# Disentangling Refractive Index Contributions in Transient Absorption Spectroscopy of Two-Dimensional Halide Perovskites

## TL;DR

This paper clarifies how refractive index changes affect the interpretation of transient absorption data in 2D halide perovskites.

## Contribution

A Kramers–Kronig constrained analysis method is introduced to separate refractive index effects in TA spectroscopy.

## Key findings

- Photoinduced changes in the dielectric function's real and imaginary parts are of similar magnitude.
- Reflectivity effects do not significantly impact TA spectra or kinetics in 2D perovskites.
- The method reconciles conflicting views in the literature on refractive index contributions.

## Abstract

Two-dimensional (2D)
halide perovskites are intensely researched
for emerging light-emitting and -harvesting technologies due to their
outstanding optoelectronic properties, strong quantum confinement,
and enhanced ambient stability over their three-dimensional counterparts.
A powerful technique for understanding their excited-state dynamics
is transient absorption (TA) (pump–probe) spectroscopy. However,
the interpretation can be complicated by simultaneous reflectivity
changes arising from their high refractive index. Here, we adopt a
Kramers–Kronig constrained variational analysis to disentangle
these effects, as demonstrated for the prototypical Ruddlesden–Popper
2D perovskite phenylethylammonium lead iodide (PEA2PbI4). We show that photoinduced changes in the real and imaginary
parts of the complex dielectric function can be of similar magnitude,
but find that reflectivity effects do not imprint significantly on
the TA spectra or kinetics. Our work clarifies the role of refractive
index contributions in the TA spectroscopy of 2D perovskites, reconciles
literature views, and provides confidence in the analysis of TA data
for these emerging semiconductors.