If you wish to arrange a meeting with Sandy, please contact Artem Bakulin a.bakulin@imperial.ac.uk
“Following the emergence of free hot carriers in lead halide perovskites with 10 fs time resolution”
Abstract:
The high energy conversion efficiency obtained in solar cells based on lead halide perovskites (LHP) has been attributed to intense absorption, long carrier diffusion lengths and slow electron-hole recombination rates. Much recent research has been concentrated on figuring out how these structurally inhomogeneous and seemingly defect laden thin semiconducting films nonetheless attain this performance. As part of this effort, exciton and free carrier dynamics have been followed using ultrafast spectroscopy on various perovskite materials, ranging from bulk thin films to single crystals of various dimensions. However almost all such studies have been conducted with pulse duration insufficient for characterizing the earliest stages of free-carrier generation.
In the lecture sub-10 fs resolution pump-probe experiments on methylammonium lead halide perovskite films will be described. Initial response 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. These findings support free carriers in these materials existing as large polarons. Finally recent results will be presented demonstrating that pump-probe data in perovskites is dominated by changes in absorption and not reflectivity.