Phonon‐Bottleneck Enhanced Exciton Emission in 2D Perovskites

Abstract Layered halide perovskites exhibit remarkable optoelectronic properties and technological promise, driven by strongly bound excitons. The interplay of spin‐orbit and exchange coupling creates a rich excitonic landscape, determining their optical signatures and exciton dynamics. Despite the dark excitonic ground state, surprisingly efficient emission from higher‐energy bright states has puzzled the scientific community, sparking debates on relaxation mechanisms. Combining low‐temperature magneto‐optical measurements with sophisticated many‐particle theory, the origin of the bright exciton emission in perovskites is elucidated by tracking the thermalization of dark and bright excitons under a magnetic field. The unexpectedly high emission is clearly attributed to a pronounced phonon‐bottleneck effect, considerably slowing down the relaxation toward the energetically lowest dark states. It is demonstrated that this bottleneck can be tuned by manipulating the bright‐dark energy splitting and optical phonon energies, offering valuable insights and strategies for controlling exciton emission in layered perovskite materials that is crucial for optoelectronics applications.