Exciton-Polaron Interactions in Metal-Halide Perovskite Quantum Dots

Metal-halide perovskites have been under intense investigation for their remarkable performance in energy conversion materials based upon their novel properties. In particular, a key aspect is their ionic lattice which features glassy anharmonicity giving rise to dynamic disorder. In this fluctuating potential energy landscape, the polaron becomes more important than the phonon. Coupling to phonons is important in covalent semiconductors in terms of the way in which the electronic system interacts with the lattice bath. Coupling to polarons becomes more important in perovskites and the observation of polaron formation and the impact of polaron formation upon optical properties is a key area of research.<br/><br/>The way in which the excitons couple to the lattice can be revealed by ultrafast electronic spectroscopies. We apply a suite of three ultrafast spectroscopies to probe the response of CsPbBr3 metal-halide perovskite quantum dots, spanning weakly to strongly confined. We exploit the state-of-the-art in electronic spectroscopies including time-resolved photoluminescence (t-PL) with 3 ps resolution, transient absorption (TA) with 100 fs resolution, and Coherent Multi-Dimensional Spectroscopy (CMDS) with 10 fs resolution to probe the hierarchy of dynamical processes that govern the function of these materials.<br/><br/>These experiments most dramatically reveal new physics of coherence in these materials in terms of exciton-lattice interactions. The t-PL experiments reveal a giant oscillator strength effect at low temperatures which we assign to spatial decoherence effects induced by lattice structural dynamics as revealed by ab initio molecular dynamics (AIMD) simulations. The CMDS experiments on the other hand reveal a coherence in time that is surprisingly long lived at 300 K. The excitonic coherence is long lived relative to exciton dephasing times and population lifetimes. The excitonic splitting observed reveals lineshape dynamics that reflect both coupling to phonons and polarons. The polaron formation dynamics reveals liquid-like behavior as the system undergoes spectral diffusion. The TA experiments reveal a breaking of phonon bottlenecks via Auger processes following the quantum dynamics of exciton-polaron coupling.<br/>This suite of three world leading ultrafast electronic spectroscopies provides deep insights into the exciton-lattice interactions that arise in the dynamically disordered potential energy landscape of metal-halide perovskite quantum dots.