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The aim of the present contribution is to give a review on the recent work concerning Cd-free buffer and window layers in chalcopyrite solar cells using various deposition techniques as well as on their adaptation to chalcopyrite-type absorbers such as Cu(In,Ga)Se 2 , CuInS 2 , or Cu(In,Ga)(S,Se) 2 . The corresponding solar-cell performances, the expected technological problems, and current attempts for their commercialization will be discussed. The most important deposition techniques developed in this paper are chemical bath deposition, atomic layer deposition, ILGAR deposition, evaporation, and spray deposition. These deposition methods were employed essentially for buffers based on the following three materials: In 2 S 3 , ZnS, Zn 1 À x Mg x O.

Monocrystalline normaln‐FeS2 (pyrite, Enormalg≈0.95 normaleV ) photoelectrodes with high photocurrent quantum efficiency (> 90%) have been obtained by improvement of the solid‐state and interfacial chemistry. During intensive illumination (4–5 W/cm2), photocurrent densities between 1 and 2 A/cm2 have been observed for single crystals with high electron mobility false(μ=180normalthinsp;cm2‐false(V‐normals)−1false) in presence of the I−/I3− redox couple. Under illumination, a charge of 623.000 C/cm2 was passed without evidence of photocorrosion. The influence of etching treatments, various redox systems, and organic electrolytes on the photochemistry of FeS2 was investigated. The formation and the dynamics of a thin oxidation layer that forms at the surface of the electrode in the presence of an acid electrolyte were studied using light reflection techniques and ESCA. FeS2 has a valence energy band with strong d‐characterlike Mo‐ and W‐dichalcogenides, that is significant for its stabilization. An unresolved problem with this photoelectrode concerning applications in solar cells is the small photopotential which up to now does not exceed 200 mV (500 mV is theoretically possible). A strong pinning of the Fermi level by surface states is evident from photoelectrochemical measurements. In addition, it is expected that the distance between the conduction band and the Fermi level in our sample will be too large. A low effective carrier density false(n=0.7×1015 cm−3false) was measured, resulting in an extended space‐charge layer, which has to be compared with the high absorption coefficient ( α>6.0×105 cm−1 for hν>1.3 normaleV ).

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Polycrystalline layers of As-doped pyrite (FeS2) have been produced in bromine atmosphere with the aim of developing this sulfide material for solar energy applications. Its photoelectrochemical behavior in contact with an aqueous I8-/I2 electrolyte was investigated. It operated as a photoelectrechemical solar cell and showed reasonably stable behavior under illumination. Optical measurements performed on FeS2 show that visible light is absorbed in an extremely thin layer of 160~ in spite of the apparently indirect gap of this semiconductor (Eg = 0.95 eV). This would make this photosensitive material an interesting candidate for thin-layer solar cells. Scanning electron micrographs of the samples reveal well-developed crystallites of about 5-10 ~m with distinct boundaries. Scanning laser spot analysis over macroscopic areas (5 mm) showed homogeneous as well as inhomogeneous regions. The photoelectric properties of these first polycrystalline pyrite samples studied are poor, but there is presently no reason to assume that they cannot be developed.Single crystals of FeS2 with pyrite structure have recently been considered in publications from our laboratory as a semiconducting material for photoelectrochemical and photovoltaic solar cells (1-3). FeS2 crystals in contact with an iodide/iodine containing electrolyte have exhibited large quantum efficiency and very high stability against photocorrosion (3). We are still far from understanding the solid-state chemistry of single-crystalline pyrite in all details. Nevertheless, it seems to be appropriate to start the development of polycrystalline pyrite at an early stage, owing to its potential advantage as a cheap material with promising photoelectrochemical properties.As a material in photoelectrochemical cells, FeS2 has the advantage of being a d band semiconductor like MoS2 or WS2 with photoexcited holes reacting from quasinonbinding d states constituting the upper edge of the valence band (3). Pyrite Preparation and PropertiesPrevious studies on pyrite synthesis.--Although large pyrite deposits have been found in the earth (4) and natural pyrite crystals of considerable size are well known, there has been no success in artificial growth of pyrite crystals in centimeter dimension. Synthesis of pyrite was first described by W6hler (5) in the last century. Reacting sulfur and Fe20~ in an open system, he succeeded in the preparation of small brass-yellow octahedra. Bouchard (6) reported on the growth of pyrite crystals with 3 mm edge length by chemical vapor transport with chlorine. Transport from hot to cold occurred in a temperature gradient from 715 ~ to 655~ Our own experiments (3) show that transport with bromine at a gradient from 650 ~ to 550~ yields crystals up to 6 mm edge length, while in the presence of iodine as transporting agent a transport rate two orders in magnitude smaller has been established. Wilke and co-workers (7) did not obtain larger crystals in growing pyrite from the solution with PbC12 as solvent.Up to now, no attempts to prepare polycrysta...

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