Disentangling Hot Carrier Decay and the Nature of Low- n to High- n Transfer Processes in Quasi-Two-Dimensional Layered Perovskites.

Quasi-two-dimensional (2D) metal halide perovskites (MHPs) are promising photovoltaic (PV) materials because of their impressive optical and optoelectronic properties and improved stability compared to their 3D counterparts. The presence of domains with varying numbers of inorganic layers between the organic spacers ( n -phases), each with different bandgaps, makes the photoinduced carrier dynamics in films of these materials complex and intriguing. Existing interpretations of the ultrafast femto- or picosecond spectroscopy data have been inconsistent, most of them focusing either on exciton/charge transfer from low- n to high- n phases or on hot carrier cooling, but not combined. Here, we present a comprehensive study of the carrier dynamics in the Dion-Jacobson type (PDMA)(MA) ( n -1) Pb n I (3 n +1) (PDMA = 1,4-phenylenedimethylammonium, MA = methylammonium) perovskite, stoichiometrically prepared as ⟨ n ⟩ = 5. Within the film, a coexistence of various n -phases is observed instead of solely the n = 5 phase, resulting in an interesting energy landscape for the motion of excitons and charge carriers. We disentangle hot carrier cooling from exciton transfer between low- n and high- n phases using ultrafast time-resolved photoluminescence and transient absorption spectroscopy. Photophysical modeling by target analysis shows that carrier cooling occurring on a subpicosecond time scale is followed by exciton transfer from low- n into high- n phases in ca. 35 ps when the film is excited by 532 or 490 nm light. Carriers in the high- n phase are much longer lived and decay in a ns time window. Overall, our results provide a comprehensive understanding of the photophysics of this material, which helps to optimize quasi-2D MHP materials for a new generation of PV devices.