Real-time observation of the large polaron buildup in α-FAPbI3

Abstract The formation of large polaron, i.e., the 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 (HIOPs). However, direct observation of the dynamical formation of large polarons occurring at time scales within hundreds of femtoseconds remains a technical challenge. Generally, to characterize the polarons in HIOPs, the detection of infrared active phonon modes resonating in the terahertz range is utilized to indicate the quasi equilibrium polarons. Here, we demonstrate the real-time observation of large polaron formation process in HIOPs by terahertz emission spectroscopy. Besides, the polaron peaked at 1THz (P1) corresponding to the carriers coupling with the inorganic sublattice vibration, the FA+ cation rotation mode related 0.45 THz polaron (P2) peak also conform to the anharmonic oscillator emission of the large polaron formation. Interestingly, with a photon excitation energy higher than 2.4eV, the relative intensity of P2 was observed to be larger than that of P1 due to the higher sub-conduction band being excited. This observation could evidence that hot carriers in HIOPs could provide a new route for polaron formation. Our results could provide new physical insight into the polaron formation dynamics and hence the long-range carrier transport in HIOPs.