Patent Application
20160190382
The present disclosure relates in general to the fields of solar cells, and more particularly to solar cell doped regions.
As solar photovoltaic technology is adopted as an energy generation solution on an increasingly widespread scale, improvements relating to solar cell efficiency and fabrication are required. Generally, solar cell structures often include passivation surfaces—for example frontside or light receiving (sunnyside) surface passivation and backside surface passivation opposite the frontside. Surface passivation and doped base and emitter formation processes are often complex and employ mechanically or thermally stressful processing.
Additionally, manufacturing cost and conversion efficiency factors are driving solar cell semiconductor absorbers ever thinner in thickness and larger in area. Thin semiconductor absorbers and corresponding thin semiconductor absorber solar cell structure aspects/components have increased fragility and are more sensitive to temperature and mechanical processing, thus, complicating and introducing challenges in the processing of these thin absorber based solar cells.
Therefore, a need has arisen to go to the highest possible efficiency which can be attained using best possible passivation, while keeping lowest possible temperatures, reducing the complexity of integration, and reducing the capital expenditure for manufacturing. In accordance with the disclosed subject matter, surface passivation and doped base and emitter formation processes are provided which may substantially eliminate or reduce disadvantage and deficiencies associated with previously developed good surface passivation but highly complex and capital intensive manufacturing methods.
According to one aspect of the disclosed subject matter, a method for passivating a silicon surface and forming doped base and emitter regions in a silicon substrate is provided. Intrinsic amorphous silicon is formed on first surface of a silicon substrate. A first doped layer is formed on the intrinsic amorphous silicon. A first laser beam is applied through the first dopant and forms a first doped region in the silicon substrate. A second dopant is formed on the intrinsic amorphous silicon. A second laser beam is applied through the second dopant and forms a second doped region in the silicon substrate.
These and other aspects 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 claimed 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 that are included within this description, be within the scope of any claims.
The features, natures, 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 aspects and identifiers 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 and components, such as a back contact back junction (BCBJ) silicon solar cell, one skilled in the art could apply the principles discussed herein to other solar cell structures (e.g., front contact or back contact front junction), fabrication processes (e.g., various deposition methods and materials such as metallization materials), as well as alternative technical areas and/or embodiments without undue experimentation.
Fabrication methods and structures are provided for the doping and passivation of solar cells. High efficiency silicon based solar cell structures and their manufacturing methods are characterized by a passivation of hydrogenated amorphous silicon or a variant produced by adding small quantities of carbon (SiCx), oxygen (SiOx), or nitrogen (SiNx), as a passivation followed by laser doping of a dopant to make diffused junctions. Note, as described herein, amorphous silicon should also be interpreted more generally to include SiOx and SiCx and SiNx variants and embodiments.
The solar cells described herein are back contact cells, however, the innovations provided herein may also be adopted for front contact solar cells. The solar cells described herein may be thin solar cells for example having a silicon absorber layer thickness in the range of 5 μm to 120 μm. Thin solar cells may be manufactured using the backplane embodiments are provided. However, the passivation and doping fabrication methods and structure should not be interpreted in a limiting sense and are applicable to thicker (e.g., thicker than 120 μm) and thinner solar cells.
While the exemplary solar cells provided use n-type starting substrates, the innovations provided herein including passivation and doping are applicable to p-type starting substrates and the flow sequence and base/emitter may be modified appropriately.
Additionally, although the solar cells described herein are single crystal solar cells, for example because of low temperature processing, the cell designs and manufacturing methods are also applicable for multi-crystalline solar cells. For multi-crystalline solar cells for example, at the fabrication onset a high temperature gettering step may be performed to increase lifetime. N-type multi-crystalline substrates in excess of 500 us are possible after gettering. Subsequently, using the fabrication processes provided the temperature of the cell is always kept relatively low (e.g. less than 350° C.), thus maintaining the lifetime which was achieved after the gettering at the onset. This is especially attractive when the multi-crystalline cell is also thinner as well as manufactured with low temperature processes which may lead to a very high efficiency multi-crystalline solar cell. In addition to multi-crystalline solar cells, the manufacturing flows and structures in this document can also be applied to epitaxially grown silicon solar cells.
Unlike a traditional silicon heterojunction solar cells which are made with amorphous silicon passivation, the amorphous silicon layer (and its variants) as provided herein is not necessarily a transport layer because of the presence of diffused junction. And while, as provided herein, hydrogenated amorphous silicon (and its variants) is a superior passivation, hydrogenated amorphous silicon (and its variants) may also be the dopant source for laser doping. This may be especially advantageous as during laser doping both amorphous silicon as well as the underlying crystalline silicon layer, to a controlled depth, are melted which, in turn, results in a high dopant solubility—thus, achieving localized doping where the laser hits the wafer without raising the temperature of the whole silicon wafer, which may preserve high quality passivation as well as bulk wafer lifetime Upon cooling and ensuing re-crystallization, the silicon is not only doped (as diffusion starts at the surface) but the contact is accessible at the surface—thus a self-aligned contact with a doped area conductive and available for contact—while areas which did not see laser remain insulating.
It is to be understood that hydrogenated amorphous silicon a-Si as a layer or film, as described herein, may also be variants of a-Si film as is common in the technical area such as amorphous SiCx, amorphous SiOx, and amorphous SiNx. Variant layers or films may include but are not limited to amorphous hydrogenated SiCx, SiOx, SiNx. In these films a small amount of carbon, oxygen, and nitrogen may be introduced, respectively. Amorphous silicon and its variants may retain high passivation quantity, may be made both n and p-type, may help increase the bandgap depending on the concentration, and in certain cases may help improve thermal stability at higher temperatures (e.g., greater than 300° C.).
The solar cell structures provided herein may retain advantages of high efficiency silicon heterojunction solar cells without processing related drawbacks. For example, the solar cell structures provided may have some or all of the following advantages: capable of very high efficiency, for example approaching 26%, especially when integrated with the back contacted architecture; do not have high temperature processing steps (e.g., no greater than 400° C.) thus maintaining the pristine lifetime of the initial substrate; retain high temperature coefficient of efficiency, for example typically less than −0.3% /C.
Additionally, as compare to silicon heterojunction solar cells, the manufacturing methods provided herein may be robust and have reduced complexity. For example, manufacturing methods provided may have some or all of the following advantages: a solar cell fill factor FF independent of the thickness of the amorphous silicon, for example because the FF is controlled by the laser doped contact, which relaxes and/or removes thickness constraints of the amorphous silicon (traditionally amorphous silicon thickness constraints have made silicon heterojunction solar cells in general and back contacted silicon heterojunction solar cells plagued by narrow process windows and FF problems); capital expenditures reduction, for example in certain fabrication embodiments provided herein only one PECVD and PVD tool is required; ITO and Ag PVD (relatively expensive materials) are not required; because of lack of full area emitter, the parasitic free carrier absorption is mitigated; and, simplified process flows are provided. Additionally, the fabrication processes provided herein may withstand higher temperatures (e.g., above 300° C.) without degrading performance. In other words, as is known in the art, it may be difficult to increase the temperature of a device having p+ amorphous silicon above 250° C. without degrading its passivation quality, while intrinsic and n+ amorphous silicon layers are more thermally stable up to about 350° C.—in several fabrication process flows provided herein, the solar cells does not require p+ amorphous silicon, thus enabling it to withstand and maintain high quality passivation up to higher temperature (e.g., up to approximately 375° C.).
Aspects of the solar cell provided herein may include, for example: no continuous emitter, so the point contacts in the form of p+ doping (e.g., for n-type substrate solar cell) should be spaced closed enough together such that there is minimal or no series resistance issues and minimal or no minority carrier lifetime issues; bulk lifetime and surface passivation (frontside passivation and backside passivation) should be high enough quality such that minority carriers (holes in the case of n-type substrate solar cells) may survive longer distances (e.g., with a bulk lifetime of 1.5 ms and relatively thicker hydrogenated a-Si, the surface recombination velocities can be as low as less than 5 cm/s); the thickness of a-Si may be in a much higher range (e.g., 10-300 nm) than the existing silicon heterojunction solar cells as the solar cell structure does not rely on this layer for current transport; in several embodiments, the emitter does not consist of a P+/intrinsic (i) amorphous silicon layer, but a diffused emitter P+ contact connecting to a metal which is otherwise insulated from the main substrate.
Solar cell backside fabrication process flows are provided which may be used to form point contacted back contact back junction solar cells having a thicker silicon thickness (e.g., a solar cell silicon substrate thickness greater than 140 μm) or may be coupled with various backend process flows including those for the formation of a solar cell utilizing a backplane (e.g., a prepreg backplane) to form a point contacted back contact back junction solar cell having a thinner silicon thickness (e.g., a solar cell silicon substrate thickness less than 120 μm). Various process flow options are provided which use depositing metal (e.g., physical vapor deposition PVD) and patterning for the cell base and emitter metallization. For example, metal patterning may be performed using laser processing—green, UV or IR nanosecond or picosecond pulsed laser may be used. Shorter wavelengths may have the advantage of being absorbed in amorphous silicon to ensure that there is minimal damage to the underlying crystalline silicon and thus retention of high lifetime. Metal patterning may also be performed using process such as: screen print resist and wet etch; metal etch paste; and print resist, laser patter resist, and wet etch of metal. These embodiments and others are implicit in the following process tables.
The following tables are provided as descriptive process flow examples for making laser doped, amorphous silicon point contacted solar cells and should not be interpreted in the limiting sense. Fabrication steps are abbreviated as follows: saw damage removal SDR; spin-on doping SOD; plasma enhanced chemical vapor deposition PECVD; physical vapor deposition PVD.
Tables 1A through 6A are distinguished by the types of dopant sources that are used for laser doping for both p and n-type doping. For example, Tables 1A and 2A use n+ doped amorphous silicon as a dopant source, Tables 3A and 4A use phosphorous or other n+ doped spin on dopants (SODs), and Tables 5A and 6A use patterned dopant sources such as a screen printed n+ phosphorous layer. For each of these base or n+ doped options, there are either spin on dopant (SOD) or patterned dopant options for p+ (boron doping). Tables 1A through 6A use metal PVD followed by patterning to create cell base and emitter metal.
Tables 1B through 6B are distinguished by the types of dopant sources that are used for laser doping for both p and n-type doping. Tables 1B through 6B are similar to Tables 1A through 6A. For example, Tables 1 and 2 use n+ doped amorphous silicon as a dopant source, Tables 3 and 4 use phosphorous or other n+ doped spin on dopants (SODs), and Tables 5 and 6 use patterned dopant sources such as a screen printed n+ phosphorous layer. For each of these base or n+ doped options, there are either spin on dopant (SOD) or patterned dopant options for p+ (boron doping). Tables 1B through 6B use patterned metal (e.g., formed using screen print or inkjet processes) to create cell base and emitter metal. Alternatively a combination of PVD and patterned metal techniques and accompanying variants may also form cell and base emitter metal.
Alternatively, another source of p+ doping may be a boron doped PECVD deposited amorphous silicon layer. P+ doping using a boron doped PECVD deposited amorphous silicon layer may be combined with various kinds of phosphorous doped n+ sources (e.g., as provided herein). Tables 7A and 7B below show descriptive process flow examples for making laser doped, amorphous silicon point contacted solar cells example where p+ amorphous silicon and n+ amorphous silicon layers are used as dopant sources. Tables 7A and 7B may be modified to dope using both n+ and p+ amorphous silicon in laser processes on top of each other and relying on counterdoping to do both base and emitter dopings simultaneously—for example by adjusting the thicknesses of the doping layers. Table 7A uses metal PVD followed by patterning to create cell base and emitter metal. Table 7B uses patterned metal (e.g., formed using screen print or inkjet processes) to create cell base and emitter metal.
Alternatively, the p+ dopant source may be aluminum metal. A technique such as laser fired contact through either a PVD AL or Al paste may be used in conjunction with the fabrication methods provided.
The solar cell structures provided have a very high efficiency potential as long as the contact area is kept low/minimal for both n and p-type contacts. Thus, a contact resistivity of less than 1 e-3 may be required and in some instances a contact resistivity more particularly in the range of less than 1 e-4 ohm-cm2. With diffused contacts, it is possible to get Jo contact (dark saturation current density under the contact area to be in 800-1200 fA/cm2). With 1% contact area, the total Jo from both contacts may be kept to less than 20 fA/cm2. With bulk lifetime and cells being thin, the bulk Jo and base Jo may be as low as less than 10 fA/cm2, while the backside thick amorphous silicon passivation is capable of less than 5 fA/cm2 Jo. Combining this may reveal a total Voc potential of greater than 730 mV.
While all the devices discussed above may be considered a class of devices where the carrier transport and photocurrent collection is done using diffused n+ and p+ layers, it is also possible to have devices where the thickness of the amorphous silicon layer is reduced to use it for partial carrier transport in conjunction with the diffused layers—in other words a hybrid device. A hybrid device may explicitly require a p+ amorphous silicon layer as the emitter along with a P+ laser doped contact. Hybrid devices may be especially applicable to cases using SiOx and SiCx where the bandgap is larger than pure amorphous silicon and hence the transport may not be adequate. In such cases, the transport may be supplemented using the diffused contacts.
Key factors increasing the solar cell efficiency of the structures provided is that, in at least in one embodiment they are: back contacted; silicon heterojunction solar cells like without suffering from the process complexities of silicon heterojunction solar cells and silicon heterojunction solar cell fabrication; and may be made ultrathin, for example having a silicon thickness as thin as 10 μm providing 800 mV Voc potential.
As noted, cell efficiency may be enhanced by and particularly advantageous when combining the passivation and doping innovations provided herein with silicon absorber thickness reduction—for example having a thickness less than 120 μm and, for practical purposes, a thickness greater than 5 μm. Thin silicon absorbers benefit from mechanical backplane support and decoupling of thermal stresses, such as for example the supportive backplane and multi-level cell metallization of the solar cell of
A thin silicon absorber based solar cell (e.g., having a silicon absorber thickness less than 120 μm) may utilize: a prepreg supporting backplane; an etch back step to thin down the wafer; monolithic isle (icell) cut technology (e.g., such as that found in U.S. Pat. Pub. 2014/0370650 published Dec. 18, 2014); and an aluminum oxide based front passivation (e.g., such as that found in U.S. Pat. Pub. 2015/0162487 published Jun. 11, 2015 which is hereby incorporated by reference in its entirety and U.S. patent Ser. No. 14/632,696 filed Feb. 26, 2015 which is hereby incorporated by reference in its entirety). Table 8 below shows a descriptive process flow example for making laser doped, amorphous silicon point contacted solar cells having a backplane and multi-level metallization structure.
Fabrication processes shown in Table 8 include laminating a backplane (e.g., prepreg) to a thicker cell, thinning the silicon absorber while it is held and supported by the backplane (e.g., prepreg), texturing the silicon absorber and applying front passivation. Subsequently, laser holes are drilled in the backplane (e.g., prepreg), for example backplane 14 shown in
Solar cell frontside (sunnyside) passivation may be formed with aluminum oxide Al2O3 which has the advantageous features of: providing very low surface recombination velocities, for example less than 10 cm/s; not absorbing readily in the visible spectrum, making it maximally transparent to the wavelengths which form useful electric current; may be stable against UV radiation and meet long term solar cell field reliability requirements.
In some instances, aluminum oxide need a slightly elevated temperature of approximately 300 to 400° C. This elevated temperature may make is necessary to ensure amorphous silicon passivations are either stable during this elevated temperature process or are deposited after Al2O3 films are deposited and activated.
Amorphous silicon passivation may also be used on the solar cell frontside (sunnyside). Amorphous silicon frontside passivation may suffer from light induced degradation, however, if an indium tin oxide ITO layer is used as an anti-reflection coating ARC it may substantially cut down the deleterious UV on the amorphous silicon frontside passivation.
Additional variations of the backside processes are possible, including but not limited to, switching the process order between different process steps such as performing a monolithic isle (icell) cut after the silicon etchback step and then performing a texture.
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.
This application claims the benefit of U.S. provisional patent application 62/036,609 filed on Aug. 12, 2014, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62036609 | Aug 2014 | US |
