Formation of Dual Bands in Mixed-Halide Perovskites During Photoinduced Halide Segregation

In mixed-halide perovskites, photoinduced halide segregation is known to occur, leading to the formation of segregated domains and, thereby, an additional low-energy photoluminescence emission. Here, we probe an archetypical mixed-halide perovskite from the perspective of band energies on the microscopic scale. Through optically coupled scanning tunneling spectroscopy, we determine the band energies of halide-segregated domains, namely, iodide-rich and bromide-rich ones. We estimate the composition of the phases and identify energy-levels associated with them as would be “seen” by charge carriers in devices. The appearance of dual bands due to the segregation process is explained in light of the molecular orbitals involved in forming the bands. Kelvin probe force microscopy upon illumination provides the local map of the contact potential difference throughout the sample surface, leading to the inference that the grain boundaries act as active sites for iodide migrations; that is, the low bandgap iodide-rich domains form at the grain boundaries.