Abstract
The recent discovery of twin domains in MAPbI3 perovskites has initiated contentious discussion on the ferroic nature of hybrid perovskites. Ferroelectric polarization is thought to facilitate the dissociation of photoinduced electron-hole pairs, helping to explain the extraordinary photovoltaic performance exhibited by this class of materials. Alternate to ferroelectricity, which has yet to be unambiguously established despite considerable efforts to do so, ferroelasticity was also proposed in these materials. Meanwhile, given the coupling of ionic states and ferroelectricity and the interconnected nature of defect chemistry and ferroelasticity, the electrochemical reactivity can no longer be ignored. In this work, using band excitation piezoresponse force microscopy, we reveal the variation in elasticity between adjacent domains, indicating the ferroelasticity and the difference in the crystallographic states of the twin domain. Moreover, using band excitation contact Kelvin probe force microscopy, we dynamically map the evolution of the twinning structure under electric bias. These results help decipher the effect of the twin domains on ionic mobility and ion diffusion pathways. Combining these results, we reveal the interaction of twin domains and ionic activity in this material. Overall, this work provides insights into the twinning structure in MAPbI3 and its potential effects on the hybrid perovskite optoelectronics.
