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

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.