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

Abstract

Cs₂TiI_yBr_6−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 Cs₂TiI_yBr_6−y do not perform well, and the reasons for their low efficiency are still unclear. Herein, hot carrier relaxation processes in Cs₂TiI_yBr_6−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 Cs₂TiI_yBr_6−y ranges from 2–3 ps, which indicates that the hot carriers within 10 nm from the Cs₂TiI_yBr_6−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 [TiI_yBr_6−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 Cs₂TiI_yBr_6−y can be adjusted by tuning the proportion of the I element.