Coating processes – Electrical product produced – Photoelectric
Reexamination Certificate
2001-12-12
2003-10-21
Meeks, Timothy (Department: 1762)
Coating processes
Electrical product produced
Photoelectric
C427S076000, C427S078000, C427S580000, C204S192380
Reexamination Certificate
active
06635307
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method for producing thin films or coatings on a solid substrate. These thin films or coatings can be used in an anti-reflection layer, light-absorbing and charge-generating active layer, window or buffer layer, and/or electrode layer in a thin-film solar cell.
BACKGROUND OF THE INVENTION
Thin-film solar cells or photovoltaic (PV) devices convert sunlight directly into DC electrical power. These multi-layer PV devices are typically configured to include an active layer, which is typically a cooperating sandwich of p-type, intrinsic (i-type), and n-type semiconductors. With appropriately located electrical contacts being included, the structure forms a working PV cell. When sunlight incident on PV cells is absorbed in the semiconductor, electron-hole pairs are created. The electrons and holes are separated by an electric field of a junction in the PV device. The separation of the electrons and holes by the junction results in the generation of an electric current and voltage. The electrons flow toward the n-type region and the holes flow toward the p-type region of the semiconductor material. Current will flow through an external circuit connecting the n-type region to the p-type region as long as light continues to generate electron-hole pairs in the PV device. Solar cells are typically arranged into PV modules by connecting the front of one cell to the back of another, thereby adding the voltages of the individual cells together. A large number of cells, typically 36 to 50, are required to be connected in series to achieve a nominal usable voltage of 12 to 18 V.
The last decade has seen a dramatic increase in commercial use and interest in thin-film PV devices. However, a wider scale commercial use of PV devices for bulk power generation remains limited primarily due to two factors: performance and cost. Recently, dramatic improvements in PV module performance have been achieved in both bulk crystalline silicon and thin film PV devices. The efficiency of laboratory scale crystalline silicon is approaching 20%, while modules with an efficiency ranging from 10 to 14% are commercially available from several vendors. Laboratory scale thin-film PV devices with efficiencies of well above 10% have been achieved with copper indium diselenide (CIS), cadmium telluride, amorphous silicon, and microcrystalline silicon. Most notably, a record efficiency of 18.8% has been achieved for copper indium gallium diselenide (CIGS). Additionally, several companies have achieved thin-film large-area modules with efficiencies ranging from 8 to 12%. Although there is still a great deal of room for further efficiency improvements, performance no longer seems to be the key limiting factor. Cost now appears to be the primary factor preventing wide-scale commercialization of PV modules for power generation.
Key areas that dictate the final product cost for a PV device includes capital equipment costs, deposition rates, layer thickness, materials costs, yields, substrates, and front and back end costs [Zweibel]. The present invention was made by taking these factors into consideration. Our work has emphasized the reduction in capital equipment cost, dramatically increased deposition rate, reduced layer thickness, reduced amount of material used, and alternative substrates. In order to illustrate why the present invention stands out as a major advancement in the field of thin-film solar cell technology, the state-of-the-art of this technology is briefly reviewed as follows:
Amorphous silicon has been made into thin film semiconductors by plasma enhanced chemical vapor deposition (PECVD), or simply plasma discharge or glow discharge, as disclosed by U.S. Pat. No. 5,016,562. Other processes used to make thin film semiconductors include cathode atomization, vapor deposition in a vacuum, high-frequency vaporization in a hydrogen-containing environment, electro-deposition, screen printing and close-spaced sublimation. The close-spaced sublimation process has been used with cadmium telluride and is performed by inserting a glass sheet substrate into a sealed chamber that is then heated. The glass sheet substrate is supported at its periphery in a very close relationship, normally 2 to 3 mm, to a source material of cadmium telluride. After the heating has proceeded to about 450° C.-500° C., the cadmium telluride begins to sublime very slowly into elemental cadmium and tellurium and, upon reaching a temperature of about 650°-725° C., the sublimation is at a greater rate and the elemental cadmium and tellurium recombines at a significant rate as cadmium telluride on the downwardly facing surface of the peripherally supported glass sheet substrate. The heating is subsequently terminated prior to opening of the chamber and removal of the substrate with the cadmium telluride deposited on the substrate. Thus, the deposition of the cadmium telluride is at a varying temperature that increases at the start of the processing and decreases at the end of the processing. Furthermore, the largest area on which such close-spaced sublimation has previously been conducted is about 100 cm
2
. Increasing the size of the substrate can cause problems in maintaining planarity since the heated substrate which is supported at only its periphery will tend to sag at the center.
Several methods have been proposed for producing the copper indium diselenide (CIS) active layer, such as a three-source simultaneous-deposition method, a spraying method, a two-stage selenidation method, a selenidation method using H
2
Se, a sputtering method, and an electro-deposition method. The three-source simultaneous-deposition method requires a deposition apparatus having a vacuum chamber which contains Cu, In, and Se evaporation sources. The three elements are evaporated simultaneously from their respective sources and deposited onto a substrate preheated to 350-400° C. Other methods for producing thin-film PV devices include solution spraying, spray pyrolysis, and combined plasma CVD and sputtering.
Optically transparent and electro-conductive electrodes can be obtained by two primary methods. The first method involves producing a metal oxide thin film, such as indium-tin oxide (ITO) or antimony-tin oxide (ATO), on a transparent glass or plastic substrate by sputtering or chemical vapor deposition (CVD). The second method involves solution-coating a transparent, electro-conductive ink on a support such as a glass substrate. The ink solution composition contains a powder of ultra-fine, electro-conductive particles having a particle size smaller than the smallest wavelength of visible rays. The ink is then dried on the support, which is then baked at temperatures of 400° C. or higher.
The first method requires the utilization of expensive devices and its reproducibility and yield are low. Furthermore, the procedure is tedious and time-consuming, typically involving the preparation of fine oxide particles, compaction and sintering of these fine particles to form a target, and then laser- or ion beam-sputtering of this target in a high-vacuum environment. Therefore, it was difficult to obtain transparent electro-conductive coatings that are of low prices. The electro-conductive film formed on the support by the second method tends to have some gaps remaining between the ultra-fine particles thereon so that light scatters on the film, resulting in poor optical properties. In order to fill the gaps, heretofore, a process has been proposed in which a glass-forming component is incorporated into the transparent, electro-conductive ink prior to forming the transparent, electro-conductive substrate. However, the glass-forming component is problematic in that it exists between the ultra-fine, electro-conductive particles, thereby increasing the surface resistivity of the electro-conductive film to be formed on the support. For this reason, therefore, it was difficult to satisfy both the optical characteristics and the desired surface resistivity conditions of the transparent, electro-conductive
Huang Wen-Chiang
Wu Liangwei
Meeks Timothy
Nanotek Instruments, Inc.
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