Electronic and structural characterisation of Cu₃BiS₃ thin films for the absorber layer of sustainable photovoltaics

Abstract Thin films of p-type Cu₃BiS₃ with an orthorhombic wittichenite structure, a semiconductor with high potential for thin film solar cell absorber layers, were synthesised by thermal annealing of Cu and Bi precursors, magnetron sputtered on Mo/glass substrate, with a layer of thermo-evaporated S. The elemental composition, structural and electronic properties are studied. The Raman spectrum shows four modes with the dominant peak at 292 cm^-1. Photoreflectance spectra demonstrate two band gaps E_gX and E_gY, associated with the X and Y valence sub-bands, and their evolution with temperature. Fitting the temperature dependencies of the band-gaps gives values of 1.24 and 1.53 eV for E_gX and E_gY at 0 K as well as the average phonon energy. Photoluminescence spectra at 5 K reveal two bright and broad emission bands at 0.84 and 0.99 eV, which quench with an activation energy of 40 meV. The photocurrent excitation measurements demonstrate a photoresponse and suggest a direct allowed nature of the band gap.