EP2462626A2

EP2462626A2 - Barrier-coated thin-film photovoltaic cells

Info

Publication number: EP2462626A2
Application number: EP10807129A
Authority: EP
Inventors: Peter Francis Carcia; Robert Scott Mclean
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 2009-08-05
Filing date: 2010-08-05
Publication date: 2012-06-13
Also published as: WO2011017479A2; JP2013501382A; KR20120055588A; EP2462626A4; CN102696116A; WO2011017479A3

Abstract

A thin-film photovoltaic cell wherein one or more layers of the cell is coated by atomic layer deposition with a barrier layer of an inorganic oxide resists attack by moisture or atmospheric gases of the water- and/or oxygen-sensitive layers of the photovoltaic cell.

Description

TITLE

BARRIER-COATED THIN-FILM PHOTOVOLTAIC CELLS

The present application claims priority benefit of US Provisional Patent Application Serial No. 61/231493, filed August 5, 2010. Each of the foregoing applications is incorporated herein in the entirety by reference thereto.

FIELD OF THE INVENTION

This invention provides a thin-film photovoltaic cell in which one or more layers are coated by atomic layer deposition with a barrier layer of an inorganic oxide to prevent attack by moisture and/or atmospheric gases of the water-sensitive and/or oxygen-sensitive layers of the cell.

BACKGROUND

Photovoltaic (PV) cells that convert solar radiation or light to electricity need to operate year round in outdoor conditions that are often harsh. To insure a lifetime of 25 years or more, solar cells need robust packaging. For integrating solar cells into buildings as a roof-top

membrane, it is also desirable that PV cells be a flexible product in roll form.

Thin-film PV cells can be fabricated as a roll product on metal foil or plastic substrates. The top or front sheet for flexible PV cells, through which solar radiation is collected, must be optically transparent, weather- resistant and soil-proof, with low permeability to moisture and other atmospheric gases.

Thin-film PV cells can be based on inorganic materials such as amorphous silicon (a-Si), cadmium telluride (CdTe) or copper indium (gallium) di-selenide (CIS/CIGS), or on emerging technologies based on dye-sensitized, organic, and nano-matehals. Moisture-sensitivity is an issue for all thin-film PV technologies, but is particularly acute for CIGS. For CIGS PV cells to achieve the desired 25-year lifetime, it is believed that a barrier must provide a water vapor transmission rate of less than 5x10^"4 g-H₂O/nn²/clay. Despite this stringent requirement, CIGS PV cells are attractive because of their high efficiency (-20% for small laboratory- size cells).

A typical packaging scheme for thin-film PV cells on flexible substrates is illustrated in Figure 1. The structure is comprised of a substrate 12, which may be metal foil or polymer on which the PV cell 10 is fabricated, an encapsulant material 14, and a transparent front sheet 16. Without a moisture barrier, this structure will have a limited lifetime, typically less than 1 year for moisture-sensitive thin-film PV cells. The front sheet provides some moisture barrier properties, and there may also be an intervening polymer sheet 18, which may comprise one or more layers (e.g., 18a and 18b) of polymers (e.g., polyesters, fluoropolymers).

However, the intrinsic permeability of polymers is, in general, too high to achieve the level of protection needed for CIGS PV cells.

Deposition of AI2O3 films via ALD processes has been disclosed for encapsulating organic light-emitting diode (OLEDs), creating potential barrier films for such devices. Water and ozone have each been used as the oxidizer in ALD processes.

Encapsulation of pentacene/Cβo heterojunction organic solar cells in a superstrate configuration with a layer Of AI₂O₃ deposited by ALD has also been disclosed. In such devices, light is collected through the glass substrate.

None of the presently used moisture barriers has provided a level of protection adequate for thin film PV cells, especially CIGS, to attain the desired functional lifetime. Thus, there remains a need for improved moisture barriers which the present invention provides.

SUMMARY OF THE INVENTION

The present invention provides, in different embodiments, a photovoltaic cell device and a process for the manufacture thereof.

In one embodiment, there is provided a thin-film photovoltaic cell device comprising a substrate, a photovoltaic cell attached to the substrate, and at least one gas permeation barrier layer formed by an ALD process employing a water vapor precursor and a trimethyl aluminum reactant. The photovoltaic cell comprises a Cu(In, Ga)Sβ2 absorber layer and a CdS window layer, and optional additional layers.

In another embodiment, there is provided a process for constructing a photovoltaic cell device that comprises: (i) providing a substrate; (ii) forming on the substrate a photovoltaic cell comprising a Cu(In, Ga)Sβ2 absorber layer and a CdS window layer; and (iii) coating the photovoltaic cell with a gas permeation barrier layer formed by an ALD process employing a water vapor precursor and a thmethyl aluminum reactant.

In a further embodiment, the ALD process of the invention is carried out in a reactor incorporating a vacuum chamber and comprises in sequence the steps of: (i) admitting a vapor of a precursor into the chamber; (ii) purging the vapor of the precursor from the chamber to leave a thin adsorbed layer of the precursor; (iii) introducing a reactant to the chamber under thermal conditions that promote a reaction with the precursor to form a sublayer of material of a desired gas permeation barrier layer; (iv) purging the chamber of the reactant and reaction products produced by the reaction; and (v) repeating the foregoing steps for a number of times sufficient to form a gas permeation barrier layer having a preselected thickness.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be more fully understood and further advantages will become apparent when reference is had to the following detailed description of the preferred embodiments of the invention and the accompanying drawings, wherein like reference numeral denote similar elements throughout the several views and in which:

Figure 1 illustrates a PV device of the prior art, comprising a metal foil or polymer substrate on which the PV cell is fabricated, an encapsulant material, and a transparent front sheet;

Figure 2 illustrates a configuration of an embodiment of an ALD coated PV cell of this invention;

Figure 3 illustrates a configuration of another embodiment of an ALD coated PV cell of this invention; Figures 4A-4D show certain configurations for chalcopyhte and CdTe solar cells;

Figure 5 illustrates a configuration for amorphous or nanocrystalline thin film silicon solar cells;

Figure 6 illustrates a top view schematic representation of the cell of the Example; and

Figure 7 is a graph of open-circuit voltage V_oc vs time, showing the stability of the coated and encapsulated CIGS PV cell of the Example at 85 ⁰C and 85% relative humidity.

DETAILED DESCRIPTION

Atomic layer deposition (ALD) is a film growth method that produces films that potentially satisfy many of the criteria for low

permeation. A description of the atomic layer deposition process can be found in "Atomic Layer Epitaxy," by Tuomo Suntola in Thin Solid Films, vol. 216 (1992) pp. 84-89. As its name implies, the ALD process permits deposition of a material layer-by-layer. In general, the process is accomplished in a chamber using a two-stage reaction, and is carried out repetitively to build up layers forming a coating of the requisite thickness. First, a vapor of film precursor is introduced into the chamber. Without being bound by any theory, it is believed that a thin layer of the precursor, usually essentially a monolayer, is adsorbed on a substrate or device in the chamber. As used herein, the term "adsorbed layer" is understood to mean a layer whose atoms are weakly bound to the surface of a substrate. Thereafter, the vapor is purged from the chamber, e.g. by evacuating the chamber or by flowing an inert purging gas, to remove any excess or un- adsorbed vapor. A reactant is then introduced into the chamber under thermal conditions that promote reaction with the adsorbed precursor to form a sublayer of the desired barrier material. The volatile reaction products and excess precursors are then pumped from the chamber.

Additional sublayers of material are be formed by repeating the foregoing steps for a number of times sufficient to form a layer having a preselected thickness. Common CVD and PVD deposition methods entail initiation and film growth at discrete nucleation sites. The PVD method is particularly prone to creation of columnar microstructures having boundaries along which gas permeation can be facile. In contrast, ALD can produce very thin films with extremely low gas permeability, making such films attractive as barrier layers for protecting sensitive electronic devices such as PV cells. ALD is a particularly attractive method for protecting moisture and/or oxygen-sensitive devices because it forms a highly conformal coating. This allows devices with complex topographies to be fully coated and protected.

One embodiment of the present invention provides PV cells comprising one or more layers that are coated with a barrier layer formed by ALD to prevent the passage of atmospheric gases. A representative embodiment of such a PV cell device is shown generally at 20 in Figure 2. A photovoltaic cell 22 is constructed atop a flexible substrate 24, which may be made of metal or polymer. A protective layer 26 is applied on cell 22 using an ALD deposition process. Layer 26 is impermeable, which is to say that it reduces the permeation of atmospheric gases, including oxygen and water vapor, which are known to degrade the performance of typical PV devices, by at least a factor of 10⁵. Further protection for both ALD layer 26 and the PV cell 22 is afforded by a weather-proof top layer 28.

Materials formed by ALD and suitable for barriers include oxides and nitrides of Groups IVB, VB, VIB, IMA, and IVA of the Periodic Table and combinations thereof. Of particular interest in this group are Siθ2 AI2O3, and Si3l\l4. One advantage of the oxides in this group is optical transparency which is attractive for optoelectronic devices, including photovoltaic cells, in which visible light must either exit or enter the device. It is to be understood that the term "visible light" as used herein includes electromagnetic radiation having a wavelength that falls in the infrared and ultraviolet spectral regions, as well as wavelengths generally perceptible to the human eye, all being within the operational limits of typical

optoelectronic devices. The nitrides of Si and Al are also transparent in the visible spectrum. The precursors and reactants used in the ALD process to form barrier materials usefully employed in the present devices can be selected from substances known to those skilled in the art and tabulated in published references such as M. Leskela and M. Ritala, "ALD precursor chemistry: Evolution and future challenges," in Journal de Physique IV, vol. 9, pp 837-852 (1999) and references therein. Water vapor or ozone is beneficially used as the precursor.

In a representative implementation, the ALD process can be described by the overall reaction:

2 AI(CHs)₃ + 3 H₂O→ AI₂O₃ + 6 CH₄.

In the actual process, the reaction proceeds in two half-reactions at the surface that may be represented as:

AI-(CH₃)* + H₂O→ AI-OH* + CH₄

AI-OH^* + AI(CHs)₃→ Al-O- AI(CHs)₂ + CH₄, with "^*" indicating a species present at the surface of the material being coated. Of course, the ALD process may be carried out with other precursors and reactants.

The present ALD barrier synthesis may be carried out with the PV cell held at a temperature within a range of about 50 ⁰C to 250 ⁰C. Too high temperature (>250 ⁰C) is found to be incompatible with processing of temperature-sensitive polymer substrates, either because of chemical degradation of the polymer(s) or disruption of the ALD coating due to large dimensional changes of the substrate. The reaction kinetics generally are found to be too slow below 50 ⁰C.

A thickness range found to be suitable for barrier films is 2 nm to 100 nm. A more preferred range is 2 nm to 50 nm. Thinner layers will be more tolerant to flexing without causing the film to crack. This is important for polymer substrates where flexibility is a desired property. Film cracking will compromise barrier properties. Thin barrier films also increase transparency. There may be a minimum thickness corresponding to continuous film coverage, for which substantially all of the imperfections of the substrate are covered by the barrier film. For a nearly defect-free substrate, the threshold thickness for acceptable barrier properties is estimated to be at least 2 nm, but may be as thick as 10 nm. It has been found that a 25 nm thick ALD barrier layer is typically sufficient to reduce oxygen transport through a polymer film to a level below a measurement sensitivity of 0.0005 g-H₂O/m²/day.

Some oxide and nitride barrier layers formed by ALD may benefit from the inclusion of a "starting layer" or "adhesion layer" to promote adhesion of the ALD layer to the PV cell requiring protection. For example, the present PV cell device may include an adhesion layer interposed between the semiconductors of the photovoltaic cell and a 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 for the adhesion layer is selected from the same group as that of the barrier material. Aluminum oxide and silicon oxide are preferred for the adhesion layer, which may also be deposited by ALD, although other methods such as chemical and physical vapor deposition or other deposition methods known in the art may also be suitable.

Another embodiment of a thin-film PV cell device is depicted generally at 30 in Figure 3. Here, a CIGS PV cell 32 is formed on a glass substrate 34 and is protected by an ALD moisture barrier coating 26. Cell 32 and coating 26 are encapsulated by an epoxy coating 36, which in turn is covered by a top layer 38, which may be a TEFLON® FEP 260C fluoropolymer.

Other useful and exemplary configurations of the present PV cell device comprising an ALD barrier layer are illustrated in Figures 4 and 5. In general, each cell device comprises a substrate, a transparent conductive oxide (TCO) layer forming a front contact (f-contact), one or more absorber layers, and a layer for a back contact (b-contact). Electric power is extracted in a conventional manner from the PV cell through connections to the f- and b-contacts, as shown by "+" and "-" indicators. Some cell device embodiments further comprise one or more layers selected from window layers, buffer layers, and interconnect layers, and combinations thereof. In general, 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 (e.g., PET, PEN), polyamides, polyacrylates and polyimides. When the substrate is flexible and permeable either to atmospheric gases or a source of diffusing ions that can degrade PV cell performance, an 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 used to enhance the PV device's optical, electrical, or mechanical properties.

The TCO layer typically comprises mixtures or doped oxides of In₂O₃, SnO₂, ZnO, CdO, and Ga₂O₃, and provides a conductive pathway through which current generated by substantially the full active area of the PV cell can flow. Common examples in PV cells include ITO (In₂O₃ doped with about 9 atomic % Sn) and AZO (ZnO doped with 3-5 atomic % Al).

The absorber layer absorbs light from the incident light spectrum (400 - 1200 nm). Suitable absorber materials include ternary

chalchopyrite compounds such as CuInSe₂, CuInS₂, CuGaSe₂, CuInS₂, CuGaS₂, CuAISe₂, CuAIS₂, CuAITe₂, CuGaTe₂ and combinations thereof, and CdTe and related compounds.

The window layer is a thin semiconductor film (an n-type if the absorber is a p-type, or a p-type if the absorber is an n-type) that forms a heterojunction with the absorber layer, by which electric charges are separated by the built-in electric field at the junction. Suitable materials for the window layer include CdS, ZnS, ZnSe, In₂S₃, (Zn₁Cd)S, and Zn(O₁S) for a chalcopyrite absorber, and ITO, CdS and ZnO for a CdTe absorber. In some implementations, the foregoing p-n semiconductor junction structure includes an intervening i-type semiconductor, forming a p-i-n configuration.

The layer for b-contact is typically either a TCO layer or a metal. The buffer layer is typically consists essentially of a transparent, electrically insulating dielectric. Suitable materials include ZnO, Ga₂O₃, SnO₂, and Zn₂SnO₄, and mixtures thereof. In the configuration of Figure 4A, the top of the PV cell device is configured to accept light incident in the direction indicated by the arrow onto a transparent substrate 42, which thus may be termed a superstrate because of its top location. TCO layer 44 provides the positive f-contact. A window 46 lies between TCO 44 and absorber 48. A metal layer 50 provides the negative b-contact, on which an ALD barrier is coated to protect the PV cell against harmful moisture and gas permeation.

In the configurations of Figures 4B and 4C, an ALD barrier is coated on the metalized TCO and/or the buffer layer. Alternatively, an ALD layer can itself be used for the buffer layer. The Figure 4B

configuration includes a top TCO layer 44 with electrodes 58 (typically formed by screen printing and firing a metal-powder paste). The active semiconductor window 46 and absorber layer 48 are separated from TCO 44 by buffer layer 52. A bottom TCO layer 54 provides the b-contact and is formed atop substrate 56. By use of a transparent substrate 60, the configuration of Figure 4C can accept back illumination.

ALD barrier layers are also beneficial in tandem configurations, which use multiple absorbers in a stacked configuration, generally to improve conversion efficiency of the device over the entire incident spectrum. An ALD barrier layer can again be coated on the metalized TCO and/or buffer-window layers. The Figure 4D tandem configuration is constructed on substrate 56 and includes first absorber 64 and second absorber 72 in conjunction with respective buffer-window layers 62 and 70. The two absorber/buffer-window layers provide sensitivity over different spectral ranges. Light is incident through first TCO layer 44

(functioning as front contact) and first impinges on first absorber 64. Light that is not absorbed continues to propagate and reaches second absorber 72. A series electrical connection is provided by interconnect layer 66 connecting absorber 64 to second TCO layer 68. The back side of second absorber 72 is connected to 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 illustrates one form of a single junction solar cell, but double and triple junction cells are also known. One or more ALD layers are beneficially employed in each.

Amorphous or nanocrystalline Si used in PV applications is usually an alloy with hydrogen, denoted as a-Si:H or nc-Si:H. Doping to produce n-type or p-type can be accomplished using the same dopants commonly used for crystalline Si. Suitable p-type dopants include Group III elements (e.g., B). Suitable n-type dopants include Group V elements (e.g., P). Alloying with Ge or C can also be used to change the optical absorption characteristics and other electrical parameters.

Thin film a-Si and nc-Si silicon solar cells typically comprise a sequence of layers including a TCO layer 44, a p-i-n semiconductor structure 80 with a p-type Si alloy layer 82, an i-Si alloy layer 84, and an n- type Si alloy layer 86, a buffer layer 88, and a metal layer 90 for the b- contact, all formed on substrate 92. The same substrates and TCO materials used in the Figure 4 configurations are suitable. Tandem cells with higher efficiency are produced by repeating the layers of

semiconductor structure 80 of the basic cell one or more times and optimizing the absorption of the stack.

In a single p-i-n cell, an ALD barrier layer on the metalized TCO layer can prevent moisture ingress into the PV cell. In a tandem cell, an ALD barrier layer can be coated on the metalized TCO and/or buffer layers. Alternatively, an ALD layer can be used for one or all of the buffer layers.

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

EXAMPLE

ALD Barrier Deposited Directly on a CIGS Photovoltaic Cell.

A photovoltaic (PV) cell device was fabricated on a 2 inch x 2 inch glass substrate using methods well-known in the art of CIGS cell fabrication. A top view schematic representation of the cell device 100 before the ALD deposition is shown in Figure 6. The sequence of layers included a molybdenum metal layer on the glass substrate 102; an absorber layer of Cu(In, Ga)Sβ2 (CIGS), a thin window layer of CdS, a thin insulating buffer layer of ZnO, a transparent conducting oxide layer (TCO) of indium-tin oxide (ITO) 104, and a metal grid electrode of a Ni/AI alloy 106. The cell size (1 cm²) was defined by the ITO layer 104 which was deposited through a 1 cm x 1 cm shadow mask.

A 1 -2 mm wide portion 108 of the Ni/AI top electrode 106 was masked near the edge of the glass for subsequent electrical contact, and the masked CIGS PV cell was placed in a reactor (Cambridge Nanotech Savannah 200) for carrying out the ALD process. The reactor was continuously purged with nitrogen gas at 20 seem and pumped with a small mechanical pump to a background pressure (no reactant or precursor) of about 0.3 Torr. Nitrogen gas was used both as a carrier for the reactants and as a purging gas. The reactant trimethyl aluminum vapor and precursor water vapor were introduced sequentially into the reactor. More specifically, for each deposition step in the sequence, the CIGS PV cell was dosed with water vapor carried by nitrogen gas for 15 milliseconds, followed by purging of the reactor with flowing nitrogen for 30 seconds. The PV cell was then dosed for 15 milliseconds with trimethyl aluminum vapor carried by nitrogen gas, followed by a 15 second purge of flowing nitrogen. This reaction sequence produced a layer of AI2O3 on the PV cell. This deposition step was repeated sequentially for 500 times

(cycles), with the cell being held at 120 ⁰C. The thickness Of AI₂O₃ formed was determined optically on a Si witness slide to be about 55 nm, corresponding to an ALD deposition rate of about 0.11 nm per cycle.

Following ALD deposition of the AI₂O₃ barrier layer, a film of Teflon® FEP 200C (0.002 inches thick) was attached to the photovoltaic cell with a uv-curable epoxy encapsulant, leaving a space at the edges of the cell for attaching electrical leads. The Teflon FEP® acts as a weathering layer that prevents condensation of water vapor on the ALD AI₂O₃ barrier and cell, adding further protection against degradation of the PV cell during end use.

Electrical contact to the masked region of the top Ni/AI electrode was made by soldering a wire. Contact to the back Mo electrode, away from the cell area, was made by mechanically scratching through the thin top layers of AI2O3, ZnO, CdS, and Cu(In, Ga)Se2, and then soldering.

To test the barrier properties of the barrier layers (i.e., the ALD- derived AI₂O₃ and the film of Teflon® FEP 200C), the encapsulated PV cell was placed in an environmental chamber and aged at 85 ⁰C and 85% relative humidity (RH), while simultaneously being exposed to constant illumination at 1000 VWm² from a solar simulator. During this test, the open circuit voltage was monitored as a function of time, yielding the results depicted in the graph of Figure 7. It is seen that even after exposure for 1000 hrs under these conditions, no measurable change in open circuit voltage could be detected, indicating that the ALD-dehved AI₂O₃ together with the film of Teflon® FEP 200C coating protected the photovoltaic cell from the expected degradation due to moisture and other atmospheric gases. It is particularly notable that the CIGS PV cell performed satisfactorily, notwithstanding the cell's exposure to water vapor as the precursor during the deposition of the ALD barrier layer.

Having thus described the invention in rather full detail, it will be understood that such detail need not be strictly adhered to, but that additional changes and modifications may suggest themselves to one skilled in the art. It is to be understood that the present PV cell and its manufacture may be implemented in various ways, using different equipment and carrying out the steps described herein in different orders. All of these changes and modifications are to be understood as falling within the scope of the invention as defined by the subjoined claims.

Claims

CLAIMS What is claimed is: 1. A thin-film photovoltaic cell device comprising:

(a) a substrate;

(b) a photovoltaic cell attached to said substrate and comprising a Cu(In, Ga)Sβ2 absorber layer and a CdS window layer and front and back electrical contacts; and

(c) at least one gas permeation barrier layer formed on said

photovoltaic cell by an ALD process employing a water vapor precursor and a trimethyl aluminum reactant.

2. The thin-film photovoltaic cell device as recited by claim 1 ,

wherein said photovoltaic cell further comprises a transparent conducting oxide layer through which electric current generated by said cell is conducted, said transparent conducting oxide layer being disposed to provide at least one of said front and back electrical contacts.

3. The thin-film photovoltaic cell device as recited by claim 1 ,

wherein said gas permeation barrier layer has a thickness ranging from about 2 to 100 nm.

4. The thin-film photovoltaic cell device as recited by claim 2,

wherein said transparent conducting oxide layer provides said front electrical contact and said device further comprises an insulating buffer layer disposed between said front-contact transparent conducting oxide layer and said window layer.

5. The thin-film photovoltaic cell device as recited by claim 4,

wherein said insulating buffer layer consists essentially of ZnO.

6. The thin-film photovoltaic cell device as recited by claim 1 ,

further comprising a fluoropolymer top layer.

7. The thin-film photovoltaic cell device as recited by claim 1 ,

wherein said substrate consists essentially of a glass.

8. The thin-film photovoltaic cell device as recited by claim 1 ,

wherein said substrate consists essentially of a polymer.

9. The thin-film photovoltaic cell device as recited by claim 1 , wherein said substrate consists essentially of a metal.

10. The thin-film photovoltaic cell device as recited by claim 1 , further comprising an adhesion layer interposed between said gas permeation barrier layer and said photovoltaic cell.

11. The thin-film photovoltaic cell device as recited by claim 10, wherein said adhesion layer is applied by an ALD process.

12. The thin-film photovoltaic cell device as recited by claim 1 , further comprising at least one gas permeation barrier layer formed on said substrate by an ALD process.

13. A process for constructing a photovoltaic cell device,

comprising:

(a) providing a substrate;

(b) forming on said substrate a photovoltaic cell comprising a Cu(In, Ga)Se₂ absorber layer and a CdS window layer; and

(c) coating said photovoltaic cell with a gas permeation barrier layer formed by an ALD process employing a water vapor precursor and a trimethyl aluminum reactant.

14. The process as recited by claim 13, wherein said ALD process is carried out in a reactor and comprises in sequence the steps of:

(a) admitting water vapor into said chamber to form an adsorbed layer on said cell;

(b) purging said chamber;

(c) introducing a trimethyl aluminum reactant to said chamber under thermal conditions that promote a reaction of said trimethyl aluminum with said adsorbed water;

(d) purging said chamber of volatile reactants and reaction

products produced by said reaction; and

(e) repeating said steps (a), (b), (c), and (d) for a number of times sufficient to form said gas permeation barrier layer having a preselected thickness.

15. The process as recited by claim 13, wherein said thermal conditions comprise holding said photovoltaic cell at a

temperature ranging from about 50 ⁰C to about 250 ⁰C.

16. The process as recited by claim 14, wherein said purging

comprises evacuating said chamber.

17. The process as recited by claim 14, wherein said purging

comprises flowing inert gas through said chamber.

18. The process as recited by claim 14, wherein said preselected thickness ranges from about 2 nm to about 100 nm.

19. The process as recited by claim 13, further comprising forming an adhesion layer interposed between said photovoltaic cell and said gas permeation barrier layer.

20. The process as recited by claim 19, wherein said adhesion layer is formed by an ALD process.

21. The process as recited by claim 13, further comprising forming a at least one gas permeation barrier layer on said substrate by an ALD process.