Size Dependence of Charge Carrier Dynamics in Organometal Halide Perovskite Nanocrystals: Deciphering Radiative Versus Nonradiative Components

In this work, we have synthesized and characterized three differently sized (3.1, 5.7, and 9.3 nm) methylammonium lead bromide (CH3NH3PbBr3) perovskite nanocrystals (PNCs) and passivated using (3-aminopropyl)triethoxysilane and oleic acid as capping ligands. These PNCs show size-dependent absorption and photoluminescence (PL) with the middle-sized PNCs, exhibiting the highest PL quantum yield (similar to 91%). The effect of size on their exciton/charge carrier dynamics is studied using transient absorption spectroscopy and time-resolved PL. The middle-sized PNCs show slower early time recombination compared to that of the larger and smaller PNCs, suggesting optimized passivation of surface trap states. The observed PL lifetime and QY are analyzed to determine the size dependence of the radiative and nonradiative decay components. The radiative lifetime is found to decrease with decreasing PNC size, which seems to be primarily determined by the PNC core, while the nonradiative lifetime is the longest for the middle-sized PNCs, which is strongly influenced by the presence of band gap states that depend on surface passivation. A kinetic model is proposed to explain the observed dynamics results. This study demonstrates the competing effect between size and surface properties in determining the dynamics and optical properties of PNCs.