Photophysics of two-dimensional perovskites -- learning from metal halide substitution

TL;DR

This study investigates how substituting different metal halides in 2D perovskites affects their photophysical properties, revealing complex excitonic behaviors and the influence of phonons and defects, thus deepening understanding of their photophysics.

Contribution

It provides a comparative analysis of PEA$_2$PbBr$_4$, PEA$_2$PbI$_4$, and PEA$_2$SnI$_4$ using optical spectroscopy, uncovering new excitonic features and the role of phonons and defects.

Findings

Identification of multiple excitonic transitions at low temperature

Discovery of vibronic progression breaking Kasha's rule

Highlighting phonons inaccessible via simple Raman spectroscopy

Abstract

Whereas their photophysics exhibits an intricate interplay of carriers with the lattice, most reports have so far relied on single compound studies. With the exception of variations of the organic spacer cations, the effect of constituent substitution on the photophysics and the nature of emitting species, in particular, has remained largely under-explored. Here PEA$_2$PbBr$_4$, PEA$_2$PbI$_4$, and PEA$_2$SnI$_4$ are studied through a variety of optical spectroscopy techniques to reveal a complex set of excitonic transitions at low temperature. We attribute the emergence of weak high energy features to a vibronic progression breaking Kasha's rule and highlight that the responsible phonons cannot be accessed through simple Raman spectroscopy. Bright peaks at lower energy are due to two distinct excitons, of which the upper is a convolution of a bright exciton and a localised state, whereas the lower is attributed to shallow defects. Our study offers deeper insights into the photophysics of two-dimensional perovskites through compositional substitution and highlights critical limits to the communities' current understanding of the photophysics of these compounds.