CN102696116A - Barrier-coated thin-film photovoltaic cells - Google Patents

Abstract

The present invention discloses a thin film photovoltaic cell which resists the intrusion of moisture or atmosphere to the water sensitive layer and/or the oxygen sensitive layer of the photovoltaic cell, wherein one or more layers of the cell are deposited by atomic layer Coated with an inorganic oxide barrier layer.

Description

Barrier-coated thin-film photovoltaic cells

This patent application claims priority to US Provisional Patent Application 61/231,493, filed August 5, 2010. Each of the foregoing patent applications is hereby incorporated by reference in its entirety.

field of invention

The present invention provides thin film photovoltaic cells wherein one or more layers are coated by atomic layer deposition with an inorganic oxide barrier layer to prevent ingress of moisture or atmosphere to the water sensitive and/or oxygen sensitive layers of the photovoltaic cell.

Background of the invention

Photovoltaic (PV) cells, which convert solar radiation or sunlight into electrical energy, need to operate year-round in often harsh outdoor conditions. To ensure a lifetime of 25 years or more, solar cells require robust and durable packaging. In order to integrate solar cells into buildings as rooftop membranes, photovoltaic cells are also expected to be flexible products in the form of rolls.

Thin film photovoltaic cells can be processed as roll products on metal foil or plastic substrates. The top or front sheet of a flexible photovoltaic cell through which solar radiation is collected must be optically transparent, weather and scale resistant, and have low permeability to moisture and other atmospheres.

Thin-film photovoltaic cells can be based on inorganic materials such as amorphous silicon (a-Si), cadmium telluride (CdTe) or copper indium (gallium) diselenide (CIS/CIGS), or on emerging technologies based on dye-sensitized , organic and nanomaterials. Moisture sensitivity is a problem for all thin-film photovoltaic technologies, but especially for CIGDS. For a copper indium gallium diselenide photovoltaic cell to achieve the desired lifetime of 25 years, it is believed that the barrier layer must provide a water vapor transmission rate of less than 5×10 ⁻⁴ gH ₂ O/m ² /day. Despite this stringent requirement, copper indium gallium diselenide photovoltaic cells are attractive due to their high efficiency (~20% for small lab scale cells).

A typical packaging scheme for thin-film photovoltaic cells on flexible substrates is shown in Fig. 1. The structure includes a substrate 12, which may be a metal foil or a polymer, on which a photovoltaic cell 10 is processed, an encapsulation material 14, and a transparent front sheet 16. Without a moisture barrier, the structure would have a limited lifetime, typically less than 1 year for moisture sensitive thin film photovoltaic cells. The front panel provides some moisture barrier properties, and there may also be an intervening polymer sheet 18, which may comprise one or more layers (eg, 18a and 18b) of a polymer (eg, polyester, fluoropolymer). However, the inherent permeability of polymers is often too high to achieve the level of protection required for CIGDS photovoltaic cells.

The deposition of _Al2O3 thin films by means of atomic layer deposition methods has been disclosed for the encapsulation of organic light emitting _diodes (OLEDs), resulting in potential barrier films for such devices. In atomic layer deposition methods, both water and ozone have been used as oxidizing agents.

The encapsulation of pentacene/C ₆₀ heterojunction organic solar cells in superstrate configurations with Al ₂ O ₃ layers by atomic layer deposition has also been disclosed. In such devices, light is collected through a glass substrate.

None of the currently used moisture barriers provide a level of protection sufficient for the desired functional lifetime for thin film photovoltaic cells, especially copper indium gallium diselenide. Accordingly, there remains a need for improved moisture barriers provided by the present invention.

Summary of the invention

In various embodiments, the present invention provides photovoltaic cell devices and methods of making the same.

In one embodiment, there is provided a thin-film photovoltaic cell device comprising: a substrate, a photovoltaic cell bonded to the substrate, and at least one passthrough using a water vapor precursor and a trimethylaluminum reactant. Gas permeation barrier layer formed by atomic layer deposition method. The photovoltaic cell includes a Cu(In,Ga) _Se2 absorber layer and a CdS window layer, and optionally additional layers.

In another embodiment, there is provided a method of constructing a photovoltaic cell device, the method comprising: (i) providing a substrate; (ii) forming a Cu(In,Ga) _Se absorber layer and a CdS window layer on the substrate a photovoltaic cell; and (iii) coating the photovoltaic cell with a gas permeation barrier formed by an atomic layer deposition process using a water vapor precursor and a trimethylaluminum reactant.

In another embodiment, the atomic layer deposition method of the present invention is carried out in a reactor equipped with a vacuum chamber and comprises the following steps in sequence: (i) allowing precursor vapor to enter said chamber; (ii) removing from said chamber Sweeping the precursor vapor to leave a thin precursor adsorbed layer; (iii) introducing reactants into the chamber under thermal conditions that promote reaction with the precursor to form the desired gas permeation barrier material sublayer; (iv) purging the reactants within the chamber and reaction products resulting from the reaction; and (v) repeating the preceding steps a sufficient number of times to form a gas permeation barrier layer having a preselected thickness.

Brief description of the drawings

The invention will be more fully understood and other advantages will become apparent when reference is made to the following detailed description of preferred embodiments of the invention and accompanying drawings, wherein like reference numerals indicate like elements throughout the several views, and in which:

Figure 1 shows a prior art photovoltaic cell device comprising a metal foil or polymer matrix on which photovoltaic cells are constructed, an encapsulating material and a transparent front sheet;

Figure 2 shows the configuration of one embodiment of an atomic layer deposition coated photovoltaic cell of the present invention;

Figure 3 shows the configuration of another embodiment of an atomic layer deposition coated photovoltaic cell of the present invention;

Figures 4A-4D illustrate certain configurations of chalcopyrite and CdTe solar cells;

Figure 5 shows the configuration of an amorphous or nanocrystalline thin film silicon solar cell;

Figure 6 shows a schematic top view of an example battery; and

7 is a graph of open circuit voltage V _oc versus time showing the stability of an example coated and encapsulated copper indium gallium diselenide photovoltaic cell at 85° C. and 85% relative humidity.

Detailed description of the invention

Atomic layer deposition (ALD) is a thin film growth method for preparing thin films that potentially meet many criteria for low permeability. A description of the atomic layer deposition method can be found in Tuomo Suntola, "AtomicLayer Epitaxy", Thin Solid Films, Vol. 216 (1992), pp. 84-89. As the name implies, the atomic layer deposition method deposits material layer by layer. In general, the process is accomplished indoors using a two-step reaction that is repeated to build up layers to form a coating of desired thickness. First, film precursor vapor is introduced into the chamber. Without being bound by any theory, it is believed that a thin precursor layer, usually substantially monolayer, is adsorbed on the substrate or device within the chamber. As used herein, it is understood that the term "adsorption layer" refers to a layer whose atoms are weakly bonded to the surface of the substrate. Thereafter, any excess or non-adsorbed vapor is removed by purging the vapor from the chamber, for example by evacuating the chamber or by flowing an inert purge gas. The reactants are then introduced into the chamber under thermal conditions that promote reaction with the adsorbed precursor to form a sublayer of the desired barrier material. Volatile reaction products and excess precursor are then pumped from the chamber. Additional sub-layers of material are formed by repeating the preceding steps a sufficient number of times to form a layer having a preselected thickness.

Common chemical vapor deposition and physical vapor deposition methods require initiation and film growth at discrete nucleation sites. Physical vapor deposition methods are particularly prone to producing cylindrical microstructures with boundaries along which gases can readily permeate. In contrast, atomic layer deposition can produce very thin films with extremely low gas permeability, making such films attractive as barrier layers for protecting sensitive electronic devices such as photovoltaic cells. Atomic layer deposition is a particularly attractive method for protecting moisture and/or oxygen sensitive devices because it forms highly conformal coatings. This allows devices with complex topography to be adequately coated and protected.

One embodiment of the invention provides a photovoltaic cell comprising one or more layers coated with a barrier layer formed by atomic layer deposition to prevent the passage of atmospheric gases. A representative embodiment of such a photovoltaic cell device is shown generally at 20 in FIG. 2 . Photovoltaic cells 22 are constructed atop a flexible substrate 24, which may be made of metal or polymer. The protective layer 26 is applied to the cell 22 using the atomic layer deposition method. Layer 26 is impermeable, ie it reduces the penetration of atmospheric gases including oxygen and water vapor, which are known to degrade the performance of typical photovoltaic devices, by a factor of at least 10 ⁵ . Further protection to both the ALD layer 26 and the photovoltaic cell 22 is provided by a weatherproof top layer 28 .

Materials formed by atomic layer deposition and suitable for barriers include oxides and nitrides of Groups IVB, VB, VIB, IIIA, and IVA of the Periodic Table of the Elements, and combinations thereof. Of particular interest in this group are SiO ₂ , Al ₂ O ₃ and Si ₃ N ₄ . One advantage of oxides in this group is attractive optical transparency for optoelectronic devices, including photovoltaic cells, where visible light must exit or enter the device. It should be understood that the term "visible light" as used herein includes electromagnetic radiation having wavelengths falling within the infrared and ultraviolet spectral regions and wavelengths generally detectable by the human eye, all within the operating limits of typical optoelectronic devices. Nitrides of silicon and aluminum are also transparent in the visible spectrum.

The precursors and reactants used in the atomic layer deposition process to form barrier materials effective for use in the devices of the present invention can be selected from those known to those skilled in the art and tabulated in published references, so References mentioned, for example, "ALD precursor chemistry: Evolution and future challenges" by M. Leskela and M. Ritala, Journal de Physique IV, Vol. 9, pp. 837-852 (1999) and references therein. Water vapor and ozone are advantageously used as precursors.

In a representative implementation, the atomic layer deposition process can be described by the following overall reaction:

2Al(CH ₃ ) ₃ +3H ₂ O→Al ₂ O ₃ +6CH ₄ .

In practice, the reaction proceeds on the surface as two half-reactions that can be expressed as:

Al-(CH ₃ ) ^* +H ₂ O→Al-OH ^* +CH ₄

Al-OH ^* +Al(CH ₃ ) ₃ →Al-O-Al(CH ₃ ) ₂ +CH ₄ ,

Where "*" indicates a substance present on the surface of the coated material. Of course, the atomic layer deposition method can be practiced with other precursors and reactants.

The ALD barrier complex of the present invention can be practiced with the photovoltaic cell maintained at a temperature in the range of about 50°C to 250°C. Excessively high temperatures (>250°C) were found to be incompatible with the processing of temperature-sensitive polymeric substrates due to chemical degradation of one or more polymers or atomic layer deposition due to large dimensional changes in the substrate Coating cracked. Reaction kinetics were generally found to be too slow below 50°C.

A thickness of 2 nm to 100 nm was found suitable for the barrier film. A more preferable range is 2nm to 50nm. Thinner layers will be more resistant to flexing without breaking the film. This is important for polymer matrices where flexibility is a desired characteristic. Film breakage will compromise barrier properties. Thin barrier films also increase transparency. There may be a minimum thickness corresponding to the percentage coverage of the continuous film for which substantially all of the substrate imperfections are covered by the barrier film. For an almost perfect substrate, the threshold thickness for acceptable barrier performance is estimated to be at least 2 nm, but can be as thick as 10 nm. It has been found that a 25 nm thick ALD barrier layer is generally sufficient to reduce oxygen transport through the polymer film to a level below the measurement sensitivity of 0.0005 _gH2O / ^m2 /day.

Some oxide and nitride barrier layers formed by ALD may benefit from the inclusion of an "initiation layer" or "adhesion layer" to facilitate the adhesion of the ALD layer to the photovoltaic cell for the desired protection. For example, photovoltaic cell devices of the present invention may include an adhesion layer interposed between the semiconductor of the photovoltaic cell and the protective ALD gas permeation barrier layer. The thickness of the adhesion layer is in the range of 1 nm to 100 nm. Preferably, the material used for the adhesion layer is selected from the same group as the barrier material. Aluminum oxide and silicon oxide are preferred for the adhesion layer, which can also be deposited by atomic layer deposition, although other methods such as chemical and physical vapor deposition or other deposition methods known in the art may also be suitable.

FEP 260C含氟聚合物。Another embodiment of a thin film photovoltaic cell device is depicted generally at 30 in FIG. 3 . Here, a CIGDS photovoltaic cell 32 is formed on a glass substrate 34 and protected by an ALD moisture barrier coating 26 . The cell 32 and coating 26 are encapsulated by an epoxy coating 36 which is in turn covered by a top layer 38 which may be TEFLON

FEP 260C Fluoropolymer.

Other useful and exemplary configurations of photovoltaic cell devices of the present invention comprising ALD barrier layers are shown in FIGS. 4 and 5 . In general, each battery device includes a substrate, a transparent conductive oxide (TCO) layer forming the front contact (f-contact), one or more absorber layers, and a substrate for the rear contact (b-contact). layer. Power is derived conventionally from photovoltaic cells by connection to the front and rear contacts as indicated by the "+" and "-" indicator lights. Some battery device embodiments also include one or more layers selected from the group consisting of window layers, buffer layers, and interconnect layers, and combinations thereof.

Generally, the substrate consists essentially of metal, polymer, or glass. Thin metal and polymer substrates have the advantage of being flexible; glass and some polymers have the advantage of being transparent or translucent. Suitable polymers include polyesters (eg, PET, PEN), polyamides, polyacrylates, and polyimides. When the substrate is flexible and permeable to the atmosphere or sources of diffuse ions that can degrade photovoltaic cell performance, the ALD layer can be coated on one or both sides of the substrate. In addition to ALD coatings, the substrate may also include other functional coatings for enhancing the optical, electrical or mechanical properties of the photovoltaic device.

The transparent conductive oxide layer typically includes a mixture of In ₂ O ₃ , SnO ₂ , ZnO, CdO, and Ga ₂ O ₃ or a doped oxide and provides a conductive path through which the current generated by substantially all of the active area of the photovoltaic cell can flow the pathway. Common examples in photovoltaic cells include ITO ( _In2O3 doped with about 9 atomic % tin) and AZO (ZnO doped with about 3-5 atomic _% aluminum).

The absorbing layer absorbs light from the incident spectrum (400-1200nm). Suitable absorbing materials include ternary chalcopyrite compounds such as CuInSe ₂ , CuInS 2 , CuGaSe ₂ , CuInS ₂ , CuGaS ₂ , CuAlSe ₂ , CuAlS _{2 ,} CuAlTe ₂ , and CdTe and related compounds. CuGaTe ₂ and their combinations.

The window layer is a thin semiconductor film that forms a heterojunction with the absorber layer (n-type if the absorber layer is p-type, or p-type if the absorber layer is n-type), whereby charges pass through the Built-in electric field separation. Materials suitable for window layers include CdS, ZnS, ZnSe, In ₂ S ₃ , (Zn,Cd)S, and Zn(O,S) for chalcopyrite absorbers, and ITO, CdS for CdTe absorbers and ZnO. In some implementations, the aforementioned pn semiconductor junction structure includes an intervening i-type semiconductor, thereby forming a pin configuration.

The layer used for the back contact is usually a transparent conductive oxide layer or a metal.

The buffer layer typically consists essentially of a transparent, electrically insulating dielectric. Suitable materials include _{ZnO , Ga2O3} , _SnO2 and _Zn2SnO4 , and mixtures thereof.

In the configuration of FIG. 4A , the top of the photovoltaic cell device is configured to receive light incident on the transparent substrate 42 in the direction indicated by the arrow, which can therefore be referred to as a top liner due to its top location. Transparent conductive oxide layer 44 provides the positive front contact. Window 46 is located between transparent conductive oxide 44 and absorber 48 . Metal layer 50 provides a negative back contact to which an ALD barrier layer is applied to protect the photovoltaic cell against harmful moisture and gas infiltration.

In the configurations of Figures 4B and 4C, the ALD barrier layer is coated on the metallized transparent conductive oxide layer and/or the buffer layer. Alternatively, the ALD layer itself can be used for the buffer layer. The FIG. 4B configuration includes a transparent conductive oxide top layer 44 with electrodes 58 (typically formed by screen printing and firing a metal powder paste). Active semiconductor window 46 and absorber layer 48 are separated from transparent conductive oxide 44 by buffer layer 52 . A bottom layer of transparent conductive oxide 54 provides a back contact and is formed on top of substrate 56 . By utilizing a transparent substrate 60, the configuration of FIG. 4C can accommodate backlighting.

ALD barriers are also beneficial in tandem configurations, which utilize multiple absorbers in a stacked configuration, generally to improve the conversion efficiency of the device across the entire incident spectrum. The ALD barrier layer can again be coated on the metallized transparent conductive oxide and/or buffer-window layer. The tandem configuration of FIG. 4D is constructed on a substrate 56 and includes a first absorber 64 and a second absorber 72 in combination with respective buffer-window layers 62 and 70 . Two absorber/buffer-window layers provide sensitivity in different spectral ranges. Light is incident through the first transparent conductive oxide layer 44 (serving as a front contact) and first strikes the first absorber 64 . The unabsorbed light continues to propagate and reaches the second absorber 72 . The series electrical connection is provided by an interconnect layer 66 connecting the absorber 64 to the second transparent conductive oxide layer 68 . The back side of the second absorber 72 is connected to the metal layer 50, which provides the back contact.

ALD layers can also be used to protect amorphous or nanocrystalline thin-film silicon (a-Si, nc-Si) solar cells. Figure 5 shows one form of a single junction solar cell, but double and triple junction cells are also known. One or more atomic layer deposition layers are beneficially used in each battery.

Amorphous or nanocrystalline silicon used in photovoltaic applications is usually alloyed with hydrogen, denoted a-Si:H or nc-Si:H. Doping to make n-type or p-type can be accomplished with the same dopants normally used for crystalline silicon. Suitable p-type dopants include Group III elements such as boron. Suitable n-type dopants include Group V elements such as phosphorus. Alloying with germanium or cesium can also be used to modify optical absorption characteristics and other electrical parameters.

Thin-film amorphous silicon and nanocrystalline silicon solar cells typically comprise a layer sequence comprising a transparent conductive oxide layer 44, a p-i-n semiconductor structure 80 with a p-type silicon alloy layer 82, an i-Si alloy layer 84 and an n-type silicon alloy Layer 86 , buffer layer 88 , and metal layer 90 for the back contact, all formed on substrate 92 . The same substrate and transparent conductive oxide materials used in the configuration of Figure 4 are suitable. Tandem cells with higher efficiencies are prepared by repeating the semiconducting structural layer 80 for alkaline cells one or more times and optimizing the absorption of the stack.

In a single p-i-n cell, an ALD barrier layer on a metallized transparent conductive oxide layer prevents moisture from intruding into the photovoltaic cell. In tandem cells, the ALD barrier layer can be coated on the metallized transparent conductive oxide layer and/or the buffer layer. Alternatively, atomic layer deposition layers may be used for one or all of the buffer layers.

In some embodiments, for example, as shown in FIG. 3, the ALD coating can also protect the edges of the layers of the photovoltaic cell.

Example

Atomic layer deposition barriers deposited directly on copper indium gallium diselenide photovoltaic cells.

Photovoltaic (PV) cell devices were fabricated on 2 inch by 2 inch glass substrates using methods well known in the art of CIGS cell fabrication. A top schematic view of the battery device 100 prior to atomic layer deposition is shown in FIG. 6 . The sequence of layers includes molybdenum metal layer on glass substrate 102; Cu(In,Ga) _Se2 (CIGS) absorber layer, CdS thin window layer, ZnO thin insulating buffer layer, indium tin oxide (ITO) transparent conductive oxide layer (TCO) 104, and a metal grid electrode 106 of nickel/aluminum alloy. The cell size (1 cm ² ) is defined by an indium tin oxide layer 104 deposited through a 1 cm x 1 cm shadow mask.

A 1-2 mm wide portion 108 of the nickel/aluminum top electrode 106 was masked near the edge of the glass for subsequent electrical contact, and the masked CIGDS photovoltaic cell was placed in a reactor (CambridgeNanotech Savannah 200) for the implementation of the atomic layer deposition process. The reactor was continuously purged with nitrogen at 20 seem and pumped with a small mechanical pump to a back pressure of about 0.3 Torr (no reactants or precursors). Nitrogen was used both as a carrier for the reactants and as a purge gas. The reactant trimethylaluminum vapor and the precursor water vapor are successively introduced into the reactor. More specifically, for each deposition step in the sequence, the CIGDS photovoltaic cell was first impregnated with nitrogen-carried water vapor for 15 milliseconds, followed by purging the reactor with flowing nitrogen gas for 30 seconds. The photovoltaic cell was then impregnated with trimethylaluminum vapor carried by nitrogen for 15 milliseconds, followed by a 15 second flush with flowing nitrogen. This reaction sequence produces _{an Al2O3} layer on the photovoltaic cell. This deposition step was successively repeated 500 times (cycles), wherein the cell was kept at 120°C. The thickness of _the formed _Al2O3 was measured visually on a silicon sight glass to be about 55 nm, corresponding to an atomic layer deposition rate of about 0.11 nm/cycle.

FEP 200C薄膜(0.002英寸厚)连结到光伏电池，在电池边缘处留下空间以用于连结电触头。Teflon FEP

用作风化层，其阻止在原子层沉积Al₂O₃阻挡层和电池上的水蒸汽冷凝，另外还保护光伏电池在最终使用期间不劣化。After atomic layer deposition of _{the Al2O3} barrier layer, Teflon was encapsulated with a UV-curable epoxy

A film of FEP 200C (0.002 inches thick) was bonded to the photovoltaic cell, leaving space at the edge of the cell for bonding electrical contacts. Teflon FEP

Acts as a weathering layer _which stops water vapor condensation on the ALD _Al2O3 barrier and on the cell, and additionally protects the photovoltaic cell from degradation during end use.

Electrical contacts were made to the masked area of the nickel/aluminum top electrode by welding wires. Contacts to the rear molybdenum electrode away from the cell area were made by mechanically scribing through a thin top layer of _Al2O3 , _ZnO , CdS and Cu(In,Ga) _Se2 , followed by soldering.

FEP 200C薄膜)的阻隔性能，将包封的光伏电池置于环境室中并在85℃和85％的相对湿度(RH)下老化，同时暴露于来自于太阳能模拟器的1000W/m²的恒定照明。在该测试期间，监测开路电压作为时间的函数，得到图7图线中描绘的结果。已看出，即使在这些条件下暴露1000小时之后，仍不会检测到可测量的开路电压的改变，这表明源自原子层沉积的Al₂O₃与TeflonFEP200C涂层一起保护光伏电池免受由于湿气及其它大气而导致的预期劣化。尤其显著的是铜铟镓二硒光伏电池令人满意地运行，尽管电池在原子层沉积阻挡层沉积期间暴露于作为前体的水蒸汽。In order to test the barrier layers (i.e. Al ₂ O ₃ and Teflon from ALD

FEP 200C film), the encapsulated photovoltaic cells were placed in an environmental chamber and aged at 85°C and 85% relative humidity (RH) while exposed to a constant 1000W/ ^m2 from a solar simulator. illumination. During this test, the open circuit voltage was monitored as a function of time, resulting in the results depicted in the graph of FIG. 7 . It has been seen that even after 1000 hours of exposure under these conditions, no measurable change in open _circuit voltage is detectable, suggesting that _Al2O3 from atomic layer deposition with Teflon Together, the FEP200C coating protects the photovoltaic cell from expected degradation due to moisture and other atmospheres. It is particularly notable that the copper indium gallium diselenide photovoltaic cell operates satisfactorily despite the cell being exposed to water vapor as a precursor during deposition of the ALD barrier layer.

Although the present invention has been described in some detail, it is to be understood that such details need not be strictly observed and that additional changes and modifications will occur to those skilled in the art. It should be understood that the inventive photovoltaic cell and its manufacture can be performed in a variety of ways, using different equipment and performing the steps described herein in a different order. All such changes and modifications are to be understood as falling within the scope of the present invention as defined by the appended claims.

Claims (21)

1. Thin film photovoltaic cell device, described thin film photovoltaic cell device comprises: (a) substrate; (b) a photovoltaic cell bonded to the substrate and comprising a Cu(In,Ga) _Se2 absorber layer and a CdS window layer and front and rear electrical contacts; and (c) at least one gas permeation barrier formed on the photovoltaic cell by atomic layer deposition using a water vapor precursor and a trimethylaluminum reactant. 2. The thin film photovoltaic cell device according to claim 1, wherein said photovoltaic cell further comprises a transparent conductive oxide layer through which electric current generated by said cell is conducted, said transparent conductive oxide A layer is arranged to provide at least one of said front and rear electrical contacts. 3. The thin film photovoltaic cell device of claim 1, wherein the gas permeation barrier layer has a thickness in the range of about 2 to 100 nm. 4. The thin film photovoltaic cell device of claim 2, wherein said transparent conductive oxide layer provides said front electrical contact, and said device further comprises an insulating buffer layer disposed on said front contacts between the transparent conductive oxide layer and the window layer. 5. The thin film photovoltaic cell device of claim 4, wherein the insulating buffer layer consists essentially of ZnO. 6. The thin film photovoltaic cell device of claim 1 further comprising a fluoropolymer top layer. 7. The thin film photovoltaic cell device of claim 1, wherein the substrate consists essentially of glass. 8. The thin film photovoltaic cell device of claim 1, wherein the substrate consists essentially of a polymer. 9. The thin film photovoltaic cell device of claim 1, wherein the substrate consists essentially of metal. 10. The thin film photovoltaic cell device of claim 1, further comprising an adhesion layer interposed between the gas permeation barrier layer and the photovoltaic cell. 11. The thin film photovoltaic cell device of claim 10, wherein the adhesion layer is applied by atomic layer deposition. 12. The thin film photovoltaic cell device of claim 1, further comprising at least one gas permeation barrier layer formed on the substrate by atomic layer deposition. 13. A method for constructing a photovoltaic cell device, the method comprising: (a) provide the substrate; (b) forming a photovoltaic cell comprising a Cu(In,Ga) _Se2 absorber layer and a CdS window layer on said substrate; and (c) coating the photovoltaic cell with a gas permeation barrier formed by atomic layer deposition using a water vapor precursor and a trimethylaluminum reactant. 14. The method of claim 13, wherein the atomic layer deposition method is implemented in a reactor and comprises the following steps in sequence: (a) allowing water vapor to enter the chamber to form an adsorption layer on the cell; (b) purging the chamber; (c) introducing a trimethylaluminum reactant into said chamber under thermal conditions that promote the reaction of said trimethylaluminum with said adsorbed water; (d) purging the chamber of volatile reactants and reaction products resulting from the reaction; and (e) repeating said steps (a), (b), (c) and (d) a sufficient number of times to form said gas permeation barrier layer having a preselected thickness. 15. The method of claim 13, wherein the thermal conditions include maintaining the photovoltaic cell at a temperature in the range of about 50°C to about 250°C. 16. The method of claim 14, wherein the purging includes evacuating the chamber. 17. The method of claim 14, wherein the purging includes flowing an inert gas through the chamber. 18. The method of claim 14, wherein the preselected thickness is in the range of about 2 nm to about 100 nm. 19. The method of claim 13, further comprising forming an adhesion layer interposed between the photovoltaic cell and the gas permeation barrier layer. 20. The method of claim 19, wherein the adhesion layer is formed by an atomic layer deposition method. 21. The method of claim 13, further comprising forming at least one gas permeation barrier layer on the substrate by atomic layer deposition.

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