Publication informationChemSusChem, 4 (4): 474-479Publisher WileyLink to online version http://dx

In May 2010 the United States National Science Foundation sponsored a two-day workshop to review the state-of-the-art and research challenges in photovoltaic (PV) manufacturing. This article summarizes the major conclusions and outcomes from this workshop, which was focused on identifying the science that needs to be done to help accelerate PV manufacturing. A significant portion of the article focuses on assessing the current status of and future opportunities in the major PV manufacturing technologies. These are solar cells based on crystalline silicon (c-Si), thin films of cadmium telluride (CdTe), thin films of copper indium gallium diselenide, and thin films of hydrogenated amorphous and nanocrystalline silicon. Current trends indicate that the cost per watt of c-Si and CdTe solar cells are being reduced to levels beyond the constraints commonly associated with these technologies. With a focus on TW/yr production capacity, the issue of material availability is discussed along with the emerging technologies of dye-sensitized solar cells and organic photovoltaics that are potentially less constrained by elemental abundance. Lastly, recommendations are made for research investment, with an emphasis on those areas that are expected to have cross-cutting impact.

Effects of nitrogen doping and illumination on lattice constants and conductivity behavior of zinc oxide grown by magnetron sputteringThe behavior of nitrogen in ZnO thin films grown by high-vacuum plasma-assisted chemical vapor deposition is examined. Highly oriented ͑002͒ films doped with 0 -2 at. % N were characterized by x-ray photoelectron spectroscopy, x-ray diffraction ͑XRD͒, Seebeck, and Hall measurements. XRD measurements revealed that the zinc oxide lattice constant decreased systematically with nitrogen doping. The as-deposited films were p-type at high doping levels, as confirmed by both Seebeck and Hall measurements. However, it was observed that hole conduction decreased and films reverted to n-type conductivity in a period of several days. This change was accompanied by a simultaneous increase in the lattice constant. The transient electrical behavior may be explained by compensation caused either by hydrogen donors or through defect formation processes common to analogous II-VI semiconductors.

Pyrite (FeS(2)) thin films were synthesized using a H(2)S plasma to sulfurize hematite (Fe(2)O(3)) nanorods deposited by chemical bath deposition. The high S activity within the plasma enabled a direct solid-state transformation between the two materials, bypassing S-deficient contaminant phases (Fe(1-x)S). The application of plasma dramatically enhanced both the rate of conversion and the quality of the resulting material; stoichiometric FeS(2) was obtained at a moderate temperature of 400 °C using a chalcogen partial pressure <6 × 10(-5) atm. As the S:Fe atomic ratio increased from 0 to 2.0, the apparent optical band gap dropped from 2.2 (hematite) to ~1 eV (pyrite), with completely converted layers exhibiting absorption coefficients >10(5) cm(-1) in the visible range. Room-temperature conductivity of FeS(2) films was on the order of 10(-4) S cm(-1) and approximately doubled under calibrated solar illumination.

Polycrystalline thin film copper chalcogenide solar cells show remarkable efficiencies, and analogous but less-explored semiconducting materials may hold similar promise. With consideration of elemental abundance and process scalability, we explore the potential of the Cu-Sb-S material system for photovoltaic applications. Using a high-throughput combinatorial approach, Cu-Sb-S libraries were synthesized by magnetron co-sputtering of Cu 2 S and Sb 2 S 3 targets and evaluated by a suite of spatially resolved characterization techniques. The resulting compounds include Cu 1.8 S (digenite), Cu 12 Sb 4 S 13 (tetrahedrite), CuSbS 2 (chalcostibite), and Sb 2 S 3 (stibnite). Of the two ternary phases synthesized, CuSbS 2 was found to have the most potential, however, when deposited at low temperatures its electrical conductivity varied by several orders of magnitude due to the presence of impurities. To address this issue, we developed a self-regulated approach to synthesize stoichiometric CuSbS 2 films using excess Sb 2 S 3 vapor at elevated substrate temperatures. Theoretical calculations explain that phase-pure CuSbS 2 is expected to be formed over a relatively wide range of temperatures and pressures, bound by the sublimation of Sb 2 S 3 and decomposition of CuSbS 2. The carrier concentration of CuSbS 2 films produced within this regime was tunable from 10 16 − 10 18 cm −3 through appropriate control of Sb 2 S 3 flux rate and substrate temperature. CuSbS 2 displayed a sharp optical absorption onset indicative of a direct transition at 1.5 eV and an absorption coefficient of 10 5 cm −1 within 0.3 eV of the onset. The results of this study suggest that CuSbS 2 holds promise for solar energy conversion due to its tolerant processing window, tunable carrier concentration, solar-matched band gap, and high absorption coefficient.