Locally resolved surface photo voltage spectroscopy on Zn-doped CuInS2 polycrystalline thin films

Incorporation of small amounts of Zinc (< 1 at.%) in polycrystalline CuInS₂ thin films for solar cells leads to an increased open circuit voltage. Here we investigate the optoelectronic effect of Zn doping by local surface photovoltage spectroscopy (SPS). SPS is measured using Kelvin probe force microscopy (KPFM) to obtain the surface photovoltage (SPV) and SPS with high lateral resolution, and thereby study the homogeneity of the doping. In our KPFM experimental setup, illumination is realized by a Xe arc lamp and monochromator in the visible spectrum range by means of an optical fiber into the UHV system of the KPFM.We compare CuInS₂ thin film samples with and without Zn doping. The pure CuInS₂ samples show a sharp onset of SPV at the band gap of 1.48 eV, whereas for Zn-doped CuInS₂ we observe a two step onset, with a steep increase of SPV at 1.48 eV. However, already below this band gap, we observe a slight SPV response, even down to about 1.40 eV. This indicates the presence of states in the band gap, likely resulting from disorder induced by the Zn-doping. The absence of lateral differences in the observed SPV spectra favors an explanation by Urbach-tails over the possible existence of a Zn foreign phase. These results are in agreement with transmission/absorption measurements.