Excited State Dynamics of Metal Halide Perovskite Nanocrystals - Role of Surface States and Inter-Particle Interactions

Ultrafast spectroscopy has been successfully employed in the investigation of carrier dynamics in metal halide perovskite nanocrystals (NCs). While most of these studies have been performed primarily on perovskite NCs that are mostly fabricated as colloidal suspensions, recently a new synthesis route has emerged in which the NCs are grown within silica scaffolds. This method enables fabrication of formamidinium lead bromide (FAPbBr3) NCs directly in a device compatible solid-state architecture, while keeping the effects of quantum confinement intact. Here we provide a comprehensive photo-physical characterization of these material architectures of FAPbBr3 ranging from a bulk system to strongly quantum confined dot like structures. We employ two-photon excitation spectroscopy and temperature dependent ultrafast transient absorption spectroscopy to probe the effect of quantum confinement and surface states on the excited-state energy landscape, carrier thermalization and recombination. We also probe the nature of coherent nonlinear interactions, inter-particle and electron-phonon coupling through two-dimensional coherent electronic spectroscopy. We demonstrate that the NCs grown within the scaffold are subject to a large size distribution with substantial contribution from optically dark surface states in the carrier dynamics. However, we provide evidence for greater inter-particle coupling and optimal interconnectivity between the NCs, thus offering an effective strategy to control energy and charge transport.