High-harmonic generation was used to generate a broadband femtosecond XUV probe to perform pump-probe spectroscopy. The relaxation of carriers after above-band excitation at 3.1 eV in CH3NH3PbI3 perovskite was investigated. Perovskite solar cell materials are a strong contender for next generation devices, but their underlying photophysics are not yet fully understood. Transient XUV spectroscopy is a unique probe for observing photoexcited hole and electron occupations independently over time. It probes transitions (~50 eV) from a shared inner shell orbital (I4d) to the valence and conduction bands, separating the hole and electron signals by the energy of the band gap. The conduction band feature includes both band filling and band gap renormalization and thus requires more advanced modeling to extract the electron distribution. Focusing on the valence band holes, carrier-carrier scattering happens within our ~70 fs instrument response resulting in a hot thermal distribution. A simple Fermi filling model was used to extract the carrier temperature of the hole distribution in the valence band. With an initial carrier density of 4.2E19 cm^-3, the holes lose their excess energy via carrier-phonon interactions resulting in rapid cooling to below our instrumental detection limit in ~400 fs. At 1.4E20 cm^-3, Auger heating becomes a dominant contribution to the carrier energy and the hot-phonon bottleneck is reached. The hole distribution thus cools with a single exponential time constant of ~0.6 ps plus the added Auger heat per timestep from recombination.

Hot-hole cooling in CH3NH3PbI3 perovskite probed via transient XUV spectroscopy

256th National Meeting (2018)

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