Herein we propose a mechanism that provides a driving force for ion migration when an organic metal halide perovskite material is photo-excited in the presence of oxygen. By analysis of the chemical changes that occur at the semiconductor surface when the semiconductor is photo-excited under controlled atmosphere in an X-ray photoelectron spectroscopy chamber, we find direct evidence of the formation of a superficial layer of negatively charged oxygen species capable of repelling the halide anions away from the surface and towards the bulk. Not only the reported photoluminescence transient dynamics, but also the partial recovery of the initial state when photoexcitation stops, the eventual degradation after intense exposure times, and the phase segregation observed in mixed halide perovskites, can be rationalized on the basis of the proposed mechanism.

Simultaneous exposure of CH₃NH₃PbX3 perovskite films to oxygen and light with energy above the electronic bandgap yields the formation of anionic oxy-gen species that accumulate as a negatively charged layer on the semiconductor surface. These reactive species both provide the electrostatic driving force necessary to induce ion migration and, simultaneously, initiate degradation of the lattice. As a result of the repulsive (attractive) coulombic force between halide ions (halide vacancies) and surface oxygen anions, a momentary reconstruction of the imperfect bulk lattice occurs, which explains the increase of PL reported. At the same time, those superficial radical anions start degrading the semiconductor surface, a phenomenon that becomes dominant and gradually extends to the bulk of the material until it causes its decomposition. The mechanism herein proposed provides a satisfactory explanation to seemingly contradictory effects, such as the initial enhancement of the photoemission and the subsequent degradation of the samples. At the same time, it is consistent with the reported light-induced phase segregation (or Hoke effect) that occurs in mixed halides (typically bromide-iodide) perovskites.

(1) M. Anaya, J. F. Galisteo-López, M. E. Calvo, J. P. Espinós, H. Míguez, "Origin of Light Induced Ion Migration in Organic Metal Halide Perovskites in the Presence of Oxygen", submitted.

(2) J. F. Galisteo-López, Y. Li, H. Míguez, "Three-Dimensional Optical Tomography and Correlated Elemental Analysis of Hybrid Perovskite Microstructures: An Insight into Defect-Related Lattice Distortion and Photoinduced Ion Migration", J. Phys. Chem. Lett., 2016, 7, 5227.

(3) Galisteo-López, F. J.; Anaya, M.; Calvo, M. E.; Miguez, H., "Environmental effects on the photophysics of organic-inorganic halide perovskites", J. Phys. Chem. Lett., 2015, 6, 2200.