Spatially resolved studies of the phases and morphology of methylammonium and formamidinium lead tri-halide perovskites

TL;DR

This study investigates the spatial inhomogeneities and phase transitions in methylammonium and formamidinium lead tri-halide perovskites using temperature-dependent micro-photoluminescence, revealing non-uniform phase behavior related to microcrystalline grains.

Contribution

It provides detailed spatially resolved insights into phase domains and inhomogeneities in lead halide perovskites, highlighting the non-uniform phase transition behavior and the influence of microstructure.

Findings

Phase transition around 160K is non-uniform across samples.

Microcrystalline grains influence local phase behavior.

FAPbBr3 shows large-scale domain structures.

Abstract

The family of organic-inorganic tri-halide perovskites including MA (MethylAmmonium)PbI$_{3}$, MAPbI$_{3-x}$Cl$_{x}$, FA (FormAmidinium)PbI$_{3}$ and FAPbBr$_{3}$ are having a tremendous impact on the field of photovoltaic cells due to their ease of deposition and efficiencies, but device performance can be significanly affected by inhomogeneities. Here we report a study of temperature dependent micro-photoluminescence which shows a strong spatial inhomogeneity related to the presence of microcrystalline grains, which can be both light and dark. In all of the tri-iodide based materials there is evidence that the tetragonal to orthorhombic phase transition observed around 160K does not occur uniformly across the sample with domain formation related to the underlying microcrystallite grains, some of which remain in the high temperature, tetragonal, phase even at very low temperatures. At low temperature the tetragonal domains can be significantly influenced by local defects in the layers. In FAPbBr$_{3}$ a more macroscopic domain structure is observed with large numbers of grains forming phase correlated regions.