Real-Time Observation of the Buildup of Polaron in α-FAPbI3

The formation of polaron, i.e., the strong coupling process between the carrier and lattice, is considered to play a crucial role in benefiting the superior photoelectric performance of hybrid organic-inorganic halide perovskites (HOIPs). However, direct observation of the dynamical formation of polarons occurring at time scales within hundreds of femtoseconds remains a technical challenge. Generally, to characterize the polarons in HOIPs, the detection of infrared active phonon modes resonating in the terahertz range is utilized to reflect the metastable polaron modes. Here, by terahertz emission spectroscopy, we demonstrate the real-time observation of polaron formation process in HOIPs. Besides, the polaron peaked at ~1THz (P1) is connected with the carriers coupling with the inorganic sublattice vibration, while the ~0.4 THz polaron (P2) peak could be connected with the FA+ cation rotation mode. The terahertz emission at the two polaron peaks is interpreted with the anharmonic coupling emission model during the polaron formation process. Interestingly, with a photon excitation energy higher than 2.4eV, the relative amplitude of P2 was observed to be further strengthened than that of P1 due to the higher sub-conduction band being excited. This observation could evidence that hot carriers in HOIPs could provide a new route for polaron formation. Our results could open a new door for THz emission spectroscopy to be a powerful tool in studying polaron formation dynamics in HOIPs.