Effects of Ge substitution on morphology and electrical properties of Cu2Sn(S,Se)3 bulk at a fixed Se/[Se+S] composition

Ge-substituted Cu 2 Sn(S,Se) 3 bulks with Cu 2 (Sn 1- x Ge x )(S,Se) 3 formula at x = 0, 0.25, 0.50, 0.75, and 1.0 were synthesized by reactive liquid phase sintering , where the heating process was held at 200   °C for 20   min before reaching the sintering temperature of 600   °C for 30   min. In the presence of volatile Sb 2 S 3 sintering aid , the effects on densification , grain boundary, crystalline size, and electrical properties of Ge-substitution were investigated. Cu 2 (Sn 1- x Ge x )(S,Se) 3 showed p-type behavior for all x values. It was found that the mobility was increased sharply and carrier concentration was decreased rapidly as the x -value was increased. The electrical conductivities were found to be 14.0, 22.7, 36.8, 126, and 153   S/cm at x = 0, 0.25, 0.50, 0.75, and 1.0, respectively. X-ray diffraction (XRD) peaks were shifted to the higher angle and band gaps increased upon successive increasing of the Ge/[Ge+Sn] fraction being 1.19, 1.21, 1.22, 1.24, and 1.26   eV at x = 0, 0.25, 0.50, 0.75, and 1.0, respectively. The studies of Ge-x-CTSSe bulks were based on Ge Sn defects and supported by the data of morphological, structural, electrical, and optical properties .