Emergence of Free Carriers and Electron-Phonon Coupling in Hybrid Lead Halide Perovskite

Lead halide perovskites have emerged as promising materials for low cost and high efficiency solar cells. Here, sub-10 fs resolution pump-probe experiments with high signal to noise ratio are carried out to investigate free carrier emergence and electron-phonon coupling in methylammonium lead halide perovskite films. Analysis of the results suggests that, the initial photoexcitation of the films result in localized hot excitons which subsequently dissociates into free carriers. This is demonstrated by band integrals of the transient absorption spectral data, which show an abrupt rise in the photoinduced bleaching due to the appearance of free carriers ~20 fs after the laser excitation. Later stages of spectral evolutions are consistent with the carrier cooling during which the band edge bleach grows more than double due to state filling of the excitonic and interband transitions. Additionally, we have identified spectral modulations in the pump-probe data which indicates electron coupling to the optic phonons in these materials. The phonon frequencies were obtained from the Fourier analysis of the spectral modulations. The frequency ~100 wavenumbers is assigned to the inorganic Pb-halide motions whereas the frequency ~300 wavenumbers is assigned to the torsions of organic cations. The amplitude of the spectral modulations allowed us to estimate the coupling strength which found to be in the weak regime.