The effect of S/Se ratio on the properties of Cu2CdGe(SxSe1−x)4 microcrystalline powders for photovoltaic applications

Abstract

The morphology, structural and compositional properties of Cu2CdGe(SxSe1−x)4 (x = 0; 0.05; 0.1; 0.15; 0.2; 0.4; 0.6; 0.8; 1) microcrystalline powders and solar cells based on these powders are investigated by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy as well as current-voltage and external quantum efficiency (EQE) methods. XRD analysis showed that all Cu2CdGe(SxSe1−x)4 solid solutions synthesized at 700 °C crystallize in orthorhombic structure (Pmn21) regardless of S/Se ratio. Cu2CdGe(SxSe1−x)4 solid solutions (x = 0; 0.05; 0.1) synthesized at 500 °C have a tetragonal structure (I-42m). Cu2CdGe(SxSe1−x)4 solid solution with x = 0.15 has a mixture of two crystal structures. Lattice parameters decrease by increasing the S-content in the solid solutions. The median crystal size decreased with the increasing S content in the Cu2CdGe(SxSe1−x)4 powders about two times.According to EQE measurements of the solar cells, the band gap of the Cu2CdGe(SxSe1−x)4 absorber material shifted towards shorter wavelengths with increasing S content, growing from 1.27 eV for x = 0 to 2.04 eV for x = 1. In this study, Cu2CdGe(SxSe1−x)4 solid solution with x = 0.2 enabled the highest power conversion efficiency of 6.4% for monograin layer solar cells with parameters VOC = 724 mV, JSC = 18.8 mA/cm2 and FF = 46.9%.