Fabrication of phase-pure and large-grained Cu3BiS3 films by a two-stage process for thin film solar cells

Abstract

Copper-bismuth-sulfide (Cu3BiS3) has gained great interest as solar cell absorber owing to the advantages of being cheap, abundant, and non-toxic. Fabrication of phase-pure and large-grain absorbers are desirable for their integration in solar cells. Herein, we have used a sequential evaporation and chalcogenization (350–450 oC) process to fabricate phase-pure and large-grain Cu3BiS3 films. X-ray diffraction indicated a CuS secondary phase at 350 °C and single-phase Cu3BiS3 at 400 and 450 °C in the sulfurized films. Elemental analysis revealed a near-stoichiometric Cu3BiS3 composition for all the films. X-ray photoelectron spectroscopy was used to assess the valence states of elements. Microstructural analysis confirmed the formation of compact and large-grained Cu3BiS3 films with an average grain size of 5 μm at 350 °C. The grain size decreased with increasing sulfurization temperature (400–450 °C). The direct bandgap decreases from 1.42 eV to 1.32 eV with increasing sulfurization temperature from 350 to 450 °C. Hall measurements indicated decreased electrical resistivity and increased hole mobility and carrier concentration with increasing the sulfurization temperature. From this study, the Cu3BiS3 absorbers produced at 400 oC exhibited superior phase purity and good quality suitable for the solar cells.