Non-Equilibrium Carrier Transport in Strongly Coupled Quantum Dot Solids and Heterostructures

Abstract

Understanding and controlling carrier dynamics in colloidal quantum dot (CQD) solids is crucial for unlocking their full potential for optoelectronic applications. The recent development of solution-processing methods to incorporate CQDs into high-mobility semiconducting matrices opens new routes to control simultaneously electronic coupling and packing uniformity in CQD solids. However, the fundamental nature of carrier transport in such systems remains elusive. Here we report the direct visualisation of carrier propagation in metal-halide exchanged PbS CQD solids and quantum-dot-in-perovskite (QDiP) heterostructures via transient absorption microscopy. We reveal three distinct transport regimes: an initial band-like transport persisting over hundreds of femtoseconds, an Auger-assisted sub-diffusive transport before thermal equilibrium is achieved, and a final hopping regime at longer times. The band-like transport was observed to correlate strongly with the extent of carrier delocalisation and the degree of energetic disorder. By tailoring the perovskite content in heterostructures, we obtained a band-like transport length of 90 nm at room temperature and an equivalent diffusivity of up to 106 cm2 s-1 – which is four orders of magnitude higher than the steady-state values obtained for PbS CQD solids. These findings not only shed light on the non-equilibrium dynamics in CQD solids and their influence on carrier transport, but also introduce promising strategies to harness non-equilibrium transport phenomena for more efficient optoelectronic devices.