Various explanations have been invoked to explain this phenomenon. These include the coexistence of different crystal phases, [17] recombination from shallow defect-related excitonic states, [18] a slightly indirect bandgap, [19][20][21] photon recycling, [22,23] and different states in bulk and at the surface. [24,25] The large discrepancy in explanation is mainly because of their complicated near-band edge photophysical characteristics, and little information is currently available about the dynamics of dual emission states. [26] Moreover, the additional emission usually appears as a shoulder in the PL profile on the low-energy side and strongly depends on the sample and the point of excitation. Although dual emission has been frequently reported, a clear physical scenario of the dynamics of dual emission is still missing. [27][28][29][30] Time-resolved spectroscopy is a power tool for providing dynamic information of emission state. [31] Therefore, an experimental probe of the ultrafast dual emission evolution is required to understand the intrinsic optoelectronic structure of metal halide perovskites.Herein, we investigate the recombination dynamics of the dual emission states in lead bromide perovskite (APbBr 3 ) single crystals with time-resolved photoluminescence spectra. We observe dual-band emission across broad temperature regions in single crystals of MAPbBr 3 (where MA is methylammonium; CH 3 NH 3 ). Using time-resolved PL spectroscopy, two communicating states are unambiguously resolved from picosecond to microsecond scale. The emission dynamic demonstrates the electronic state transition from the high-energy state to the lowenergy state at cryogenic temperatures, providing new and vital spectroscopic information to shed light on the nature of the dual emission. Dual emissions from lead bromide perovskites with different A-cation, namely, FAPbBr 3 (where FA is formamidinium, CH(NH 2 ) 2 ) and CsPbBr 3 , furthermore show that this is a general characteristic of APbBr 3 perovskites.
Results and DiscussionWe start with investigating the dual emission from MAPbBr 3 crystals at room temperature. The crystals are grown from solution by inverse temperature crystallization (ITC), according to previous reports. [25,32] The photoluminescence spectra from freshly cleaved crystals are measured in a reflection geometry, in which the same lens (focus length: f = 150 mm) is Metal halide perovskites exhibit intriguing properties for optoelectronic and electronic applications. Understanding their intrinsic photoexcitation and band-tail dynamics is essential for exploring novel functionality and improving device performance. This report addresses the band-tail luminescence of lead bromide perovskite (APbBr 3 ) single crystals, where A is methylammonium, formamidinium, and cesium. Using time-resolved photoluminescence (PL) spectroscopy, the dynamics of well-defined dual emission peaks are unambiguously resolved from the picosecond to the microsecond scale. Whereas the long-lived states exhibit similar recombination dy...