Charged Exciton Formation in Compact Polycrystalline PerovskiteThin Films

Metal halide perovskites are interesting materialswith various promising applications due to their morphologicallyand energetically tunable properties. However, fundamentalunderstanding of the relationship between their polycrystallinemicrostructures and associated carrier behaviors remains elusive.Here, we use an excitation density-resolvedfluorescence method toinvestigate thefluorescent properties in hybrid perovskitefilms. Itis found that in isolated crystal grains, the carriers and excitonsexist in a dynamical balance following the model described by theSaha-Langmuir equation. However, with the formation of acompact polycrystallinefilm, the morphology breaks thecoexistence of the two species and induces an additional emissivechannel at high excitation intensities, which can be assigned tocharged excitons or trions. The compact morphology keeps a gross shape of the grains, while deforming internal structures inperovskitefilms of the tetragonal phase. This study is beneficial for understanding the mechanism of the applications where highexcitation densities are involved. It demonstrates that excitonic emission in perovskitefilms can be altered by squeezing crystalgrains