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

Cs(2)TiI(y)Br(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 Cs(2)TiI(y)Br(6−y) do not perform well, and the reasons for their low efficiency are still unclear. Herein, hot carrier relaxation processes in Cs(2)TiI(y)Br(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 Cs(2)TiI(y)Br(6−y) ranges from 2–3 ps, which indicates that the hot carriers within 10 nm from the Cs(2)TiI(y)Br(6−y)/TiO(2) 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 Cs(2)TiI(2)Br(4) compared with that in Cs(2)TiBr(6) and Cs(2)TiI(6) 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(y)Br(6−y)](2−). The simulation results indicate that Cs(2)TiI(2)Br(4) should be better than Cs(2)TiBr(6) and Cs(2)TiI(6) to act as a light absorption layer based on the hot carrier energy loss, and the hot electron relaxation time in Cs(2)TiI(y)Br(6−y) can be adjusted by tuning the proportion of the I element.