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E7.05 - Overflux Growth Approach to High-Quality CuSbS2 Thin Films for Photovoltaic Absorber Applications 
April 23, 2014   11:30am - 11:45am

Strict level control of stoichiometry for compound semiconductor materials is challenging but necessary for photovoltaic (PV) and other electronic applications, with rare exceptions (ODCs in CIGS and ZnS in CZTS). Stoichiometry control can be achieved by an overflux method if one component is in gas phase at the growth temperature, such as arsenic in GaAs or oxygen in ZnO. Here, we apply this overflux growth strategy to the thin films of CuSbS2 grown on heated substrates by RF sputtering from Cu2S and Sb2S3 targets. The target materials have large difference in vapor pressures which has been exploited to grow films with near stoichiometric cation ratios (Cu/Sb=1.0). During the synthesis, the overflux of vapor phase Sb2S3 is such that at ambient substrate temperature the bulk equilibrium composition would be CuSbS2 + Sb2S3 impurities. However, high substrate temperatures ensure that Sb2S3 remains in the vapor phase, thereby preventing precipitation of Sb2S3 impurities in the film. In this way, Sb2S3 is incorporated into the growing film only in the case that it reacts with Cu2S to form the ternary CuSbS2 compound.The critical substrate temperature for the overflux growth, above which Sb2S3 remains in gas phase was identified using high-throughput combinatorial approach with spatially-resolved characterization techniques. Phase identification was carried out by X-ray diffraction (XRD), stoichiometry and thickness was verified by X-ray fluorescence (XRF) confirmed by other techniques, and conductivity was determined from four point probe sheet resistance measurements. Below the critical temperature, conductivity of the samples varied by 6 orders of magnitude across a narrow range of composition around the CuSbS2 due to presence of either Sb2S3 (n-type) or Cu12Sb4S13 (metallic) impurity phase controlled by relative Cu2S and Sb2S3 fluxes. Above the critical temperature, the resulting phase-pure CuSbS2 samples grown by Sb2S3 overflux approach had uniform and consistent hole concentration of 10^17 cm-3 and hole mobility of 0.2 cm2/V-s (as determined by Hall effect) and steep optical absorption onset at 1.5 eV with no signs of sub-gap absorption caused by metallic secondary phases (as determined by optical spectroscopy). Our progress towards integration of the resulting phase-pure high-quality CuSbS2 samples into solar cell prototypes will be reported.In summary, we demonstrate the application of the overflux growth approach to growth of stoichiometric high-quality CuSbS2 thin films.This technique may also offer some control of grain orientation and dopant level off-stoichiometry in this materials. In addition, it may be applicable to other chemically similar Cu-S based ternary materials, such as Cu2SnS3.

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Keynote Address
Panel Discussion - Different Approaches to Commercializing Materials Research
Business Challenges to Starting a Materials-Based Company
Fred Kavli Distinguished Lectureship in Nanoscience
Application of In-situ X-ray Absorption, Emission and Powder Diffraction Studies in Nanomaterials Research - From the Design of an In-situ Experiment to Data Analysis