Charge Transport in MAPbI₃ Pellets across the Tetragonal-to-Cubic Phase Transition: The Role of Grain Boundaries from Structural, Electrical, and Optical Characterizations

Methylammonium lead iodide (MAPbI 3 ) is a very promising semiconducting material for photovoltaic applications. Despite extensive research and tremendous progress, basic charge transport properties are still being debated. Combining first-principles calculations and macroscopic and local measurements, we have investigated the structural, optical, thermal, and electrical transport (ac/dc) properties of MAPbI 3 hot-pressed pellets through the tetragonal-to-cubic phase transition. Thermal analysis and X-ray diffraction experiments confirm the tetragonal-to-cubic phase change around T S = 56 °C, which is often close to the working temperature of photovoltaic devices. The ac/dc electrical resistivities of the tetragonal phase indicate a metallic-like behavior as a function of temperature followed by an abrupt decrease in the cubic phase just above T S . In contrast to the abrupt changes observed in the electrical properties, the bandgap energy is barely affected across the phase transition. Similarly, local measurements obtained by means of nuclear magnetic resonance confirm a continuous variation in the lattice parameters and site symmetry ( 207 Pb and 127 I) across the structural phase transition. Density functional theory calculations combined with electrical characterizations indicate that iodine and/or unintentionally incorporated hydrogen interstitials influence decisively the charge transport activation energy in the cubic phase. In light of these findings, the unusual electrical resistivity behavior across the phase transition is discussed taking the grain boundary effects into consideration.