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

Abstract

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.

Preparation of Cu2SnS3 thin film by sol-gel dip coating

“…The peaks at 294 and 350 cm −1 were consistent with those of previously reported monoclinic CTS samples. 10,28,29) This is consistent with the identification of these peaks as the main A′ modes of monoclinic Cu 2 SnS 3 . 27) The peak at 335 cm −1 is consistent with that of tetragonal CTS.…”

Temperature-Dependent Ultrafast Charge Carrier Dynamics in Amorphous and Crystalline Sb2Se3 Thin Films

“…This throws some open challenges for the development of cost-effective solar cells which are composed of nontoxic and earth abundance elements. Among many alternatives, metal sulfides and selenides such as CuInS 2 , CuInSe 2 , Cu 2 ZnSnSe 4 , Cu 2 SnS 3 , CuSbS 2 , SnSe, and Sb 2 X 3 (X = S, Se) have emerged as promising candidates. − …”

Effect of rapid thermal annealing on sprayed Cu2SnS3 thin films for solar-cell application

“…47) Moreover, the absorption spectra of 1.4 Cu exhibited high absorption tailing compared to the samples with other Cu ratios. 48)…”