Abstract
Halide perovskites such as methylammonium lead iodide (MAPbI3) currently attract considerable attention because of their excellent optoelectronic properties and performance in solar cell devices. Despite tremendous research efforts to elucidate their fundamental properties, ion migration with the presence of ionic defects is still not fully understood. Here, various types of ionic defects for specific (100) and (112) lattice facets in single-crystal MAPbI3 have been investigated systematically. Our measurements reveal significant anisotropic properties. Photoluminescence (PL) and electrical transport measurements show that the (100) facet has higher PL intensity and over 1 order lower trap density compared to that of the (112) facet. We find that the facet-dependent variations of contact potential difference measured with Kelvin probe force microscopy under different bias voltages and light illuminations provide insights into different types of ionic defects on the surface of MAPbI3 single crystals. We also observe a completely different ion migration behavior on specific crystal facets through nanoscale scanning probe microscopy investigations. Our results indicate that the (100) facet exhibits an n-type behavior dominated with I– vacancies, whereas the (112) facet exhibits a p-type behavior with MA+ or Pb2+ vacancies. The findings on the facet-dependent configuration of ionic defects provide deeper understanding on facet-dependent optoelectronic properties in single-crystal MAPbI3.