Enhancement of low energy photon absorption in Bi2S3 based Ti replacement: Prospect for high performance solar cells

Full-potential (linearized) augmented plane wave plus local orbitals (FP-LAPW+lo) scheme as implemented in WIEN2K code is used to study the effect of intermediate band (IB) formation in chalcogenide-type semiconductor at single level of Ti substitution. By using Tran-Blaha's modified Becke and Johnson with generalized gradient approximation/ local density approximation (mBJ-GGA/LDA) energy functionals; structural, electronic, and optical properties, including the dielectric function, refractive index, extinction coefficient, absorption spectrum, and optical reflectivity for the pure Bi2S3 and two slabs model Bi4Ti4S12 and Bi12Ti4S24 are calculated and analysed. Our calculated electronic properties show how the effects of interaction between the d electrons of Ti and the p electrons of the parent atoms do influence the formation of IB when Ti is substituted Bi at a 25% level. In addition, the parent structure has shown a direct band gap and high absorption coefficient α(ω), and other calculated optical properties indicate Bi2S3 to be a promising material for photovoltaic application. Moreover, at 25% substitution the enhancement in the absorption due to the intermediate band appears mainly in the most intense part of the sun spectrum.