Discerning between Exciton and Free-Carrier Behaviors in Ruddlesden–Popper Perovskite Quantum Wells through Kinetic Modeling of Photoluminescence Dynamics

Two-dimensional (2D) Ruddlesden–Popper (RP)-layered lead halide perovskites have recently emerged as a more stable alternative to their three-dimensional (3D) counterparts while also exhibiting intriguing photophysical properties. Although time-resolved photoluminescence (TRPL) is an excellent diagnostic tool for the photophysics of these luminescent semiconductors, the most common approaches to analyzing TRPL transients do not discriminate between the mechanisms responsible for charge carrier dynamics, namely, the behavior of excitons, which is more relevant for optoelectronic applications, and the behavior of electrons and holes, which is most relevant for solar conversion. Here, we develop a kinetic approach to systematically resolve exciton and free-carrier dynamics across a series of (PEA)2(CH3NH3)n−1PbnI3n+1 RP perovskite single crystals (PEA = phenethylammonium, n = 1, 2, 3, 4, and ∞). Our approach uses repetitive excitation that builds up a steady state of free carriers, even when the exciton binding energies are large. Within the time scale of a TRPL experiment, the rapid changes in the total photoexcited carrier density cause the carrier dynamics to change from exciton-dominated to free-carrier-dominated, as expected from the Saha equilibrium. Thus, despite only measuring excitonic emission, we can analyze the dynamic and steady-state dynamics of the TRPL transients to also resolve the dynamics of free carriers. We obtain an Arrhenius-like relationship between the exciton dissociation rate constant and the exciton binding energy (determined by the quantum well thickness, n) and compare the trends of the exciton annihilation and the electron–hole recombination rate constants as a function of n. Moreover, we examine the influence of excitation power and trap filling on the observed value of each parameter. The computational framework developed for this study provides more insights into the photophysics of the 2D-RP perovskites and can be used more generally to obtain a mechanistic understanding of semiconductor carrier dynamics.