be used as first metal layers as well for low resistance contacts; however, their minor quality may not be adequate and the process is more complex.
Both PVD and printing processes for metal allow optionally for the deposition of stacked metal layers. In a PVD based process, aluminum deposition may be followed by an adhesion improving nickel vanadium (NiV) or nickel (Ni) layer, Ni often preferred because of the lower stress. This may be followed by a tin (Sn) layer, which allows for plating further on in the process flow. An alternative to this stack is Al, followed by tantalum (Ta). Other layer combinations are also possible. For simplicity of processing and for good performance as a minor layer for the later introduced process of laser via opening, Al alone may be used as the first layer metal. When a plated layer is later used for the next metal layer and aluminum is the only base metal layer, then aluminum needs special surface treatment, such as zincation or double zincation.
The metal or metal stack needs to be chosen with several properties in mind, namely it needs to provide good adhesion, firstly to the underlying oxide or glass layer on the epi, secondly between the metals of a stack, and thirdly between the top metal on the stack and the backplane or, to be precise, to the adhesive component of the backplane.
For that, if aluminum is the first deposited metal and if the glass layer near the top also serves as a dopant source, for instance phosphorus silicate glass (PSG) as the n-type dopant source for the base contact, then it is typically helpful to retain the phosphorus content in the PSG at or below approximately 6% and/or cap the PSG layer with an undoped glass layer. Optional treatments of the metal during and after deposition can serve to improve subsequent adhesion. Such treatments include thermal annealing, laser annealing, surface roughening and others. For deposited materials, aluminum tends to provide good adhesion also to backplane materials presented here.
Printed metals normally require one or more thermal steps, for baking out solvents and optional sintering and/or drive-in steps. If more than one metal is printed, it is conceivable to do one thermal step for all or to have one or more thermal steps in between printing of metals. Printing of metals also allows for selective thickening of metal in areas where beneficial, such as in areas that serve as contact areas to the next layer metal at a later point in time. One method for selective thickening when using screen printed metal is to do more than one print while using different screen structures.
The surface of the deposited metal or metal stack is optimized to allow for a large process window for the metal ablation which is employed in conjunction with a PVD based process. For both PVD and printed metals, it may be beneficial for the top metal of the metal stack (or the surface of the metal)—if only one metal is used for the first metal layer—is chosen or engineered such that it provides sufficient thickness and high reflectance to a laser beam that is employed at a later point in time to drill vias through a backplane material, where these vias have the function to provide next level metal contact to the first metal layer. For such via drilling, CO2 lasers may, for example, be used and aluminum, copper, silver and several other metals tend to provide good reflectance in the long infrared wavelength range of the CO2 lasers.
In the case of printed metals, it may be beneficial to locally thicken the metal and/or to locally add another metal print in the area of the future vias. This may serve both to increase the process window for via hole drilling as well as to provide a good metal area for the second layer metal to make contact to.
Prior to lamination, which is the next main process step after the first layer metal and its patterning and treatment, it may be advisable to provide the epitaxial layer with an oversized cut while it is on the template, so as to provide a known breakage location during the release of the backplane reinforced epitaxial cell structure.
Process flow for the pluto structure at lamination. The material chosen as the backplane material to be laminated to the thin film epitaxial solar cell structure (TFSS) which contains the patterned first layer metal is chosen with several important properties in mind, some of which are presented as follows: first the material has to be suitably matched with respect to its coefficient of thermal expansion with respect to silicon. Secondly the material has to either by itself, or with the help of a blanket or patterned adhesive layer, exhibit good adhesion to the TFSS and provide this adhesion throughout the temperature, pressure and humidity ranges that are required for the manufacturing of the backplane reinforced TFSS into a finished solar cell and that are required of solar cells in modules throughout their useful lifespan. Thirdly, the backplane reinforced TFSS needs to be able to withstand the chemistry, the gas environment and all handling steps throughout the manufacturing into a solar cell and into a module. Fourthly, the material needs to be cost-competitive, non-toxic and readily available.
The foregoing description focuses on the embodiments of a prepreg backplane in conjunction with silicon as the active absorber material. The same concepts apply for the use of Silicon with heterojunction materials such as Ge,SiGe, SiC, SiGeC, a-Si or a-SiGe, as well as for the use with III-V materials such as GaAs or the combination of GaAs with Si or Ge or its alloys. An attractive example material family to fulfill such requirements are prepregs that are used in similar formulations in the printed circuit board industry. Such prepregs are available with woven and non-woven fibers of different kinds, such as aramid, Kevlar or glass fibers in a matrix of resin.
Such sheets are laminated to the TFSS while it is on the template. The reinforcement can consist of a single sheet or more than one sheet, where different pretreatments or different fibers, fiber content percentage and resin type and content percentage are all employed to optimize adhesion as well as CTE mismatch.
As pointed out earlier, prior to lamination of the prepreg it may be advantageous to print an additional adhesive onto the cell. This adhesive may be thermally or UV curable and may cover either the whole area (as shown in
Other backplane reinforcement materials options include materials similar to those used in solar module encapsulation, such as EVA or Z68. In examples below, whenever prepreg material treatment is mentioned explicitly, this should be understood to cover the use of other suitable backplane materials as well.
The materials chosen can include, depending on process flow and material formulation, the option of having very compliant or flexible cell structures, as well as enabling non-flat cell surfaces which enable further architectural solutions for applications such as non-flat solar modules.
Optionally, the prepreg area in contact with the TFSS may be covered at the time of lamination using a protective sheet which suppresses moisture or chemical uptake of the prepreg sheet during subsequent processing of the backplane reinforced TFSS such as texture and post-texture clean, as well as plating and plating surface preparation. Examples for such cover sheets are mylar or other PEN based materials which are chemically resistant.
Typical parameters to govern the lamination process itself are the use, extent and timing of pressure, temperature, temperature differential and ramping rates, resin and fiber type and content percentage, an optional pre-tacking or pre-treatment of the prepreg lamination sheet or sheets, the process time and time at temperature, the application and level of vacuum. Full curing of the prepreg through the lamination or at least prior to exposing it to water and wet chemistry may be advantageous.
After cooling down from the lamination step, the laminated TFSS on template is unloaded from the lamination tool and subsequently released from the template whether mechanically or other means such as etch. In general, the top side of the backside reinforced TFSS and the template are chucked and separated, either by the use of direct pulling, by peeling or by a pulsated pulling force, such as the force generated by the pulsated application of a vacuum on one or both sides of the structure.
The release may be optionally be assisted by the use of sonic or ultrasonic mechanical force, such as that administered by a piezo actuator that is coupled to the plates that are used to chuck the topside of the reinforced TFSS and/or the template. Also, immediately prior to the release a laser cutting step can be employed around the TFSS area to provide a preferred boundary within or along which the release takes place.
After releasing, the edges of the backplane reinforced TFSS are trimmed to a size suitable for further processing or even to the final size. In general, the trimming process may be carried out by the use of either a mechanical trim by cutting, shearing or sawing or by the use of one or more lasers, such as a CO2 laser or a pulsed YAG laser or similar, or by a combination of mechanical trimming and laser trimming.
Several options exist for aligning the trimming cut to the structure, depending on the geometry and setup of the cutting and whether the cut is initiated from the backplane side or from the TFSS side. Among the alignment options are the use of visible or infrared cameras (the latter in the case that buried alignment targets are to be used. Markings in the release layer residue may reflect the process of laser processing on the backside of the TFSS—such carried-through markings can serve as directly visible alignment targets.
Before or after the edge trimming, in any case before exposure to large capacity automated wet chemistry tools and processes and depending on the material and process chosen and the resulting flatness after releasing the backplane reinforced TFSS from the template, an optional thermal and pressure treatment of the released backplane reinforced TFSS may help provide optimized flatness of the layer which is advantageous for subsequent processes. The backplane reinforced TFSS contains remnants of the release layer including the reflowed top of the release layer. This layer is highly defective and also acts as a gettering site. It is either removed in the following texture step or with a separate step prior to the texturing. Post texture cleaning is employed to remove metallic and optionally organic residue prior to passivation. There are several options for passivation and anti reflection coating that are compatible with the temperature range of the backplane materials and typically these steps can be restricted to temperatures below 200-250° C.
The initial passivation layer in contact with the textured surface may be an oxide, such as a silicon dioxide or a silicon-sub-oxide, i.e. a silicon oxide with a stoichiometric ratio between oxygen and silicon of less than two, where any such oxide layers are deposited or grown via chemical vapor deposition (CVD) or wet chemistry. Alternatively, the initial passivation layer can also be an oxynitride, for instance deposited via CVD or an amorphous silicon (a-Si) layer, which is intrinsic or optionally doped. This layer is for instance deposited using CVD or PVD. The anti-reflection coating may be performed using silicon nitride, even though aluminum oxide is also an option, especially for p-type base cells. This layer is for instance deposited using CVD.
After deposition of the top surface layer or layers or alternatively in between the depositions, an anneal can be employed, in order to reduce front surface recombination velocity (FSRV) and also back surface recombination velocity (BSRV). Such annealing is to be controlled in such a way that it is compatible with the thermal budget range that the device, especially the backplane allows. Suitable processes for such anneals include forming gas anneals or anneals in air or in an inert ambient as well as laser anneals that are tuned suitably to deposit their energy close enough to the surface and/or for short enough times so as to not exceed the allowable thermal budget of the backplane materials. An example for laser annealing processes for this application is pulsed laser anneals in the visible or near infrared wavelength range.
To protect the front surface during subsequent processes and for improved handling, it can be advisable to attach an optically transparent protective layer to the front surface. Such layers can be either thermoset or thermoplastic materials such as EVA or PE based materials such as a Z68 or z68 like material. The latter can later be reflowed and used for attachment of the cell to glass in the module assembly part of the process.
In order to prepare the structure for later second level metallization, an optional step may be inserted to prepare the backside surface for good adhesion. Such a step may comprised mechanical roughening of the surface with processes such as grit blasting or sanding. Alternatively, a chemical treatment or a plasma treatment of the surface can be employed which promotes adhesion. It is to be noted that such treatments can also be carried out prior to texturing, if desired.
The next set of process steps serves to establish contact to the first metal layer which is so far protected underneath the backplane material. This contact opening can be accomplished by laser based via drilling. An example laser employed for this process is a CO2 laser, although other lasers such as pulsed UV, visible or IR YAG lasers may be employed as well to ablate the backplane material. Holes can be drilled by direct pulsing at the same spot, using single or repeated pulses or by trepanning the area with multiple pulses, depending among others on the via hole size desired and the laser pulse energy available. For best selectivity of the laser drilling process to the underlying first layer metal, the underlying metal should be very reflective to the laser beam, such as for example aluminum and silver are very reflective at CO2 laser wavelengths. Depending on its absorption characteristics for CO2 laser wavelengths, it may be advantageous to have a dye in the material to be drilled (e.g. the prepreg). This dye is to serve to increase the drilling speed in the backplane (e.g. prepreg) and thereby increase the selectivity to the underlying metal. The dye may also have a visual function of providing cells with darker sidewalls for an overall dark appearance of the cells in the module.
The laser drilling process may also be combined with other processes, such as plasma etching of residue in the opened via hole, or organic cleanups of the via holes using for instance hydrogen peroxide, or changing between different types of lasers or parameter settings of lasers between onset and finish of the via drilling process.
A potential plasma etching, if employed, may be implemented immediately prior to the next level metal deposition, especially if this deposition occurs in vacuum such as when using PVD. The use of molecule radicals is also envisioned for a cleanup process immediately prior to next level metal deposition.
In this implementation of the process flow, the via holes need to be aligned to the underlying structures on the TFSS, especially to the patterned metal fingers from the first layer metal. If an additional metallic contact is printed on top of the first layer metal underneath the via, in order to increase the laser process window or to promote good adhesion and electrical contact to the next layer, then the via holes also have to be aligned to this layer.
For alignment structures or targets on the TFSS it is conceived that either during one of the on-template patterning or patterned deposition processes the alignment targets may be laid down, or else the structure itself, especially where it breaks the symmetry, such as the edges of the active area, may be employed to provide alignment without using up active area for alignment targets. Since the reinforcement material in general may not be transparent, the alignment to the targets on the TFSS for the via hole drilling process may be accomplished in several ways: First by having window cutouts in the reinforcement backplane material prior to lamination. These windows need to contain some resin that reflows into the windows during lamination and that is transparent enough to allow for visual recognition of the alignment targets. Or second, the alignment targets can be viewed using a camera with suitable sensitivity wavelength, such as an infrared camera that locates the targets either through the backplane material or through the thin silicon.
Using an infrared camera which locates the targets in the laser drilling tool by transmission infrared (IR) illumination through the TFSS has the advantage that with suitable instrumentation no movement has to occur between locating the targets and drilling the vias. After drilling and optional cleanup of the vias, the backside reinforced TFSS is now ready for second layer metal formation to contact the first layer metal.
Before describing the second layer metal formation, another closely related embodiment is described. It is to be noted it is also possible to drill the via holes into the backplane material prior to the lamination. This process is subsequently called pre-drilling of vias. Pre-drilling may be advantageous for the overall drilling process window. If the contact holes are predrilled, then the requirements to selectivity to underlying first layer metal material is removed or greatly relaxed. For pre-drilling, if more than one sheet of backplane reinforcement material such as prepreg is used, it may be advisable to tack the sheets prior to pre-drilling using a tacking lamination at suitably low temperature. Furthermore, during the via pre-drilling the laser can cause local curing at the edges of the vias. This may serve to reduce outflow of resin that tends to close the opened holes. Since the predrilled holes after lamination would not necessarily protect the underlying first layer metal suitably during the wet chemistry process for texturization and post texture clean, it may be useful to add an undrilled protective sheet of the above mentioned mylar, teonex or other PEN or PET based materials. Similarly to the above described via drilling process where the vias are drilled after drilling the area, albeit this time with a much less stringent requirement on process selectivity. Such gains in process selectivity may potentially eliminate the need of other post via drilling hole cleanup steps. As an alternative to the application of an undrilled protective sheet it is also possible to locally cover the first layer metal that is to be contacted with a suitable dielectric such as a glass or polymer which is chemically resistant enough to withstand the texture and post texture clean processes, but which may be removed prior to second layer metal contact formation to the first layer metal. In the case where the wet processing is not carried out by immersion but by single side wet chemistry application, a protective sheet may not be required when using a pre-drilled sheet.
When the predrilled reinforcement backplane is laminated to the TFSS which is at that time supported by the template, the application of the backplane sheet or sheets with the TFSS on template for the lamination has to be done with an alignment. To secure that an aligned position is retained during the lamination, the sheet or sheets can be pretacked to the surface using a laser or other local heat source. Alternatively, as part of the first layer metal formation, the area of the vias may be built up with a taller, preferably printed, metal region. Such local pillars, when dimensioned properly, may serve to secure the predrilled sheet in place during the lamination. The above mentioned optional local protective material is in that case applied on top of such pillars. Such pillars can be employed in a very sparse pattern, so as to save on material usage for said pillars.
Second layer metal formation. The second layer of metal is preferably structured in an essentially orthogonal relation to the first layer metal fingers, with the potential exception of one or more bus bar strips for each terminal. Orthogonal relation may greatly relaxe the requirements for patterning the second layer metal. For instance, if the patterning requirements of the first layer metal are in the hundred or hundreds of micrometers, the patterning requirements of the second layer metal are in the millimeters to centimeters range. This in turn enables the use of very economic patterning techniques such as simple shadow masks or very cheap printing , roller coating or spraying applications. Also it allows for the use of stamped out large dimension metal fingers. This relaxation is enabled by the concept that for the orthogonal relation geometry, the distance that current has to travel in each first layer metal finger is suitably short prior to reaching a via for extraction.
Various process flow options for forming the second layer metal have been disclosed including the embodiments and alternatives following. If aluminum is the contact metal to the second layer, then a zincation process, preferably a double zincation is advantageous for reliable plating on top of aluminum. If a PVD process is to follow, zincation may be circumvented by doing a pre-sputter etch cleanup.
A suitable PVD process for contacting the first layer metal may then start with a pre-sputter etch, Al, follow by Ni or NiV deposition, followed by optional Sn deposition. This PVD process may be performed using a shadow mas, thereby enabling patterned metal deposition. Alternatively, the metal may be patterned after the deposition using laser ablation, similar to the patterning for the first layer metal. The deposited metal or metal stack may optionally be annealed after deposition to tune its properties.
Alternatively the vias may be filled or partially filled first by printing, for instance stencil printing a conductive paste such as aluminum, copper, nickel or silver paste. A seed metal or metal stack can then be deposited also using PVD or screen printing, on top of the metal that is used to at least partially fill the vias. Printed pastes can be baked and/or annealed after application.
On top of this seed, the remainder of the metal may be plated. And alternatively, the whole necessary thickness of the orthogonal metal fingers of the second layer metal can be printed using a suitable paste. In the case of plating, the deposition of the seed metal may be performed in a patterned way, as described above, or as a blanket layer which is subsequently patterned using a resist structure which separates emitter from base plated areas. After plating, the resist is stripped and the seed layer is etched back in the areas that were protected using the resist. A typical sequence of plating starts with Ni, followed by copper (Cu) and ending with Sn for solderability. Alternatively and depending on the seed material, Cu can be plated directly. Sn can also be applied locally after plating, using printing, in areas where it is required for soldering. In the case of a printed seed, it is also possible, if affordable, to print up the whole second level metallization using for instance screen or ink jet printing.
The structure for the second layer metal may either have a single or multiple bus bars per terminal or it can contain only metal fingers. In case of a plating process for the second layer metal, the number of contact points required for module integration scales with the number of independent bus bars at the time of plating. Contacts in the module from cell to cell can be accomplished using dog bone shaped contact fingers. For a finger-only structure, the dog bone contact points per side need to equal the number of second layer metal fingers per terminal. Minimizing the area of the bus bar, up to the point where no bus bar is employed, serves to maximize the overall active area on the cell that current can be drawn from by in turn minimizing the area of electrical shading underneath the bus bars.
The contact metal strips between cells can consist of Cu with solder or solderable aluminum, such as Al with thin Ni and Sn or tin-bismuth (SnBi) coating. In the areas that are visible in the module, the strips can be painted in black locally to add to the all-black look of the panel. Such paint coating can act as a dielectric as well, allowing for tight arrangement of cells within a module.
Manufacturing the Oasis structures.
Oasis formation embodiments.
Second, for the attachment between the TFSS with the patterned first layer metal fingers and the large metal fingers of the backplane embodiments include the use of a combination of printed dielectric adhesive and conductive adhesive or epoxy versus the use of a dielectric sheet such as prepreg which may then in turn vary from being drilled prior to lamination or after lamination. In the case of pre-lamination drilling, the CA posts may be printed onto either the TFSS side of the structure or to the backplane side of the structure, if the backplane is manufactured separately. For a single step lamination using pre-drilled prepreg, the CA posts are printed onto the metal fingers on the TFSS.
Oasis lamination using a dielectric adhesive. In a process flow embodiment, such as that depicted in
These structures are aligned and laid up on top of each other, optionally covered with release sheets or with non-sticky surfaces on both sides, and then laminated together. With the right choice of material and lamination conditions, such as suitable evacuation, temperature range, ramping and lamination pressure, the dielectric material flows and planarizes the structure. Areas where planarization is not desired, such as in the backside contact areas, may be kept open by providing suitably shaped lamination contact chucks or by pre-curing the edges of the contact holes (using for instance increased laser power during the cutting of these holes) to prevent outflow of material from the edge to close the holes.
A B-stageable or at least partially reflowable, i.e. thermoplastic, printed dielectric adhesive is used as the adhesive is then applied to the backplane or (not shown) to the TFSS. Further, a conductive adhesive may be printed on either side. Dielectric and conductive adhesive each receive suitable optional thermal treatment after printing. To keep costs low, the overall area of the conductive bumps is to be kept low, preferably below 2% of the overall cell area. Prior to lamination, the TFSS may be pre-cut in a region just outside the active area to provide a designated breakage point of the epi layer upon the release that is to occur after the lamination. After that, the backplane and the TFSS on template are laminated together. In this process, the electrical contact between the metal fingers on the TFSS and the large metal fingers on the backplane is also established. After the lamination, the structure is released by mechanical release, similar to the release described for the pluto structure. The edge of the released and backplane reinforced device may then be trimmed, also similar to the trimming described for the pluto structure. Preferably, the edges of the backplane structure where the trimming occurs are sealed by the suitably chemically resistant dielectrics. Afterwards, also similar to the pluto structure, on the sunny side of the TFSS the residue of the release layer is cleaned off, the surface is textured, post-texture cleaned and passivated. As a final process for the cell, the contact access points to the large metal fingers of the backplane are opened, for instance by laser drilling of the cover sheet material.
Conductive solder bumps may be placed, or the solder from a stringer that is used for module assembly manufacturing may be used, to establish the contact to the cell. The cell receiving its own solder bumps may have the advantage that the individual cell may be tested and passing cells may subsequently be assembled into a module; however, such testing may also be accomplished using suitable probe card arrangements.
Lamination using a dielectric sheet. As an alternative to the lamination using a printed dielectric adhesive process described above, a dielectric sheet, for example a prepreg material, which is pre-drilled may also be used as the adhesive between large metal finger containing backplane and the TFSS. The cross-sectional diagrams of
Single step lamination process for Oasis structure. With proper thermal budget and thermal sequencing during lamination it is possible to attach all components of the Oasis structure in a single step, rather than having a separate step for backplane lamination and lamination of the backplane to the TFSS on template.
The cross-sectional diagrams of
Process flow for the Pluto-Oasis Hybrid structure.
Alternatively to depositing the metal fingers through a shadow mask, which may be possible due to the rather coarse dimensional requirements for the fingers (millimeters to centimeters). The metal may also be deposited as a blanket metal and afterwards patterned, for example using laser ablation.
On top of the large width metal fingers which run orthogonally to the metal fingers on the TFSS and which contain optionally one or several bus bars per polarity, as explained for the pluto structure, an array of conductive bumps or epoxy are printed. Similar to the oasis structure, there is an additional backplane with large metal fingers, for instance made of solderable Al, for instance with a Ni and/or Sn coating, which may be either pre-manufactured and then laminated to the already reinforced TFSS, or which may be laminated in a single step. The backplane itself consists for example of large width metal fingers which are held in place by a dielectric adhesive which in turn may have a backing plate, for example of glass, polymer, ceramic or metal. For contacting the cell to other cells or in general within the module, it may be advantageous to either have holes in the layer above the large width metal fingers or to have the large width metal fingers extend beyond the edge of the cell. Such metal fingers may be generated in a similar way as for the oasis structure, for example by EDM, stamping, slit cutting or suitable etching after a definition of the etch areas using a mechanical or laser marking. From a structural point of view, it may be advantageous to retain the structure throughout the process in such a way that the area that becomes the bus bars is connected to both polarities, and only prior to cell assembly each side of the contact polarities is cut off. This is an especially straightforward process if the large width metal finger grid is chosen to be oversized compared to the cell.
As another alternative, such metal connections may also be integrated into the module assembly where then a large area of metal fingers may be processed and laminated in parallel. This is possible since the initial metal of the reinforced cell already enables testing and sorting of the cell.
It is to be noted that for the hybrid structures the orthogonal transfer of the metal lines between the on-cell thin fingers and the on-backplane wide fingers may be implemented either from the on-cell metal fingers to the second layer deposited or printed metal or from the printed metal to the backplane aluminum foil fingers. With the latter, it may be advantageous to implement another dielectric between the second layer deposited or printed metal and the aluminum foil fingers.
Process flow for the immersion contact bonding structure. The immersion contact bonding structure is processed similar to an oasis type structure. Main process step differences are depicted in
As described above, the disclosed subject matter relates to novel structures and methods for the metallization of solar cells, and specifically multiple (two or more) levels of metallization associated with an active semiconductor photon absorber for low-loss interconnection and efficient photo-generated electricity collection from a solar cell. In some solar cell embodiments, the semiconductor photon absorber may be crystalline silicon, including but not limited to mono-crystalline silicon solar cells with back-contact metallization architecture. Additionally, although primarily described with reference to dual layer metallization in conjunction with a supporting electrically insulating backplane, any number of metallization layers (for instance, using 2, 3, 4, or higher levels of metallization) may be utilized (even including a single metallization layer comprised of “stacked” metallization materials) on the backside (or the side opposite the sunnyside) of a solar cell comprising an electrically insulating layer (for 2-level metallization) or layers (for metallization comprising more than 2 levels of metallization) used as inter-level metallization electrical insulator in accordance with the disclosed subject matter. Further, in some embodiments the metal or electrical interconnect layer(s) on each level may be independently patterned using several available techniques, such as but not limited to blanket metal deposition (for example, by a Physical-Vapor Deposition or PVD technique such as plasma sputtering, evaporation, thermal or arc plasma spray, or ion beam deposition; or by using an electrochemical deposition process such as plating) followed by pattern formation using pulsed laser metal ablation or a combination of lithography (for instance, using screen printing of a patterned etch resist layer) and subsequent etching of metal and stripping of the resist layer. Alternatively, the metallization pattern may be formed during and by the metallization process itself (called in-situ patterning). Examples of in-situ patterning include screen printing of a metallization paste (forming the desired pattern), PVD using in-situ shadow masking, etc. The metal layers may be separated by electrically insulating dielectric layers (using one electrically insulating layer for two levels of metallization, and in general N-1 layers of electrically insulating layers for N levels of metallization, wherein N is an integer equal to or greater than 2) and connected together using specific patterns of inter-level via holes formed through the electrically insulating layer(s) (vias), either partially or fully filled with an electrically conductive material formed through the dielectric layers (hence, forming electrically conductive via plugs). The electrically conductive via plugs (connecting two adjacent metallization levels according to a pre-specified interconnection pattern) may be formed using the same metallization material and process utilized to form the higher metallization level. Each metal layer may be composed of similar or disparate metal types, such as Al, Cu, Ag, Ni, Sn, or a combination of metals—for example, low-cost high-conductivity metallization materials composed of aluminum and/or copper—and may be patterned to different dimensions. And while the following is described with reference to aluminum (Al), any electrically conductive metallization material may be a viable material choice in some instances (including copper, zinc, or even silver although silver is expensive and may be less desirable than much lower cost high conductivity material options such as aluminum and copper for solar cell metallization).
In one embodiment the metallization structure may utilize dual level metallization structure wherein the first level (or lower level) metallization closer to the solar cell absorber substrate is referred to as metal 1 or M1 and the second level (or upper level) metallization on top of M1 and, in some embodiments separated from the first level by an electrically insulating layer, is referred to as metal 2 or M2. For example, patterned M1 may be formed directly on the solar cell substrate underneath the electrically insulating layer or sheet separating M1 and M2, while M2 is formed on top of the electrically insulating layer or sheet attached to the solar cell substrate (in other words the electrically insulating layer or sheet is sandwiched between M1 and M2, and M1 is sandwiched between the solar cell substrate and the electrically insulating layer or sheet). The combination of the electrically insulating layer(s) or sheet(s) with the metallization structure above M1 may be referred to as the solar cell backplane. Advantages of a dual layer (bi-layer) metallization structure over a single metal structure for back-contact solar cells include, but are not limited to, the following:
Current solar cell metallization designs often use a single metallization level adjacent to and connected to the active absorber (e.g., fired paste metallization on silicon solar cells). In a traditional front contacted solar cell architecture, each side of the solar cell has metallization layer with the front side/sunnyside (solar cell side facing sunward) metal patterned (typically a screen printed silver paste metallization) to let the light through to the cell and the backside metallization/non-sunnyside (solar cell side opposite the side facing sunward) patterned or non-patterned (typically a screen printed aluminum paste metalliation). For example, a typical front-contact crystalline silicon solar cell may have one patterned emitter metallization layer (often comprising silver) on the cell sunnyside and one blanket base metallization layer (often comprising aluminum which also serves as the back-surface field or BSF layer), or screen printed aluminum with optional localized back-surface field or BSF, on the cell backside with a patterned dielectric layer. In back contacted back junction (also known as interdigitated back contact or IBC) solar cells, a single metallization layer may be patterned (e.g., as an IBC pattern) on the non-sunnyside and no sunnyside metallization—thus no sunnyside optical shading for an unobstructive and ideally maximum coupling of the sunlight. The multi-level (for example bi-layer or two-layer) metallization embodiments disclosed herein, while applicable to any solar cell architecture such as front junction/back-contact or back-contact/back junction solar cells, are described with reference to back contact/back junction (BC/BJ) crystalline silicon architecture (also known as IBC solar cells).
Multi-level metallization schemes of at least two levels of solar cell metallization, and for example designed in an orthogonal M2-M1 pattern (alternative metallization layers aligned orthogonally or the adjacent metallization levels with interconnect fingers substantially perpendicular to each other) and separated by electrically insulating layer(s), one electrically insulating layer between M1 and M2 for the two-level metallization scheme, provide numerous metallization, efficiency, and cell processing advantages over known solar cell metallization structures and manufacturing methods. For example, in a bi-layer (also known as two-level) metallization embodiment where the two metallization levels M1 and M2 are separated by one electrically insulating layer or sheet, the combination of M2 and the electrically insulating layer or sheet (also known as the solar cell backplane in this invention) may serve as a reinforcement and support structure for the semiconductor absorber, an advantage particularly applicable to ultrathin solar cell absorbers. Such backplane reinforcement and support structure may be made rigid (for instance using a non-polymeric insulator such as a glass or a ceramic layer) or pliable/flexible (for instance, using a polymeric material such as a prepreg material). Additional benefits and advantages of multi-level metallization, particularly with respect to bi-layer metallization for high-efficiency crystalline semiconductor solar cells (particularly, back-contact/back-junction solar cells), include but are not limited to:
The M1 metal may be made much thinner (e.g., in the range of 100's to 1000's of nm thick) as compared to known on absorber metallization patterns (which use 10's of microns metal thickness for back-contact/back-junction cells). Thinner M1 metallization creates less stress from M1 applied onto the silicon absorber, a very desirable advantage, particularly for ultrathin cells with absorber thickness below about 100 microns and/or large-area cells with cell area larger than 125 mm×125 mm. Larger area scaling is pssobile because the multi-level metallization schemes in accordance with the disclosed subject matter decouple the M1 thickness requirement from the cell area since M1 is used for contact metallization and localized areal power collection instead of global power collection. Thus, the stress reduction of a multi-level metal configuration (for example a two-level metal configuration with a thinner M1 layer on the cell—and the M1 metal thickness being essentially independent of the cell dimensions or area) may allow the silicon (or any crystalline semiconductor) substrate to be scaled to much larger cell areas (e.g., larger than 125 mm×125 mm, for instance, up to or larger than cell area of 1000 cm2) and much thinner in absorber thickness (e.g., thinner than 100 microns, and in the range of about 1 micron to about 100 microns). Multi-level metal configuration allows for a thinner M1 as the M1 layer serves as contact metallization for areal extraction of the solar cell electrical power and is not required to carry the cell current over a long distance over relatively low electrical conductivity lines. For example, the electrical current (or power) is locally pulled up vertically throughout the area of the solar cell from M1 through conductive plugs to the next upper metal level (which may be far less resistive by virtue of being much thicker and/or wider), and in one embodiment current or power is pulled from M1 to M2 through periodic via holes in a dielectric layer formed between M1 and M2 and filled (or partially filled) with conductive via plugs. Via plugs/holes along the M1 lines may be positioned with spacing such that the ohmic losses of the M1 line segments are negligible and do not have a significant detrimental impact on Fill Factor of the solar cell.
In example embodiments of multi-level metallization for solar cells, the metallization layers on different levels may be oriented to run along the same direction or independently in different directions such as orthogonal or perpendicular to each other). For example, the thicker and higher conductivity M2 may run perpendicular to the thinner on-cell M1 or may run parallel to M1. An advantage of M2 running perpendicular to M1 (referred to as orthogonal transformation of M2 with respect to M1) is the width as well as pitch dimensions of M2 may be much coarser and larger than M1, resulting in a fewer number of M2 electrodes than the number of M1 electrodes (by a factor in the range of about 5 to 50 depending on the specific design requirements and specifications). This metallization architectural attribute, the combination of two-level cell metallization in conjunction with orthogonal interconnect transformation, may provide an advantage by decreasing the restraints of deposition and patterning of M2. The much coarser dimensions on thicker M2 opens the door to depositing M2 using relatively cheap/simple direct write techniques such as direct write thermal spray, screen and stencil printing metallic pastes such as Cu or aluminum paste, and inkjetting or depositing by aerosol printing metallic inks such as copper or nickel inks. Alternatively, M2 may be formed by a combination of formation of a seed layer by one of the above techniques (such as PVD) and electroplating of a high conductivity metal such as copper. Structural considerations for the pitch and width of the M2 may be dictated by the resistance of M1 which in-turn may dictate the spacing of vias to draw the current upward.
Cost and efficiency are important metrics when selecting a method for creating multi-level metallization in solar cell manufacturing. Several methods for forming multi-level metallization for high-efficiency, cost-effective solar cells are described below in relation to dual level metallization having a sandwiched electrically insulating or dielectric layer positioned between M1 and M2; however, these methods may also extended to multi-level metallization and are equally applicable to both conventionally thick solar cells as well as very thin silicon solar cells. The electrically insulating layer may also serve as part of the reinforcement and support structure of the solar cell. Very thin solar cells utilizing the methods and structures disclosed herein include ultrathin crystalline silicon solar cells with crystalline silicon layers in the thickness range of about one to 100 microns and which may formed by wire saw, epitaxial lift-off, proton implant and exfoliation, stress-induced peeling, laser wafering, or other thin silicon slicing techniques. And while the described metallization methods may be integrated, combined, or arranged in alternative orders, for descriptive purposes the methods for forming dual level metallization may be organized into the following four categories: (1) Methods for depositing and patterning M1; (2) Methods for forming an electrically insulating dielectric layer or sheet on top of M1 (for example by deposition, lamination, etc.); (3) Methods for forming via holes through the dielectric to connect M1 with M2 and subsequent cleaning; and (4) Methods for depositing and/or patterning M2.
For step 1 relating to methods for depositing and patterning M1, techniques such as plasma sputtering (or evaporation) followed by patterning using laser ablation or wet etching, or patterned screen printing (or stencil printing or inkjet printing or aerosol printing) may be used to deposit M1. For blanket deposition techniques such as PVD (plasma sputtering or evaporation), subsequent patterning may be performed using laser metal ablation or using standard lithography and etching techniques (such as with screen printing of a resist followed by wet or dry etching). Further, if using screen printing or inkjet printing (or stencil printing, aerosol jet printing), the M1 layer may be directly formed as a patterned metal layer using an appropriate metal paste or metal ink. It should be noted, the choice of M1 metal should be made to ensure minor quality and low resistivity contacts to both n and p-type diffusions in silicon. For example, a metallic material for M1 comprises aluminum (or an alloy of aluminum and silicon) because of its high electrical conductivity, high IR reflectance, good ohmic contacts without shunts, and low cost. Alternatively, silver and thin nickel, followed by aluminum may be used as an M1 metal stack; however, silver is typically associated with a higher cost, for example as compared to aluminum thus aluminum may be used for fabrication lower costs and to provide good ohmic contacts to both base and emitter regions.
For step 2, related to forming an electrically insulating dielectric layer or sheet between M1 and M2, design considerations include the choice of material as well as the method of deposition or formation. Material choice considerations should include material cost as optimally the dielectric material will only be a fraction of the conventional metallization cost. For example, several plastics and polymeric materials meet these cost targets including but not limited to the prepreg materials which serve as adhesive and structural support layers in conventional printed circuit (PC) boards. Further dielectric material choice considerations and constraints may depend on which step during the process flow the dielectric material is inserted formed, and if it serves additional functions beyond serving as an electrical isolation interlayer between M1 and M2. For example, for thin silicon cells (such as crystalline silicon cells with thickness of less than 100 microns), this dielectric layer may also serve as a permanent reinforcement/support layer to handle and support thin silicon during processing of the cell as well as during the photovoltaic (PV) module lamination process and for the PV module lifespan in the field. In this example, the dielectric layer may be inserted midstream in the solar cell formation process by laminating a sheet of insulating material to the thin silicon solar cell absorber layer, which puts additional constraints on the choice of the material to ensure compatibility with downstream solace cell formation process steps as well as dexterity to support thin silicon. In another embodiment related to conventional thick silicon, constraints on the choice of dielectric material may be significantly relaxed as the dielectric layer may be formed at the end of the line after completion of the main solar cell fabrication process steps up to the metallization stage. Further, as an additional function of the dielectric material, active components and electrical components to build cell level intelligence may be positioned on the dielectric (for example a prepreg style dielectric material). In general, the electrically insulating layer may be pliable/flexible (such as prepreg or other polymeric materials) in order to produce flexible solar cells and flexible solar modules, or rigid for rigid solar cells and PV modules (such as glass or other ceramic materials).
Several methods for forming this electrically insulating layer are described herein. Formation methods include depositing the dielectric layer, for example using direct write techniques such as but not limited to thermal spray, dielectric spin-on, screen printing, or stencil printing. Alternatively, formation methods include laminating a cheap dielectric thin sheet such as a polymeric or plastic sheet (e.g., 25 microns to 200 microns thick) on the back surface of the solar cell comprising the M1 level (opposite the sunny side) for example by applying a combination of pressure and temperature whereby the electrically insulating dielectric film or laminant is reflowed to substantially conform to the M1 topography and completely encapsulate it. In a one embodiment, this laminate may be a prepreg material made of aramid fibers and resin commonly utilized as the building block layer for certain PC boards. Other prepreg materials may also be used.
For step 3, related to forming via holes connecting M2 with M1, techniques such as conventional masked and dry plasma etching may be used. Alternatively, via holes may be formed by mechanical formation such as by mechanically punching of dielectric; however, care must be taken to avoid damaging the underlying silicon, especially in the case for thin silicon cells but also for conventionally thick solar cells. In yet another embodiment, laser processing may be utilized to drill the via holes using a pulsed laser beam. For example, a cost effective and fast laser such as a CO2 ˜10 micron wavelength laser may drill holes through a prepreg laminant sheet at a very fast speed covering the entire cell substrate dielectric sheet in a few to tens of seconds or less. The choice of the laser and the underlying M1 metal may be made to have intrinsic compatibility, as the laser should stop at the underlying metal cleanly without punching through it. One solution uses a laser that is highly reflected by the M1 metal—thus the laser energy is not absorbed into the M1 metal—which serves as an end-point detection and self-limiting end to the drill process. In yet another embodiment, stop layer thick metal pads (for example made of aluminum and/or silver paste) may be printed before the dielectric is deposited/laminated only where the via holes are drilled. Care must be taken to ensure that the contact resistance of these metal pads to both M1 and the subsequently deposited M2 is low enough to contribute negligibly toward the total series resistance of the solar cell, for example conductive epoxy pads may be printed on top of M1 and served as the laser drill stop layer. Epoxy for the metal pads may be applied using known methods such as screen printing. In yet another embodiment, Al metal paste pads with a thickness in the range of about 10 microns to 40 microns may be used on top of Al paste metal lines (with thickness in the range of about 5 microns to 40 microns).
A fourth method for formation of the via holes utilizes direct printing of a suitable patterned electrically insulating dielectric layer by screen printing or stencil printing of a dielectric paste in which the patterned printing includes the via holes in the dielectric layer. Direct writing of the patterned dielectric layer including via holes eliminates the need for subsequent formation of holes using lithography and etch or laser drilling.
If the dielectric is polymeric or plastic based (for example formed from a range of low cost materials comprising polymers, plastics, prepreg materials, etc.), the via holes may be cleaned after drillling to ensure a clean contact with low contact resistance between M2 and M1. Various cleaning methods include but are not limited to: 1) wet organic clean; 2) dry etch clean with highly oxidizing plasma such as that produced using an oxidizing gas ambient (for example oxygen or nitrous oxide); 3) plasma sputter etch; 4) ozonated treatment; 5) a subsequent laser step to burn off the carbon; or 6) hot metal deposition to burn through the residual contaminant layer.
For step 4, relating to M2 deposition and patterning (or alternatively direct write deposition of a patterned M2 layer), conventional methods may be used such as plating on top of a seed layer formed for example by plating, inkjet printing, screen printing, or patterned PVD layer, or alternatively direct writing techniques may be used such as, but not limited to, screen printing, stencil printing, thermal (or arc or plasma) metal spray, inkjet or aerosol printing. Direct patterned metal foil attachment, or foil attachment and subsequent patterning by cut design, may also be used and particularly for an orthogonal structure, foil attachment may be preceded by depositing a conductive layer in the via holes separating M1 and M2. A metal foil M2 in an orthogonal bi-layer metallization structure provides a planar and mechanically reproducible structure due to large in-plane M2 pattern dimensions. For example, the M2 metal foil finger width may be much larger than the metal foil layer thickness (for example greater by a factor of 10).
A specific process flow embodiment is detailed below for forming dual level metallization, and although described in the context of back-contact/back-junction thin monocrystalline silicon solar cells using epitaxial silicon lift-off methods, the metallization methods and structures described herein may be applicable to solar cells of any thickness including standard crystalline silicon wafer-based cells (for example in the thickness range of 100 microns to 200 microns using CZ or FZ wafers).
A thin M1 of, for example, aluminum or an alloy comprising aluminum and silicon, is deposited on the backside solar cell substrate surface. Patterned deposition may use screen or stencil printing of Al paste, and/or a myriad other direct pattern writing techniques for metal deposition such as inkjet and aerosol printing or PVD followed by laser ablation. The thickness of the screen printed Al/Al—Si metal may range from about 5 microns to about 40 microns depending on the conductivity requirements of the solar cell design. Alternatively, Aluminum/Aluminum Silicon may be deposited as M1 using physical vapor deposition techniques such as sputtering. In the case of PVD Al or AlSi deposition for M1, post deposition the blanket metal layer may be subsequently patterned using a pulsed picosecond laser (for example with a wavelength in the near infrared range of about 1 micron wavelength) which ablates metal lines to pattern and electrically isolate the emitter and the base polarity. In one embodiment, the M1 pattern for the back-junction/back-contact monocrystalline silicon cell may be an array of straight-line (rectangular, triangular or trapezoidal) interdigitated base and emitter fingers separated by isolation regions with a relatively large metal coverage area ratio, at least 70% and up to over 90% for enhanced rear minor IR reflectivity in conjunction with the rear dielectric layer. Importantly, there are no busbars on M1 in order to eliminate busbar induced electrical shading effects and to maximize the solar cell efficiency. In the case of patterned Al screen print to form M1, a follow up Al paste may be optional. The Al paste may be printed as a periodic pattern of pads along M1 to serve as a laser stop layer as described above. Alternatively, where the first Al paste line conductivity is not sufficient, the second print when used in the same pattern as the first print (in contrast to a second print of pads) may also serve to reduce M1 resistance and serve as a laser stop layer.
In the case PVD such as plasma sputtering is used to deposit M1, the laser ablation metal patterning may be optionally followed by optional screen printing of reflective conductive epoxy pads (for example using an aluminum and/or silver paste) where via hole drills would be positioned/land on M1. This may be required due to the lack of laser stopping ability if a small thickness of sputtered M1 is applied. Sputter deposited M1 thickness may be kept thin for lower costs and to increase ease of patterning, for example using a pico second IR laser. The Ag and/or Al based conductive epoxy provides better reflective-pad stopping power to the CO2 laser than the thin M1 layer by itself.
Subsequently, a thin polymeric (e.g., a suitable prepreg) sheet, for example having a thickness in the range of about 50 up to 500 microns (in some cases 50 microns to 200 microns) is laminated, for example using a pressure-thermal lamination process, on top of M1 and cell substrate backside (in other words the side opposite the cell sunnyside). Because the electrically insulating dielecytric sheet is attached to thin silicon and is inserted midstream during cell processing (for instance, just prior to the lift-off process for epitaxial silicon cells) to also serve as a permanent reinforcement layer and carrier for thin silicon cell in this embodiment, dielectric choice may be dictated by the following additional attributes in addition to cost considerations. First, the dielectric should be a material conducive to being drilled using laser drilling such that M1 connections may be accessed and M2 may be deposited on top of the dielectric (for example prepreg). Second, it should support effective adhesion of both M1 and M2 as well as the cell isolation regions (typically covered by silicon oxide or silicon nitride and/or aluminum oxide) on the cell backside. Third, it should have a relatively well-matched Coefficient of Thermal Expansion (CTE) compared to silicon (for example a low CTE of well below 10 ppm/degree C. such as a CTE in the range of about 0 to 5 ppm/degree C.) to ensure that at lamination and subsequent solar cell processing temperatures, as well as during the long-term field operation, there is no silicon cracking due to CTE mismatch between silicon and the laminant. Fourth, in a BC/BJ architecture such as that described, the solar cell sunnyside processing steps such as texturing and front passivation may follow after the dielectric layer is formed (in one embodiment the step after laminating the dielectric layer is the lift off and release of the thin silicon/dielectric sheet laminate from the reusable template which exposes the sunnyside of the cell for completion of the cell sunnyside processing), thus the backside passivation layer may be exposed to wet chemicals used for texturing and cleaning of the front surface or cell sunnyside. In this case, the dielectric layer should serve as an effective sealant to protect the cell backside, including the M1 layer, during texturing and post-texture cleaning of the cell. Fifth, the dielectric layer must not substantially bow the solar cell due to stresses (for example the overall bowing of the cells should be limited to less than about 2 or 3 mm for a 156 mm×156 mm solar cell). Sixth, the lamination process should meet the throughput speed required for high productivity solar cell manufacturing. And finally, the dielectric material should have high thermal stability (for example up to at least 200 degrees C. and in some cases up to at least 300 degrees C.) to be able to sustain high process temperatures to ensure excellent front side passivation using PECVD passivation processes which normally utilize substrate heating in the range of 150 degrees C. up to 400 degrees C., depending on the passivation process used—a pivotal process/structure for some BC/BJ solar cell embodiments.
Subsequent to dielectric layer lamination, the solar cell may go through a number of process steps unique to this thin silicon process flow embodiment. These steps, as outlined above, may include mechanical release of a thin silicon substrate from a template along a porous sacrificial layer, frontside texture and post texture clean, frontside silicon nitride passivation, and the formation of the M2 layer. Following completion of the frontside texture and passivation process steps, via holes are drilled in the laminated dielectric sheet to connect M2 and M1 through M1-M2 conductive via plugs using laser drilling, for example using a CO2 ˜10 micron wavelength laser, to drill holes stopping on M1 (or alternatively, on conductive epoxy pads printed on M1). Pre-established fiducials may be used to align the via holes to the pads. Further, laser drilling holes through a dielectric laminate may create carbon residue inside the holes which may be subsequently cleaned using, for example, plasma sputter etch or directly with hot metal during the hot M2 deposition.
In a variation of this process flow for thin crystalline semiconductor (e.g., thin crystalline silicon) solar cells, the M1 layer may be screen printed (or formed by inkjet printing, aerosol printing, stencil printing) using a suitable paste, for example a paste comprising mostly aluminum and some silicon to prevent junction spiking, and fired instead of PVD deposited and laser patterned. This direct write method may provide further manufacturing cost reduction compared to PVD and patterning methods. Dual level metallization, M1 directly on the cell and M2 separated from the cell by the dielectric sheet, may also be formed as is on thicker conventional silicon solar cells. This means that the multi level metallization embodiments in accordance with the disclosed subject matter may be used with thin semiconductor absorber solar cells using thin substrates not formed by conventional wire sawing from ingots or cast bricks of silicon and also with solar cells made on standard thickness wafers formed from ingots using the wire saw process. Because BC/BJ solar cells require high electrical conductivity to reduce line resistance, and as cell area becomes larger, metallization designs have to use thicker metal layers to provide lower sheet resistance and enable higher current carrying capability. The multi-level metallization structures and methods provided are an enabling technology for scaling up the area of BC/BJ cells to 156 mm×156 mm and well beyond (for instance to cell areas up to and larger than 1000 cm2) while allowing the use of cell absorbers over a wide range of thicknesses, from about one micron up to hundreds of microns depending on the cell design and manufacturing process.
Importantly, while structures and formation methods for multi-level metallization are detailed in this disclosure, various aspects of each structure and process flow may be combined and/or altered in accordance with the disclosed subject matter.
The metallization contacts shown in
The mostly-processed back-contact, back-junction backplane-reinforced large-area (for instance, a solar cell area of at least 125 mm×125 mm or larger up to or larger than 1000 cm2) solar cell is then separated and lifted off from the template along the mechanically-weakened sacrificial porous silicon layer (for example through a mechanical release or MR process) while the template may be re-used many times to further minimize solar cell manufacturing cost. Final cell processing may then be performed on the solar cell sunny-side which is exposed after being released from the template. Sunny-side processing may include, for instance, completing frontside texturization (for example by wet etch or laser texturing processing), passivation (for example using a PECVD process), and anti-reflection coating deposition process.
Backplane vias may then be formed in the backplane, for example using a CO2 laser, and Metal 2 seed is deposited on the backplane, for example using PVD of aluminum and nickel. Metal 2, for example tin and/or copper plating, is then formed on the backside of the solar cell.
The backplane material may be made of a thin (for instance, about 50 to 250 microns), flexible, and electrically insulating polymeric material sheet such as an inexpensive prepreg material commonly used in printed circuit boards (PCB) which meets the process integration and reliability requirements. Generally, prepregs are reinforcing materials pre-impregnated with resin and ready to use to produce composite parts (prepregs may be used to produce composites faster and easier than wet lay-up systems). Prepregs may be manufactured by combining reinforcement fibers or fabrics with specially formulated pre-catalyzed resins using equipment designed to ensure consistency. Covered by a flexible backing paper, prepregs may be easily handled and remain pliable for a certain time period (out-life) at room temperature. Further, prepreg advances have produced materials which do not require refrigeration for storage, prepregs with longer shelf life, and products that cure at lower temperatures. Prepreg laminates may be cured by heating under pressure. Conventional prepregs are formulated for autoclave curing while low-temp. prepregs may be fully cured by using vacuum bag pressure alone at much lower temperatures.
The viscosity of a prepreg resin affects its properties, and it is affected by temperature: At 20° C. a prepreg resin feels like a ‘dry’ but tacky solid. Upon heating, the resin viscosity drops dramatically, allowing it to flow around fibers, giving the prepreg the necessary flexibility to conform to mold shapes. As the prepreg is heated beyond the activation temperature, its catalysts react and the cross-linking reaction of the resin molecules accelerates. The progressive polymerization increases the viscosity of the resin until it has passed a point where it will not flow. The reaction then proceeds to full cure. Thus prepeg material may be used to “flow” around and in gaps/voids in the M1 metallization pattern.
Further, PCBs are alternating layers of core and prepreg where core is a thin piece of dielectric with copper foil bonded to both sides (core dielectric is cured fiberglass-epoxy resin) and prepreg is uncured fiberglass-epoxy resin. Prepreg will cure and harden when heated and pressed. In other words, prepregs are rolls of uncured composite materials in which the fibers have been pre-impregnated (combined) with the resin. During production, the prepreg sandwich is heated to a precise temperature and time to slightly cure the resin and, therefore, slightly solidify through crosslinking. This is called B-Staging. Care must be taken to insure that the sandwich is not heated too much, as this will cause the prepreg to be too stiff and seem “boardy.” The solvent is removed during B-Staging so that resin is relatively dry of solvent. Typical thermoset resins and some thermoplastic resins are commonly used in prepregs. The most common resin is epoxy as the major markets for prepregs are in aerospace, sporting goods, and electrical circuit boards where excellent mechanical, chemical, and physical properties of epoxies are needed. Typically, prepregs have a thickness in the range of as little as about 1 mil (˜25 μm) up to a multiple of this amount.
Further, prepegs may be made of thermoplastics (not as common as thermosets). Thermoplastic prepegs are often used for their toughness, solvent resistance, or some other specialized purpose. Most of the thermoplastics used are very high performance resins, such as PEEK, PEI, and PPS which would compete with 350° F. cure epoxies in aerospace applications. Some new applications such as automotive body panels which depend up special properties, such as toughness, are using thermoplastics either alone or mixed with thermosets.
For example, interdigitated back contact IBC M2 metallization (metal layer 2) conductivity requirements for a dual busbar M2 metallization pattern (in other words no on-cell distributed busbars or no mini cells), for a cell such as that shown in
In another example, interdigitated back contact IBC M2 metallization (metal layer 2) conductivity requirements for an on-cell distributed busbar M2 metallization pattern (in other words mini-cells), for a cell such as that shown in
Designing dual metallization structures using an orthogonal interconnect pattern (in other words an orthogonal transformation of M2 emitter/base fingers as compared to M1 emitter/base fingers) such as that shown in
Two sets of integral equations will be used solve the total base and emitter ohmic losses due to on-cell metal: One integral equation set for the edge base and emitter fingers and another integral equation set for the rest of the base and emitter backplane fingers (all fingers excluding the edge fingers). Total loss (Ptot) is the sum of the above for all the base and emitter fingers, and the solution may be simplified into an analytical solution Ptot=(ρ/t).[(F+3)/96].(Imp2/F3).[(WBM+WEM)2/(WBM·WEM)].
Assuming an emitter/base pitch of 800 μm and a large (≧95%) M1 on-cell Al metallization area coverage (i.e., base-emitter metal gap≦40 μm), the following may be calculated:
As can be seen in Table 1, for a given on-cell base metal ratio WBM/(WBM+WEM) and metal gap≦5% of base+emitter pitch, the loss factor [(WBM+WEM)2/(WBM·WEM)] is independent of pitch (e.g., same for 800 μm and 1,300 μm pitch). Further, for an on-cell base metal ratio in the range of 35.0% to 47.5%, the factor [(WBM+WEM)2/(WBM·WEM)] in the Ptot solution is on the order of ˜4.0 to 4.4. The smallest factor (4.0) is obtained for on-cell base metal ratio of 50%. And a base metal ratio as small as 35% results in an acceptable small increase to 4.40. As a design rule, a base metal ratio on the order of 35% (for example WBM=280 μm for 800 μm pitch) may be used.
Table 2 below compares the number of backplane orthogonal Al-foil finger pairs (M2) to on-cell Al (M1) sheet resistance and thickness.
ρ = 3
ρ = 45
ρ = 30
Tighter
More Relaxed
μΩ · cm
μΩ · cm
μΩ · cm
Loss Spec
Loss Spec
The above calculations may lead to the following orthogonal backplane design conclusions:
Table 3 below summarizes results for the number of backplane orthogonal finger pairs (M2) as compared to on-cell metallization thickness (M1).
0.10 μm
F = 6 Pairs of
F = 4 Pairs of
1.5 μm
0.20 μm
F = 4 Pairs of
F = 3 Pairs of
3.0 μm
0.40 μm
F = 3 Pairs of
F = 3 Pairs of
6.0 μm
As can be seen from Table 3, in one embodiment F=3 in conjunction with 3 μm cured Al ink (or 0.2 μm PVD Al) may be chosen for a less or equal to 0.50% loss and in another embodiment F=4 in conjunction with 3 μm cured Al ink (or 0.2 μm PVD Al) may be chosen for a less than or equal to 0.25% loss.
In operation, the multi-level metallization structures, for example dual level metallization comprising M1 and M2 including orthogonal and non-orthogonal transformations, disclosed herein provide the following M1/M2 design flexibility advantages relating to performance, cost, mechanical yield, and architecture:
Further, and orthogonal pattern may allow for the insertion of active components such as bypass diodes and MPPT electronics components.
While the embodiments described herein have been largely explained in conjunction with back-contact/back-junction crystalline silicon solar cells using very thin (e.g., from about one micron up to about 100 microns) mono-crystalline silicon absorber layers supported on flexible or rigid backplanes, it should be understood that the aspects of the disclosed subject matter may be applied in some instances to other solar cell and module implementations by one skilled in the art, including but not limited to the following: front contact solar cells and PV modules comprising such cells; non-crystalline silicon solar cells and modules such as those made from crystalline GaAs, GaN, Ge, and/or other elemental and compound semiconductors; and, wafer-based solar cells including back-contact/front-junction, back-contact/back-junction and front-contact solar cells made from crystalline semiconductor wafers (such as crystalline silicon wafers).
As mentioned previously, silicon is currently the dominant absorber material of choice for the mainstream high efficiency solar cells. This disclosure provides solutions in the form of solar cell fabrication processing and structures, driving improvements, such as improved cost per watt, in high efficiency solar cells. Solutions include, but are not limited to: The use of significantly less silicon material. Process methods and structures providing for the use of significantly less silicon material, for example cells formed by epitaxial deposition on a re-usable template with subsequent release (or lift off separation) of epitaxial films by means of a designated sacrificial release layer or region (for example formed by porous silicon, stress induced splitting, laser cleaving, or other release methods, such as hydrogen implantation).
Second, process capabilities for high efficiency, back contacted wafer based cells are provided including: multi-level metallization comprising an affordable second layer metal structure (for example made of aluminum and/or copper formed by thermal spray and/or twin-wire arc spray) which reduces overall metallization cost, enables the use of larger cell substrates and the extraction of larger cell currents, the decoupling of higher conductivity thicker metal (metal 2) stresses from the active thin cell layer, and the use of thinner absorber cells. The use of an affordable inter-metal dielectric or electrically insulating layer positioned between a first layer metal and a second layer metal which may also serve as a permanent mechanical support is provided which may also further reduce cell substrate thickness.
Third, the use of heterojunction designs and structures as well as formation methods which may be integrated and used in conjunction with the disclosed high efficiency capable cell structures further extends the efficiency and cost per watt capabilities of back contacted, back junction cell concepts.
Fourth, aspects of the disclosed subject matter are also directly applicable to cells formed from non-silicon material—for example direct bandgap materials such as GaAs. For example, cells utilizing these materials may also benefit from a mechanically supportive backplane and an affordable release layer or cleaving region.
Fifth, the use of thermally sprayed metal (or metal deposited using low-cost atmospheric twin-wire arc spray)—for example pure Al or Al-containing metal alloy such as Al/Zn, Al/Cu, etc.—may reduce metallization fabrication equipment and processing cost. Further, the spraying of metal (for example by thermal spray and/or twin-wire arc spray) may be used for an all-dry as well as direct-write metallization process which may reduce propensity of metallization to cause contamination and absorber material lifetime reduction as compared to wet plating processing as metals (such as copper) may reach the silicon area of the solar cell (often accidentally) during or after the wet plating process.
The disclosed subject matter provides structures and processing methods for high efficiency solar cell structures, particularly back-junction back-contacted cells, embodiments including, but not limited to:
Importantly, aspects of the described exemplary process flows and structures presented embodiments are provided as illustrative examples and are not exclusive, but rather may be combined in numerous and various manners and permutations in accordance with the disclosed subject matter. For example, heterojunction passivation on the cell front side may be combined with heterojunction application to the cell back side, as well as implementation of either frontside heterojunction passivation or backside heterojunction passivation. Further, generally the aspects of the described exemplary process flows and structures may be combined to form processes and structures for high efficiency solar cells, particularly cells utilizing thin semiconductor layers (for example cells with semiconductor layers comprising silicon and/or gallium arsenide and/or other semiconductor material layers with the active absorber layer thickness in the range of below 1 micron up to approximately 100 microns). And while the solar cells disclosed herein are described using silicon as the active light absorber material—and corresponding dopant sources and glasses suitable for silicon are detailed—aspects and concepts provided in the detailed structures and process flows are transferable to other light absorbing materials, such as compound semiconductors (for example GaAs, etc.), the use of release layers such as porous silicon, backplane reinforcements, back contact, laser ablation, and metallization schemes. Thus, the disclosed solar cell structures and fabrication methods may utilize and are applicable to alternative light absorbing materials and solar cell fabrication materials and processes.
Additionally, although the following solar cells and process flows are generally described for n-type cells having, for example, a phosphorus base, and subsequent emitter formation by borosilicate glass deposition(s) (BSG), phosphosilicate glass (PSG) deposition, and subsequent diffusion to generate emitter and base contacts, respectively, the disclosed subject matter is transferable to a p-type base cell. For example, a p-type base cell having, for example, a boron doped base, and emitter formation by PSG and BSG deposition and diffusion to generate n-type emitter and p-type base contacts to the p-type cell, respectively.
The following outlines various partial structures and processing methods for the manufacturing of a variety of solar cell structures by a variety of methods. Alternative options for the partial structures and methods may be further detailed herein. And wherever not explicitly excluded, the exemplary list of partial structure and method options provided below may be combined. Importantly, processes methods may also occur at different points in the overall solar cell fabrication process flow and this disclosure is intended to cover all such permutations. Partial structures and methods include, but are not limited to:
Relating to steps 1A1, 1A2, 1A3, 1A4, 1B1 and 1B2 in relation to the process flow embodiments of
1A1. In the case of epi (short for epitaxy) based cells, a reusable template (for example made of p-type silicon doped with boron) may be utilized. After cell process cycles, the starting or re-used template may receive a bevel grinding to remove deposition residue of the epitaxial layer from the bevel as well as to form a bevel which lowers the propensity of template chipping or breakage during on-template processing and subsequent reuse cycles. Further, after cell process cycles, templates may also undergo a removal step removing epitaxial layer residue on areas detrimental for subsequent processing. Example residue removal processes may used known methods as well as grinding steps, either closed to the edge, such as bevel or near bevel grinding, or area grinding or lapping, optionally followed by polishing, wherever needed, if—mainly due to defectiveness—epitaxial layer residue is found in the active area.
1A2. Subsequent template cleaning steps may include the removal of porous silicon layer residue, silicon etching, as well as the removal of metal and optionally of organic residues from the template.
1A3. A porous silicon layer structure may be formed by electrochemical etching. For example, a porous silicon layer comprising at least two zones with different porosity values may be formed, where a lower porosity zone is formed (for example having a porosity value less than approximately 35%) on which the epi layer is to be deposited on, and a high porosity zone is formed (for example having a porosity value greater than approximately 50%) in the region closer to the template to aide in epi layer release. In a pre-epitaxial deposition hydrogen bake the top porous layer (the layer having a lower porosity) then reforms to close the pores to constitute an effective starting monocrystalline seed layer for subsequent epitaxial layer formation, and the buried high porosity layer reforms to form a designated mechanically weak layer (for subsequent on-demand lift-off/release). For example, at step 8 after the backplane reinforced epitaxial film may be split from the template over the entire cell area, and processed further to complete formation of a solar cell and template may be re-used in an additional epi layer formation process after above described epitaxial residue and porous silicon residue removal (template reconditioning) and template surface cleaning. Inspection steps are implicitly included in this process loop.
The epitaxial layer deposition may be performed in an atmospheric pressure high-productivity batch or in-line epitaxial reactor using, for example, low cost and abundant silicon containing precursors such as trichlorosilane (TCS), dichlorosilane (DCS), silane (SiH4) or silicon tetrachloride (STC).
1A4. During the silicon deposition, the film may be in-situ doped, for example with phosphorus for n-type back-contact/back-junction solar cells. For an n-type base solar cell the dopant atom may be phosphorus, for example incorporated by controlled addition of PH3 during the epitaxial layer formation. Further, this addition of PH3 may be varied during the deposition process to form zones of higher and lower doping or graded doping, for instance to reduce the base resistance by incorporating one or more thin higher doped layers (for example with doping in the range of 5×1015 cm−3 to 5×1018 cm−3in conjunction with a lower doped base region with an in-situ base doping in range of 5×1014 cm−3 to 5×1015 cm 3). In a suitable depletion mode reactor, gas flow direction may be switched several times, for example at least once per designated doping zone, for form a designated doping profile
Doping profiles may provide several advantages, including: A higher doping level close to the sunny side of the device (in other words the first part of the epi deposition in the case of a back-contact/back-junction cell) may be used to generate an in-situ front surface field (FSF) while at the same time reducing the effective base resistance. And a higher doping level at the back side of the device (in other words the last part of the epi deposition) may be used to reduce the depletion region width of the junction by producing a back-surface field (BSF) layer. Further, a buried layer having a higher doping level (for example one or multiple layers embedded throughout the base region) may reduce the effective base resistance while allowing for a relatively low base doping and very high minority carrier bulk lifetime (over at least 100 microseconds up to over 1 ms). Both BSF and FSF layers may be used to reduce the base resistance.
1B1. In contrast to the vapor-phase epitaxial cell flow, a wafer based flow starts with a sliced wafer. Starting wafers may be any shape such as full squares, with just a minor chamfer at the edges, or pseudo-squares, having sizes in the range of 125 mm by 125 mm or 156 mm by 156 mm on a side, or as large as 210 mm by 210 mm and larger.
1B2. The starting wafer may be cleaned and saw damage removed. Saw damage removal may be performed using an acidic chemistry (such as hydrofluoric acid and nitric acid) or an alkaline chemistry (such as heated potassium hydroxide (KOH)). High temperatures and high concentrations of KOH may make the process less faceted as the KOH process gets more isotropic in these ranges. Subsequent processes may benefit if such wafers, as part of the saw damage removal, also receive a chemical polishing, such as with a concentrated mixture of hydrofluoric acid and nitric acid (HF/HNO3). In some instances, chemical polishing may applied only to the back side of the wafer at this time to consume as little silicon as possible at this time to retain the wafer mechanical stability and strength to reduce mechanical breakage yield during wafer processing. Alternatively, chemical polishing may be two sided.
HF/HNO3 saw damage removal and chemical polishing steps may be followed by KOH based removal of surface stains of porous silicon generated during this etch. After KOH application, the surface may be treated for metal removal using chemistries such as those containing HCl or mixtures of HCl with HF, or HCl with hydrogen peroxide (H2O2). In some instances for a hydrophobic layer prior subsequent processing steps, the wafer clean and saw damage removal may end by exposing the wafer surface to HF, either by immersion, spraying or application of vapor. A final rinse with ultra-dilute hot HF (for example with HF concentrations well below 1%) may also be applied at this point.
The prepared CZ or continuous growth CZ starting wafers (formed from a continuous growth ingot) may then have an essentially uniform starting base doping (for example a wafer sliced from a CZ ingot or from a seeded cast monocrystalline silicon brick). Continuous growth CZ may then be applied, for example in the case of n-type cells, to generate silicon wafers with suitably tight distribution of resistivity to enable both a high lifetime as well as a suitably low base resistance, which may be particularly applicable when the wafers are thinned as part of the overall cell process.
Where needed, it is also possible to overcome non-uniformity of doping across an ingot by applying a heavier n-doped layer to the back of the wafer in order to have comparable base resistance. Such heavier n-doped layers may be applied by using a dopant precursor such as POCl3, PSG or a phosphorus based implant with subsequent anneal. In this case, a lower bulk doping may be used which generally enables higher lifetimes.
2. After epi deposition (or after saw damage removal and optional backside chemical polishing for wafer based cells), a sequence of process steps is employed to prepare and structure the emitter junction and doped contact regions on the cell backside (for back-contact/back-junction cells). These processes are shown as steps 2, 2B and 3, 3B1 and 3B2 in
Cells and cell processing may differ substantially depending on whether the backside structure (including emitter and base contact region) is formed by homojunctions or heterojunctions. It is to be noted in the following process flows that the thermal anneal step (icon 3) is omitted in the case of the heterojunction back contact region.
Homoiunction backside contacts.
Cell fabrication processing to generate the structures shown in
In one uniform homojunction emitter formation embodiment (which may be referred to herein as a cascaded or cascade process as the deposition alternates or cascades between glass deposition steps and laser patterning steps), boron doped silicate glass (BSG) is deposited on the cell backside to form the precursor for the boron doped emitter, and the base contact region may then be defined by laser ablation (for example using a picosecond (ps) laser) of the BSG layer above the selective base contact regions.
In a selective homojunction emitter formation embodiment, the main lightly doped emitter junction region is defined by depositing a lower boron concentration or thickness BSG glass, which then may be followed by laser ablation (for example using a ps laser) of the BSG glass and local deposition of thicker or more heavily boron doped glass, then the base contact region may be defined in a similar manner as described above for the uniform emitter. For example, base contact regions may formed as regular parallel lines, or discrete or isolated (or interrupted base) islands in various shapes in various shapes which may in some instances reduce electrical shading underneath the base contact region as carriers generated underneath the base contact regions may have on average a closer path to the emitter for collection.
After the base contact region is defined, a phosphorus doped silicate glass (PSG) may be deposited in the opened regions.
And while the process flow above are based on forming an main emitter junction region using a BSG layer (followed by laser patterning and furnace annealing), it is also possible to form the emitter region directly by in-situ boron doping (changing the doping from phosphorus for the base to boron for the emitter junction) during the final stage of silicon epitaxy (followed by laser ablation of p+ doped emitter to pattern the emitter region).
And in the case of the wafer based cells for which the silicon is not to be thinned after lamination, it is also possible to use PSG or POCl3 as a dopant on the front side for the formation of an ex-situ FSF, if previously an essentially single sided texturing step has been applied—as depicted as a process flow option in
The cell may then be thermally annealed (for example in a tube furnace), which serves to drive in the dopant underneath the glass regions. Introduction of an oxidizing ambient during at least part of the anneal may generate a thin thermally grown passivation oxide at the original silicon oxide interface for high-quality backside passivation and low back surface recombination velocity (BSRV). The annealing process may also act as an impurity gettering step.
And in a case where the frontside has an FSF dopant recursor, this FSF may be driven in at the same time as the dopant.
For wafer based cells, as an alternative to the PSG or POCl3 precursors, the FSF dopant may be supplied by ion implantation of an n-type dopant (such as As, P, or Sb in the case of Si) into the textured frontside of the wafer. The implanted dopant may then be annealed and activated in the same step that drives in the junctions on the backside of the device.
Hetero backside contact formation.
As an alternative to depositing/growing layers of tunnel nitride or oxide (or oxynitride), as well as the a-Si and the poly-SiGe, in the epitaxial reactor, these layers (or a portion thereof such as the a-Si and doped SiGe layer) may be deposited in an atmospheric pressure chemical vapor deposition (APCVD) reactor (in one instance without breaking the controlled atmosphere between depositions). And particularly, but not exclusively, relevant for wafer based cells, such deposition/growth processes may also be performed in plasma-enhanced CVD (PECVD) or low pressure CVD (LPCVD) systems. Alternatively, the tunnel dielectric may be formed in situ and at the end of the epitaxal growth process in the silicon epitaxial reactor followed by formation of a-Si and doped SiGe layers in an APCVD reactor. With the described processes concluded, the base contact and emitter regions on the backside for the various options have been formed and the cell is ready for subsequent processing.
Relating to the patterns and the structure of the contact openings (formed, for example, using a laser processing, such as picoseconds laser) shown as step 4 in the process flows of
After contact opening, the first level metal may deposited (shown as steps 5A1, 5A2, 5B1 and 5B2). First level metal deposition options include, but are not limited to, physical vapor deposition (PVD) metallization (for example plasma sputtering or evaporation) of metals such as Al or Al with an Si component (to reduce junction spiking) and provide, in some instances, improved ohmic contact to both n and p-type silicon, followed by Ni or NiV and optionally Sn (or another solder material). After the first level metal deposition, the relatively thin first metal layer (for example the stack having a thickness less than approximately 2 microns and in some instances less than approximately 1 micron) may be patterned by ablation, for example using a pulsed ps laser. Optionally, after this patterning, additional metal may be locally applied, for example by screen printing of conductive epoxy, in the areas where contact to the second level metal will be made.
As an alternative to a PVD metallization scheme, the first level metal may be printed (for example by ink jet, aerosol jet, or stencil or screen printed) using a metal paste of pure Al or an alloy of Al with an Si content to reduce spiking. The paste may be dried after application, and optionally may be applied in a single or dual print step where a second layer is only printed where desired (for example in an area around the designated contacts to the second level metal). The paste may or may not contain frit, depending on whether the etching assistance of the frit is required for good contact resistance. In some instances the paste does not contain frit so a nested design is not required. For example, a nested design is a design where the extent of each polarity metal (base contact metal and emitter contact metal) does not exceed the extent of the respective base or emitter contact region. The paste final anneal to form a low series and contact resistance metal layer may optionally be performed in more than one heating step. The deposited first level metal may also be annealed using lasers, for examples using nanosecond (ns) or ps lasers, or an annealing step may combine laser and thermal anneal processing. Relating to laser annealing, different laser conditions may be utilized for different areas on the cells—for example to drive Al deeply into the silicon using a higher laser fluence in the designated emitter contact areas to form Al—Si eutectic, thus forming localized heavily doped p++ emitter contacts in conjunction with lightly doped selective emitter junction structures.
Further, first level metal designs and patterns may comprise a busbar-less pattern (to eliminate electrical shading) of interdigitated fingers (long parallel fingers of alternating contact polarity) which may different shapes (such as an interleaved multiple “H”-bar shape which allowing for a more relaxed (wider) second level metal design).
Relating to process flow steps 6, 7A and 7B: After first level metal formation is completed, the backplane may be applied (for example by lamination or thermal spraying, printing such as screen printing, or other coating method). In some instances, particularly relating to an epitaxial layer based cell flow, before backplane application an optional pre-lamination cut, for example in the peripheral areas of the device or even outside of the active device, may be performed. In the case of lamination of a backplane in sheet format, backplane materials include materials such as prepreg and fiber enforced resin sheets. For printed backplanes, materials include mixtures of resins and fillers such as small glass beads or fibers. In both cases the material mix may optimized for adhesion, Coefficient of thermal expansion (CTE) matching, and chemical and thermal resilience. Further, in the case of a printed backplane material (such as a screen printable dielectric paste) the backplane pattern may be directly printed with via holes open or partially open for contact between the first metal level and a subsequent second level metal. Direct backplane pattern printing may be especially applicable when front side texture is performed prior to backplane lamination—such as when using a pre-textured template which transmits texturization to the conformally deposited eip layer. However, if needed single sided texturization may be performed using a single sided texturing machine. And for unpatterned backplane material embodiments (laminated or otherwise applied), the backplane itself may acts as a protection layer during subsequent texturing.
Relating to step 8 in the disclosed process flows: After the lamination, in the case epitaxial wafer based cells, the backplane reinforced epitaxial cell structure is released and lifted off from the template. The template may then returned to the beginning of the template reuse cycle for the fabrication of multiple cells. The backplane reinforced epitaxial cell structure, also called thin film solar substrate (TFSS), may then be trimmed (for example by laser) around the edges to define cells outer periphery geometric outline.
Importantly, alternatively to the use of porous silicon as a designated release layer, the active cell area may be cleaved or released from a template or carrier wafer utilizing alternative cleaving mechanisms or separation methods which may be readily integrated with the described fundamental process flows and structures. For example, the formation of a mechanically weak layer using the implantation of hydrogen ions (or proton implantation) to a controlled depth in order to release the structure along the line defined by the implant depth. Alternatively, laser processing may be used to cleave/separate a controlled thickness from a source wafer. For example, one or a plurality of closely focused laser beams (focus in the um range) are scanned across the wafer from the side opposite of the backplane material. These laser beams are focused at a depth that defines the thickness of the layer to be cleaved off. Similar to the use of porous silicon as the release layer, after this release cleaving step the source wafer may undergo a re-use process sequence and the released backplane reinforced cell structure is processed further. Other release/separation methods include stress-induced splitting and exfoliation of the thin semiconductor layer from a reusable wafer using a thick metal layer to produce a controlled stress field.
Relating to step 9 in the disclosed process flows: in the case of excessive cell bow after lamination (and after cell release), and particularly relevant to cells that utilize releasing from a reusable template or are based on a source wafer, a flattening step (for example a thermally assisted flattening step) may be employed to flatten the TFSS and reduce cell bow for improved handling through high volume, high throughput subsequent process tools.
In subsequent texture and post texture cleaning steps, shown as step 10 in the disclosed process flows, the epitaxial cell based TFSS may undergo selective wet removal of the remnants/residue of the porous silicon layers, namely any remnant of the high porosity layer and the quasi monocrystalline silicon (QMS) layer which has formed after the reflow bake of the porous silicon prior to the epitaxial deposition. For monocrystalline semiconductor material, alkaline texturing process will remove the reside layers and also may result in randomized pyramidal texture formation resulting in improved solar cell light trapping and reduction of the optical reflection losses. Alternatively, a designated etch-back step may be employed to remove the QMS layer and part of the starting epitaxial layer which is then followed by a texturing process. Alternatively, dry texturing steps such as laser texturing or plasma texturing (with or without a final post-texture wet cleaning step to remove any residual dry texture damage) may be employed. Before and after the texturing organic residue may be removed from the cell surface, for example using chemistries such as mixtures of potassium hydroxide (KOH) and hydrogen peroxide (H2O2). After the texturization and prior passivation, a metal removal step may be performed to remove metal contamination from the surface, for example utilizing hydrochloric acid (HCl). Further, in some instances prior to passivation it may be desired to leave the surface hydrophobic. In the case of crystalline silicon based cells, this be accomplished by various implementations of hydrofluoric acid (HF) application, such as a dip, an ultra-dilute dip, a combination with HCl, a vapor-HF step or a drying using ultra-dilute HF.
Wafer based cells also may undergo texturing and subsequent clean at this point in the fabrication process (unless performed earlier in the process flow as shown in steps 1B2 of
Optionally, and as shown in step 10 in
After texture and post texture clean steps, front surface passivation steps may be performed—shown as steps 11A, 11B and 12 in the disclosed process flows. For both a wafer based cell and an epitaxial layer based cell, passivation steps may comprise the deposition of thin intrinsic amorphous silicon (a-Si), or a-Si with sub-stoichiometric oxygen content (a-Si—O), or a-Si with sub-stoichiometric nitrogen content (a-Si—N), or a-Si with sub-stoichiometric carbon content (a-Si—C), or a or a-Si with sub-stoichiometric oxygen and/or nitrogen and/or carbon content (a-Si—ONC). This deposition may be optionally followed by an n-doped a-Si layer forming a heterojunction front side passivation and generating a minority carrier repelling field. The total thickness of the amorphous silicon frontside passivation layer may be less than 100 angstroms. In either case, a subsequent (for example in the same tool) silicon nitride layer may deposited which may serve as donor of hydrogen for further passivation and provide fixed positive charges to repel minority carriers (holes) from the surface, as well as forming and anti-reflective coat (ARC) to help couple light into the cell (reduce optical reflection losses). Alternatively to silicon nitride, other combinations, such as SiCxNy, may also be used as an ARC.
An optional flattening step, shown as step 13 in the disclosed process flows, combined with a thermal anneal may be employed after ARC layer deposition (or alternatively after any subsequent process step). Vias in the backplane providing contact to the first layer metal may then be formed by drilling vias through the backplane material (as shown in step 14). Patterned vias may drilled by ablating backplane material using lasers (for example CO2 lasers) to expose the underlying first level metals.
After exposure of the first level metal (for example via drilling through the backplane) process, a second level metallization may be formed. A two-level metal architecture formed in conjunction with a backplane supported thin cell enables the fabrication of low-cost, large-area, high-efficiency solar cells. Second level metal formation is shown as steps 15A and 15B1 and 15B2 in the disclosed process flows. The second level metal, for example an interdigitated pattern of rectangular or tapered fingers may be patterned orthogonally to the first level metallization—in other words the interdigitated fingers of M1 and M2 are positioned substantially perpendicular—which allows for a substantially less number of conductor fingers on the second metal level and relaxed design rules for the second level metal (millimeters or centimeters versus hundred(s) of micron for the first level metal).
Relating to the second level metallization: The second level metal may be formed using a metal plating sequence comprising a seed layer deposition (for example using PVD or electroless plating) and an optional pre-PVD surface clean etch to clean the surface of the first level metal exposed at the bottom of the via hole. For example, a PVD layer stack may comprise Al followed by NiV and Sn. After seed layer deposition, the areas to be plated may be defined using for example screen printing of a sacrificial resist. In subsequent plating, metal (for example copper Cu) followed by Sn may deposited by electroplating. To protect the front (sunny) side of the cell during plating, the front side may be covered with a temporary strippable or permanent transparent protection layer which is removed/stripped off after the plating. Alternatively, cell fixturing protecting the cell active side from plating chemistry exposure may be employed during plating. In some instances, a second level metal pattern retracted from the edge further than the first level metal may be advantageous for such fixturing. Dual layer (multi-layer) metallization allow for both fabrication process flexibility and cell mechanical flexibility. Following the electroplating process, the resist, the optional front side protection layer, and subsequently the exposed seed layer metal are stripped, and after an optional anneal the cell may then be ready for electrical testing and module integration.
As an alternative to a PVD seed for plating, the seed may also be prepared by a screen or ink jet printed metal paste or ink. With suitable fixturing, the screen print patterning for the plating may be circumvented, if the seed layer is applied in a patterned form. Alternatively, the second level metal may be formed as a thermally sprayed metal, for example using a direct write metallization process, to further reduce manufacturing costs by using a low capital and consumable cost approach.
Backplane reinforcement in conjunction with back contact architecture allows for comparatively relaxed second level metallization requirements in terms of pattern design and fabrication processing. Thus, the equipment required for second level metallization, and particularly when used in conjunction with the relaxed geometries, may have a relatively low level of complexity. M2 metallization materials, to transport/carry the cell currents, include, but are not limited to aluminum or a material containing a high percentage of aluminum—thus resulting in a relatively low cost versus conductivity ratio. Alternative M2 metallization materials include, for example, copper, layers of aluminum with copper, and/or zinc or alloys of aluminum with copper and/or zinc.
Prior to thermally spraying (using flame spray and/or arc spray) the second layer metal, an optional cleanup and surface preparation step may be employed that prepares the bottom of the drilled via for a low resistance contact. Such surface preparation may include plasma steps (such as plasmas of oxygen and/or hydrogen) to remove carbon residue from the backplane material, as well as hydrogen or forming gas plasma (for example at atmospheric pressures) to remove native oxides, such as those found on Al paste or Al with Si or other metals after CO2 laser drilling. Oxide removal may be performed using lasers such as pulsed nanosecond or picosecond lasers. Alternatively wet chemical dips or chemical vapors may be employed to remove such oxides. In some instances, oxide removal may performed in an integrated tool without breaking the controlled atmosphere an ensuing flame spray deposition is carried out in. Metals materials to be thermally sprayed include, for example Al and alloys and/or layered stacks of Al and Zn (or Al and Cu or Al and Cu and Zn) to allow subsequent contacting (particularly at the module level). Pre-cleaning in a hydrogen plasma may be combined with metal spray deposition. In addition, the deposition may begin with one metal, for instance pure Al being sprayed by a twin or two wire ARC spray (TWAS), and using the same machine an additional metal or alloy may be deposited using a simpler single wire fed spray.
Second level metallization patterns include, for example, alternating/interdigitated linear parallel strips of metal with or without bus bar connections on the edges of the wafer. If the second level metal is without bus bar connections on the wafer, then the bus connection may be part of the module stringing or each strip segment may connected one by one to the neighbor cell in the module.
Thermally sprayed patterning embodiments include, for example: blanket metal deposition with subsequent masking and etching to generate the metallization pattern. Alternatively and in some instances reducing manufacturing cost, the second level metal may be deposited through a shadow mask comprising the metallization pattern information. In this case, the mask may be cleaned of deposited metal residue after every or several depositions or may consist of a disposable low cost material (for example a thermally and chemically resistant paper or film).
In certain second level metallization patterns having relaxed pattern geometries (in the range of several mm to cm range) the metallization pattern may be formed be direct write—for example using a plurality of nozzles or by using a scanning mechanism for either the nozzle or the cell to be coated.
In some instances, for example when pure Al is sprayed onto the cell, an optional second level metallization step may comprise localized printing or spraying of solderable contacts, such as copper and/or Ni and/or Sn and/or Zn or other metals or alloys, for example deposited in the same tool or without breaking the controlled atmosphere. For instance, in one embodiment Al (or an alloy of Al with another element such as Zn or Cu) may be used as the main metal 2 structure and a thin layer of Cu or Sn may be deposited on top for solderability.
A controlled atmosphere for thermal or arc plasma spray process or processes include oxygen reduced or oxygen free atmosphere, such as nitrogen, forming gas, hydrogen, or an inert ambient such as argon or helium (with closed loop capture and recovery of the gas if desired). After an optional final anneal to optimize especially the front side passivation as well as the second level metal layer and contact, shown as step 16 in the disclosed process flows, the cells may then be ready for test the sort, shown as step 17 in the disclosed process flows. Alternatively, a final anneal may be employed where needed at any time after the passivation layer has been applied.
Thermal Spray integration schemes for high efficiency solar cell manufacture. The following relates directly to the concept of thermal or arc plasma spraying on back contacted solar cells, particularly used in conjunction with integral second level metal. Thermal spray or arc plasma spray (flame, arc or twin arc) has been demonstrated for front sided crystalline, multi-crystalline and polycrystalline solar cells where the molten species of metal is directly deposited onto the silicon cell and utilized as an ohmic contact, or as a diffusion source, and the dopant may be the metal species itself or a metal alloy where the incorporated species is the dopant. However, thermal spraying suffers from challenges when applied to silicon solar cell manufacture and particularly back contact solar cells, including: 1)Thermal spray directly onto Si may not offer a controlled heat zone and hence does not offer a controlled intentional or unintentional diffusion front; and 2) Patterning of metal on front sided silicon solar cells requires fine geometries in order to reach high efficiency by reducing reflective areas and hence optical shading. Utilizing metal masks or direct write with fine thermal sprays soon reaches the limit of the critical dimensions required to support such fine geometries. Recent progress on high efficiency solar cells requires further improvement of reducing metal coverage on the front surface to zero metal (with back contact metal only). Other areas for high efficiency include reducing the thickness of crystalline silicon, to allow lower tolerances on material lifetime and advanced surfaces, minimizing surface recombination and maximizing anti reflection coatings. Back contact metallization, thinner silicon absorber thickness and especially two-level metal cell architecture open up new possibilities for thermal spray metallization. The rear two-level metal scheme relaxes the critical dimensions for metal printing. Thin silicon requires mechanical support provided, as disclosed herein, by bonded low cost substrates—thus the thermal sprayed material may be predominantly located on the support material, buffering the Si from the deposition heat zone.
High efficiency solar cells, which offer peak currents around 10 A for 156×156 mm square cells, require low contact resistance and high conductivity metallization to transport large currents to the remaining cells in the module. In some instances, the application of pure aluminum, zinc, copper and tin and their corresponding alloys, often utilizes PVD and plating which may be considered expensive for solar cell manufacturing. And in some instances, the use of pastes, where glass frits provide etching through oxide layers, may lead to good contact resistance but may be compromised by poor bulk conductivity.
The back/rear contacted silicon cells as disclosed herein allow for relaxed metallization geometries and patterns—particularly relating to the second level metal. For example the two-level metallization scheme with backplane reinforcement material of the thin cell also serving as the inter-metal dielectric and the orthogonal metal 1 metal 2 pattern transformation. This structure enables a coarse metal 2 pattern which in turn allows the use of thermal or arc plasma spray through metal masks for direct write metallization, or using conformal coating followed by etch back methods. And while described with reference to a crystalline silicon solar cell, the structures and processing methods disclosed herein are not limited to this material set or state. Importantly, this scheme may be applied to different crystalline states of silicon, including multi and poly-crystalline as well as semiconductor systems such as GaAs, CIGS and CdTe. Further, the two-level metallization scheme may be applied to any silicon absorber thickness including a standard thickness silicon (120-160 um or thicker) or thin, supported silicon (5 to 50 microns).
For thicker, unsupported silicon the thermal spray material may sit on top of thermal or deposited oxide and electrically contact silicon through open windows in the oxide or via paste pillars. Further, standard thickness silicon may also benefit from having a backplane which enables a second layer metal at relaxed pitch. The cell cross sectional and top view diagrams of
For backplane supported thin silicon, the backplane acts as a dielectric. Vias/holes are formed in the dielectric to gain access either directly to or to the underlying metal 1 layer and the deposited second level metal M2 may be patterned on the top surface of the backplane. The first metal pattern M1 under the dielectric (which may be orthogonal or parallel) to a thermally sprayed second level metallization M2 pattern deposited on top of the dielectric.
The foregoing description of the exemplary embodiments is provided to enable any person skilled in the art to make or use the claimed subject matter. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the innovative faculty. Thus, the claimed subject matter is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
It is intended that all such additional systems, methods, features, and advantages that are included within this description be within the scope of the claims.
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/619,300, filed Apr. 2, 2012, which is hereby incorporated by reference in its entirety. This application is a continuation-in-part of U.S. patent application Ser. No. 13/731,112 filed Dec. 31, 2012, Ser. No. 13/807,631 filed Dec. 28, 2012, and Ser. No. 13/204,626 filed Aug. 5, 2011, which are hereby incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61619300 | Apr 2012 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13855657 | Apr 2013 | US |
| Child | 15478140 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13731112 | Dec 2012 | US |
| Child | 13855657 | US | |
| Parent | 13807631 | Sep 2014 | US |
| Child | 13731112 | US | |
| Parent | 13204626 | Aug 2011 | US |
| Child | 13807631 | US |
