Hot carrier relaxation in Cs2TiIyBr6-y (y=0, 2 and 6) by a time-domain ab initio study

Cs2TiIyBr6-y is a potential light absorption material for all-inorganic lead free perovskite solar cells due to its suitable and tunable bandgap, high optical absorption coefficient and high environmental stability. However, solar cells fabricated based on Cs2TiIyBr6-y do not perform well, and the reasons for their low efficiency are still unclear. Herein, hot carrier relaxation processes in Cs2TiIyBr6-y (y = 0, 2 and 6) were investigated by a time-domain density functional theory combined with the non-adiabatic molecular dynamics method. It was found that the relaxation time of the hot carriers in Cs2TiIyBr6-y ranges from 2-3 ps, which indicates that the hot carriers within 10 nm from the Cs2TiIyBr6-y/TiO2 interface can be effectively extracted before their energy is lost completely. The carrier-phonon non-adiabatic coupling (NAC) analyses demonstrate that the longer hot electron relaxation time in Cs2TiI2Br4 compared with that in Cs2TiBr6 and Cs2TiI6 originates from its weaker NAC strength. Furthermore, the electron-phonon interaction analyses indicate that the relaxation of hot electrons mainly comes from the coupling between the electrons distributed on the Ti-X bonds and the Ti-X vibrations, and that of hot holes can be attributed to the coupling between the electrons distributed on the X atoms and the distortions of [TiIyBr6-y](2-). The simulation results indicate that Cs2TiI2Br4 should be better than Cs2TiBr6 and Cs2TiI6 to act as a light absorption layer based on the hot carrier energy loss, and the hot electron relaxation time in Cs2TiIyBr6-y can be adjusted by tuning the proportion of the I element.