Colloidal Cu2Zn(Sn1−xGex)S4 nanocrystals: electrical properties and comparison between their wurtzite and kesterite structures

Abstract

Wurtzite colloidal Cu₂Zn(Sn_1−xGe_x)S₄ nanocrystals with a nearly monodispersed size distribution and tunable compositions were prepared. The optical band gaps linearly tune from 1.57 to 2.13 eV as the parameter x increases from 0 to 1. Kesterite nanocrystals were also obtained by replacing thiourea with sulfur powder. Enhanced zinc content accompanied with increasing germanium (Ge) incorporation is found for both hexagonal and tetragonal crystalline samples. The electrical properties of the multinary compounds indicate a decline in dark current density as the Ge content rises; however, the maximum illuminated current density comes from the sample with a Ge/(Ge + Sn) ratio of 0.28. The higher dark and illuminated current densities, as well as higher photon current/dark current ratios observed in nanocrystals with a wurtzite structure were further demonstrated by a comparison with the properties of kesterite compounds.