Unraveling Exciton-Lattice Dynamics on the Emission from Excitons to Multiexcitons in Metal-halide Perovskite Quantum Dots

Metal-halide perovskites have been under intense investigation for their promise in a variety of energy conversion applications that arise from their unique exciton-lattice interactions. Quamtum dots of these perovskites further enable exciton-lattice interactions and exploiting quantum size effects. These materials are attractive for light emission due to their high quantum yields, fast radiative recombination, and defect tolerance. They show efficient single photon emission with long coherence times, and a puzzling giant oscillator strength effect at low temperature. Like II-VI quantum dots, they show promise for optical gain and lasing. All these processes arise from the excitonics of the system and the way in which the excitonic system couples to the lattice bath.<br/><br/>Here, we apply time-resolved photoluminescence (t-PL) spectroscopy with 3 ps resolution to probe the excitonics if light emission in CsPbBr3 metal-halide perovskite quantum dots spanning weakly to strongly confined. This unprecedented time resolution enables, improving our prior work with 100 ps resolution, us to make first observations of key processes from low temperature superradiance, to multiexciton formation dynamics, to non-equilibrium exciton-phonon interactions, to strong exciton-lattice coupling that breaks the near universal Condon approximation. These ultrafast t-PL measurements reveal the richness of the system-bath interactions in metal-halide perovskite quantum dots. These measurements provide a first glimpse into light emission from high quality model system perovskite QD.<br/>Key results are highlighted below:<br/><br/>The striking result is that the radiative rate constant of the single exciton increases at low temperatures with an exponential functional form, suggesting quantum coherent effects with dephasing at high temperatures. The opposing directions of the radiative and non-radiative decay rate constants enable enhanced brightening of PL from excitons to biexcitons due to quantum effects, promoting a faster approach to the quantum theoretical limits of light emission. <i>Ab initio</i> quantum dynamics simulations reproduce the experimental observations of radiation controlled by quantum spatial coherence enhanced at low temperatures.<br/><br/>The presence of higher fine structure states, let alone non-equilibrium processes within the fine structure, and multiexcitonic fine structure remains poorly understood due to a lack of experimental probes. The simple and immediate observation from temperature dependence is a previously unobserved fine structure to the multiexcitons. The bandwidth trajectories reveal the presence of a previously unobserved fine structure in excitons as well as multi-excitons. The bandwidth trajectories reveal a complex history, from multiexciton recombination to exciton thermalization to Auger heating to lattice thermalization.<br/><br/>Comparing the same size of QD reveals that perovskite QD have a large radiative rate constant for emission from X than CdSe QD due to a larger oscillator strength. The multiexciton (MX) regime reveals that perovskite QD emit brightly and with more focused bandwidth than equivalent sized CdSe QD enabling more spectrally pure brightness. The MX kinetics reveals that perovskite QD maintain efficient radiative decay, effectively competing with Auger recombination. These experiments reveal that strongly confined QD of perovskites can be efficient multiexcitonic emitters, such as in high brightness light emitting diodes, especially in the blue.