Reversible Decomposition of Single-Crystal Methylammonium Lead Iodide Perovskite Nanorods

Perovskite solar cells offer remarkable performance, but further advances will require deeper understanding and control of the materials and processing. Here, we fabricate the first single crystal nanorods of intermediate phase (MAI-PbI₂-DMSO), allowing us to directly observe the phase evolution while annealing in situ in a high-vacuum transmission electron microscope, which lets up separate thermal effects from other environmental conditions such as oxygen and moisture. We attain the first full determination of the crystal structures and orientations of the intermediate phase, evolving perovskite, precipitating PbI₂, and e-beam induced PbI₂ during phase conversion and decomposition. Surprisingly, the perovskite decomposition to PbI₂ is reversible upon cooling, critical for long-term device endurance due to the formation of MAI-rich MAPbI₃ and PbI₂ upon heating. Quantitative measurements with a thermodynamic model suggest the decomposition is entropically driven. The single crystal MAPbI₃ nanorods obtained via thermal cycling exhibit excellent mobility and trap density, with full reversibility up to 100 °C (above the maximum temperature for solar cell operation) under high vacuum, offering unique potential for high-performance flexible solar cells.