International Journal of Energy Research, Vol.45, No.5, 8049-8060, 2021
First-principles study of Cs2Ti1-xMxBr6 (M = Pb, Sn) and numerical simulation of the solar cells based on Cs2Ti0.25Sn0.75Br6 perovskite
The impact of partial substitution by replacing 'Ti' atom with Pb and Sn respectively in vacancy ordered perovskites Cs2Ti1-xMxBr6 (M = Pb, Sn) has been studied using density functional theory calculations. Our results indicate the unexpected presence of intermediate bands (IBs) in the forbidden gap of electronic structure for lead substituted compounds (x = 0.25, 0.5, 0.75) Cs2Ti1-xPbxBr6. These IBs located above the valence band could be tuned from 0.60 to 0.45 eV with appropriate Pb doping percentage. The nature of the band gap for the Sn substituted Cs2Ti0.25Sn0.75Br6 compound was found to become direct as the highly dispersive Sn impurity bands from the conduction band minimum. Also, the negative formation energy of Pb and Sn doped compositions ensures the stable nature of these compounds. The tunable nature of IBs in the Pb substituted Cs2Ti1-xPbxBr6 compounds suggests that they could become potential solar absorber semiconductor for IB solar cells (IBSCs), however, further tuning could be required to realize them. Also, the electronic band structure of lead-free composition Cs2Ti0.25Sn0.75Br6 exhibits a direct band gap of similar to 1.50 eV, which could be a promising candidate for single-junction caesium-titanium perovskite-based solar cells. Based on this result, the efficiency of perovskite solar cell based on the lead-free composition Cs2Ti0.25Sn0.75Br6 has been calculated by solar cell capacitance simulator (SCAPS)-1D software. The effect of various physical parameters on the photovoltaic performance of Cs2Ti0.25Sn0.75Br6 solar cells has been investigated to obtain the highest efficiency of the solar cells. The optimized power conversion efficiency of the solar cell based on the planar device configuration FTO/c-TiO2 (20 nm)/Cs2Ti0.25Sn0.75Br6 (600 nm)/Cu2O (200 nm)/Au is similar to 22.13% with V-oc = 1.02 V, J(sc) = 26.24 mA/cm(2), and FF = 82.24%. These results could pave the way towards environmentally friendly perovskite solar cells.