In the same way as electron transport is crucial for information technology, ion transport is a key phenomenon in the context of energy research. To be able to tune ion conduction by light would open up opportunities for a wide realm of new applications, but it has been challenging to provide clear evidence for such an effect. Here we show through various techniques, such as transference-number measurements, permeation studies, stoichiometric variations, Hall effect experiments and the use of blocking electrodes, that light excitation enhances by several orders of magnitude the ionic conductivity of methylammonium lead iodide, the archetypal metal halide photovoltaic material. We provide a rationale for this unexpected phenomenon and show that it straightforwardly leads to a hitherto unconsidered photodecomposition path of the perovskite.

By applying a multitude of experimental techniques including 1H, 14N, 207Pb NMR and 127I NMR/NQR, tracer diffusion, reaction cell and doping experiments, as well as stoichiometric variation, conductivity, and polarization experiments, iodine ions are unambiguously shown to be the mobile species in CH3NH3PbI3, with iodine vacancies shown to represent the mechanistic centers under equilibrium conditions. Pb2+ and CH3NH3 + ions do not significantly contribute to the long range transport (upper limits for their contributions are given), whereby the latter exhibit substantial local motion. The decisive electronic contribution to the mixed conductivity in the experimental window stems from electron holes. As holes can be associated with iodine orbitals, local variations of the iodine stoichiometry may be fast and enable light effects on ion transport.

Thermodynamics shows halide perovskites to be highly unstable against oxygen. Light accelerates O2 interaction kinetics. The materials stays metastable in the dark.

This contribution discusses the chemical stability of methylammonium (MA) halide perovskites (MAPbI 3 , MAPbBr 3 , and MAPbCl 3 ); it considers degradation processes relevant for devices (vs T, O 2 , H 2 O, voltage, illumination) by outlining their thermodynamic constraints and linking them to experimental observations. Thermodynamic considerations indicate that degradation under O 2 is highly favored, albeit in principle preventable by encapsulation. The same is true for H 2 O exposure. Intrinsic degradation is unavoidable in devices, and under real conditions, it can be thermodynamically favored, as is the case for photodecomposition. The Gibbs energies of the decomposition reactions show strong dependences on the A-cation (decomposition vs T), on the halide (vs illumination), or on both (vs O 2 , H 2 O). The stability trends vs composition change with degradation pathway, but MAPbI 3 often appears as the most unstable. These serious stability issues appear to have no master solution, though some approaches (e.g., encapsulation, exploiting lower dimensionality, and alternative contact phases) have so far shown promise.