Crystal orientation and grain size: do they determine optoelectronic properties of MAPbI3 perovskite?

TL;DR

This study investigates whether crystal orientation and grain size influence the optoelectronic properties of MAPbI3 perovskite films, finding that these structural features do not significantly affect device-relevant properties.

Contribution

The paper demonstrates that large, oriented grains in MAPbI3 films do not necessarily improve optoelectronic performance, challenging assumptions about the importance of crystal orientation and grain size.

Findings

Large grains with specific orientation do not enhance optoelectronic properties.

Similar photoluminescence and charge mobility are observed despite differences in grain size.

Optoelectronic quality is independent of crystal orientation and grain size.

Abstract

It is thought that growing large, oriented grains of perovskite can lead to more efficient devices. We study MAPbI3 films fabricated via Flash Infrared Annealing (FIRA) consisting of highly oriented, large grains. Domains observed in the SEM are often misidentified with crystallographic grains, but SEM images don't provide diffraction information. We measure the grain size, crystal structure and grain orientation using Electron Back-Scattered Diffraction (EBSD) and we study how these affect the optoelectronic properties as characterized by local photoluminescence (PL) and time-resolved microwave conductivity measurements (TRMC). We find a spherulitic growth yielding large (tens of micron), highly oriented grains along the (112) and (400) planes in contrast to randomly oriented, smaller (400 nm) grains observed in films fabricated via conventional antisolvent (AS) dripping. We observe a local enhancement and shift of the photoluminescence emission at different regions of the FIRA clusters, but these can be explained with a combination of light-outcoupling and self-absorption. We observe no effect of crystal orientation on the optoelectronic properties. Additionally, despite a substantial difference in grain size between our FIRA sample and a conventional AS sample, we find similar photoluminescence and charge carrier mobilities and lifetime for the two films. These findings show that the optoelectronic quality is not necessarily related to the orientation and size of crystalline domains in perovskite films indicating that fabrication requirements may be more relaxed for perovskites.