A front sheet having a transparent, amorphous barrier layer made by an atomic layer deposition process. It is especially useful when applied to thin-film photovoltaic cells.
The permeation of O2 and H2O vapor through polymer films is facile. To reduce permeability for packaging applications, polymers are coated with a thin inorganic film. Aluminum-coated polyester is common. Optically transparent barriers, predominantly SiOx or AlOy, made either by physical vapor deposition (PVD) or chemical vapor deposition (CVD), are also used in packaging. The latter films are commercially available and are known in the industry as “glass-coated” barrier films. They provide an improvement for atmospheric gas permeation of about 10×, reducing transmission rates to about 1.0 cc O2/m2/day and 1.0 ml H2O/m2/day through polyester film (M. Izu, B. Dotter, and S. R. Ovshinsky, J. Photopolymer Science and Technology., vol. 8 1995 pp 195-204). While this modest improvement is a reasonable compromise between performance and cost for many high-volume packaging applications, this performance falls far short of packaging requirements in electronics. However, 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. Electronic packaging usually requires at least an order of magnitude longer desired lifetime than, for example, beverage containers. As an example, flexible displays based on organic light emitting polymers (OLEDs) fabricated on flexible polyester substrates need an estimated barrier improvement of 105-106× for exclusion of atmospheric gases. Such gases can seriously degrade both the light-emitting polymer and the water-sensitive metal cathode, which can frequently be Ca or Ba. Thin-film photovoltaic cells projected to have long lifetimes (˜25 years) need a barrier improvement of 104-106×.
Because of their inherent free volume fraction, the intrinsic permeability of polymers is, in general, too high by a factor 104-106 to achieve the level of protection needed in electronic applications, such as flexible OLED displays or photovoltaic cells. Only inorganic materials, with essentially zero permeability, can provide adequate barrier protection. Ideally, a defect-free, continuous thin-film coating of an inorganic should be impermeable to atmospheric gases. However, the practical reality is that thin films have defects, such as pinholes, either from the coating process or from substrate imperfections which compromise barrier properties. Even grain boundaries in films can present a pathway for facile permeation. For the best barrier properties, films should be deposited in a clean environment on clean, defect-free substrates. The film structure should be amorphous. The deposition process should be non-directional, and the growth mechanism to achieve a featureless microstructure would ideally be layer-by-layer to avoid columnar growth with granular microstructure.
Atomic layer deposition (ALD) is a film growth method that satisfies many of these criteria for producing low permeation films. 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, films grown by ALD form by a layer-by-layer process. ALD is in contrast to growth by common CVD and PVD methods where growth is initiated and proceeds at finite numbers of nucleation sites on the substrate surface. The latter technique can lead to columnar microstructures with boundaries between columns along which gas permeation can be facile. ALD can produce very thin films with extremely low gas permeability, making such films attractive as barrier layers for packaging sensitive electronic devices and components built on plastic substrates.
Photovoltaic (PV) cells that convert solar radiation or light into electricity need to operate year round in harsh outdoor conditions. To insure longevity of 25 years or more, solar cells need robust packaging. For integrating solar cells into building materials such 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 flexible substrates. The top or front sheet for flexible PV cells that principally collects solar radiation should be optically transparent, weather-resistant, and soil-resistant, with low permeability for moisture and other atmospheric gases. When the PV cell is fabricated on a partially transparent cell substrate, a transparent backsheet with a moisture barrier also can improve cell performance by collecting reflected light, while the barrier simultaneously protects the PV cell from moisture ingress.
Thin-film PV cells are based on amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium (gallium) di-selenide/sulfide (CIS/CIGS), and dye-sensitized, organic and nano-materials. Moisture sensitivity is an issue for all thin-film technologies, and is particularly acute in CIGS cells. To achieve a 25-year lifetime, a CIGS cell needs a barrier with a water vapor transmission rate <5×10−4 g-H2O/m2day. Nonetheless, CIGS PV cells are attractive because of their high efficiency (˜20% for small laboratory-size cells).
A typical packaging scheme for thin-film cells uses glass as the front and back sheets. This rigid structure can be impermeable with long lifetime. Alternatively the structure can be flexible, consisting of a metal foil or polymer substrate, on which the PV cell is fabricated, an encapsulant material, and a flexible transparent frontsheet, typically a polymer. However, without including a moisture barrier, the thin film cells with a flexible, transparent, polymer front sheet will have a limited lifetime.
There remains a need for flexible front sheet structures that meet the packaging needs for thin-film PV cells, especially CIGS cells.
The invention describes a multilayer article comprising:
The invention further describes a process for making a multilayer article comprising:
The atomic layer deposition process employed in the foregoing process may comprise:
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:
Atomic layer deposition (ALD) is a film growth method wherein a vapor of film precursor is adsorbed on a substrate in a reaction chamber. The vapor is then purged from the chamber, leaving an adsorbed layer of precursor, which may be a monolayer, on the substrate. The purging can be carried out by evacuation or by flowing inert gas through the chamber, or any combination thereof. As used herein, the term “adsorbed layer” is understood to mean a layer whose atoms are bound to the surface of a substrate. A second precursor is then introduced into the chamber under thermal conditions, which promote reaction with the adsorbed layer of precursor forming a layer of the desired material. The reaction products are pumped from the chamber. Subsequent layers of material can be formed by again exposing the substrate to the precursor vapor and repeating the deposition process for a number of times sufficient to form a layer having a preselected thickness. Transparent, amorphous barrier layers are formed as described above.
Described herein are transparent, amorphous barrier layers formed by ALD on plastic substrates and useful for preventing the passage of atmospheric gases. The substrates with barrier layer(s) are used as front sheets or back sheets in photovoltaic cells.
In one embodiment, a multilayer article is described having:
The plastic substrates of this invention are optically transparent and flexible, and include a general class of polymeric materials, such as those described in Polymer Materials, (Wiley, New York, 1989) by Christopher Hall or Polymer Permeability, (Elsevier, London, 1985) by J. Comyn. Common examples include polyethylene terephthalate (PET), poly(propylene terephthalate (PTT) and polyethylene naphthalate (PEN), which are commercially available by the roll as film base. For application as a back sheet for photovoltaic devices, the plastic substrate can be optically transparent as described above, but, also, may include non-transparent flexible substrates, such as translucent substrates (e.g., polyimides). The plastic substrates may include concentrations of chemical additives, that absorb uv radiation and/or reduce water absorption. Additives could improve durability of the polymer substrate in an application as a front or back sheet in a photovoltaic device.
The materials formed by ALD, suitable for barrier layers, include oxides and nitrides of Groups IVB, VB, VIB, IIIA, and IVA of the Periodic Table and combinations thereof. Of particular interest in this group are SiO2, Al2O3, TiO2, ZrO2, HfO2, and Si3N4. One advantage of the oxides in this group is optical transparency, which is attractive for electronic displays and photovoltaic cells where visible light must either exit or enter the device. The nitrides of Si and Al are also transparent in the visible spectrum. 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 precursors used in the ALD process to form these barrier materials can be selected from precursors 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. The preferred range of substrate temperature for synthesizing these barrier coatings by ALD is 50° C.-250° C. Too high temperature (>250° C.) is incompatible with processing of temperature-sensitive plastic substrates, either because of chemical degradation of the plastic substrate or disruption of the ALD coating because of large dimensional changes of the substrate. The reaction kinetics generally are found to be too slow below 50° C.
In a representative implementation, the ALD process can employ trimethyl aluminum and water, whose overall reaction can be specified as:
2 Al(CH3)3+3 H2O→Al2O3+6 CH4.
In the actual process, the reaction proceeds in two half-reactions at the surface that may be represented as:
Al—(CH3)*+H2O→Al—OH*+CH4
Al—OH*+Al(CH3)3→Al—O—Al(CH3)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.
In an embodiment, a process is described for making a multilayer article by:
In an embodiment, the atomic layer deposition process employed may comprise:
A thickness range found to be suitable for barrier films on the plastic substrate is 2 nm-100 nm. A more preferred range is 2-50 nm. Thinner layers will be more tolerant to flexing without cracking. This is extremely important for polymer substrates where flexibility is a desired property. Film cracking will compromise barrier properties. Thin barrier films also increase transparency in the cases of electronic devices where input or output of light is important. 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 was estimated to be at least 2 nm, but may be as thick as 35 nm and all thicknesses found within this range are included herein. 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 plastic substrates coated by ALD may require a “starting layer,” also known as a “nucleation layer,” to promote continuous ALD film growth on the plastic substrate or the article requiring protection. The preferred thickness of the nucleation layer is in the range of 1 nm-100 nm. Materials for the nucleation layer will generally be selected from the same group of materials to be used for the barrier layer(s). Aluminum oxide, silicon oxide, and silicon nitride are preferred for the nucleation layer, which may also be deposited by ALD, although other methods such as chemical and physical vapor deposition methods may also be suitable. Surface treatment of the plastic surface can also be used to promote nucleation of the ALD barrier layer on plastic and reduce the ALD threshold thickness for good barrier properties. Suitable surface treatments include chemical, physical, and plasma methods.
In one embodiment, the basic building block of a plastic substrate with barrier layer is a barrier layer coated by ALD on one side of a plastic substrate, where the substrate has an optional nucleation layer and/or has optionally been surface treated. In one embodiment, the basic building block is a barrier layer coated by ALD on each side of a plastic substrate, where the substrate has an optional nucleation layer and/or has optionally been surface treated. These basic building blocks can then be combined in any number of combinations by lamination to form multiple, independent barrier layers.
The plastic substrate coated with at least one barrier layer described above is particularly useful for a front sheet for copper indium gallium (di)selenide (CIGS) photovoltaic cells and other thin-film photovoltaic cells that are found in the photovoltaic commercial market, such as nanocrystalline Si, amorphous silicon (a-Si), cadmium telluride (CdTe), dye-sensitized, and organic materials. The photovoltaic cell to receive a front sheet with an ALD barrier coating(s) can be of any of several configurations and comprises a cell substrate, a metal layer for back contact, one or more absorber layers, a window layer, a transparent conducting oxide TCO layer, and a metal grid top contact layer. Some embodiments also contain one or more layers selected from window layers, buffer layers, and interconnect layers.
In general, the cell substrate on which the photovoltaic cell is fabricated is made from metal, polymer, or glass. 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, but are not limited to polyesters (e.g., PET, PEN), polyamides, polyacrylates and polyimides.
The TCO layer typically comprises mixtures or doped oxides of In2O3, SnO2, ZnO, CdO, and Ga2O3. 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 solar spectrum (400-1200 nm). Suitable absorber materials include ternary chalchopyrite compounds such as CuInSe2, CuInS2, CuGaSe2, CuInS2, CuGaS2, CuAlSe2, CuAlS2, CuAlTe2, 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. In this description, n-type refers to semiconductors in which electrical conduction is predominately by electron carriers, and p-type refers to semiconductors wherein electrical conduction is predominately by hole carriers. Suitable materials for the window layer include CdS, ZnS, ZnSe, In2S3, (Zn,Cd)S, and Zn(O,S) for a chalcopyrite absorber, and ITO, CdS and ZnO for a CdTe absorber.
The layer for back-contact is typically either a TCO layer or a metal.
The buffer layer is typically a transparent, electrically insulating dielectric. Suitable materials include ZnO, Ga2O3, SnO2, and Zn2SnO4.
The front sheet with the barrier layer(s) can also be used to protect amorphous or nanocrystalline thin-film silicon (a-Si, nc-Si) solar cells. The structure of a-Si and nc-Si solar cells is commonly p-i-n for a single cell, wherein “n” refers to n-type Si, “i” refers to insulating Si, and “p” refers to p-type Si. Tandem cells with higher efficiency are produced by stacking this basic cell and optimizing the absorption of the stack.
Thin-film silicon solar cells typically comprise a TCO layer, a p-type Si alloy layer, an i-Si alloy layer, an n-type Si alloy layer, a buffer layer, a metal layer and a substrate.
Amorphous or nanocrystalline Si is usually an alloy with hydrogen, i.e., a-Si:H or nc-Si:H. Doping n-type or p-type can be accomplished using common dopants 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.
Referring to
The Al2O3 barrier layer 16 was prepared by atomic layer deposition. The precursors used were trimethyl aluminum (TMA) vapor and water vapor. The precursors were introduced sequentially into a reactor (Cambridge Nanotech Savannah 200 made by Cambridge Nano Tech, Cambridge, Mass.). The reactor was continuously purged with nitrogen gas at 20 sccm and pumped with a small mechanical pump to a background pressure (no reactant or precursor) of about 0.3 Torr. The nitrogen gas was used as a carrier for the TMA and H2O precursors, and also as a purging gas. More specifically, the PET substrate 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 substrate 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 Al2O3 on the substrate. The reaction sequence was repeated 250 times, which formed an Al2O3 barrier layer approximately 25 nm thick on the PET substrate. Because of the high diffusivity of precursors in atomic layer deposition, a portion of the surface of the PET facing the bottom of the reactor was also coated with a layer of Al2O3. The portion of the surface facing the bottom appeared to be coated with the layer of Al2O3 near the edges of the PET substrate.
To test the barrier properties of the flexible front sheet, a uv-curable epoxy 20 (˜0.150 mm thick, ELC-2500, sold by Electro-Lite Corp, Danbury, Conn.) used as an encapsulant layer was coated onto the exposed PET substrate side of the front sheet. The epoxy coated side was then laminated to a side of a glass substrate 24 having discrete squares of a thin (˜60 nm thick) semi-transparent Ca layer 22. The squares were defined and formed by evaporation through a shadow mask. The deposition of the Ca layer 22 and the lamination were done in a nitrogen atmosphere because of the extreme air-sensitivity of Ca. The Ca was used in place of a thin-film PV cell in the test for barrier properties, because Ca is even more moisture sensitive than a typical thin-film PV cell. It, therefore, allows more rapid evaluation of the effectiveness of the flexible front sheet regarding the exclusion of water vapor ingress and mechanical integrity.
The same experiment was also carried out on a control cell having the same structure, except that the ALD-coated front sheet was replaced by a glass front sheet. The resulting data are shown as trace 32.
The optical data shown in
The structure discussed in this example is illustrated in
Data indicative of the permeation of water vapor were collected using the same optical transmission technique employed for Example 1. As shown in
To test the barrier properties of the flexible front sheet, a uv-curable epoxy 20 (˜0.150 mm thick, ELC-2500, Electro-Lite Corp, Danbury, Conn.) was coated onto the Al2O3 side of the FEP coated front sheet. The epoxy coated side was then laminated to a glass substrate 24 forming a flexible front sheet structure. The laminated side of the glass substrate had a thin (˜60 nm thick) semi-transparent Ca layer 22 deposited thereon. The deposition of the Ca layer and the lamination were done in a nitrogen atmosphere because of the extreme air-sensitivity of Ca. The Ca was used in place of a thin-film PV cell in the following test for barrier properties, because Ca is more moisture sensitive than a typical thin-film PV cell. It allows for more rapid evaluation of the effectiveness of the flexible front sheet regarding the exclusion of water vapor ingress and mechanical integrity.
The same optical transmission technique employed for Examples 1 and 2 was repeated with the
The benefit of improved resistance to gas permeation provided by ALD barrier layers, as demonstrated by Examples 1-3 above, was confirmed by testing of an actual thin-film photovoltaic cell device, as illustrated in
The Al2O3 barrier films 80a-d were prepared by the process of atomic layer deposition. The precursors used were trimethyl aluminum vapor and water vapor. The precursors were introduced sequentially into a reactor (Cambridge Nanotech Savannah 200). The reactor was continuously purged with a nitrogen gas at 20 sccm and pumped with a small mechanical pump to a background pressure (no reactant or precursor) of about 0.3 Torr. The nitrogen gas was used as a carrier for the reactants and also, as a purging gas. More specifically, the PEN substrate 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 substrate 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 Al2O3 on the substrate. The reaction sequence was repeated 250 times, which formed an Al2O3 barrier layer approximately 25 nm thick on the PEN substrate. Because of the high diffusivity of precursors in atomic layer deposition, the surface of the PEN in contact with the bottom of the reactor was also coated with a layer Al2O3. Coating both sides can further reduce the gas permeation compared to one barrier layer. A further advantage is that adhesion to an encapsulant or other layer is improved with an oxide coated surface.
To test the barrier properties of the flexible front sheet, a thermoplastic encapsulant 86, which was 0.018″ (18 mils) thick, was used to bond the flexible front sheet to a thin-film photovoltaic cell 88 with a Cu(In, Ga)Se2 (CIGS) absorber. This CIGS PV cell 88 with barrier front sheet was aged for 43 days at 85° C. and 85% relative humidity, while simultaneously exposing the PV cell to constant illumination at 1000 W/m2 from a solar simulator. Photovoltaic properties were measured before and after the 43 days of aging to assess the effectiveness of the barrier front sheet.
The photovoltaic (PV) cell 88 was fabricated on a 2 inch×2 inch glass substrate using a structure and methods well known in the art of CIGS cell fabrication. The layers consisted of a Mo metal layer on glass; an absorber layer of Cu(In, Ga) Se2, a thin window layer of CdS, a thin insulating buffer layer of ZnO, a transparent conducting oxide which was indium-tin oxide (ITO), and a metal grid electrode of a Ni/Al alloy with a Ni/Al tab electrode. The cell size of 1 cm2 was defined by the ITO layer, which was deposited through a shadow mask, 1 cm×1 cm. To improve adhesion to the encapsulant, the entire surface of the CIGS PV cell was coated with a thin (25 nm) insulating and passivating layer of ZnO. Electrical contact to the bottom Mo layer and the top Ni/Al tab electrode was made through vias that penetrated the ZnO passivation layer. The front sheet barrier, thermoplastic encapsulant, and the CIGS PV cell were laminated together by applying pressure to the stack at 150° C. for 10 minutes in a pressure laminator.
Before aging, this PV cell had an open circuit voltage (Voc) equal to 0.566 V. After aging for 43 days at 85° C. and 85% relative humidity with simultaneous solar illumination, the open circuit voltage was 0.547 V, a reduction of only about 3%, which demonstrates the effectiveness of the front sheet with ALD barrier to exclude moisture which can damage the CIGS photovoltaic cell. It is notable that the CIGS cell, which is known to be highly moisture sensitive, performed well, notwithstanding the use of water vapor as a reactant in the ALD process.
The present application claims benefit of U.S. Provisional Patent Application Ser. No. 61/236,177, filed Aug. 24, 2009, which is incorporated herein in the entirety by reference thereto.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US10/46457 | 8/24/2010 | WO | 00 | 2/23/2012 |
Number | Date | Country | |
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61236177 | Aug 2009 | US |