Temperature-Dependent Ultrafast Spectral Response of FAPb(Br_0.4I_0.6)₃Nanocrystals

We employed transient absorption spectroscopy to investigate the temperature-dependent carrier dynamics of FAPb(Br0.4I0.6)3 nanocrystals (NCs). The carrier dynamics involved the hot-carrier (HC) cooling process, carrier-carrier interactions, and carrier recombination. With an increase in the absorbed photon flux or a decrease in the temperature, the HC cooling process slowed down and other carrier dynamics processes accelerated simultaneously. Moreover, the decrease in temperature caused the FAPb(Br0.4I0.6)3 NCs to pass through a cubic α-phase (Pm3¯ m, #221) to a tetragonal β-phase (P4/mbm, #127) at around 175 K, and then retain the tetragonal β-phase (P4/mbm, #127) until 77 K. The β-phase showed a narrowing band gap, a small longitudinal-optical (LO) phonon energy, and a reduction in shallow defects compared with the α-phase. These factors together influence the carrier dynamics and the carrier temperature (TC) cooling process in different crystal structures. These results demonstrate that temperature-dependent spectral analysis is essential for unraveling carrier dynamics in organic-inorganic halide perovskites.