Inorganic Lattice Fluctuation Induces Charge Separation in Lead Iodide Perovskites: Theoretical Insights

The high photoconversion efficiency of lead halide perovskites-based photovoltaic devices may be attributed to the remarkable carrier properties in this class of materials, e.g., long carrier lifetime and long carrier diffusion length. These properties should be ascribed to the efficient charge separation; electrons and holes are well separated so that the recombination is suppressed.
However, the mechanism of such efficient charge separation is still under debate. Especially, role of the molecular cation component of the perovskites is a controversial problem in this research field.
In this work, through first-principles molecular dynamics simulations, we demonstrated that the structural fluctuation of the inorganic (lead iodide) part of the perovskites enhances the charge separation.
Also, the mechanism of the structural fluctuation-induced charge separation is discussed; it is shown that the charge separation is caused by the electrostatic potential fluctuation coupled to the inorganic lattice dynamics, based on both simple tight-binding model-based analyses and first-principles calculations.
These results suggest that the molecular cations are unlikely to be essential for the excellent carrier properties in these materials.