Observation of abnormal photoluminescence upon structural phase competence and transition-induced disorder of stable α-FAPbI3

Abstract

Black-phase formamide lead iodide (α-FAPbI3), considered one of the most important materials for a solar cell application, is generally poorly stable in air, which leads to not only the inability of photovoltaic devices but also the lack of its fundamental optics research. In this paper, we synthesized a stable α-FAPbI3 film by modifying a previously reported method and investigated its temperature- and excitation intensity-dependent photoluminescences (PLs). It is found that at low temperatures, the crystal phase competition process is unusually complicated and out of order. The temperature range of the biphasic coexistence is at least 30 K, and during this process, the PL intensity of either the high-temperature phase (cubic) or low-temperature phase (tetragonal) changes chaotically. After the complete transition to the tetragonal phase, compared with the cubic monophase, PL resulting from the crystal defects is obviously enhanced. Our findings provide a deeper understanding of the complex structural phase transition of halide perovskite and valuable insights into the fundamental optics of α-FAPbI3.