Abstract

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.

© 2017 IEEE