Inhibition of light emission from the metastable tetragonal phase at low temperatures in island-like films of lead iodide perovskites

Photonic applications based on halide perovskites, namely CH3NH3PbI3 (MAPbI3), have recently attracted remarkable attention due to the high efficiencies reported for photovoltaic and light emitting devices. Despite these outstanding results, there are many temperature-, laser excitation power-, and morphology-dependent phenomena that require further research to be completely understood. In this work, we have investigated in detail the nature of exciton optical transitions and recombination dynamics below and above the orthorhombic/tetragonal ('O'-/'T'-) temperature phase transition (∼150 K) depending on the material continuity (continuous-like) or discontinuity (island-like) in MAPbI3 films. At low temperatures, continuous thin films of the perovskite can exhibit strain inhomogeneities associated with the formation of different 'T'-defective domains leading to an energy spread of states over more than 200 meV. On the other hand, a single photoluminescence line peak related to the perovskite 'O'-phase (associated with the distortion of the [PbI3]- anion) is observed in the island-like sample that we attribute to strain relaxation for this morphology. Moreover, the predominantly radiative recombination dynamics of the continuous-like sample mainly originates from nongeminate electron-hole formation of excitons in the 'O'-phase and the internal dynamics with carrier trapping levels. This observation is in strong contrast to the free exciton recombination dominantly found in the island-like sample.