<p>Polaron formation in a halide perovskite is analyzed <italic>via</italic> nanometre-scale quantum mechanical molecular dynamics simulations.</p>

The high performance of recently emerged lead halide perovskite based photovoltaic devices has been attributed to remarkable carrier properties in this kind of material: long carrier diffusion length, long carrier lifetime, and low electronhole recombination rate. However, the charge separation mechanism underlying such carrier properties is still debated in this research field. In this work, using first-principles molecular dynamics simulations, we have demonstrated that the charge separation is induced by the structural fluctuation of the inorganic lattice, assuming that the charge carriers occupy the band edge. It is shown that the charge separation is attributed to the electrostatic potential fluctuation coupled to the inorganic lattice dynamics, on the basis of both simple tight-binding-model-based analyses and first-principles calculations. These results suggest that the organic cations, which are often used as components of lead halide perovskites, are unlikely to be essential for the above-mentioned carrier properties. Hence, it is expected that all-inorganic lead halide perovskite based photovoltaics might be able to rival organicinorganic lead halide perovskite based ones in performance.

The trapping of charge carriers at defects on surfaces or grain boundaries is detrimental for the performance of perovskite solar cells (PSCs). For example, it is the main limiting factor for carrier lifetime. Moreover, it causes hysteresis in the current voltage curves, which is considered to be a serious issue for PSCs' operation. In this work, types of surface defects responsible for carrier trapping are clarified by a comprehensive first-principles investigation into surface defects of tetragonal CH3NH3PbI3 (MAPbI(3)). Considering defect formation energetics, it is proposed that a Pb-rich condition is preferred to an I-rich one; however, a moderate condition might possibly be the best choice. Our result paves the way for improving the performance of PSCs through a rational strategy of suppressing carrier trapping at surface defects.