Lattice distortions drive electron-hole correlation within micrometer size lead-iodide perovskite

Manipulation of perovskite crystallinity in order to obtain the desired optoelectronic properties is a critical issue. Perovskite structural characterization has been carried out applying several tools, which usually interrogate bulky portions of the material, averaging out over the inhomogeneities at the sub-micron scale [1]. Preliminary results obtained through photoluminescence microscopy across the crystal grains [2] showed that a clear understanding of the entangled relationship between structural and physical properties of perovskites is still missing. Ultrafast transient absorption (TA) experiments on macroscopic CH3NH3PbI3 films previously demonstrated that the mesoscale film morphology impacts on the electron-hole interactions [3]. In small crystals (<50 nm), the dynamics are dominated by a population of free carriers, while larger crystals (∼1um) sustain the formation of a stable exciton. These results were interpreted in terms of the different lattice dielectric constant, which is responsible for tuning the electron-hole interaction, through the organic cations: if they are frozen, a long-range order is established, leading to the formation of stable excitons. On the contrary, when the organic dipoles respond in a dielectric manner they screen the exciton formation, leading to a predominance of free carriers.