Unique Optical Properties of Methylammonium Lead Iodide Nanocrystals Below the Bulk Tetragonal-Orthorhombic Phase Transition.

Methylammonium (MA) and formamidinium (FA) lead halides are widely studied for their potential as low-cost, high-performance optoelectronic materials. Here, we present measurements of visible and IR absorption, steady state, and time-resolved photoluminescence from 300 K to cryogenic temperatures. Whereas FAPbI nanocrystals (NCs) are found to behave in a very similar manner to reported bulk behavior, colloidal nanocrystals of MAPbI show a departure from the low-temperature optical behavior of the bulk material. Using photoluminescence, visible, and infrared absorption measurements, we demonstrate that unlike single crystals and polycrystalline films NCs of MAPbI do not undergo optical changes associated with the bulk tetragonal-to-orthorhombic phase transition, which occurs near 160 K. We find no evidence of frozen organic cation rotation to as low as 80 K or altered exciton binding energy to as low as 3 K in MAPbI NCs. Similar results are obtained in MAPbI NCs ranging from 20 to over 100 nm and in morphologies including cubes and plates. Colloidal MAPbI NCs therefore offer a window into the properties of the solar-relevant, room-temperature phase of MAPbI at temperatures inaccessible with single crystals or polycrystalline samples. Exploiting this phenomenon, these measurements reveal the existence of an optically passive photoexcited state close to the band edge and persistent slow Auger recombination at low temperature.