Competing Ultrafast Excitonic- and Quasi-Free-Carrier-Driven Decay of Optical Excitations in CVD-Grown 2D Hybrid Halide Perovskites

Two-dimensional (2D) organic–inorganic Ruddlesden–Popper (RP) halide perovskites emphasize a strong excitonic contribution, which enhances light-emission properties but decreases the photovoltaic efficiency. Decay dynamics using pump–probe spectroscopy provides insight into the initial makeup of the excited states. The decay of the ground state bleaching peak at 2.3 eV from chemical vapor deposited (CVD) films of phenylethylammonium lead iodide (PEA2PbI4) with and without self-trapped excitons is investigated. Below a critical excited carrier density, which is in the low 1012 cm–2 and varies slightly in the presence of self-trapped excitons, the decay is dictated by both excitons and quasi-free carriers. At these carrier densities, the Saha equilibrium condition predicts only 5% of free carriers, suggesting that a small fraction of free carriers may catalyze entropy-driven exciton ionization, resulting in a strong contribution from Auger recombination. Beyond the critical excited carrier density, the system is primarily excitonic. A Sn-based CVD-grown 2D perovskite shows decay dynamics similar to those of the lead-based sample; however, it has a slower carrier cooling time of ∼1 ps compared with the fast cooling time in PEA2PbI4.