Seminar on "Ultrafast Photophysics of Hybrid Lead Halide Perovskites Using Sub-10 Femtosecond Pump-Probe Spectroscopy"

Hybrid lead halide perovskites (LHPs) have been emerged as an efficient material for superior solar energy conversion during the last decade, due to their following unique properties: large absorption coefficient in the visible, low charge carrier (electron/hole) recombination rates, and sufficiently long carrier diffusion length.1-3 Understanding fundamental photophysics behind such high power conversion efficiency requires a thorough understanding of the following phenomena: dissociation excitons to free carriers, hot carriers cooling, and recombination dynamics. We use sub-10 femtosecond pump-probe spectroscopy to uncover early stages of carrier evolution in methylammonium lead iodide (MAPbI3) perovskite thin films. 4,5 Results show that photoexcitation with laser pulses centered ~530 nm (~0.7 eV excess to that of the band gap of MAPbI3) generates localized exciton (e-h pair), which subsequently dissociate to free carriers within first 20 fs. In later stages, these hot free carriers cool to the band edge by emitting optic phonons, with a time constant ~0.4 ps. Using a very high signal-to-noise ratio (S/N), we are able to detect faint periodic spectral modulation in the transient signals due to electron-phonon coupling in the material. The Fourier transform of the periodic modulation results in mainly two frequencies: ~100 and ~240 cm-1 assigned to the stretching of Pb-I bonds in the inorganic cage and torsions of CH3NH3 + cation, respectively. The amplitude of the spectral modulations has been used to estimate the electron-phonon coupling strength. The estimated coupling strength falls in the weak regime and hence suggest the formation of large polarons.