Free Carrier Emergence and Onset of Electron-Phonon Coupling in Methylammonium Lead Halide Perovskite Films

Sub-10 fs resolution pump-probe experiments on methylammonium lead halide perovskite films are described. Initial response to photoexcitation is assigned to localized hot excitons which dissociate to free carriers. This is attested to by band integrals of the pump-probe spectra where photoinduced bleaching rises abruptly 20 fs after photoexcitation. Later stages of spectral evolution are consistent with hot carrier cooling, during which state filling induced bleaching of interband and exciton transitions curiously more than doubles. Electron coupling to optical phonons is observed as periodic spectral modulations in the pump-probe data of both films. Fourier analysis identifies active phonons at ∼100 and 300 wavenumbers pertaining to the lead-halide framework and organic cation motions, respectively. Coupling strengths estimated from the depth of these modulations are in the weak coupling limit, in agreement with values extracted from temperature dependent emission line shape analysis. These findings support free carriers in these materials existing as large polarons. Accordingly, these modes are probably not dictating the moderate carrier mobility in this material.