The Optoionics behind (reversible) “photo-demixing”
Light-induced defect formation drives phase segregation in hybrid mixed halide perovskites
Shining light on semiconducting materials has some predictable consequences, one of which is that their electrical conductivity increases. Knowing this allows us to build devices such as cameras and solar panels. When shining light on lead halide perovskites, a recently developed class of materials in the field of photovoltaics, their conductivity does indeed increase, but something else unexpected happens. While for common semiconductors the number of electronic charge carriers is enhanced by light, in lead halide perovskites electronic carriers and also ionic defects that are able to conduct electrical current increase in concentration under illumination. This striking finding, first reported by our group in 2018 , has now been investigated as a function of the material composition .
Lead halide perovskites have the chemical formula ABX3. For example, in methylammonium lead iodide (MAPI), methylammonium (MA) represents the A cation, lead is the B cation and iodide is the halide X. Fixing lead as B constituent, leaves freedom in the choice of the A cation and X (halide) anion constituent. In our study, with Gee Yeong Kim as key person, we show that substitution of MA as the A cation with slightly larger or smaller cations can vary the magnitude of the electronic and ionic conductivities of lead halide perovskites, however it does not significantly vary the enhancement in the extracted value of ionic conductivity under light compared to the case under dark (Fig. 1(a)). On the other hand, moving gradually from iodide- to bromide-based perovskites shows a significant decrease in the light-effect described above (Fig. 1(b)).
The difference in ionic conductivity enhancement between perovskites using iodide or bromide is the most striking outcome of this study, also providing an explanation for another debated and unexpected behavior of these materials. It has been reported that perovskite films made from mixtures of iodide and bromide precursors undergo a reversible phase separation process when exposed to light. From a homogeneous mixture in the dark, the material segregates into iodide- and bromide-rich domains when light is shone on it. This surprising phenomenon has puzzled the community for several years and has attracted a lot of scientific attention, given the potential of varying the bandgap (and color) of halide perovskites through these mixing strategies. Based on our results, the origin of this light induced phase instability observed in perovskites with mixed halide compositions could therefore be related to the difference in defect chemical behavior of iodide and bromide under light. The formation of iodide rich and bromide rich domains in the materials allows for the photo-effect to take place in the iodide phase, making this counterintuitive demixing process favorable.
Our work takes us a step closer to understanding the behavior of halide perovskites but also opens a new perspective towards devices, beyond cameras and solar cells, that can make use of optoelectronic but also of optoionic functions.
 G. Y. Kim, A. Senocrate, T.-Y. Yang, G. Gregori, M. Grätzel, J. Maier
Large tunable photoeffect on ion conduction in halide perovskites and implications for photodecomposition
Nature Materials 17, 445–450 (2018)
 G. Y. Kim, A. Senocrate, Y.‐R. Wang, D. Moia, J. Maier
Photo‐Effect on Ion Transport in Mixed Cation and Halide Perovskites and Implications for Photo‐Demixing
Angewandte Chemie International Edition 60, 820-826 (2021)