Microscopic understanding of the phonon bottleneck in 2D perovskites

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Perovskites constitute a remakable semiconducting material system, which is in many aspects very different from the well-known and understood family of epitaxial semiconductors. The huge interest in 3D perovskites triggered an investigation of their lower dimensional forms, such as nanocrystals and two-dimensional (2D) perovskites. The latter are known for their superior emissive properties exhibiting a quantum emission efficiency that is one or two orders of magnitude higher than in conventional semiconductors. The microscopic origin of this technologically important characteristics is still the subject of ongoing research and has been controversially discussed in literature. The soft lattice results in a specific exciton-phonon interaction in perovskite materials, in particular, excitons are known to a couple very weakly to acoustic phonons. It was proposed that this leads to extremely reduced phonon assisted scattering into the dark exciton states explaining very intense luminescence from perovskites nanocrystals. However, a clear evidence for this phonon bottleneck between bright and dark exciton states in 2D perovskites is still missing and an advanced microscopic modelling of exciton relaxation dynamics is needed. In this joint experiment-theory project, we will investigate the microscopic origin of the phonon bottleneck in 2D perovskites and its impact on their optical properties. These technologically promising nanomaterials constitute an unprecedented playground to study the phonon bottleneck between bright and dark exciton states as they offer the possibility for independent modification of the excitonic fine structure and the phonon spectrum. This provides an opportunity not only to thoroughly understand phonon-mediated exciton relaxation, but also to control and overcome the phonon bottleneck for a deterministic design of novel highly efficient light emitters. By varying the 2D perovskite quantum well thickness, chemical composition and organic spacer molecule we will be able to tune the spectral difference between the dark-bright exciton splitting and the optical phonon energy revealing the conditions for enhanced or suppressed phonon bottleneck effect. To realize the general objective of this project we will join forces of experimental and theoretical groups. Merging expertise in the field of optical spectroscopy with advanced exciton dynamics modelling we will provide microscopic insights into the elementary processes behind the phonon-bottleneck in 2D perovskites and we will reveal strategies of how to control this effect and optimize the technologically crucial quantum emission efficiency of 2D perovskites.