Delayed Exciton Formation Involving Energetically Shallow Trap States in Colloidal CsPbBr3 Quantum Dots

We report the occurrence of delayed exciton formation in highly emissive CsPbBr3 quantum dots, which results in anomalously slow buildup kinetics that are readily observed in a time-resolved photoluminescence trace. It is inferred from the dependence of the buildup kinetics on nanoparticle size, temperature, and excitation fluence that the delayed exciton formation originates from multiple carrier trapping and detrapping events between the band edge state and energetically shallow structural disorder states. A kinetic model that incorporates these carrier pathways produces fits that are in excellent agreement with the time-resolved data. Importantly, the kinetic model allows for the determination of photoluminescence quantum yield values that closely match those obtained from integrating sphere measurements, whereas merely accounting for the pathways associated with the decay kinetics produces large discrepancies. This work highlights the crucial role played by delayed exciton formation in the photoluminescence dynamics of CsPbBr3 QDs.