Temperature-Dependent Ultrafast Spectral Response Of Fapb(Br0.4i0.6)(3) Nanocrystals

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 a-phase (Pm (3) over barm, #221) to a tetragonal beta-phase (P4/mbm, #127) at around 175 K, and then retain the tetragonal beta-phase (P4/mbm, #127) until 77 K The beta-phase showed a narrowing band gap, a small longitudinal-optical (LO) phonon energy, and a reduction in shallow defects compared with the alpha-phase. These factors together influence the carrier dynamics and the carrier temperature (T-C) 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.