Inhomogeneous Trap-State-Mediated Ultrafast Photocarrier Dynamics in CsPbBr₃ Microplates

Quantitatively elucidating photocarrier dynamics mediated by trap states in perovskites is crucial for establishing a structure-performance relation and understanding the interfacial photocarrier transport mechanism. Here, trap-state-mediated photocarrier dynamics in defect-rich CsPbBr₃ microplates are noninvasively investigated by ultrafast laser spectroscopy. Time-resolved photoluminescense (TRPL) measurements as a function of sample thickness indicate that trap densities of surface and bulk regions are inhomogeneous, leading to fast and slow decay components of TRPL, respectively. Fast and slow PL lifetimes present the same decreasing trend as the thickness is decreased from 5 to 0.1 μm, suggesting that both surface and bulk trap densities dramatically increase in sub-micrometer thick microplates. Furthermore, dynamical competition of ultrafast photocarrier energy relaxations between surface and bulk regions is studied in a 1.6 μm-thick sample by temporally correlating pump fluence-dependent TRPL with transient absorption signals. Strikingly, long-lived hot carriers (20 ps) are observed and complete filling of mid-gap trap states in the surface region can markedly enhance PL emission in the bulk region. By control measurements, we attribute these anomalous phenomena to the polaron-assisted ultrafast energy transfer process across the surface-bulk interface. Our results provide new insights into dynamical photocarrier energy relaxations and interfacial energy transport for inorganic perovskites.