Inhomogeneous\nTrap-State-Mediated Ultrafast Photocarrier\nDynamics in CsPbBr<sub>3</sub> Microplates

Quantitatively elucidating photocarrier\ndynamics mediated by trap\nstates in perovskites is crucial for establishing a structure–performance\nrelation and understanding the interfacial photocarrier transport\nmechanism. Here, trap-state-mediated photocarrier dynamics in defect-rich\nCsPbBr<sub>3</sub> microplates are noninvasively investigated by ultrafast\nlaser spectroscopy. Time-resolved photoluminescense (TRPL) measurements\nas a function of sample thickness indicate that trap densities of\nsurface and bulk regions are inhomogeneous, leading to fast and slow\ndecay components of TRPL, respectively. Fast and slow PL lifetimes\npresent the same decreasing trend as the thickness is decreased from\n5 to 0.1 μm, suggesting that both surface and bulk trap densities\ndramatically increase in sub-micrometer thick microplates. Furthermore,\ndynamical competition of ultrafast photocarrier energy relaxations\nbetween surface and bulk regions is studied in a 1.6 μm-thick\nsample by temporally correlating pump fluence-dependent TRPL with\ntransient absorption signals. Strikingly, long-lived hot carriers\n(20 ps) are observed and complete filling of mid-gap trap states in\nthe surface region can markedly enhance PL emission in the bulk region.\nBy control measurements, we attribute these anomalous phenomena to\nthe polaron-assisted ultrafast energy transfer process across the\nsurface–bulk interface. Our results provide new insights into\ndynamical photocarrier energy relaxations and interfacial energy transport\nfor inorganic perovskites.