Center for Nanoscale Materials, Argonne National Laboratory , 9700 S. Cass Avenue, Lemont, Illinois 60439, United States.
Department of Chemistry, Northwestern University , 2145 Sheridan Road, Evanston, Illinois 60208, United States.
Nano Lett. 2018 Feb 14;18(2):846-852. doi: 10.1021/acs.nanolett.7b04099. Epub 2018 Jan 12.
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.
甲脒（FA）和甲胺（MA）卤化铅是广泛研究的低成本，高性能光电材料。在这里，我们展示了从 300 K 到低温的可见和红外吸收，稳态和时间分辨光致发光的测量结果。虽然 FAPbI 纳米晶体（NCs）的行为与报道的体相行为非常相似，但 MAPbI 的胶体纳米晶体表现出与体材料低温光学行为的偏离。通过光致发光，可见和红外吸收测量，我们证明与单晶和多晶薄膜不同，MAPbI 的 NCs 不会经历与体四方到正交相转变相关的光学变化，该转变发生在接近 160 K 的温度下。我们没有发现有机阳离子旋转冻结到 80 K 以下或激子结合能改变到 3 K 以下的证据在 MAPbI NCs 中。在尺寸为 20 至超过 100 nm 的 MAPbI NCs 中以及在包括立方体和板的形态中都获得了类似的结果。因此，胶体 MAPbI NCs 为在单晶或多晶样品无法达到的温度下研究与太阳能相关的室温相 MAPbI 的性质提供了一个窗口。利用这一现象，这些测量结果揭示了在低温下接近能带边缘的光致激发态和持续缓慢的俄歇复合的存在。
