Ultralong Charge Carrier Recombination Time in Methylammonium Lead Halide Perovskites

Due to their exceptional photovoltaic properties, metal halide perovskites (MHPs) are extensively studied for their potential applications in solar cells. In recent years, the power conversion efficiencies of MHPs-based solar cells rapidly increased from the initial few % toward more than 25% for single-junction devices. Therefore, also taking into account their low costs and ease of manufacturing, MHPs-based solar cells have become the fastest-advancing photovoltaic technology. In this regard, much of the recent work has been dominated by absorber materials based on methylammonium MHPs, such as MAPbX3, where MA = CH3NH3 and X = Cl, Br, and I. Here, we present the results of contactless time-resolved photoconductivity measurements in an exceptionally wide range of temperatures of 4 to 290 K that were performed for the various crystalline forms of the three parent MAPbX3, that is, MAPbCl3, MAPbBr3, and MAPbI3. This approach was made possible by the use of a high quality factor (Q) microwave resonator, which cooperated with a commercially available microwave bridge equipped with an automatic frequency control (AFC) and a helium gas-flow cryostat. The structural phase transitions from orthorhombic to tetragonal are found to drastically affect the transient photoconductivity signal, and we also observe ultralong charge carrier recombination times approaching 70 its at low temperatures. The difference caused by morphology on the photophysical properties is supported by a marked difference between rapidly cooled (quenched) and slowly cooled samples. The sensitive technique also allowed to observe differences between samples with different morphologies and crystallite sizes.