Spectroscopic ellipsometry study of Cu2Zn(GexSi1-x)Se4 bulk poly-crystals

The present study addresses to the synthesis and determination of the dielectric function of Cu2Zn(GexSi1-x)Se4 solid solutions with x = 0.4 and 0.8 over the range 1–4.5 eV by spectroscopic ellipsometry analysis, with the aim to achieve a suitable band gap tuning. The dielectric function of the samples is determined using the Adachi model. From the analysis the lowest E0 transition and high energy E1A and E1B transitions are identified. It is found that the band gap varies nonlinearly on composition in the Cu2Zn(GexSi1-x)Se4 alloys and band gap values as large as 1.87 eV are obtained. These results are essential for the design of efficient tailored photovoltaic solar cells and show the high potential of the kesterite compounds for the development of low-cost sustainable future solar cells.