2, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
The twin domain in methylammonium lead triiodide (MAPbI3) has drawn extensive research efforts, starting the discussion on its ferroic nature. Given the coupling of defect chemistry and ionic states with ferroelectricity/ferroelasticity, the research efforts should be extended to its chemical behavior. Our earlier investigations revealed the ion segregation correlating to the ferroelastic domain contrast. To follow up, we systematically investigate the chemical evolution of the ion migration in the MAPbI3 twin domains in this work. Using Band Excitation Contact Kelvin Probe Force Microscopy (BE-cKPFM), we reveal the absence of ferroelectric polarization switching in this material, as our data indicate an underlying electrochemical effect regarding ion migration. This disproves the ferroelectric origin of the previously observed butterfly and hysteresis loops in Switching Spectroscopy piezoelectric force microscopy (SS-PFM). In addition, Band Excitation Scanning Kelvin Probe Force Microscopy (BE-scKPFM) measurement, which was utilized to study the electrochemical activities in the twin domain, indicates that the difference in ion migration and/or surface charging effect in the adjacent domains. This result implies a different ionic conductivity and a variation of electronic properties in adjacent domains. Combining Band Excitation PFM (BE-PFM), nanoscale infrared spectroscopy (Nano IR), and scanning probe microscope (SEM), we clarify the correlation between ionic diffusion, electronic properties, and chemical segregation. We reveal that the methylammonium segregation leads to a decrease of electronic conductivity and an increase of ionic conductivity. Overall, this work provides new insights into understanding the role of the twin domain in photovoltaic action.