Material properties of CuIn(Se,S)2 thin films prepared by the thermal diffusion of sulfur into CuInSe2

In this paper, we demonstrate that device quality chalcopyrite thin films can be produced by a simple and reproducible processing technology. CuInSe2 thin films were prepared by the thermal evaporation of InSe/Cu/InSe precursors, followed by a selenization step in elemental Se vapour. This technique produced large-grained, single-phase CuInSe2 thin films with a high degree of in-depth compositional uniformity. The samples were subsequently sulfurized by the controlled solid-state thermal diffusion of sulfur into the CuInSe2 thin films. The sulfurization temperature, reaction period and position of the elemental sulfur source with respect to the CuInSe2 films were optimized in this study. Under optimized conditions, sulfurization resulted only in a marginal increase in the surface roughness, x-ray diffraction studies confirmed single-phase CuIn(Se, S)(2) material and x-ray fluorescence in-depth profiling revealed uniform Cu/In atomic ratios and a systematic increase in the sulfur concentration towards the top surface region of the samples. The increase in bandgap of the absorber layers after sulfurization was also confirmed by ail increase in the open-circuit voltage of completed solar cell devices.