Indirect-to-direct band gap transition and optical properties of metal alloys of Cs 2 Te 1- x Ti x I 6 : a theoretical study.

In recent years, double perovskites have attracted considerable attention as potential candidates for photovoltaic applications. However, most double perovskites are not suitable for single-junction solar cells due to their large band gaps (over 2.0 eV). In the present study, we have investigated the structural, mechanical, electronic and optical properties of the Cs 2 Te 1- x Ti x I 6 solid solutions using first-principles calculations based on density functional theory. These compounds exhibit good structural stability compared to CH 3 NH 3 PbI 3 . The results suggest that Cs 2 TeI 6 is an indirect band gap semiconductor, and it can become a direct band gap semiconductor with the value of 1.09 eV when the doping concentration of Ti 4+ is 0.50. Moreover, an ideal direct band gap of 1.31 eV is obtained for Cs 2 Te 0.75 Ti 0.25 I 6 . The calculated results indicate that all the structures are ductile materials except for Cs 2 Te 0.50 Ti 0.50 I 6 . Our results also show that these materials possess large absorption coefficients in the visible light region. Our work can provide a route to explore stable, environmentally friendly and high-efficiency light absorbers for use in optoelectronic applications.