Excitonic and Band‐to‐Band Transitions in Temperature‐Dependent Optical Absorption Spectra of Cu2SnS3 Thin Films

Temperature-dependent optical absorption spectra from transmittance and reflectance measurements of Cu2SnS3 (CTS) thin films which are promising material for solar cells were investigated. Thin film CTS samples with Cu-poor, near-stoichiometric, and Cu-rich compositions were prepared on glass substrates by thermal co-evaporation and sulfurization. At low temperature, three band-to-band (BB) transitions that are allowed between triple upper valence bands and a single lower conduction band were observed for all samples. The square of the product of the absorption coefficient and the photon energy was plotted to estimate the band gap energy for three bands of the Cu-poor sample. On the other hand, excitonic (EX) transitions that corresponded to three bands were observed for the Cu-rich and near-stoichiometric samples. Temperature-dependent optical absorption spectra with the triple BB and EX transitions were resolved using a simplified fitting equation with Lorentzian functions as the EX transitions. The band gap, EX transition, and exciton binding energies for the lowest energy band of the Cu-rich sample were determined to be 0.945 eV, 0.936 eV, and 8.9 meV at 6 K, respectively. In the low-temperature region, anomalous blue-shifts of the estimated band gap energy with increasing temperature were obtained for three bands of all samples.