WO2015148637A1

WO2015148637A1 - Thin film solar cells with metallic grid contacts

Info

Publication number: WO2015148637A1
Application number: PCT/US2015/022442
Authority: WO
Inventors: Xavier Multone; Jerome Steinhauser; Evelyne Vallat-Sauvain; Johannes Meier
Original assignee: TEL Solar AG
Current assignee: TEL Solar AG
Priority date: 2014-03-25
Filing date: 2015-03-25
Publication date: 2015-10-01
Anticipated expiration: 2016-09-25

Abstract

An improved solar cell is disclosed having metallic grid contacts alone, or in combination with a transparent conductive oxide (TCO) contact layer or a p-oxide contact layer. A method of forming such solar cells is also disclosed. Metallic grid contacts, with or without transparent conductive oxide (TCO) layer or p-oxide contact layer can be used with various types of thin film solar cell current-generating layer stacks, and optionally combined with textured, diffusing, or patterned or imprinted glass substrates for improved solar cell electrical and optical characteristics.

Description

THIN FILM SOLAR CELLS WITH METALLIC GRID CONTACTS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on and claims the benefit of and priority to copending U.S. Provisional Patent Application No. 61/970,264, entitled "THIN FILM

SOLAR CELLS WITH METALLIC GRID CONTACTS" (Ref. No. TES-157PROV), filed on March 25, 2014, the entire contents of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

[0002] The present invention relates to solar cells having metallic electrical contacts, and a method for forming such devices.

DESCRIPTION OF RELATED ART

[0003] Photovoltaic devices, also known as solar cells, are entering the mainstream as a reliable and cost-effective source of renewable energy. One of the main drivers of adoption of photovoltaic technology has been the decrease of price per watt (or kilowatt) that each generation of devices has been able to reach, along with increased performance manifested primarily as power generated per unit area. Therefore, active research and development efforts are directed at achieving ongoing cost reductions in all the layers that comprise a thin film solar cell and all the processes of forming thereof.

[0004] Solar cells of all kinds employ transparent electrical contacts (i.e. electrodes) that sandwich the current-generating layer stack, for tapping generated electrical energy while allowing light to pass through to the current-generating layer stack. In thin film solar cells, transparent conductive oxide (TCO) materials, such as zinc oxide (ZnO), tin oxide (Sn0₂), or indium tin oxide (ITO), etc., are used in the form of a transparent layer as electrical contacts. A particularly critical layer is the front electrical contact which faces the light source, i.e the sun. One disadvantage of these TCO materials is their relatively high cost, particularly if they contain indium (In), but zinc-containing LPCVD precursors, like diethyl-zinc can be also be expensive. Besides having a relatively high cost, TCO materials also in some cases limit the current density

[mA/cm²] that the solar cell can achieve. Therefore, there exists a need for less expensive electrical contacts for solar cells, preferably with electrical and optical performance superior to TCO materials. Various alternative electrical contact structures considered in the past have comprised carbon nanotubes, graphene, various organic conducting materials, etc. Still, many of these alternative materials fall short of the electrical performance of TCOs, which has limited their adoption in mainstream photovoltaic devices, unlike display devices where their use is more widespread. This is because unlike in display devices, solar cells require that the incoming light be adequately scattered prior to entering the current-generating layer stack, for maximum utilization of incoming solar (light) energy. Of currently used TCOs, zinc oxide (ZnO) in particular has a favorable crystal structure with an approximately 50° angle between crystal facets, and is thus a very effective scatterer of light. It is deposited typically via a low pressure chemical vapor deposition (LPCVD) or sputtering process in a layer 1 to 2μΐη thick, and is highly transparent (>95%) in the 400 to 800nm light wavelength range, thus suitable for solar energy capture. Every alternative electrical contact solution would thus need to address light scattering as well as electrical performance.

SUMMARY OF THE INVENTION

[0005] An aspect of the invention includes a thin film solar cell, comprising a substrate and a front electrode layer adjacent the substrate, the front electrode layer comprising a metallic grid. The substrate can comprise glass, which may be diffusive, or textured, or nanotextured. In an embodiment, the glass may comprise a

nanoimprinted surface. The solar cell further comprises a current-generating layer stack, which may comprise multiple photovoltaic conversion units, such as p-i-n or n-i-p photovoltaic conversion units, which may comprise amorphous and/or crystalline silicon absorbers. Alternative embodiments may include photovoltaic conversion units comprising copper indium selenide (CIS), copper indium gallium selenide (CIGS), dye solar cells, or may comprise a hybrid of multiple aforementioned types of photovoltaic conversion units. In an embodiment, a back electrode layer is formed on the opposite side of the current-generating layer stack, wherein the back electrode layer can also comprise a metallic grid. In a further embodiment, the front electrode layer has an optical transmission greater than about 95%, and a sheet resistance of less than about 30 Ω per square, preferably less than about 20 Ω per square. [0006] Another aspect of the invention includes a front, or back, or both electrode layers that comprise a transparent conductive oxide (TCO) layer and/or a p-oxide contact layer in addition to the metallic grids.

[0007] Yet another aspect of the invention includes the process of forming front and back electrode layer metallic grids using rolling mask lithography (RML), to pattern the metallic grid, wherein the patterning step is followed by an etch and/or liftoff step to pattern a metallic layer from which the metallic grid is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] A more complete appreciation of the invention and many of the attendant advantages thereof will become readily apparent with reference to the following detailed description, particularly when considered in conjunction with the accompanying drawings, in which:

[0009] FIG. 1 shows a cross section of an exemplary photovoltaic device in accordance with an embodiment of the invention.

[0010] FIG. 2 shows a view of the metallic grid contact layer in accordance with an embodiment of the invention.

[001 1] FIG. 3 shows a schematic of the rolling mask lithography (RML) process.

[0012] FIG. 4 shows a comparison of optical and electrical performance

characteristics of a number of alternative contact materials.

[0013] FIG. 5 shows photovoltaic device current density for various types of contact structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0014] Embodiments of the present invention relate to design of and method of forming a photovoltaic device, i.e. a solar cell.

[0015] In the following description, in order to facilitate a thorough understanding of the invention and for purposes of explanation and not limitation, specific details are set forth, such as particular geometries of a photovoltaic device, and descriptions of various components and processes. However, it should be understood that the invention may be practiced in other embodiments that depart from these specific details.

[0016] In the description to follow, the term photovoltaic device, which represents a device capable of conversion of incoming light energy into electrical energy may be used interchangeably with terms such as cell, solar cell, solar panel, etc., the design and method of forming of all of which falls within the scope of the claimed invention. Furthermore, the invention is also applicable to other types of devices that require or can optionally have optically transparent or semi-transparent electrical contacts, such as for example various display devices, liquid crystal display (LCD) devices, light emitting diodes (LED) and similar lighting devices, etc.

[0017] Reference throughout this specification to "one embodiment" or "an

embodiment" means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, but do not denote that they are present in every embodiment. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily referring to the same

embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more

embodiments.

[0018] Various operations will be described as multiple discrete operations in turn, in a manner that is most helpful in understanding the invention. However, the order of description should not be construed as to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation. Operations described may be performed in a different order than the described embodiment. Various additional operations may be performed and/or described operations may be omitted in additional embodiments.

[0019] FIG. 1 shows a cross section of a thin film solar cell 100 in accordance with an embodiment of the invention. The solar cell is formed on a transparent substrate 1 10, typically made of glass or other suitable transparent material. The lower side of the substrate is the side from which light is incident onto the solar cell (not shown) and may be coated with an a nti reflective coating 105, to reduce reflections and maximize coupling of the incident irradiation.

[0020] On the opposite (upper) side of the substrate 1 10, a current-generating layer stack 155 is formed comprising (in this example) a tandem of two p-i-n silicon

photovoltaic conversion units 150 and 160. The first p-i-n photovoltaic conversion unit

150 may be an amorphous silicon photovoltaic conversion unit. The second p-i-n photovoltaic conversion unit 160 may be a crystalline photovoltaic conversion unit.

Alternately, the current-generating layer stack 155 may comprise additional, i.e. 3 or more photovoltaic conversion units, and the photovoltaic conversion units themselves do not need to comprise silicon, but may comprise copper indium selenide (CIS), copper indium gallium selenide (CIGS), dye photovoltaic conversion units, etc.

Furthermore, the photovoltaic conversion units may be n-i-p photovoltaic conversion units and/or the current-generating layer stack 155 may comprise a hybrid of multiple types of photovoltaic conversion units, e.g. a silicon and a CIS/CIGS photovoltaic conversion unit. The makeup of the current-generating layer stack will not be discussed in further detail here, and the reader is directed to copending U.S. Patent Application No. 14/159,002, entitled "SYSTEM AND METHOD FOR TRAPPING LIGHT IN A SOLAR CELL", and copending U.S. Provisional Patent Application No. 62/001 ,722, entitled "MONOLITHIC MULTI-JUNCTION SOLAR CONVERSION DEVICE WITH A NONSILICON SOLAR SUB-CELL AND A THIN FILM SILICON SOLAR SUB-CELL", both incorporated by reference here in their entirety, for further details of current- generating layer stacks.

[0021] With further reference to FIG. 1 , a front electrode layer is formed between the substrate 1 10 and the first photovoltaic conversion unit 150. The front electrode layer comprises a metallic grid 140 formed on the surface 120 of substrate 1 10. The surface 120 of substrate 1 10 may be polished flat or it may be textured to increase light scattering into the photovoltaic conversion units 150 and 160. For example, the glass can be textured by etching, scribing, or any other suitable method known to those skilled in the art. Furthermore, the surface 120 may be nanotextured or may comprise a nanoimprinted layer to form a texture or features for increased light scattering.

[0022] The metallic grid may be formed from a metal such as silver (Ag), aluminum (Al), copper (Cu), or other suitable metal. FIG. 2 shows a cut-away view of thin film solar cell 100 of FIG. 1 with just the substrate 1 10 and metallic grid 140 shown formed thereupon, for clarity. The metallic grid 140 comprises grid elements 142 that enclose openings 145, wherein the size of the openings 145, the cross sections of grid elements 142, are selected such that a high optical transmission of the metallic grid 140 is achieved, in the range of 90% or higher, typically around 95%, and to preserve the current carrying capability of the metallic grid (i.e. having a low resistance). FIG. 2 shows a grid with a square pattern, but other patterns such a rectangular or hexagonal (honeycomb) patterns may be used. [0023] With reference again to FIG. 1 , the front electrode layer may further comprise an optional front contact layer 130 in addition to the metallic grid 140. The front contact layer 130 may comprise a thin layer of transparent conductive oxide (TCO) or a p- doped oxide layer, wherein the front contact layer 130 serves to provide additional scattering of light (as in the case of a TCO) and/or to protect the first photovoltaic conversion unit 150 from diffusion of species from metallic grid 140 and substrate 1 10. Suitable transparent conductive oxides (TCOs) include but are not limited to zinc oxide (ZnO), tin oxide (Sn0₂), and indium tin oxide (ITO). The p-oxide layer may comprise a ρ-μο3ίΟ/ρ-μο3ί layer stack, for example. The thickness of the front contact layer 130 may be greater than about 5nm, for example.

[0024] A similar contact structure may be formed on the back side of the current- generating layer stack 155. While it is not critical for a metallic grid to be used as a back contact, because less light coupling therethrough, it is still preferable from the standpoint of maximizing solar cell performance, and from a cost perspective. For example, a back electrode layer may be formed atop the second photovoltaic conversion unit 160 (or atop a stack comprising more than two photovoltaic conversion units), the back electrode layer comprising a metallic grid 170. Metallic grid 170 may be similar to metallic grid 140, of FIGs. 1 and 2, or it may be of a different geometry and comprising a different material. With the back electrode layer formed, a reflector 190 is typically added to the solar cell 100, to reflect transmitted light back into solar cell so it can be fully utilized in energy conversion.

[0025] Similar to front electrode layer, the back electrode layer may also comprise a back contact layer 180 comprising a thin layer of transparent conductive oxide (TCO), to protect the second photovoltaic conversion unit 160 from diffusion of species from metallic grid 170. An effective thickness of the back contact layer 180 may be about 5nm, or higher.

[0026] In terms of achievable performance, metallic grids 140 and 170 have a clear advantage over a number of other alternative materials to TCOs, as can be seen in plot 400 of FIG. 4. FIG. 4 shows the achievable range 410 of optical transmission and sheet resistance for metallic grids formed by a rolling mask lithography (RML) process commercialized by Rolith, Inc., of 5880 W. Las Positas Blvd #51 , Pleasanton, CA 94588, United States. Optical transmission in excess of 95% can be achieved with metallic grid geometries and dimensions similar to those shown in FIGs. 1 and 2, with a sheet resistance of less than 10 Ω per square - all marked improvements over traditional TCOs. Typical performance may include an optical transmission in the 95% range, a wavelength passband from 350nm to 1200nm, and a sheet resistance of 10 to 30 Ω per square.

[0027] Because of the increased contact optical and electrical performance, it is expected that the total photocurrent can be significantly increased due to reduced absorption losses in wavelength ranges inaccessible with the use of typical TCOs, i.e. blue, violet, and UV with wavelengths less than about 400nm, and red and infrared with wavelengths greater than about 800nm. All that may be achieved at a lower per-area cost, provided an inexpensive patterning and metal deposition solution is used, compared to traditional TCO layer forming.

[0028] FIG. 5 shows actual measured current densities vs total thickness for traditional TCOs, in a smooth state (plot 520) and rough condition for improved light trapping (plot 510). A theoretical current density curve 530 is plotted for the case with no TCO, and a plot 540 is provided calculated based on anticipated performance of aforementioned metallic grids 140 and 170 used in lieu of TCO contacts, without the use of optional additional front and back contact layers 130 and 180. A photocurrent improvement in the range of 2.5 to 3 mA/cm² is readily apparent over traditional TCOs, allowing a higher conversion efficiency to be achieved in a thin film solar cell.

[0029] While the metallic grid 140 does provide some light scattering into the current- generating layer stack 155, it is advantageous to use additional methods of inducing light scattering, particularly in the case where a TCO front contact layer 130 is not used. Methods to induce light scattering may include using a diffuse glass substrate 1 10, or a substrate 1 10 where the surface 120 is textured or nanotextured, or where it has a pattern formed thereupon, for example using nanoimprinting or other patterning methods. With the additional scattering provided by the substrate 1 10 itself, a higher solar cell performance can be achieved, manifested by a higher open circuit voltage Voc and a higher fill factor FF. Furthermore, the conditioning of the substrate 1 10 and its surface 120 also can be tailored to improve index matching, to reduce unwanted internal reflection losses. [0030] A design of a thin film solar cell with metallic grids 140 and 170, and in particular the decision whether to include TCO front and back contact layers 130 and 180 will involve making design tradeoffs. For example, a TCO-free thin film solar call would be expected to exhibit a higher level of reliability, due to absence of TCO-related problems, such as moisture-, or UV-induced degradation, or anodic degradation. Also, the use of metallic grids only allows the decoupling of electrical and optical functions of the contact scheme. On the other hand, a TCO front and back contact layers introduce additional light scattering, which is beneficial, and they also serve as diffusion barriers. Lastly, the introduction of metallic grids may require changes in bus bar design and scribing processes. Overall, the design will necessarily become a result of an optimization process, with many parameters taken into account.

[0031] It was previously explained that the success of the introduction of metallic grid contacts will depend on the ability to form the metallic grids at a competitive cost, on large area substrates (greater than 1 m²). One particularly promising technique involves the use of the process of rolling mask lithography (RML), marketed by Rolith, Inc. The RML patterning process is well suited for processing of large area substrates at high rates of throughput (in excess of 3m² per minute). FIG. 3 depicts a schematic of the patterning process 300, in which a substrate 1 10 is moved through a patterning process that begins with dispensing photoresist from a nozzle (or set of nozzles) 310, to form a photoresist layer 320. Patterning of the photoresist layer 320 is accomplished by exposure to UV light from a UV source 340 placed inside a rolling mask 330. The rolling mask 330 comprises a pattern that is to be formed in the photoresist layer 320. Unlike traditional stepping exposure processes, the RML process proceeds at a continuous high rate, forming a latent image of the pattern inside photoresist layer 320. The exposed photoresist layer 320 is then subjected to a development process wherein developer is dispensed onto the photoresist layer 320 from nozzle(s) 350 to form a pattern 370. The process completes with a rinse step in which a rinse liquid is dispensed from rinse nozzle(s) 360, to clean the pattern 370 atop substrate 1 10, and prepare it for further processing steps.

[0032] The forming of a metallic grid itself can proceed with a number of variations.

In an embodiment, the pattern 370 can be formed as a negative of the metallic grid pattern, and the metallic grid can be formed by simple metal deposition into the pattern openings and across the negative pattern 370. This step is followed by a liftoff etch process in which the negative pattern 370 is lifted off along with the overlying metal layer, to expose a metallic grid. In other embodiments, a more traditional approach can be taken, in which a metallic layer is formed prior to RML patterning, the pattern 370 being a positive pattern formed thereupon, and used as a mask for a subsequent etch step in which openings 145, for example, of metallic grid 140 are formed by etching the metal. Both processes can be followed by further cleaning steps to remove the lifted-off or etched material and photoresist from the substrate 1 10.

[0033] Once a metallic grid, for example metallic grid 140 is formed, the usual process of forming a current-generating layer stack 155, the back electrode layer, and reflector 190, can proceed, as known to those skilled in the art.

[0034] Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above teaching. Persons skilled in the art will recognize various equivalent combinations and substitutions for various components shown in the figures. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

Claims

What is claimed is:

1 . A thin film solar cell, comprising:

a substrate comprising a first surface; and

a front electrode layer adjacent the first surface, the front electrode layer comprising a first metallic grid.

2. The cell of claim 1 , wherein the substrate comprises glass.

3. The cell of claim 1 , wherein the substrate comprises diffusive glass.

4. The cell of claim 1 , wherein the first surface is textured.

5. The cell of claim 1 , wherein the first surface is nanotextured.

6. The cell of claim 1 , wherein the first surface comprises a nanoimprinted surface.

7. The cell of claim 1 , further comprising:

an anti reflective coating applied to a second surface of the substrate opposite the first surface.

8. The cell of claim 1 , further comprising:

a first p-i-n photovoltaic conversion unit adjacent the front electrode layer.

9. The cell of claim 1 , further comprising:

a first contact layer disposed between the front electrode layer and the first p-i-n photovoltaic conversion unit,

wherein the first contact layer comprises at least one of a p-oxide layer and a transparent conductive oxide (TCO) layer.

10. The cell of claim 4, wherein the first p-i-n photovoltaic conversion unit comprises an amorphous silicon absorber.

1 1 . The cell of claim 5, further comprising:

a second p-i-n photovoltaic conversion unit adjacent the first p-i-n photovoltaic conversion unit.

12. The cell of claim 1 1 , wherein the second p-i-n photovoltaic conversion unit comprises a microcrystalline silicon absorber.

13. The cell of claim 1 1 , further comprising:

a back electrode layer adjacent the second p-i-n photovoltaic conversion unit, the back electrode layer comprising a second metallic grid.

14. The cell of claim 13, further comprising:

a second contact layer disposed between the back electrode layer and the second p-i-n photovoltaic conversion unit, wherein the second contact layer comprises a transparent conductive oxide (TCO) layer.

15. The cell of claim 13, further comprising:

a reflector adjacent the back electrode layer.

16. The cell of claim 1 , wherein the front electrode layer has an optical transmission greater than about 95%.

17. The cell of claim 1 , wherein the front electrode layer has a sheet resistance less than about 30 Ω per square.

18. The cell of claim 1 , wherein the front electrode layer has a sheet resistance less than about 20 ohms per square.

19. The cell of claim 1 , wherein the first metallic grid comprises a metallic grid deposited using a rolling mask lithography method.