Phonon-assisted recombination kinetics in two-dimensional lead halide perovskites

We report the coupling of radiative excitonic states to a vibronic band structure in 2D PEA₂Pb(I_1–xBr_x)4 (x = 0, 0.37, 0.53, 0.76, 1.0) thin films resolvable using photoluminescence measurements at temperatures below 300 K. We show that the photoluminescence profile becomes increasingly broad through the inclusion of bromine into the lattice and conduct low temperature steady state photoluminescence measurements to show that the origin of the emission profile is coupling of bound excitons to a vibronic band structure with energies between, E = 36.4 - 62.9 meV. This work illuminates the origins of the broad steady state radiative recombination that occurs in mixed halide 2D perovskites.