Photoluminescence characterization of Cu2Sn1-xGexS3 bulk single crystals

Cu2Sn1-xGexS3 (CTGS) which is composed of earth-abundant and non-toxic elements is a promising material for the absorber layer of thin-film solar cells. In this study, the optical properties of CTGS bulk single crystals with varying germanium content were investigated by temperature and excitation power dependent photoluminescence (PL) measurements. At low-temperature, excitons and broad defect-related bands were observed in the PL spectra from all samples. These PL bands indicate a blue-shift with an increase in the germanium content, which suggested the formation of solid-solutions in CTGS. The broad band was dominated by donor-acceptor pair (DAP) recombination luminescence. The DAP bands were due to the transition of carriers between shallow acceptors and relatively deep donors for any alloy composition. Band-to-band (BB) recombination luminescence was also observed from all samples at room temperature. The band gap energies were varied from 0.933 to 1.544 eV with an increase in the germanium content, which was determined by spectral fitting of the BB bands. In addition, a small optical bowing parameter b, of ca. 0.1 eV was determined, which indicates that the band gap energy of CTGS can be controlled almost linearly by varying the alloy composition. Therefore, the optimum band gap energy for single-junction solar cells can be achieved by control of the CTGS alloy composition.