WO2011129979A2

WO2011129979A2 - Method for depositing barrier layers on substrates for high quality films

Info

Publication number: WO2011129979A2
Application number: PCT/US2011/029870
Authority: WO
Inventors: Kaushal K. Singh; Gaurav Saraf; Chris Eberspacher; Klaus Schuegraf; Daniel Severin
Original assignee: Applied Materials, Inc.
Priority date: 2010-04-12
Filing date: 2011-03-24
Publication date: 2011-10-20
Also published as: WO2011129979A3

Abstract

Methods of forming barrier films for thin-film photovoltaic devices are provided. The barrier films are deposited at interfaces of the device, such as between conductor layers and layers adjacent thereto, and between photoelectric layers and layers adjacent thereto. The barrier films use dopants to tune refractive index to an intermediate value between the refractive indices of the adjacent layers to provide desired optical properties. The barrier films may also include nucleation species, such as nucleation films or nucleation sites, to stimulate growth of high quality layers adjacent to the barrier films. The barrier films may be bilayer films with a high-density portion for good barrier properties and a low-density portion into which nucleation species may be added by various implantation methods.

Description

METHOD FOR DEPOSITING BARRIER LAYERS ON SUBSTRATES

FOR HIGH QUALITY FILMS

FIELD

[0001] Embodiments described herein relate to manufacturing thin-film photovoltaic devices. More specifically, this application describes methods for forming a front contact structure for a thin-film photovoltaic device.

BACKGROUND

[0002] Photovoltaic energy generation was the fastest growing energy source in 2007. In 2008, installed photovoltaic energy generation capacity increased approximately 2/3 to about 15 GW. Nearly 6 GW/yr of photovoltaic cell manufacturing capacity was added in 2008. 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 0.015% of these available resources could replace fossil fuels and nuclear energy as sources of electrical power. The U.S. Energy Information Administration predicts total global energy consumption will reach 22.7 TW in 2030. This is less than 0.02% of available solar energy incident on the earth.

[0003] With so much potential, researchers 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, optical and electrical losses at interfaces between device layers due to refractive index transitions and structural transitions between the layers. Thus, there is a continuing need for methods of reducing or avoiding optical and electrical losses in thin-film photovoltaic devices. SUMMARY

[0004] Embodiments described herein provide a method of forming a photovoltaic device by forming a doped barrier layer on a structural substrate, and forming a thin-film photovoltaic device over the doped barrier layer.

[0005] Other embodiments provide a method of forming a front contact layer for a thin-film photovoltaic device by forming a barrier layer on a structural substrate, adding nucleation sites to the barrier layer, and using the nucleation sites to form a metal oxide layer on the barrier layer.

[0006] Other embodiments provide a photovoltaic device with a doped barrier layer between a structural substrate and a metal oxide layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] 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.

[0008] Figure 1 a flow diagram summarizing a method according to one embodiment.

[0009] Figure 2 is a flow diagram summarizing a method according to another embodiment.

[0010] Figure 3 is a flow diagram summarizing a method according to another embodiment.

[0011] Figure 4 is a schematic cross-sectional view of a photovoltaic device according to another embodiment. [0012] Figure 5 is a schematic cross-sectional view of a photovoltaic device according to another embodiment.

[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 provide methods of forming thin-film photovoltaic devices, and the devices resulting from the methods. Figure 1 is a flow diagram summarizing a method 100 according to one embodiment. The method of Figure 1 may be used to form a doped barrier layer on a substrate, the doped barrier layer having a crystal structure compatible with a crystal structure of a conductive oxide or semiconductive material. The method of Figure 1 may be practiced in a physical vapor deposition chamber such as the ATON™ 5.7 PVD system, available from the Applied Films division of Applied Materials, Inc., located in Santa Clara, California, and in other PVD chambers including chamber from other manufacturers, or in a plasma-enhanced chemical vapor deposition chamber or system such as the CENTURA^® or PRODUCER^® CVD systems, also available from Applied Materials, Inc.

[0015] At 102, with a solar substrate positioned in a chamber for processing, a gas mixture comprising a barrier precursor, a dopant precursor, and an inert gas is provided to the chamber. In a PVD process, the gas mixture has the inert gas as a sputtering agent for sputtering the barrier precursor from a sputtering target into the gas mixture as suspended particles. Argon or helium may be used as convenient inert gases for sputtering. Oxygen may be added to the gas mixture either from the target material or through an oxidizing agent added to the mixture separately. In a PECVD process, the gas mixture has an inert gas such as argon or helium as a carrier for the barrier and dopant precursors, and an oxidizing gas such as oxygen (0₂), ozone (0₃), carbon dioxide (C0₂), carbon monoxide (CO), or nitrous oxide (N₂0) is added to the gas mixture. Barrier precursors are generally silicon or aluminum precursors. Dopants such as carbon (C), hydrogen (H), water (H2O), silver (Ag), aluminum (Al), aluminum oxide (Al₂0₃), boron (B), zinc (Zn), erbium (Er), europium (Eu), francium (Fr), and combinations thereof may be used.

[0016] The barrier precursor is generally selected based on composition of adjacent layers. Aluminum precursors may yield barrier layers having generally better barrier properties than those formed from silicon precursors, especially relative to silicate dopants such as boron, phosphorus, while silicon precursors may offer good barrier properties against components of sodalite glass, metal oxide, or doped metal oxide layers. Barrier precursors generally comprise materials selected from the group consisting of silicon, oxygen, nitrogen, aluminum, and combinations thereof. Useful barrier layers may comprise silicon dioxide (Si0₂), nitrogen-doped silicon dioxide (Si02(N)), silicon nitride (Si3N₄), silicon oxynitrides (SiO_xN_y, where the ratio of y/x varies from about 0.1 to about 10, such as between about 0.2 to about 5, for example between about 0.5 and about 2), and alumina (aluminum oxide, AI2O3) or aluminum oxynitrides (AIO_xN_y, where the ratio of y/x varies from about 0.1 to about 10, such as between about 0.2 to about 5, for example between about 0.5 and about 2).

[0017] In a PVD process, silicon based barrier layers may be deposited by sputtering a silicon target in a reactive sputtering process, wherein a sputtering gas comprising argon and oxygen (e.g. 500 seem argon and 100 seem 0₂ with 10 kW RF power) is used to sputter the silicon target. Nitrogen may be added instead of, or in addition to, oxygen to form a silicon nitride or oxynitride barrier layer, with any desired ratio of nitrogen to oxygen. Addition of nitrogen may be helpful in selecting a refractive index for the barrier layer, allowing for refractive index tuning of layers to minimize reflectivity, if desired. Nitrogen may also improve barrier properties of silicon-based barrier layers. Aluminum based barrier layers may be similarly formed by PVD sputtering of an aluminum target in a reactive atmosphere. Conversely, alumina or silica targets may be sputtered in an argon atmosphere or in an argon/oxygen atmosphere if excess oxygen is desired in the deposited layer. Nitrogen may be added to the atmosphere, if desired, or sputtered from a nitride target.

[0018] In a PECVD process, silicon or aluminum precursors such as silanes (silane SiH or disilane Si₂H₆), functionalized silanes (R_xSiH_4-x, where R is an alkyl group, an alkoxy group, an amino group, a phenyl group, or any substituted variety thereof, and x is an integer from 1 to 4), or functionalized aluminum compounds (R_XAIH_3-X, where R is a methyl, ethyl, or higher alkyl group, or higher functionalized amino or group, and x is an integer from 1 to 3) , are generally provided to a CVD chamber in a carrier gas with oxygen and/or nitrogen sources to deposit the desired layers (e.g. 1000 seem argon, 500 seem TEOS, 100 seem 0₂, 1 ,000 W capacitative RF power).

[0019] At 104, a doped barrier layer is formed on the solar substrate from the precursors described above. The doped barrier layer is formed with a transitional lattice structure to facilitate compatibility with the layer formed adjacent to the doped barrier layer. Such compatibility improves performance of the eventual solar device by improving light capture and by reducing recombination of charge carriers at the interface between the layers. The barrier material in the doped barrier layer prevents migration of incompatible species between layers, and the dopant in the doped barrier layer adjusts the crystal structure of the doped barrier layer to achieve the structural compatibility. The barrier material is selected based on the required barrier properties. For example, in some embodiments, sodalite glass is used as a structural substrate, and a conductive oxide layer is formed as a front contact on the sodalite glass substrate. The conductive oxide layer may be a metal oxide layer such as zinc oxide or tin oxide that may also be doped with aluminum, indium, or gallium. A barrier layer comprising silicon or aluminum may prevent migration of sodium atoms or ions from the sodalite substrate into the conductive oxide layer. The barrier material may be an oxide or nitride of the silicon or aluminum, or a mixture thereof, as described above.

[0020] The crystal structure of the doped barrier layer is tuned to facilitate compatibility with the next layer to be formed on the barrier layer. The dopant imparted to the barrier layer by the dopant precursor adjusts the crystal structure of the barrier layer in a desired fashion to facilitate forming an interface substantially free of defects to promote light capture and carrier mobility. The dopant may occupy interstitial positions in the crystal lattice to create distortions in the lattice, or the dopant may occupy crystal lattice positions, distorting the crystal lattice structure by introducing different bond lengths and angles. In a barrier material with an oxide- nitride mixture, the ratio of oxygen and nitrogen may also affect the crystal structure of the barrier layer.

[0021] Inclusion of dopants in the doped barrier layer also allows for tuning of the refractive index of the barrier layer. Tuning the refractive index of the barrier layer facilitates light capture by influencing reflectivity at the interface between the doped barrier layer and layers adjacent thereto on either side. At the front of a solar device, a large change in refractive index between two adjacent layers, for example between a front glass layer and a conductive oxide layer, causes reflectivity and reduces light transmission to the photovoltaic core of the solar device. A doped barrier layer having a refractive index between the refractive indices of the layers on either side of the doped barrier layer reduces reflectivity by incrementing the refractive index.

[0022] The dopants included in the doped barrier layer may be used to tune the refractive index of the doped barrier layer by choice of dopant, dopant concentration, or both. A target refractive index for the doped barrier layer is determined based on the refractive indices of the layers adjacent to the doped barrier layer on either side thereof. The dopant material is selected based on the barrier material used, and the ability of the dopant to tune the refractive index of the barrier material. The amount of dopant to add is determined from the target refractive index.

[0023] Increasing the ratio of nitrogen to oxygen in the barrier material generally increases refractive index of the barrier material. Adding dopants such as carbon, hydrogen, boron, and water vapor species to the barrier layer also generally increases refractive index. Carbon may be added using carbon precursors such as hydrocarbons. Some exemplary hydrocarbons include alkanes (R_aCxH_4x-a-2, x>0, a<4x-1 ), alkenes (R_aCxH x_-a-4, x>1 , a<4x-3), alkynes (R_aC_xH_4x__a-6, x>1 , a<4x-5), cyclics (R_aC_xH2x_-a, x>2, a<2x+1), polycyclics (R_aC_xH2(_X-n)-_a, x>4, n>1 , a<2(x-n)+1) and aromatics (R_aPh). Some exemplary boron precursors include boron hydrides such as borane oligomers (B_xH_2x).

[0024] In one example, a barrier layer comprising silicon, oxygen, and nitrogen may be formed between a glass substrate and a conductive oxide layer. The glass substrate may be PSG, BSG, or sodalite glass having a refractive index in the range of 1.45 to 1.55, and the conductive oxide layer may be a metal oxide such as zinc oxide or tin oxide having a refractive index in the range of 1.95 to 3.0. Adding nitrogen to the barrier layer improves barrier properties of silicon oxide against phosphorus, boron, and sodium dopants in the glass substrate, and refractive index of the barrier layer increases with nitrogen content from about 1.54 for 20 atomic percent nitrogen to about 1 .87 for 80% atomic nitrogen.

[0025] In another example, a barrier layer comprising aluminum and oxygen may be formed between a glass substrate and a conductive oxide layer. Aluminum- based materials offer good barrier properties with respect to glass dopants, and the refractive index of aluminum oxide is in the range of 1.65 to 1.8. Some aluminum- based barrier materials may include up to about 7 atomic percent nitrogen, as well. Adding Er, Eu, or Fr to silicon or aluminum based barrier materials will also increase the refractive index of the material.

[0026] At 106, a conductive oxide layer or a semiconductive layer is formed on the doped barrier layer. At least a portion of the conductive oxide or semiconductive layer has a crystal lattice structure compatible with the crystal structure of the doped barrier layer deposited at 104. Conductive oxide layers used in various embodiments include ZnO, which may be doped with aluminum, indium, gallium, tin, or combinations thereof, and SbO, which may be doped with aluminum, indium, gallium, zinc, or combinations thereof. Semiconductive layers include used in various embodiments include layers comprising elemental semiconductors such as silicon, germanium, and mixtures thereof, and compound semiconductors such as group lll/V or group ΙΙΛ/Ι semiconductors, including CIGS compound semiconductors. The semiconductive layers may be doped with p-type or n-type dopants.

[0027] In a PVD embodiment, a doped barrier material may be formed by sputtering a target containing a barrier material in an atmosphere comprising a sputtering gas, such as argon. The target may contain a dopant, or the dopant may be added to the gas mixture as a dopant precursor. For barrier materials containing oxygen and/or nitrogen, the target may contain oxygen and/or nitrogen, or the oxygen and nitrogen may be added to the gas mixture as an oxygen precursor, a nitrogen precursor, or both. Thus, a doped barrier layer may be formed in a reactive or non-reactive sputtering process. Refractive index and density or porosity of barrier layers may be tuned by doping with carbon, boron, hydrogen, or water.

[0028] In one embodiment of a PVD process for depositing a doped barrier layer on a solar substrate, the solar substrate is positioned in a PVD chamber having a target that comprises silicon. A gas mixture comprising 10 seem O2, 200 seem Ar, and 5 seem CH₄ is provided to the chamber. RF power is coupled to the target at about 5,000 W, and silicon is sputtered from the target, co-depositing with the oxygen and carbon to form a barrier layer comprising silicon oxide doped with carbon. The barrier layer is generally formed to a thickness between about 20 nm and about 120 nm. The refractive index of the barrier layer may be adjusted by using more or less CH₄ in the sputtering gas.

[0029] In other embodiments, nitrogen may be added to the doped barrier layer instead of, or in addition to, oxygen, by providing a nitrogen source, such as nitrogen gas (N₂), ammonia (NH₃), or hydrazine (H₂N₂) to the process chamber with the sputtering gas. Carbon and boron may be added by adding any of the carbon and boron sources described above. Hydrogen may also be used as a dopant by adding a hydrogen source, which may also be a source of any of the other elements to be included in the barrier film, if convenient. Hydrogen gas H₂ may be used as the hydrogen source, or as a supplement to hydrogen provided in the other sputtering gases. [0030] The doped barrier films described herein may be disposed between any two layers of a solar substrate desirous of barrier properties, refractive index matching, and a low-loss interface. In many solar substrates, such interfaces occur on the front side of the substrate between the substrate glass and the front conductive layer, between the front conductive layer and the photovoltaic layer, and on the back side of the substrate between the photovoltaic layer and the back conductive layer. Because a typical solar substrate manufacturing process features certain layers formed by a PVD process and other layers formed by a PECVD process, it may be advantageous to form the doped barrier film in a PECVD process, in some embodiments.

[0031] In a PECVD embodiment, the substrate is positioned in a plasma processing chamber, and a gas mixture is provided to the chamber. The gas mixture comprises any of the barrier or dopant precursors described above and a non-reactive carrier gas. Barrier and dopant precursors that may be readily vaporized are preferred. The gas mixture usually further comprises an oxygen containing gas, a nitrogen containing gas, or both, which may be a mixture of an oxygen containing gas and a nitrogen containing gas. In an embodiment for processing a 300 mm circular substrate, for example, a barrier precursor such as trimethylsilane or trimethylaluminum may be provided with oxygen gas, optional nitrogen gas, and a dopant precursor such as methane, or any light hydrocarbon, borane or any low oligomer of borane, hydrogen gas, or water vapor. RF power may be coupled to the gas distributor, the substrate, or both, and is generally provided at power levels between about 100 W and about 1 ,000 W to deposit the doped barrier layer to a thickness between about 20 nm and about 120 nm.

[0032] Figure 2 is a flow diagram summarizing a method 200 according to another embodiment. The method 200 may be used to form a photovoltaic device having a functionalized barrier film on the front side of the device.

[0033] At 202, a barrier layer is formed on a solar substrate, which may be a structural substrate suitable for solar panels, such as a solar glass substrate. The barrier layer generally comprises silicon, aluminum, or a combination thereof, and oxygen, nitrogen, or a combination thereof. Examples of a barrier layer for use in the method 200 include, but are not necessarily limited to, Si0₂, Si0₂(N), SiO_xN_y, where the ratio of y/x varies from about 0.1 to about 10, such as between about 0.2 and about 5, or between about 0.5 and about 2, Si₃N₄(0), Si₃N₄, and Al₂0₃. The barrier layer may be formed as described above in connection with Figure 1.

[0034] At 204, a thin nucleation layer is formed on the barrier layer. The thin nucleation layer facilitates nucleation and growth of subsequently formed layers. The thin nucleation layer generally comprises a metal such as Ag, Al, Zn, and B, and may comprise any of the barrier materials listed above. Thus, the thin nucleation layer may also be a second barrier layer, doped with a nucleation dopant, the first barrier layer and the second barrier layer defining a barrier bilayer or dual layer. The thin nucleation layer may be formed by a PVD or PECVD process, similar to that used to form the barrier layer, and the two layers may be formed in the same process chamber by sputtering targets having different composition, or in different process chambers.

[0035] In other embodiments, nucleation sites may be formed in a barrier layer by altering the crystal structure of the substrate prior to forming the barrier layer, such that the desired crystal structure is transferred to the barrier layer, or to a nucleation layer formed on the barrier layer. For example, a glass substrate may be thermally treated at a temperature just below its glass transition temperature to form a nucleation structure on the surface of the glass structure. It is thought that the thermal treatment removes embedded species and restructures the crystal matrix of the surface to form nucleation sites that can be preserved through formation of a thin barrier film.

[0036] At 206, a TCO layer is formed on the nucleation layer. The TCO layer is generally formed by sputtering a zinc target, or a zinc target with up to about 3 wt% aluminum, in an oxidative sputtering gas, for example a mixture of argon and oxygen gas. Alternately, a ZnO target with up to about 3 wt% aluminum may be sputtered using a non-reactive sputtering gas, such as argon. Methods of forming TCO layers are described in United States Patent Application Publication No. 2008/0153280, published June 26, 2008, and are incorporated by reference herein.

[0037] The thin nucleation layer generally has a thickness less than about 20 A. In some embodiments, the thin nucleation layer may be discontinuous, such that the substrate is not entirely covered by the thin nucleation layer. In some embodiments, the nucleation layer is deposited as a plurality of isolated nucleation sites. In some embodiments, the nucleation sites may be deposited in a regular pattern, for example by using a mask. The metals in the nucleation sites provided by the thin nucleation layer provide a basis for growth of subsequent layers that allows more natural lattice growth initially, resulting in fewer lattice defects, higher quality crystal structure, and better optical and electrical properties at the interface between the substrate and the TCO layer. A metal oxide layer, for example, nucleates at the nucleation sites, or on the nucleation layer, to form a high-quality crystal interface with low electrical and optical losses. A semiconductive layer may likewise nucleate at the nucleation sites or the nucleation layer to grow a high-quality crystal interface, if desired.

[0038] Figure 3 is a flow diagram summarizing a method 300 according to another embodiment. The method 300 of Figure 3 reflects disposing a barrier layer on either side of a photovoltaic device in a solar substrate. At 302, a first barrier layer having a plurality of nucleation sites is formed on a solar substrate. Forming a barrier layer with a plurality of nucleation sites promotes formation of a high quality interface between the barrier layer and the subsequent layer because the nucleation sites cause initial formation of the subsequent layer to follow the crystal structure of the nucleation sites, resulting in few lattice defects at the interface. The barrier layer may be a doped barrier layer formed according to the methods described above in connection with Figures 1 and 2.

[0039] The nucleation sites may be formed by adding species to the surface of the barrier layer that promote nucleation of the subsequent layer. Such species may be metal atoms or semiconductor atoms implanted using ion beam or plasma immersion ion implantation. Alternately, such species may be deposited in a discontinuous thin-film as described above in connection with Figure 2.

[0040] In one embodiment, implantation of nucleation species to form nucleation sites is promoted by forming a high density portion and a low density portion of the barrier film. The high density portion is formed adjacent the layer having mobile species, for which barrier properties are desired, and the low density portion is formed adjacent the layer to be deposited using nucleation sites. The reduced density facilitates implanting nucleation species in the barrier film surface. Density of the barrier film may be reduced by disposing dopant species in the barrier film to extend the lattice structure of the film. Density may also be reduced by depositing a portion of the film using low plasma power, either in the PECVD deposition or in the PVD deposition. In some embodiments, the low density portion of the barrier film may be porous.

[0041] In one embodiment, the barrier film is formed as a bilayer comprising a first layer having a high density and a second layer having a low density. The first layer may be deposited at high density by applying high power level to a sputtering target. The sputtering power for the first layer may be between about 5 kW and about 10 kW for a high density deposition. The power level for depositing the second layer is generally below about 5 kW, such as about 2 kW to about 5 kW. It is thought that the high power level during deposition of the first layer generates a high fall of material impacting the substrate, densifying the resulting layer, whereas the lower power level during deposition of the second layer generates a lower fall that allows porosity in the deposited layer to remain, resulting in lower density. A porous barrier layer may support implantation of nucleation species such as metals by ion, plasma, or thermal implantation.

[0042] At 304, a layer of a photovoltaic device is formed on the barrier layer. Formation of such layers is known to the art. The barrier layer formed at 302 prevents species such as dopants from travelling between the photovoltaic layer and other layers of the substrate, such as a TCO layer. [0043] A 306, a second barrier layer is formed on the photovoltaic layer. The second barrier layer may be similar to the first barrier layer, and the two barrier layers together prevent dopants from travelling into or out of the photovoltaic layer. The first and second barrier layers are generally formed according to the methods described above, and are generally thin layers with tuned refractive index to achieve good optical properties. The second barrier layer may be formed with nucleation sites, as described above, to facilitate formation of a high quality back conductor TCO layer at the interface with the photovoltaic layer.

[0044] At the interface between a zinc-based TCO layer and a silicon-based photovoltaic layer, an aluminum-based barrier layer, such as an aluminum oxide or aluminum oxynitride barrier layer, may be formed with zinc or other metal dopants to adjust the refractive index of the barrier layer. The metal dopants may be implanted into the surface of the barrier layer to serve as a seed material for formation of the TCO layer, as described above.

[0045] Figure 4 is a schematic cross-sectional view of a photovoltaic device 400 according to one embodiment. The device 400 incorporates a barrier film 418 such as that described above in connection with Figures 1-3. The barrier film 418 is formed on a structural substrate 1 14, which may be glass, plastic, quartz, ceramic, or other structural material. The barrier film 418 prevents species from the structural substrate 4 from migrating into the active layers of the device 400. The barrier film 418 may be a doped barrier film, such as any of the doped barrier films described in connection with Figures 1-3, or the barrier film 418 may be a dual film or a bilayer comprising a high density layer and a low density layer. A barrier film 418 with a low density layer may be doped with nucleation species, as described above, to provide a smooth structural interface between the barrier film 418 and the next layer.

[0046] The device 400 of Figure 4 comprises a photoelectric junction 414, which is separated from the barrier film 418 by a first conductor 402. The first conductor 402 is usually transparent to allow light to reach the photoelectric junction 414. In most embodiments, the first conductor 402 is a transparent metal oxide layer, such as a zinc oxide, doped zinc oxide, tin oxide, doped tin oxide, or other metal oxide layer. Dopants include aluminum, gallium, and indium, among other species. The barrier film 418 prevents mobile species of the first conductor 402 from migrating into the structural substrate 1 14 and vice versa. Additionally, if the barrier film 418 includes nucleation species or nucleation sites, as described above, the nucleation species or sites stimulate highly regular crystal growth of the first conductor 402, resulting in a high quality interface between the first conductor 402 and the structural substrate 1 14, minimizing optical and electrical losses at the interface.

[0047] The photoelectric junction 414 comprises an n-type doped semiconductor layer 404, an intrinsic semiconductor layer 406, and a p-type doped semiconductor layer 408, which may also be in the reverse order. The layers 404-408 of the photoelectric junction 414 may be elemental semiconductor, such as silicon or germanium, or a combination thereof, or compound semiconductors such as group IMA/ semiconductors, group IIA I semiconductors, CIGS (cadmium indium gallium sulfide) type semiconductors, and the like. P-type dopants may include boron, aluminum, gallium, or indium, but are most commonly boron. N-type dopants may include phosphorus, arsenic, and tin, but are most commonly phosphorus. One of the doped layers 404 and 408 usually has an amorphous morphology, while the other is microcrystalline to capture different spectral segments, but all layers may be amorphous or microcrystalline.

[0048] The device 410 may have one or more barrier films 420 and 422 adjacent to the photoelectric junction 414, as described above in connection with Figure 3. The barrier films 420 and 422 may have the same composition as the barrier film 418, or they may be different. Any of the barrier films described herein, including the bilayer barrier films and the barrier films having nucleation layers and/or nucleation sites, may be used adjacent to the photoelectric junction 414.

[0049] The device 400 is finished by a back cover 416, comprising a back conductor layer 410 and a protective layer 412. The back conductor layer 410 is usually a transparent conductor, and may be similar to, or the same as, the front conductor 402. Aluminum doped zinc oxide and indium tin oxide are both commonly used. The back conductor layer 410 may include a reflective layer to enhance light capture. The reflective layer may be a metal layer, such as aluminum or silver, or a dielectric mirror. Metal layers are commonly used as reflectors to enhance conductivity of the back conductor layer 410.

[0050] The protective layer 412 is usually a nickel-vanadium alloy layer for protection of the back conductor layer 410 from moisture and oxygen intrusion, and may include a structural backing such as glass.

[0051] Figure 5 is a schematic cross-sectional view of a tandem-junction photovoltaic device 500 according to one embodiment. The device 500 is similar to the device 400, with a second photoelectric junction 524 and a buffer layer 510 added between the first photoelectric junction 414 and the back cover 416. The second photoelectric junction 524, comprising a second n-type doped semiconductor layer 512, a second intrinsic semiconductor layer 514, and a second p-type doped semiconductor layer 516, generally similar to the corresponding first semiconductor layers 404, 406, and 408, respectively of the first photoelectric junction 414. The barrier films 418, 420, and 422, may be used singly or in any combination, as described above, in the tandem-junction device 500, as well.

[0052] 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: . A method of forming a photovoltaic device, comprising:

forming a doped barrier layer on a substrate, the doped barrier layer having a crystal structure compatible with a crystal structure of a conductive oxide or semiconductive material; and

forming a conductive oxide or semiconductive layer over the doped barrier layer.

2. The method of claim 1 , wherein the doped barrier layer comprises a dopant selected from the group consisting of carbon, hydrogen, H₂0, silver, aluminum, Al₂0₃, boron, zinc, and combinations thereof.

3. The method of claim 1 , wherein the doped barrier layer comprises one or more barrier materials selected from the group consisting of Si0₂, Si0₂(N), Si₃N , SiO_xNy, Al₂0₃, and combinations thereof, and a dopant selected from the group consisting of carbon, hydrogen, H₂0, silver, aluminum, Al₂0₃, boron, zinc, and combinations thereof.

4. The method of claim 1 , wherein forming the doped barrier layer comprises forming a first layer having a first density and a second layer having a second density less than the first density, wherein the second layer comprises a dopant selected from the group consisting of silver, aluminum Al₂0₃, boron, zinc, and combinations thereof.

5. The method of claim 3, wherein forming the doped barrier layer comprises: selecting a barrier material based on dopants in one or both layers adjacent to the doped barrier layer; determining a target refractive index based on the refractive indices of the layers adjacent to the doped barrier layer on either side; selecting a dopant material and target concentration thereof based on the barrier material and target refractive index; and disposing the target concentration of the selected dopant material in the barrier layer to form the doped barrier layer.

6. A method of forming a front contact layer for a thin-film photovoltaic device, comprising: forming a barrier layer on a substrate; forming nucleation sites on the barrier layer; nucleating a conductive oxide layer or a semiconductive layer at the nucleation sites on the barrier layer; and growing a conductive oxide layer or a semiconductive layer from the nucleation sites on the barrier layer.

7. The method of claim 6, wherein forming the nucleation sites on the barrier layer comprises forming a nucleation layer comprising a metal on the barrier layer.

8. The method of claim 6, wherein forming nucleation sites on the barrier layer comprises doping the surface of the barrier layer.

9. The method of claim 6, wherein doping the surface of the barrier layer comprises adding dopant atoms selected from the group consisting of Ag, Al, B, Zn, C, and H to the surface of the barrier layer.

10. The method of claim 6, wherein forming nucleation sites on the barrier layer comprises forming nucleation structures on the surface of the substrate and preserving the nucleation structures in the surface of the barrier layer.

1 1. The method of claim 10, further comprising heating the surface of the substrate to a temperature within about 100^°C below a glass transition temperature of the substrate.

12. The method of claim 1 1 , wherein forming the barrier layer on the substrate comprises forming a high density barrier layer and a low density barrier layer.

13. The method of claim 12, wherein adding dopants atoms to the surface of the barrier layer comprises adding dopant atoms to the surface of the low density barrier layer.

14. The method of claim 1 or claim 13, wherein the barrier layer has a refractive index between the refractive index of the substrate and the refractive index of the conductive oxide layer or the semiconductive layer.

15. A photovoltaic device, comprising: a doped barrier layer disposed between a first layer and a second layer, the doped barrier layer comprising a dopant material selected from the group consisting of C, H, B, Ag, Al, Αΐ2θ_3> Zn, and combinations thereof, and a barrier material selected from the group consisting of Si0₂, SiO₂(N), Si₃N₄, SiO_xN_y, AI₂O₃, and combinations thereof.

16. The photovoltaic device of claim 1 or claim 15, wherein a portion of the doped barrier layer is porous.