WO2011129970A2

WO2011129970A2 - Low-loss thin-film si back contact system

Info

Publication number: WO2011129970A2
Application number: PCT/US2011/029629
Authority: WO
Inventors: Chris Eberspacher; Dapeng Wang
Original assignee: Applied Materials, Inc.
Priority date: 2010-04-12
Filing date: 2011-03-23
Publication date: 2011-10-20
Also published as: WO2011129970A3

Abstract

A method and apparatus for forming a thin-film photovoltaic device is described. One or more photoconversion layers is formed on a substrate. A transparent conductor is formed over the photoconversion layer and a patterned conductor formed over the transparent conductor. The patterned conductor comprises a plurality of joined conductive segments that cover portions of the transparent conductor while leaving other portions exposed. A broad spectrum reflector is formed over the patterned conductor, and a structural back layer may be applied to the broad spectrum reflector. The thickness and conductivity of the transparent conductor and the areal coverage of the patterned conductor may be adjusted to achieve a desired bulk sheet resistance.

Description

LOW-LOSS THIN-FILM SI BACK CONTACT SYSTEM

FIELD

[0001] Embodiments of the invention relate to methods of forming thin-film photovoltaic devices. Specifically, methods of forming a low-loss back conductor for a thin-film photovoltaic device are described.

BACKGROUND

[0002] Photovoltaic energy generation was the fastest growing energy source in 2007. In 2008, installed photovoltaic capacity increased approximately 2/3 to about 15 GW. By some estimates, the global market for photovoltaic power will grow at a compound annual rate of 32% between 2008 and 2013, reaching over 22 GW, while installed capacity grows at an average rate of 20-30% per year or more, possibly reaching 35 GW by 2013. With available solar resources estimated at 120,000 TW, using less than 0.013% of these available resources could replace fossil fuels and nuclear energy as sources of electrical power. Total global energy consumption of 16 TW in 2005 is less than 0.02% of available solar energy incident on the earth.

[0003] With so much potential, countries and companies around the world are racing to increase efficiency, and lower cost of, photovoltaic power generation. Efficiency in thin-film photovoltaic devices is reduced by, among other things, transformation of photons into phonons, plasmons, and polaritons. Interfaces between metal materials and dielectric materials, in particular, are prone to plasmon losses, as photons incident at the interface energize free electrons in the metal to oscillate in a charge concentration wave that propagates through the metal, dissipating the photon energy. Thus, there is a continuing need for methods of reducing or avoiding optical losses, such as plasmon loss, in thin-film photovoltaic devices.

SUMMARY

[0004] Embodiments described herein provide methods of processing a thin-film photovoltaic substrate having a transparent conductive layer formed over a photoelectric junction, including forming a patterned conductor on the transparent conductive layer, and forming a reflective layer over the patterned conductor. The patterned conductor may have a regular or irregular pattern, and the reflective layer may be a broad spectrum reflector having low electrical conductivity.

[0005] Other embodiments described herein provide methods of forming a thin- film photovoltaic substrate, including forming a photoelectric junction on a substrate, forming a composite conductor comprising at least one patterned component on the photoelectric junction, and forming a broad spectrum reflective layer over the composite conductor.

[0006] Other embodiments described herein provide methods of forming a back conductor for a thin-film photovoltaic device, incluiding forming a conductive oxide layer on the thin-film photovoltaic device, forming a pattern of metal lines over the conductive oxide layer, and forming a low conductivity reflector over the conductive oxide layer and the pattern of metal lines.

[0007] Other embodiments described herein provide a thin-film photovoltaic device that has a photoelectric conversion layer, a compound back conductor with a low conductance layer and a patterned high conductance layer adjacent to the photoelectric conversion layer, and a broad spectrum reflector formed over the compound back conductor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] So that the manner in which the above-recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0009] Figure 1 is a flow diagram summarizing a method according to one embodiment. [0010] Figure 2A is a schematic cross-sectional view of a photovoltaic substrate according to another embodiment.

[0011] Figure 2B is a schematic top view of one embodiment of the substrate of Figure 2A.

[0012] Figure 2C is a schematic top view of another embodiment of the substrate of Figure 2A.

[0013] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.

DETAILED DESCRIPTION

[0014] Embodiments described herein generally provide methods of making a thin-film photovoltaic device having a patterned back conductor. The patterned back conductor is a composite conductor layer having at least one patterned layer and at least one unpatterned layer. The patterned layer is usually a conductor, and the unpatterned layer a semiconductor. Patterning the conductor reduces areal contact between the conductor and semiconductor, lowering the incidence of optical carrier losses at the interface between the conductor and semiconductor.

[0015] Figure 1 is a flow diagram summarizing a method 100 according to one embodiment. At 102, a photoelectric junction is formed on a substrate. The substrate may be any structurally strong substrate, such as glass, plastic, or metal. The photoelectric junction is generally formed from a semiconductor material, such as silicon, germanium, or a compound semiconductor such as a group ΙΙΙΛ/ semiconductor, a group ll/VI semiconductor, or a CIGS material. The photoelectric junction usually comprises p-type doped layers and n-type doped layers with electron deficient and electron rich dopants, respectively, to enhance charge separation and collection. The photoelectric junction may be formed by any process for forming layers on a substrate, but is commonly formed using a chemical vapor deposition process, which may be plasma enhanced. In one embodiment, a mixture of silane and hydrogen is activated by application of an electromagnetic field to deposit silicon on a substrate. Methods and apparatus for forming silicon- based photoelectric junctions are described in United States Patent Publication 2009/0020154, published January 22, 2009, and incorporated herein by reference.

[0016] At 104, a conductive oxide layer is formed on the photoelectric junction as a first component of a compound back contact structure. The conductive oxide layer is generally a metal oxide material doped with conductivity enhancing dopants. In one embodiment, zinc oxide is doped with 5% or less aluminum to form the conductive oxide layer. The conductive oxide layer is generally formed by a vapor deposition process, which may be a chemical or physical vapor deposition process, and may be plasma-enhanced. The conductive zinc oxide layer described above is formed, in one embodiment, by sputtering an aluminum-doped zinc oxide target in a PVD chamber using argon as the sputtering gas. In another embodiment, a zinc target may be sputtered using a mixture of argon, oxygen, and an aluminum compound such as trimethylaluminum in a reactive sputtering process. Methods and apparatus for forming conductive oxide layers are described in United States Patent Publication 2008/0153280, published June 26, 2008, and incorporated herein by reference.

[0017] At 106, a patterned conductor is formed on the conductive oxide layer. The patterned conductor comprises a conductive material formed as a semi- continuous layer of connected segments covering portions of the conductive oxide layer and separated by spaces through which the conductive oxide layer is exposed. The segments have a thickness between about 50 nm and about 500 nm, for example between about 75 nm and about 150 nm, and are spaced apart by a distance of about 0.1 mm to about 5 mm. The patterned conductor enhances the conductivity of the back contact structure of the solar cell, while the semi- continuous contact with the underlying conductive oxide layer reduces optical losses that may occur at the interface between the conductive oxide and the conductor. [0018] Conductivity of the compound back contact is determined by the conductivity and mass of the components. More mass of the high-conductivity back conductor allows for less conductive oxide, or lower conductivity of the conductive oxide layer. However, more mass of the high-conductivity back conductor leads to more interfacial surface area between the conductor and the conductive oxide, which leads to more optical losses at the interface. Normally, the compound back conductor has a bulk sheet resistivity between about 0.1 Ω/square and about 5 Ω/square. The unit "Ω/square" is understood in the art to refer to sheet resistivity for an arbitrary square area of the surface of a substrate, and is defined as the ratio of voltage drop per unit length to surface current per unit width of an arbitrary square area of the substrate. A desired sheet resistivity may be achieved by using a high-conductivity component and a low conductivity component in proportions that result in the desired sheet resistivity.

[0019] Reducing the area of the interface between the high-conductivity and low-conductivity components reduces optical losses at the interface. Not wishing to be bound by theory, it is thought that photons entering the interface between, for example, metals and dielectric materials can generate plasmons, and possibly phonons, that siphon energy from the photons as they are reflected. This is thought to be due to the matrix mobility of electrons within the metal reflector. Reducing the availability of metal at the interface reduces this optical loss phenomenon.

[0020] The patterned conductor may be metal in some embodiments, and may be formed in a variety of ways. In some embodiments, the conductor is formed in an explicit pattern, while in others the pattern in substantially random. Methods of forming a patterned conductor in an explicit pattern include screen printing, pad printing, ink-jet printing, syringe dispensing, roll-coating, gravure printing, and fine- line spraying. For a random pattern, a self-organizing ink may be deposited by techniques such as roller coating, spin coating, dip coating, large-area spraying, or waterfall coating, and the pattern allowed to develop as the solvent evaporates. [0021] The patterned conductor may be a metal grid of parallel lines deposited over the conductive oxide layer. The parallel lines may be connected by cross- wires at right angles to the parallel lines. The lines may be formed by scribing grooves into the conductive oxide using a laser or a diamond saw, forcing a metal- containing paste into the grooves, and fusing the metal into wires. In other embodiments, a metal layer may be deposited by any convenient means, such as sputtering, and metal removed to expose the conductive oxide in spaces of the resulting pattern.

[0022] At 108, a broad spectrum reflector is deposited on the substrate. The broad spectrum reflector comprises a material with high reflectivity for the incident wavelengths of light. In some embodiments, the broad spectrum reflector is a metal oxide material, such as titanium oxide or tantalum oxide. In other embodiments, the broad spectrum reflector may be oxides of tin, zinc, or aluminum, or the oxide may mix metals. For example, indium tin oxide may be used.

[0023] The broad spectrum reflector may be deposited over the patterned conductor and the conductive oxide layer, or the broad spectrum reflector may be deposited only in the spaces of the patterned conductor, leaving the conductive material exposed. The broad spectrum reflector may be deposited by any convenient means, such as vapor deposition, chemical or physical, thermal or plasma-enhanced. In one embodiment, a layer of titania is formed by argon sputtering a titania target onto the substrate having a patterned conductor. The titania fills the spaces of the pattern and covers the conductive lines of the pattern, coating the entire substrate. In this embodiment, the titania layer may be relatively thick, such as between about 50 μηη and about 100 μιτι.

[0024] In one embodiment, a pattern is laser scribed into a zinc oxide layer, doped with about 1% aluminum by weight, formed on a thin-film solar substrate. The zinc oxide layer is formed to a thickness of about 150 nm by sputtering an aluminum-doped zinc target using a sputtering gas comprising oxygen. The laser scribing is performed with a frequency-doubled Nd:YAG laser (λ= 064 nm) and the lines form a pattern of parallel and perpendicular segments resembling a grid. The lines are separated by a distance of about 3 mm, and are about 5 pm in width and depth. An aluminum-containing paste, for example aluminum powder dispersed in an organic solvent, is rubbed into the lines and subjected to a thermal treatment process, under an inert atmosphere such as nitrogen, argon, or helium, to yield continuous aluminum lines embedded in the doped zinc oxide layer. Excess aluminum may be removed prior to, or after, thermal treatment by polishing, if necessary. The aluminum patterned conductor and exposed doped zinc oxide are then covered with a layer of titania to serve as a broad spectrum reflector. The layer of titania is formed by spraying a fluid suspension of titania in solvent over the substrate surface and evaporating the fluid, optionally under elevated temperature. The fluid is applied to a thickness of about 10 pm, and yields a layer of titania with a uniform thickness of about 5 pm after drying. The solar substrate is finished by applying a structural back, such as glass or aluminum, over the layer of titania.

[0025] Figure 2A is a schematic cross-sectional view of a substrate 200 according to one embodiment. The substrate 200 comprises a first structural layer 202, a first conductor layer 224, a photoelectric layer 204, a second conductor layer 222, and a second structural layer 218. The first and second structural layers 202 and 218 are each generally formed from a structurally strong material, such as glass or aluminum, at least one of which is transparent. Transparent solar glasses such as BSG, PSG, or sodalite glass may be used. In the embodiment of Figure 2A, the first structural layer 202 is transparent.

[0026] The first conductor layer 224 is generally a transparent conductive material such as a metal oxide semiconductor doped with a conductive material. Zinc oxide doped with between about 0.5% and 5% by weight of aluminum, indium, gallium, silver, or tin may be used. The first conductor layer 224 is deposited to a thickness between about 50 nm and about 200 nm using any suitable deposition process, such as vapor deposition, physical or chemical, which may be plasma- enhanced.

[0027] The photoelectric layer 204 comprises a semiconductive material, such as silicon, germanium, or a compound semiconductor such as a group ΙΙΙΛ , group ll/VI, or CIGS semiconductor. The photoelectric layer 204 comprises an n-type doped semiconductor layer 206, an intrinsic semiconductor layer 208, and a p-type doped semiconductor layer 210. In alternate embodiments, the order of these layers may be reversed, such that the n-type doped semiconductor layer is formed on the first structural substrate 202. The various layers of the solar substrate may be textured in some embodiments to improve light capture characteristics. In some embodiments, buffer layers may be disposed at various interfaces to improve electrical and optical properties. Some embodiments may feature multiple photoelectric layers, such as tandem or triple junction devices.

[0028] The second conductor layer 222 is a compound conductor, comprising a low conductance layer 212 and a patterned high conductance layer 214. As described above in connection with Figure 1 , the low conductance layer 212 may comprise a semiconductive oxide material doped with a conductive material, and may be formed by any convenient method, including vapor deposition such as physical or chemical vapor deposition, which may be plasma-enhanced. In some embodiments, the low conductance layer is a conductive metal oxide layer, such as an aluminum-doped zinc oxide layer. Other metal dopants, such as gallium, indium, and tin, may be used as conductive dopants. Conductive oxide layers may be formed according to any of the methods described elsewhere herein, including those incorporated by reference. The low conductance layer 212 generally has a bulk sheet resistance greater than about 100 Ω/square.

[0029] The high conductance layer 214 may be metal or another conductive material with bulk sheet resistance generally below about 0.1 Ω/square. The high conductance layer 214 is patterned to reduce area of contact between the high conductance and low conductance material. Reducing the area of contact reduces optical losses that occur at the interface between the high conductance and low conductance layers 212 and 214. The high conductance layer 214 may be embedded in the low conductance layer 212, partially embedded in the low conductance layer 212, or deposited on the surface of the low conductance layer 212, depending on the method used to form the high conductance layer 214. For example, a pure deposition method, such as a standard lithographic patterning method, may be used to form a patterned conductor on the surface of a dielectric such as the low conductance layer 212. In other deposition techniques, the conductive material may be deposited on the surface of the substrate by a printing process, such as inkjet printing using a conductive ink that may be dried to yield conductive lines. The conductive material may be applied by syringe writing in another embodiment. Alternately, lines may be scribed into the surface of the low conductance layer 212 by any convenient means, such as laser scribing or any of the other means described herein, and conductive material disposed in the lines to form an embedded high conductance layer. The conductive material may be disposed by applying a paste or liquid containing the conductive material into the lines and then drying to coalesce the conductive material. A partially embedded high conductance layer 214 may be formed by a combination of scribing and deposition such that the lines are filled with conductive material that rises above the surface of the low conductance layer 212.

[0030] A broad spectrum reflector 216 is formed over the second conductor layer 222 to prevent loss of photons transmitted through the low conductance layer 212 of the second conductor layer 222. The broad spectrum reflector is a material that reflects photons across a broad spectrum, and may be particularly selected to have high reflectivity of photons likely to be transmitted by the second conductor layer 222. The broad spectrum reflector 216 may be a specular reflector or a diffuse reflector. In general, the broad spectrum reflector 216 will have low electrical conductivity to avoid optical losses at the interface with the second conductor layer 222. Exemplary diffuse reflector materials include metal oxides such as titanium oxide, tantalum oxide, tin oxide, zinc oxide, and combinations thereof. As described above, a metal oxide reflector may be deposited by sputtering or other convenient method.

[0031] The patterned high conductance layer 214 may have a regular or irregular pattern. Figure 2B is a schematic top view of the substrate 200 at a stage of formation before the broad spectrum reflector 216 and the second structural layer 218 are added. The view of Figure 2B shows one arrangement of the patterned high conductance layer 214 over, or embedded in, the low conductance layer 212. In the embodiment of Figure 2B, the high conductance layer 214 comprises a plurality of parallel segments 220 connected by a perpendicular segment 226 that forms a 90° angle to each of the parallel segments. The embodiment of Figure 2B thus illustrates a patterned high conductance layer arranged in a regular pattern. The parallel segments 220 are separated by a distance "d" selected to achieve a desired conductivity for the compound back conductor 222 while minimizing optical losses due to the interface between the low conductance materials 212 and the high conductance material 214. The distance "d" is generally between about 0.1 mm and about 5 mm in most embodiments, and may be between about 1 mm and about 3 mm. A plurality of perpendicular segments such as the perpendicular segment 226 may be provided to form a grid, if desired. The perpendicular segments may be spaced at any convenient spacing to achieve a desired areal coverage or bulk sheet resistivity of the compound back conductor 200. Additionally, the parallel segments 214 may form an angle other than 90° with the perpendicular segment(s) 226. Other regular patterns featuring, for example, zig-zag patterns, circular or spiral patterns, or geometric patterns other than rectilinear patterns, such as triangular or hexagonal patterns, may be used to form a patterned conductor having a desired density of conductive lines to provide a desired bulk sheet conductivity for the compound back conductor 200.

[0032] As described above, the patterned high conductance layer 214 may be formed in a regular pattern by a deposition process or by a printing or scribing process, or by a combination thereof. In one aspect, physical or chemical vapor deposition may be performed using a patterned mask. In another aspect, lines of conductive liquid or paste may be applied to the substrate by screen printing, syringe printing, gravure printing, roller printing, ink jet or ink paste printing.

[0033] Figure 2C is a schematic top view of an embodiment of the substrate 200 having a patterned high conductance layer 214 with an irregular pattern. The patterned high conductance layer 214 in the embodiment of Figure 2C comprises a plurality of conductive joined segments, separated by irregularly shaped spaces 216 through which the low conductance layer 212 is exposed, that form a conductive web or mesh across the substrate surface. The segments are of different lengths and are joined at different angles and curves to form the irregular pattern.

[0034] The irregular pattern of Figure 2C may be formed by precipitating a conductive material from a liquid. The liquid containing the conductive material may be applied by any process capable of forming a uniform thin liquid layer on a substrate, such as spin coating, roller coating, dip coating, spraying (eg. large-area spraying), waterfall coating, or other comparable processes. The liquid may be a metal containing fluid such as a metal suspension in a dielectric fluid. The liquid is evaporated to encourage the suspended metal to deposit on the surface, and the metal is then subjected to thermal treatment to drive off any remaining liquid and coagulate the metal into the joined segments of the patterned high conductance layer 214.

[0035] In some embodiments, the broad spectrum reflector 216 of Figure 2A may be formed in a pattern, as well. For example, it may be useful to form the broad spectrum reflector between the conductive lines of the patterned high conductance layer 214 to avoid any damage or disruption to the pattern. Such a result may be achieved using processes similar to those described herein in connection with forming a patterned conductive layer. In other embodiments, the broad spectrum reflector 216 may be deposited in a defined area without a defined pattern. For example, the broad spectrum reflector may be deposited over the substrate leaving an exclusion zone at the perimeter, so the conductive lines are available for module packaging.

[0036] While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof.

Claims

What is claimed is:

1. A method of forming a thin-film photovoltaic device, comprising:

forming a patterned conductor on a transparent conductive layer; and forming a reflective layer over the patterned conductor.

2. The method of claim 1 , wherein the patterned conductor comprises a plurality of connected segments covering portions of the transparent conductive layer.

3. The method of claim 2, wherein the reflective layer is deposited on the patterned conductor and fills spaces between the connected segments.

4. The method of claim 1 , wherein the reflective layer comprises a broad spectrum reflective material.

5. The method of claim 2, wherein the plurality of segments are formed by a printing process.

6. The method of claim 2, wherein the reflective layer is deposited by a physical vapor deposition process, and the reflective layer fills spaces between the connected segments.

7. A method of forming a thin-film photovoltaic device, comprising: forming a thin-film photoelectric junction on a substrate; forming a composite conductor comprising at least one patterned component on the photoelectric junction; and forming a broad spectrum reflective layer over the composite conductor.

8. The method of claim 7, wherein forming the composite conductor comprises forming a conductive oxide layer on the thin-film photoelectric junction and forming a patterned metal layer on the conductive oxide layer.

9. The method of claim 8, further comprising selecting a thickness of the conductive oxide layer that yields a sheet resistance of the composite conductor less than about 100 Ω/square.

10. The method of claim 8, wherein the patterned metal layer comprises a plurality of connected segments that cover portions of the conductive oxide layer.

11. The method of claim 9, further comprising selecting a conductivity of the conductive oxide layer based on the conductivity, thickness, and coverage of the metal layer.

12. The method of claim 11 , further comprising selecting an areal coverage of the metal layer that minimizes optical losses at the interface between the conductive oxide layer and the metal layer.

13. The method of claim 8, wherein the patterned metal is formed as a plurality of parallel segments.

14. A method of forming a back conductor for a thin-film photovoltaic device, comprising: forming a conductive oxide layer on the thin-film photovoltaic device; forming a pattern of metal lines over the conductive oxide layer; and forming a low conductivity reflector over the conductive oxide layer and the pattern of metal lines.

15. A thin-film photovoltaic device, comprising: a photoelectric conversion layer; a compound back conductor having a low conductance layer and a patterned high conductance layer adjacent to the photoelectric conversion layer; and a broad spectrum reflector formed over the compound back conductor.

16. The thin-film photovoltaic device of claim 15, wherein the low conductance layer comprises a transparent conductive oxide layer, the patterned high conductance layer comprises a metal, and the compound back conductor has a sheet resistance less than about 100 Ω/square.