Hot carrier relaxation in Cs₂TiI Br_6- (y = 0, 2 and 6) by a time-domain ab initio study

Cs₂TiI Br_6- 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 Cs₂TiI Br_6- do not perform well, and the reasons for their low efficiency are still unclear. Herein, hot carrier relaxation processes in Cs₂TiI Br_6- (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 Cs₂TiI Br_6- ranges from 2-3 ps, which indicates that the hot carriers within 10 nm from the Cs₂TiI Br_6- /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 [TiI Br_6- ]^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 Cs₂TiI Br_6- can be adjusted by tuning the proportion of the I element.