Propagation of Excitons and Trions in Two-Dimensional Hybrid Perovskites

Two-dimensional hybrid perovskites are a highly intriguing class of materials. They are considered natural quantum well systems composed of alternating inorganic and organic molecular layers. Their reduced dimensionality combined with weak dielectric screening leads to the formation of tightly bound excitons that are shown to be both efficient light emitters as well as highly mobile. However, external control of their optical response has proven difficult due to challenges to introduce electrical doping into these systems. Moreover, a key feature of some of these materials is the occurrence of a structural phase transition that can alter the electronic band gap as well as their optical response. To what extent the phase transition can affect these fundamental properties has been barely explored so far.<br/><br/>Here, we study the temperature-dependent exciton binding energy and exciton diffusion across the phase transition using a combination of time-resolved microscopy and non-linear spectroscopy. We demonstrate that neither exciton binding energy nor exciton diffusion are affected by the phase transition in contrast to initial predictions. These findings are unexpected considering the substantial changes of the free carrier masses and contrast the semi-classical understanding of their transport, highlighting the unusual behavior of excitons in 2D hybrid perovskites. To better understand excitonic equilibration and relaxation, we investigate non-equilibrium dynamics at cryogenic temperatures at both off-resonant and near-resonant excitation conditions. Beyond that, we report realization of electrically doped, ultrathin 2D perovskite layers, studied in ambipolar field-effect transistor geometries. We demonstrate formation of three-particle exciton complexes, known as trions, by detecting photoluminescence spectra as a function of gate voltage and we reveal the influence of exciton-carrier interaction on the exciton mobility. The experimental realization of both tunable and mobile trions encourages potential applications involving electrically guided charged exciton currents.