Under solar irradiation, most photons absorbed in methyl-ammonium lead iodide perovskite (MAPbI<sub>3</sub>) possess ~0.4 eV of excess energy above the 1.65 eV band gap (BG). Accordingly characterizing free carrier generation and relaxation in these materials is of central importance for understanding how perovskite photocells work, as well as designing methods of harnessing this excess energy for enhancing performance. The ultrafast timescales for dissipation of this excess has led to extensive recourse to femtosecond spectroscopy for its characterization. A number of dynamic features are common to femtosecond pump-probe studies on perovskite films: 1) A rapid buildup of Photo-Induced Bleach at the optical band-edge assigned to low energy state filling 2) a short lived photo-Induced absorption (PIA) to the “red” of the optical band edge, which decays with carrier cooling, assigned to to shifting of exciton states due to band gap renormalization, and 3) A rising low intensity broad PIA near ~650 nm.<br/>Previous pump-probe experiments on perovskite films were however incapable of recording initial carrier emergence and thermalization due to time resolution limitations. Here sub-10 fs pulse 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 (figure 1). Later stages of spectral evolution are consistent with hot carrier cooling. 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.

Monitoring free carrier emergance in hybrid lead halide perovskites with 10 fs resolution

256th National Meeting (2018)

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