Methylammonium lead iodide (MAPbI 3 ) perovskite shows an outstanding performance in photovoltaic devices. However, certain material properties, especially the possible ferroic behavior, remain unclear. We observed distinct nanoscale periodic domains in the piezoresponse of MAPbI 3 (Cl) grains. The structure and the orientation of these striped domains indicate ferroelasticity as their origin. By correlating vertical and lateral piezoresponse force microscopy experiments performed at different sample orientations with x-ray diffraction, the preferred domain orientation was suggested to be the a 1 -a 2 -phase. The observation of these ferroelastic fingerprints appears to strongly depend on the film texture and thus the preparation route. The formation of the ferroelastic twin domains could be induced by internal strain during the cubictetragonal phase transition.

Notwithstanding the success of lead-halide perovskites in emerging solar energy conversion technologies, many of the fundamental photophysical phenomena in this material remain debated. Here, the initial steps following photogeneration of free charge carriers in lead-iodide perovskites are studied, and timescales of charge carrier cooling and polaron formation, as a function of temperature and charge carrier excess energy, are quantified. It is found, using terahertz time-domain spectroscopy (THz-TDS), that the observed femtosecond rise in the photoconductivity can be described very well using a simple model of sequential charge carrier cooling and polaron formation. For excitation above the bandgap, the carrier cooling time depends on the charge carrier excess energy and lattice temperature, with cooling rates varying between 1 and 6 meV fs , depending on the cation. While carrier cooling depends on the cation, polaron formation occurs within ≈400 fs in CH NH PbI (MAPbI ), CH(NH ) PbI (FAPbI ), and CsPbI . Its formation time is independent of temperature between 160 and 295 K. The very similar polaron formation dynamics observed for the three perovskites points to the critical role of the inorganic lattice, rather than the cations, for polaron formation.

Efficient charge extraction within solar cells explicitly depends on the optimization of the internal interfaces. Potential barriers, unbalanced charge extraction, and interfacial trap states can prevent cells from reaching high power conversion efficiencies. In the case of perovskite solar cells, slow processes happening on time scales of seconds cause hysteresis in the current-voltage characteristics. In this work, we localized and investigated these slow processes using frequency-modulation Kelvin probe force microscopy (FM-KPFM) on cross sections of planar methylammonium lead iodide (MAPI) perovskite solar cells. FM-KPFM can map the charge density distribution and its dynamics at internal interfaces. Upon illumination, space charge layers formed at the interfaces of the selective contacts with the MAPI layer within several seconds. We observed distinct differences in the charging dynamics at the interfaces of MAPI with adjacent layers. Our results indicate that more than one process is involved in hysteresis. This finding is in agreement with recent simulation studies claiming that a combination of ion migration and interfacial trap states causes the hysteresis in perovskite solar cells. Such differences in the charging rates at different interfaces cannot be separated by conventional device measurements.