Light-Induced Defect Generation in CH₃NH₃PbI₃ Thin Films and Single Crystals

In a period of only a few years, the power conversion efficiency of organic–inorganic perovskite solar cells has surpassed a value of 24.2%. However, a major drawback is the lack of long-term stability, which is partially related to the dissociation of organic cations under prolonged illumination. This degradation mechanism is not limited to ambient temperatures. At low temperatures (T = 5 K), illumination of methyl ammonium lead iodide (CH₃NH₃PbI₃) thin films with a photon energy of E_ph = 3.4 eV results in the formation of localized trap states located about 100 meV within the bandgap. These light-induced defects are metastable, and annealing at T ≥ 15 K removes the localized states. Defect creation is not limited to polycrystalline perovskites but is also observed in single-crystal CH₃NH₃PbI₃. The experimental data are discussed in terms of a two-level model where the metastable state is separated from the annealed state by an energy barrier.