The present invention in general relates to semiconductor devices, and in particular to design, fabrication, and use of devices configured as arrays of dome-like (e.g., dome-like, column-like, cone-like, and hemisphere-like) structures that are operative as photovoltaic devices (e.g., as solar cells), as light detectors (e.g., photodetectors), as light producing devices (e.g., OLEDs and LEDs), as photosynthesis devices, and as charge storage devices.
Historically, photovoltaic, and light detection devices such as photodetector devices, lithium drifted silicon based detectors, and photo electron effect devices have been formed as a succession of planar layers constructed to provide a functioning device. However, in the case of photovoltaic devices, for example, the high optical reflectivity of planar substrates and poor optical path length matching, have traditionally caused planar devices to exhibit less than optimum light harvesting capabilities. In recognition of the less than optimum light harvesting capabilities, such planar light-gathering devices are preferentially etched or otherwise textured to promote internal light reflection and trapping within the light harvesting portions of the device. While surface texturing incrementally improves light harvesting efficiency on such devices, the texturing process is far from uniform and inefficient in material usage. U.S. Provisional Application Ser. No. 61/559,065 filed 26 Sep. 2011, the contents of which are hereby incorporated by reference, provides a periodic arrangement of nano-elements to create an array on a device electrode inside a cell structure, as shown in
Structures using a nano-element array to form and shape features for enhancing carrier collection and light collection management have been explored by Applicants (1-3) and, as noted above, patented by Applicants. Others have also explored similar structures (4-9), and there are a number of patent applications and patents such as those listed in (10). The common factor in all of these previous device configurations is that they are based on using an array of nano-elements inside the cell which are purposefully and actively used to define the device form and shape of subsequent material dispositions. Here the term nano-element is being used to describe a feature whose characteristic height and width are in the nano-scale or perhaps, if one adheres to the formal definition of the nano-scale, somewhat outside this scale with these characteristics in a range up to about 2000 nm.
However, the forming of these nano-element array based devices requires complex design considerations for incorporating such arrays, and in the manufacturing sequences employed to fabricate them. This is necessary to insure the positioned nano-element array will control subsequent material dispositions and thereby control the device shape and form. An alternative design and processing sequence is needed to give the flexibility of being able to avoid the necessity of using a nano-element array to control device shape and form.
A photo-active device is provided that has a cavity in an integrated, transparent mold material. An active material layer is disposed therein along with other layers disposed in and about said cavity to define a dome-array architecture. A process for forming the dome-array structure includes disposing an active layer into a series of empty periodically positioned cavities of a dome-array template working mold material. Each of the series of empty periodically positioned cavities has curvature variations of the interior surface of the dome-array cavities optimized for device efficiency, reduction of performance sensitivity to light impingement angle, or a combination thereof. At least one of contact layers, spacer/transport layers, and electrode layers are also disposed in the series of cavities.
The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
Dimensions depicted in the schematics are not necessarily to scale.
The detailed description explains the preferred embodiments of the invention.
The present invention has utility for a design and process for photo-active devices which do not utilize a nano-element array in the device to cause the formation of a repetitive, undulating structure but, instead, have a repetitive structure based on utilizing a dome-array cavity architecture. The dome-array cavity architecture has the distinct advantage of not using a nano-element array and subsequent material dispositions thereon to dictate feature form and shape. In all versions of this invention, light enters (photovoltaic and photo-detecting devices) or exits (e.g., LEDs) the active material of the dome-like features and of the inter-dome-like-feature region through the periodically undulating surface defined by the dome-array (i.e., dome-like, column-like, cone-like, hemisphere-like, etc.) features and the corresponding inter-dome-like region. In all versions of this invention, the dome-array enhances light trapping and distribution inside the device. In photovoltaic devices, for example, this may be exploited to reduce of the amount of absorber materials needed to attain or exceed the device efficiency of thicker absorber planar structures thereby giving cost savings and reducing material demands. The resulting reduction in absorber volume which can be exploited can allow photogenerated entities to be created closer to collecting surfaces thereby also contributing to enhancing performance.
Embodiments of the inventive process employ disposition of an active layer or parts thereof, possibly preceded by disposition of other layers, into the periodically patterned empty cavities of a working mold material, and subsequently forming a back contact, an electrode, and, in the case of a substrate structure, a supporting substrate all sequentially disposed onto the exposed dome-like structure material region. Following the formation of the inventive dome-array, the mold material is removed, except in the transparent, integrated mold case, and possibly a portion of the active layer and other device layers (e.g., counter contact/electrode layers) then are completed, as needed. In the case of photovoltaic devices and light detectors, embodiments of the design and process are applicable to organic and inorganic absorber materials. Embodiments of the design and process are applicable to substrate or superstrate cell configurations in one sun and concentrator uses. By way of example, photo-active devices formed according to embodiments of the present invention include solar cell photovoltaics, photosynthesis devices, light detection devices, and light producing devices.
Transparent, integrated mold variants of the inventive process employ disposition of an active layer preceded by disposition of other device layers (e.g., contact/electrode materials) into the periodically patterned empty cavities of a working integrated, transparent mold material. Subsequent formation of a back contact, an electrode, and a supporting substrate (in the substrate version) all sequentially disposed onto exposed dome-like (dome-like, cone-like, column-like, and hemisphere-like) material and inter-dome region then occur. Following this formation of the inventive dome-array, the transparent mold material is not removed in these embodiments but remains in place. It therefore becomes a part of the completed device and may function to protect the device. Further, the side of this integrated mold material facing the incoming light may also be patterned to further enhance light coupling and trapping. When the side of this integrated mold material facing the incoming light is exposed to the environment directly, its pattern may also be used to keep this surface clean. The cleaning function results from the well-known Lotus Effect (6). This embodiment is also applicable to organic and inorganic absorber materials and may be finished as a substrate or superstrate configuration. In either case, light enters through the light entry surface of the transparent, integrated mold and enters the active material volume through the periodically undulating surface defined by the dome-array (dome-like, column-like, cone-like, and hemisphere-like) features and the corresponding inter-dome-like region; i.e., light enters through the dome-array.
The basic architectures and process flow described in the present invention can be applied to photovoltaic devices, photosynthesis devices, light detection devices, and light producing devices. Embodiments of the inventive design and fabrication approach permit low cost, manufacturable devices. The inventive embodiments disclosed herein all have the following advantages: (1) the embodiments do not need a dry-etching step for cleaning the exposed inter-dome portion after dome material disposition into the cavities of the mold, (2) with the proper mold material selection, the inventive embodiments may use high processing temperatures during the process steps; (3) embodiments do not use an etching step to define the dome-array, and (4) embodiments do not need an etching step for removing any base portion of the dome-array material. The approaches disclosed herein for design and manufacturing not only simplify the processing compared to approaches requiring a positioned array of nano-elements for device shape and form control, but they also offer freedom in selecting process conditions for subsequent steps. These potential advantages combined with the capabilities of high throughput with low cost production offered by roll-to-roll processing critically contribute to low cost manufacturing by embodiments of the present invention, and offer market competitiveness advantages.
In addition, the approach disclosed herein can, as noted, use a transparent, integrated mold material thereby giving the further advantages of (1) forming and utilizing the dome-array architecture without requiring a mold material removal step, (2) offering the ability to pattern the incoming light side of the integrated mold material to further enhance light coupling, and (3) offering the options of using the integrated mold material as a cleaning layer and/or a protection layer for the device beneath. Use of the transparent, integrated mold material approach also means devices may be finished as substrate or superstrate configurations. Further, the integrated mold material approach does not need any anti-adhesion layer utilization, often important to separation processing, between the mold and the undulating dome-like (dome-like, cone-like, column-like, or hemisphere-like) structure surface. Anti-adhesion layers may be required in the non-integrated mold material approaches.
The dome-array design and fabrication control approach used in embodiments for creating effective structures for photo-active devices will be specifically discussed in the context of solar cells. However, embodiments of the present invention have application to various photo-active devices producing or utilizing light, the latter including solar cell photovoltaics, photosynthesis devices, and light detection devices. In the exemplary embodiments discussed, the working mold material arrayed, with empty cavities used for forming the device arrays, is prepared by creating a master template. If this master template is used to produce imprinting (including embossing) templates, the master template is formed using standard techniques such as lithography and etching to have what is termed the positive of the desired array pattern. The master template is used to produce the imprinting template which then is a negative. The imprint template (e.g., a roller, diaphragm, or plate tool) is forced, using conventional imprint techniques, into contact with a mold material creating the working mold material with the positive array pattern, which is the desired pattern into which material is disposed in creating the device. Alternatively, if this master template is used to produce patterned mold material directly without imprinting, it has what is termed the negative of the desired array pattern. The master template is then used to produce the working mold material, which is positive, by first positioning a liquid or vapor precursor of the working mold material into the master template. This step is followed by a curing of this precursor employing various chemical, physical, or both treatments including heating and radiation exposure (e.g., ultra violet (UV)). The resulting array of cavities in the mold material is the pattern into which material is disposed in creating the device in this molding approach.
It is noted that in either the imprinting or molding approach, the master template can be used multiple times to create multiple positive mold material arrays. In the case of molding, this merely requires reuse of the master template. In the case of imprinting, the imprinting tool can be used multiple times and, when needed, can be replaced by a new tool using a sequence of positive/negative pattern formation. Mold materials for these purposes can vary and can include a variety of materials from polymers to spin-on-glass types and to sol-gel (e.g., ZnO sol-gel, TiO2 sol-gel) materials.
The curvature variations of the interior surface of dome-array cavities may be determined by adjusting feature shaping in the master mold. The resulting dome-like, hemisphere-like, cone-like, column-like, etc. cavity curvature variations across the cavity interior surface can be optimized for (1) device efficiency and/or (2) reduction of performance sensitivity to light impingement angle. The disposing of material into the cavities of a working mold material for forming a dome-array structure includes disposing material into the cavities and onto the inter-cavity region. The materials involved include active layer materials, or their precursors and may include optical spacer/transport controlling (e.g., hole transport/electron blocking and electron transport/hole blocking) layers, contact layers, and electrode layers. It may also require other device layers. These mold cavities are generally wider at their opening than at their bottom (i.e., they have what may be termed a dome-like, hemisphere-like, cone-like, or similarly shaped features collectively referred to as dome-like herein) for facilitating disposing material into the cavities and subsequent separation, when undertaken. The working mold material is chosen to be compatible with the processing and processing temperatures to which the mold material will be exposed. Active layer materials disposed into these cavities may comprise at least one of semiconductor quantum dots, nc-Si, CIGS, CdTe, iron pyrite, dyes or other inorganic or organic absorber materials. The one or more active layers which are disposed into the empty cavity array of the working mold material consists of at least one absorber, and one or more of the layers may be doped. The active layer(s) may comprise the materials, or some of the materials, necessary to have p-i-n, p-n, surface barrier (e.g., Schottky barrier type structures), heterojunction (including bulk heterojunction), and dye sensitized collection structures. The working mold material itself is formed by imprinting or molding using a master pattern, as noted. The disposing of material into cavities as well as onto inter-cavity regions of the mold template may be accomplished by using one or more of a variety of physical and chemical deposition techniques such as physical or chemical vapor deposition (including close spaced sublimation (CSS) deposition), in situ growth, and atomic layer deposition, as well as, by employing stamping and impressing techniques (e.g., impressing quantum dots into the cavities).
The one or more active layer materials disposed into the cavities, by whatever means, essentially fill or partially fill these cavities, which are arranged in a repetitive lattice pattern (e.g., hexagonal, triangular, or square unit cells). The one or more active layer materials and other possible device layers also cover the working mold material surfaces between the cavities of the array. A device electrical contact is disposed onto the entire region of exposed material. Subsequently, the electrical contact material may be further built-up in thickness, or another material may be disposed. In either case an electrode is finally formed. This interconnected contact base provides an electrode, as well as mechanical and electrical continuity. The contact base may function as the substrate or may be attached to a substrate so that, in either case, a mechanically stable platform is created. All material dispositioning processes, contact base processing, and the processing needed to complete the device, may be batch processing but are amenable to being done in continuous processing fashion (e.g., roll-to roll).
The dome-array (i.e., arrayed dome-like, hemisphere-like, cone-like, or similarly shaped features) resulting from embodiments of the inventive design and process flow has at this point a supporting mechanical substrate, but is otherwise covered with the mold material. This mold material is then removed by means of chemical phenomena (e.g., etching or dissolution), physical phenomena (e.g., mechanical separation), or both. This removal may also involve external energy sources such as heat, ultra violet (UV) radiation, etc. If the mold material has been selected to function as an integrated, transparent mold material, then it is not removed and remains as an integral part of the device and may have functions such as device protection, enhancing light entry and trapping, and/or enhancing surface cleaning.
With the removal of mold material in the non-permanent mold designs, there may be a cleaning step included. At this point, the now exposed dome-like structures of the dome-array may have disposed on them the final layers necessary to complete the cell fabrication. These final layers may include a dopant layer, a spacer layer for aiding in controlling light distribution and in forming selective Ohmic contacts, a light absorber layer, a conductor, or a top (i.e., counter) electrode layer. The result is the dome-array device such as that shown in
Additive processes such as physical positioning, physical vapor deposition, chemical vapor deposition or some combination are used to dispose all of the active layer components including at least one of absorber material and of other layers for doping, spacer/transport control, and the top contact layer/electrode into the/empty cavity and the regions between cavities on the mold material, before back contact/back electrode/substrate formation. The result is 17 of
If imprinting is employed in the array formation, the process flow starts with a mold material 12 on a working substrate 10 as seen in
Whether formed by imprinting or molding, the cavities 16 in the mold material 12 of
The absorber copper indium gallium selenide (CIGS) is disposed to form a heterojunction solar cell device. However, the design and sequencing are not limited to that material system, nor to its specific deposition methods. The present example has two variants. In one variant, the CIGS material is deposited (e.g., by co-evaporation) into the empty cavities and onto the regions between these cavities of the working mold material, and a back contact is formed. This back contact may not be flat (planar), depending on the disposition techniques and parameters. If planarization is needed for 17 and/or 18, it may be attained using the approaches discussed in Example 1. After back contact formation, that material or another disposed material may serve as the back electrode, as needed, and a substrate is attached (e.g., bonded) or deposited (e.g., vacuum deposition, laser ablation, or electroplating). The resulting dome-like, hemisphere-like, cone-like, or similarly shaped features, with CIGS and these other materials, are then separated from the mold material. A cadmium sulfide (CdS) layer is then formed (e.g., by chemical-bath-deposition (11)) on the free surface of these arrayed CIGS dome-like, hemisphere-like, or cone-like features and the exposed CIGS planar surfaces between these. This step may require a CIGS surface cleaning/conditioning prior to CdS layer formation. Subsequently, a sputtered resistive/conductive zinc oxide (ZnO) bi-layer may be deposited (11). It may be advantageous to replace the ZnO/CdS system with another (e.g., Zn1−xMgxO (12)). In any case, the resulting device is shown schematically in
In the other variant, CIGS material is deposited into the empty cavities, and onto the regions between these cavities of the mold material, after positioning into the cavities of one or both of the top layers (e.g., either one or both of ZnO/CdS or Zn1-xMgxO, etc.). The top (undulating) electrode material may be deposited similarly or after mold removal. The back contact is then formed and the mold material is removed and processing concluded, as needed, producing the device shown schematically in
In the current example, the process flow may again start with the working mold material 12 on a working substrate 10 as shown in
Whether formed by imprinting or molding, the cavities 16 in the mold material may be of various dome-like, hemisphere-like, cone-like or similarly shaped features with a maximum dimension d at their base, a maximum depth h, and array spacing L. These parameters d, h, and L are selected such that the final thickness height h and final width d along with thickness t and array spacing L seen in
The sequencing in
In this example, the mold material key to forming these dome-array architectures, which are all distinguished by having mold cavities control solar cell features, is transparent and integral to the final device; i.e., it is permanent and not removed. The mold materials in this case may include glass-like materials, polymers, transparent conductive oxides, or any material amenable to the process flow and transparent to light across the spectrum of interest. The material may be chosen from among insulators, semiconductors, or conductors. It may or may not be or be a part of the contiguous electrode. Physical positioning, physical vapor deposition, chemical vapor deposition (includes CSS here and elsewhere) or some combination is used to dispose device layer components including the top contact layer/electrode (on side of incoming light) and at least one absorber material and other layers as needed for doping as well as spacer/transport control into the empty cavity and the regions between cavities on the mold material, before back contact/back electrode/substrate formation. The result is 17 of the schematic of
The cavities 22 in
Whether formed by the imprinting or molding approaches discussed in examples 1 and 2, the cavities 22 used to define this dome-array in the mold material 21 may be of various dome-like, hemisphere-like, cone-like, or similar shapes with a maximum dimension d at their base, a maximum depth h, and array spacing L and collectively termed dome-like herein. The parameters d, h, and L are selected such that the final thickness height h and final width d along with thickness t and array spacing L after device fabrication and back contact formation maximize device performance.
This example exploits the situation in which the cavity-inter-cavity region does not have a flat active layer, exposed surface after active layer dispositioning. This example specifically utilizes one or more of d, h, aspect ratio h/d, material type, and disposition type and parameters to adjust the thicknesses of the material layers disposed on the inter-cavity region 30, on the side walls of a cavity 31, and on the bottom of a cavity 32 in
In this specific example, a CdS/CdTe heterojunction solar cell is disposed by first RF sputtering CdS into the empty working mold cavities and, of course, onto the adjacent, inter-cavity regions. This is followed by RF sputtering of CdTe into the working mold cavities and onto the adjacent, inter-cavity regions. The result, after the deposition of these two films using the parameters of Table 1, is seen in the field emission scanning electron micrograph (FESEM) cross-section given in
If it is necessary to adjust the active material thickness in the inter-cavity region, the thickness t of the layers in this region may be tailored using the well-known techniques employed in planarization such as CMP and/or etching. This may be done before or after nano-element disposition.
If the device and the processing are designed to give a non-flat active layer surface resulting in voids in cavity regions as demonstrated in
Any patents or publications mentioned in this specification are indicative of the levels of those skilled in the art to which the invention pertains. These patents and publications are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference.
The foregoing description is illustrative of particular embodiments of the invention, but is not meant to be a limitation upon the practice thereof. The following claims, including all equivalents thereof, are intended to define the scope of the invention.
This application claims priority of U.S. Provisional Patent Application Ser. No. 61/806,622 filed Mar. 29, 2013; and 61/826,707 filed May 23, 2013, and which are incorporated herein by reference.
Number | Date | Country | |
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61806622 | Mar 2013 | US | |
61826707 | May 2013 | US |