Soft-X-ray spectroscopy of ultrafast dynamics in materials for solar energy applications.

Metal halide perovskites have intriguing properties, such as high absorption, tunable bandgaps, and long charge carrier lifetimes. In particular, perovskite solar cells (PSCs) recently received attention due to their high conversion efficiencies of up to 25.5%. However, the success of PSCs has been hindered by their rapid degradation, the presence of lead, a toxic constituent, and the presence of moving defects affecting the dynamics of carriers. Understanding these dynamics on their natural time-scale is then an essential prerequisite for establishing reliable technology based on perovskites. In this respect, I will show how ultrafast soft-X ray spectroscopy enables the investigation of the processes involving interactions between electrons, phonons, and photons with an unprecedented temporal resolution, down to the attosecond time scale ($10^{-18}$ s). Moreover, since photon absorption occurs locally at the atomic cores, it has the further advantage of being site-selective and oxidation- and spin-state specific. Due to the high temporal resolution and high sensitivity to the chemical composition, this technique promises to stimulate potential new solutions for future photovoltaics.