  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, we elucidate the origin of the bright
exciton emission in perovskites by tracking the thermalization of dark and
bright excitons under a magnetic field. We clearly attribute the unexpectedly
high emission to a pronounced phonon-bottleneck effect, considerably slowing
down the relaxation towards the energetically lowest dark states. We
demonstrate 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.
