Solar cells convert sunlight directly into electricity. These electronic devices are commonly fabricated on silicon (Si) wafers. However, the cost of electricity generated using silicon-based solar cells is rather high, compared to electricity generated by the traditional methods, such as fossil-fuel-burning power plants. To make solar cells more economically viable, low-cost, thin-film growth techniques that can deposit high-quality light-absorbing semiconductor materials need to be developed. These techniques need to employ cost-effective approaches to grow solar cell absorbers on large-area substrates with high throughput and high materials utilization. Therefore, low-cost and high-efficiency thin film solar cell fabrication requires: i) a solar cell absorber material that fundamentally has the capability to yield high-efficiency devices, ii) a low-cost deposition technique that can deposit this absorber material in the form of a high-quality thin film on a low-cost substrate. Both of these ingredients are necessary for manufacturing high-efficiency, low-cost solar cell structures. When solar cells formed on large-area substrates are interconnected, modules with higher voltage and power output are obtained.
Copper-indium-gallium-sulfo-selenide, Cu(In,Ga)(S,Se)2, compounds are excellent absorber materials for thin-film solar cell structures provided that their structural and electronic properties are good. An important compositional parameter of Cu(In,Ga)(S,Se)2 thin films is the metals molar ratio of Cu/(In+Ga). The typically acceptable range of this molar ratio for high-efficiency solar cell absorbers is about 0.70–1.0, although in some cases when the compound is doped with a dopant such as sodium (Na), potassium (K) or lithium (Li), this ratio can go even lower. If the Cu/(In+Ga) molar ratio exceeds 1.0, however, a low-resistivity copper selenide, sulfide or sulfo-selenide phase precipitates and deteriorates the performance of the device due to electrical shorting paths it creates through the absorber. Therefore, control of the Cu/(In+Ga) ratio is important for any technique that is used for the preparation of Cu(In,Ga)(S,Se)2 films for radiation detector or solar cell applications. The Ga/(In+Ga) ratio is also important to control since this ratio determines the bandgap of the absorber, which can be varied from about 1 eV (for CuInSe2) to 2.43 eV (for CuGaS2). In principal the Ga/(In+Ga) ratio may vary from zero in CuIn(S,Se)2 to 1.0 in CuGa(S,Se)2. However, laboratory experience to date has shown that best device efficiencies are obtained for Ga/(In+Ga) ratios in the range of 0.1–0.3.
Although important to control, the Cu/(In+Ga) and Ga/(In+Ga) ratios are not the only factors that influence the electronic properties of Cu(In,Ga)(S,Se)2 compound thin films. The compositional ratios of a compound film may be within the acceptable ranges, but solar cells fabricated on this film may still have poor light-to-electricity conversion efficiencies. Cu(In,Ga)(S,Se)2 compound thin films used in high-efficiency solar cell structures, besides having the right composition, also need to have good morphology and large-grain structure. For example, a typical high-quality Cu(In,Ga)(S,Se)2 thin film is 1.0–3.0 μm thick; it is dense and it has columnar grains with widths of at least 0.5 μm.
One approach that yielded high-quality Cu(In,Ga)Se2 films for solar cell applications is co-evaporation of Cu, In, Ga and Se onto heated substrates in a vacuum chamber [see for example, Bloss et al., “Thin Film Solar Cells”, Progress in Photovoltaics, 1995, vol. 3, page 3]. Absorbers grown by this technique are typically dense and they have large columnar grains. The Cu/(In+Ga) ratio and the Ga/(In+Ga) ratio are closely controlled during deposition by monitoring and controlling the individual evaporation rates of Cu, In and Ga. Consequently, Cu(In,Ga)Se2 solar cells fabricated on co-evaporated absorbers yielded small, laboratory-size solar cells with close to 19% conversion efficiency. Although there are now concentrated efforts to apply this technique to the fabrication of large-area Cu(In,Ga)Se2 modules, the method is not readily adaptable to low-cost production of large-area films, mainly because control of Cu/(In+Ga) and Ga/(In+Ga) ratios by evaporation over large-area substrates is difficult, materials utilization is low and the cost of vacuum equipment is high.
Another technique for growing Cu(In,Ga)(S,Se)2 type compound thin films for solar cell applications is the two-stage processes where at least two components of the compound are first deposited onto a substrate, and then reacted with each other and/or with a reactive atmosphere in a high-temperature annealing process. U.S. Pat. No. 4,798,660 issued to J. Ermer et al. teaches a method for fabricating a CuInSe2 film comprising sequentially depositing a film of Cu on a substrate by DC magnetron sputtering and depositing In on said film of Cu and heating the composite film in the presence of Se to form the compound. This approach is schematically depicted in FIG. 1, as it would be applied to the growth of a Cu(In,Ga)Se2 thin film. In FIG. 1, a Cu sub-layer 12 is first deposited on a substrate 10 which has a metallic film 11, such as molybdenum (Mo) on its surface. Then a (In+Ga) sub-layer 13 is deposited over the Cu sub-layer 12 to form the composite layer 14. Later in the process, the complete structure of FIG. 1 is heated in the presence of Se vapors to convert the composite layer 14 into a Cu(In,Ga)Se2 compound layer.
Karg et al. in U.S. Pat. No. 5,578,503 teach an alternate two-stage technique where a Cu film, an In or Ga film and a S or Se film are deposited on a substrate to form a stacked layer and then this stacked layer is heated rapidly to form the compound. The stacked structure of this prior art approach is depicted in FIG. 2, as it would be applied to the growth of a Cu(In,Ga)Se2 solar cell absorber. In FIG. 2, a Cu sub-layer 22 is first deposited over the metal film 21 which is previously coated on substrate 20. This is followed by the depositions of a (In+Ga) sub-layer 23 and a Se sub-layer 24. All the sub-layers form the stacked layer 25 which is then converted into a Cu(In,Ga)Se2 compound layer when the whole structure of FIG. 2 is rapidly heated in a rapid thermal processing (RTP) furnace.
Yet another two-stage processing approach is taught in European Patent application No EP0838864A2 by K. Kushiya et al. In that method a stacked precursor comprising a Cu—Ga sub-layer and an In sub-layer was employed. This method is schematically shown in FIG. 3 as it would be applied to the growth of a Cu(In,Ga)Se2 absorber. According to FIG. 3, a Cu—Ga sub-layer 32 is first deposited over the metal contact layer 31 which was previously deposited on the surface of substrate 30. An In sub-layer 33 is then deposited over the Cu—Ga sub-layer 32. The structure of FIG. 3 is then annealed in the presence of Se vapors to convert the multi-layer 34 into a Cu(In,Ga)Se2 layer. Further annealing in H2S yields a Cu(In,Ga)(S,Se)2 film.
All of the prior art two-stage techniques reviewed above yielded high-quality compound films in terms of their structural and electronic properties. Large-scale manufacturing, however, also requires strict control of the material composition over large-area substrates. This means that in the two-stage processes that utilize a stack of various sub-layers, the uniformity and thickness of each sub-layer have to be individually controlled over large-area substrates. This is very difficult. DC magnetron sputtering techniques which are commonly used to deposit the sub-layers of FIGS. 1, 2 and 3 are expensive vacuum techniques with low materials utilization. Therefore, their cost is high.
Since compositional control, especially the control of the Cu/(In+Ga) ratio is important for Cu(In,Ga)(S,Se)2 compounds, attempts have been made to fix this ratio in an initial material, before the deposition process, and then transfer this fixed composition into a thin film formed using this initial material. T. Arita et al. in their 1988 publication [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 CuInSe2 phase. Solar cells fabricated on the sintered layers had very low efficiencies because the structural and electronic quality of these absorbers were poor.
Thin layers of CuInSe2 deposited by a screen printing method were also reported by A. Vervaet et al. [9th European Communities PV Solar Energy Conference, 1989, page 480]. In that work a CulnSe2 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 a suitable fluxing agent for dense CulnSe2 film formation. Therefore, solar cells fabricated on the resulting layers had poor conversion efficiencies.
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, where IB=Cu, Ag, Au, IIIA=In, Ga, Al, Tl, and VIA=S, Se, Te. 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 invention the source material includes Group IB-IIIA alloy-containing particles having at least one Group IB-IIIA alloy phase, with Group IB-IIIA alloys constituting 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 CuInSe2 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. Also small grains limit the conversion efficiency of solar cells.
PCT application No. WO 99/17889 (Apr. 15, 1999) by C. Eberspacher et al. describes methods for forming solar cell materials from particulates where various approaches of making the particulates of various chemical compositions and depositing them on substrates are discussed.
As the above brief review of prior art demonstrates, there have been attempts to use Cu(In,Ga)Se2 compound powders, oxide containing particles, and Cu—(In,Ga) alloy powders with (In,Ga)-rich compositions, to form precursor layers which were then treated at high temperatures to form Cu(In,Ga)Se2 compound films. These techniques were successful in demonstrating compositional control. In other words the overall composition of the powder was directly transferred into the precursor layer and then into the compound layer. As discussed previously however, composition is only one of the important parameters of high-quality solar cell absorbers. The other important parameters are the morphology and the grain size, which directly influence the electronic properties of these films. Solar cell absorbers need to be dense layers with large grain size. This requires precursor layers that are dense and compositionally uniform both in micro-scale and macro-scale. Repeatability and the overall yield of the process further requires the quality of the source material or the initial powder material to be repeatable. This means that the chemical composition and the phase content of the individual particles comprising the powder need to be well controlled and repeatable.
To this end, there is a need for a low-cost method that has the capability to form Cu(In,Ga)(S,Se)2 thin films over large-area substrates with controlled compositional uniformity, good structural and electronic properties in a repeatable manner with high yield.