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Formation of compound film for photovoltaic deviceRelated Patent Categories: Batteries: Thermoelectric And Photoelectric, PhotoelectricFormation of compound film for photovoltaic device description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060207644, Formation of compound film for photovoltaic device. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATION [0001] This application is a continuation-in-part of commonly-assigned, co-pending application Ser. No. 11/081,163, entitled "METALLIC DISPERSION", which was filed on Mar. 16, 2005, the entire disclosures of which are incorporated herein by reference. This application is also related to commonly-assigned, co-pending application Ser. No. 10/782,017, entitled "SOLUTION-BASED FABRICATION OF PHOTOVOLTAIC CELL" which was filed Feb. 19, 2004 and published as US Patent Application Publication 20050183767, and to commonly-assigned, co-pending application Ser. No. 10/943,658 entitled "FORMATION OF CIGS ABSORBER LAYER MATERIALS USING ATOMIC LAYER DEPOSITION AND HIGH THROUGHPUT SURFACE TREATMENT," which was filed Sep. 18, 2004 and published as US Patent Application Publication 20050186342, the entire disclosures of both of which are incorporated herein by reference. FIELD OF THE INVENTION [0002] This invention is related to formation of nanoparticles and more particularly to formation of photovoltaic cells using nanoparticle inks for the fabrication of IB-IIIA-VIA absorber layers and where the absorber layers have a graded bandgap. BACKGROUND OF THE INVENTION [0003] Solar cells convert sunlight into electricity. These electronic devices have been traditionally fabricated using silicon (Si) as a light-absorbing, semiconducting material in a relatively expensive production process. To make solar cells more economically viable, solar cell device architectures have been developed that can inexpensively make use of thin-film, light-absorbing semiconductor materials such as copper-indium-gallium-sulfo-selenide, Cu(In,Ga)(S,Se).sub.2, also termed CI(G)S(S). Solar cells of this class typically have an absorber layer sandwiched between an electrode layer and a junction partner layer. The electrode layer is often Mo, while the junction partner is often CdS or ZnS. A transparent conductive oxide (TCO) such as zinc oxide (ZnO) is formed on the junction partner layer is typically used as a transparent electrode. CIGS-based solar cells have been demonstrated to have power conversion efficiencies exceeding 19%. [0004] A central challenge in constructing a CIGS-based solar cell is that the components of the CIGS layer must be within a narrow stoichiometric ratio in order for the resulting cell to be highly efficient. Achieving precise stoichiometric composition over relatively larger substrate areas is however difficult using traditional vacuum-based deposition processes. For example, it is difficult to deposit compounds and/or alloys containing more than one element by sputtering or evaporation. Both techniques rely on deposition approaches that are limited to line-of-sight and limited-area sources, tending to result in poor surface coverage. Line-of-sight trajectories and limited-area sources can result in the non-uniform three-dimensional distribution of elements in all three dimensions and/or poor film-thickness uniformity over large areas. These non-uniformities can occur over the nano-meso, and/or macroscopic scales. Such non-uniformity also alters the local stoichiometric ratios of the absorber layer, decreasing the potential power conversion efficiency of the complete device. [0005] Alternative approaches to vacuum-based deposition techniques such as sputtering and evaporation have been developed. In particular, production of solar cells on flexible substrates using semiconductor printing technologies provides a highly cost-efficient alternative to conventional vacuum-deposited solar cells. For example, T. Arita and coworkers [20th IEEE PV Specialists Conference, 1988, page 1650] described a screen printing technique that involved mixing and milling pure Cu, In and Se powders in the compositional ratio of 1:1:2 and forming a screen printable paste, screen printing the paste on a substrate, and sintering this film to form the compound layer. They reported that although they had started with elemental Cu, In and Se powders, after the milling step the paste contained the CuInSe.sub.2 phase. However, solar cells fabricated using the sintered layers had very low efficiencies because the structural and electronic quality of these absorbers were poor. [0006] Screen-printed CuInSe.sub.2 deposited in a thin-film was also reported by A. Vervaet et al. [9th European Communities PV Solar Energy Conference, 1989, page 480], where a CuInSe.sub.2 powder was used along with Se powder to prepare a screen printable paste. Layers formed by screen printing were sintered at high temperature. A difficulty in this approach was finding an appropriate fluxing agent for dense CulnSe.sub.2 film formation. Solar cells made in this manner also had poor conversion efficiencies. [0007] U.S. Pat. No. 5,985,691 issued to B. M. Basol et al describes another particle-based method to form a Group IB-IIIA-VIA compound film. The described method includes the steps of preparing a source material, depositing the source material on a base to form a precursor, and heating the precursor to form a film. In that method the source material includes Group IB-IIIA containing particles having at least one Group IB-IIIA phase, with Group IB-IIIA constituents present at greater than about 50 molar percent of the Group IB elements and greater than about 50 molar percent of the Group IIIA elements in the source material. The powder is milled to reduce its particle size and then used in the preparation of an ink which is deposited on the substrate in the form of a precursor layer. The precursor layer is then exposed to an atmosphere containing Group VIA vapors at elevated temperatures to convert the film into the compound. The precursor films deposited using this technique were porous and they yielded porous CuInSe.sub.2 layers with small-grain regions as reported by G. Norsworthy et al. [Solar Energy Materials and Solar Cells, 2000, vol. 60, page 127]. Porous solar cell absorbers yield unstable devices because of the large internal surface area within the device, and small grains limit the conversion efficiency of solar cells. Another key limitation of this method was the inability to effectively incorporate gallium into the material. The properly-distributed presence of gallium in a CIS film serves to potentially broaden the bandgap of the semiconductor material, thereby increasing the open circuit voltage of the solar cell, and to promote the adhesion of the CIGS layer to a (Mo) electrode, providing a back surface electric field which can improve the collection of carriers. The absence of gallium decreases the potential power conversion efficiency of the solar cell. In practice, while gallium oxide particles can easily be produced, it is very difficult to reduce gallium oxide, even at relatively high temperatures, and in the absence of reduction, gallium oxide cannot be effectively used as a precursor material for gallium in the final film. Accordingly, in addition to poor stability, solar cells made using the approach of Basol et al. had sub-optimal power conversion efficiency. [0008] Eberspacher and Pauls in U.S. Pat. No. 6,821,559 describe a process for making phase-stabilized precursors in the form of fine particles, such as sub-micron multinary metal particles, and multi-phase mixed-metal particles comprising at least one metal oxide. The preparation of particulate materials was described using a range of methods including laser pyrolysis, atmospheric arc evaporation, solution precipitation, chemical vapor reactions, aerosol pyrolysis, vapor condensation, and laser ablation. In particular, aerosol pyrolysis was used to synthesize mixed-metal particulates comprising metal oxides formed as substantially solid and spherical particulates. These particulate precursor materials were then deposited onto large-area substrates in thin layers using any of a variety of techniques including slurry spraying methods such as pneumatic spraying with a pressurized gas nozzle, hydraulic spraying with a pressurized slurry expelled through an orifice, and ultrasonic spraying with a rapidly vibrating atomization surface. A disadvantage of solar cell devices comprised of thin-film absorber layers formed in this manner was the poor reproducibility of the resulting device performance, and the porous form of the absorber layer, which tends to result in poor device stability. [0009] Bulent Basol in U.S. Published Patent application number 20040219730 describes a process of forming a compound film including formulating a powder material with a controlled overall composition and having particles of one solid solution. The powder material is deposited on a substrate to form a layer on the substrate, and this layer is reacted in at least one suitable atmosphere to form the compound. According to one preferred embodiment of that process, the compound film has a Cu/(In+Ga) compositional range of 0.7-1.0 and a Ga/(In+Ga) compositional range of 0.05-0.3. Due to the improved process window made available by the phase space of a solid solution, the use of nanoparticles comprised of a solid solution may improve the repeatability and the overall yield of the thin-film deposition and solar cell production process. [0010] Using the solid-solution approach, gallium can be incorporated into the metallic dispersion in non-oxide form--but only with up to approximately 18 relative atomic percent (Subramanian, P. R. and Laughlin, D. E., in Binary Alloy Phase Diagrams 2.sup.nd Edition, edited by Massalski, T. B. 1990. ASM international, Materials Park, Ohio, pp 1410-1412; Hansen, M., Constitution of Binary Alloys. 1958. 2.sup.nd Edition, McGraw Hill, pp. 582-584.). The lack of a means to incorporate additional Ga beyond that possible through a solid-solution (containing either Cu+ Ga or In+Ga) restricts the potential performance of a device constructed by this method. In particular, since the presence of additional gallium in the light absorbing film can serve both to widen the bandgap of the semiconductor material and to increase the open circuit voltage of the solar cell, a lack of additional gallium in the light-absorbing thin film tends to decrease the potential power conversion efficiency of solar cells created in this manner. Efficient CIGS solar cells benefit from achieving a gallium ratio of up to 40 relative atomic percent. Furthermore, it would be simpler to directly work with elemental metallic nanoparticles rather than solid-solution metallic nanoparticles in that the elements can be optimized individually and they are more readily available in elemental form. However, no technique was known in the prior art to create gallium nanoparticle powders sufficient and adequate for semiconductor applications, in part because gallium is molten near room temperature and therefore does not lend itself to common techniques for creating nanoparticles in the form of powders that are then dispersed in solution (as is commonly done with the other elements). As a result, it was not possible to directly incorporate gallium (or incorporate gallium in a high percentage) into a metallic dispersion used to print the CIG precursor of a CIGS solar cell. [0011] Robinson and Roscheisen, in commonly-assigned, co-pending, prior U.S. patent application Ser. No. 11/081,163, recently developed a technique to incorporate any desired amount of gallium into a nanoparticulate mixture used to form a compound film in a photovoltaic device. In this approach, a mixture of elemental nanoparticles composed of the IB, the IIIA, and, optionally, the VIA group of elements is combined with a suspension of nanoglobules of gallium to form a dispersion. The dispersion may be deposited onto a substrate to form a layer on the substrate. The layer may then be reacted in a suitable atmosphere to form the compound film that can be used as a light-absorbing layer in a photovoltaic device. However, this approach results in a compound film without an intentionally graded bandgap. [0012] It would be highly desirable to grade the bandgap of a CIGS absorber layer by varying its composition as a function of depth, since there are numerous advantages to varying the relative concentrations of the components of the CIGS absorber layer. These advantages include (1) improved open circuit voltage; (2) improved short circuit current density; and (3) improved optoelectronic quality in the absorber layer. A detailed discussion of these and other advantages may be found in Olle Lundberg in "Band Gap Profiling and High Speed Deposition of Cu(In, Ga)Se.sub.2 for Thin Film Solar Cells", Comprehensive Summaries of Uppsala Dissertations From the Faculty of Science and Technology 903, Acta Universitatis Upsaliensis, Uppsala, Sweden 2003, which is incorporated herein by reference. [0013] In particular, the presence of higher concentrations of Ga at the back of the absorber layer can also act as a carrier reflector, directing carriers forward to the junction at the front of the absorber layer. In addition, higher amounts of Ga deposited at or near the back contact (e.g. near the Mo interface) of the CIGS cell tend to improve device function by forming smaller grains in the presence of Ga near the back contact region, where these smaller grains are less-mechanically stressed, thus improving the mechanical stability of the cell. [0014] Further, a relatively high level of Ga in the middle of the CIGS absorber layer tends to negatively impact device function, as small CuGaSe.sub.2 grains form. These small grains tend to have a high defect density and may act as sites for charge recombination in the absorber layer. [0015] Finally, high amounts of Ga deposited at or near the front contact (e.g. near the TCO layer) of the CIGS cell promote improved device function in two ways: (1) a higher bandgap (e.g. about 1.35 eV) near the front contact sets the voltage of the cell at a relatively higher value than would otherwise exist, allowing the absorption of more photons than would otherwise be possible, thus further increasing the efficiency of light harvesting, and (2) such a higher voltage couples with a lower current, resulting in fewer I.sup.2R losses. [0016] In the prior art, graded bandgap devices with graded concentration profiles have been prepared using co-evaporation in a vacuum from elemental and/or alloy sources, and have produced the best performing CIGS solar cells recorded to date. For example, Ramanathan and coworkers at the National Renewable Energy Laboratory showed a cell having a 19.2% conversion efficiency with a fill factor of 78.12%, Jsc=35.71 mA/cm.sup.2, and an open-circuit voltage of 0.69 V using this approach (see K. Ramanathan et al., "Properties of 19.2% Efficiency ZnO/CdS/CuInGaSe.sub.2 Thin-Film Solar Cells", Progress in Photovoltaics: Research and Applications. Vol. 11 2003, pp. 225-230). However, this device was formed on a relatively small substrate, and the compound film was formed over a relatively long time. This fabrication method does not provide an efficient approach for high-volume solar cell production. [0017] Further, as described above, there are several challenges and disadvantages associated with evaporation or other vacuum-based deposition techniques for the CIGS absorber layer, including but not limited to (a) relatively high production cost, (b) relatively poor spatial and chemical uniformity of deposited compound films, and (c) relatively low throughput, limiting the potential for high-volume production. Moreover, the creation of a bandgap graded absorber layer using evaporative sources requires a relatively expensive real-time monitoring system to assess the relative composition of the absorber layer as it is being constructed. [0018] Thus, there is a need in the art for a method of forming a material comprised of gallium-containing nanoparticulate CIGS precursor materials, where the precursor materials can be reproducibly, uniformly, and densely printed over large substrate areas to form the absorber layer of a thin-film CIGS solar cell, and where the absorber layer has a graded bandgap. BRIEF DESCRIPTION OF THE DRAWINGS [0019] The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: [0020] FIGS. 1A-1E are a sequence of schematic diagrams depicting the formation of a composition of matter according to an embodiment of the present invention. Continue reading about Formation of compound film for photovoltaic device... 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