Exotic optoelectronic behaviors in CH₃NH₃PbCl₃perovskite single crystals: Co-existence of free and bound excitons with structural phase transitions

CH3NH3PbCl3 (MAPbCl3) perovskite single crystal is attractive for the ultra-violet detector due to its wide bandgap and comparative stability over iodine or bromine systems. Single crystals of MAPbCl3 perovskite are also intriguing owing to its generic type of materials for fundamental photophysical properties and excitonic behaviors for its use in devices. Furthermore, recent progress using crystal-based device fabrication will shed light on semiconducting devices like III-V compounds. In this study, a structurally well-defined crystal is grown and examined to reveal free and bound excitonic behaviors depending on the structural phase transition. We classified the free and bound excitonic behaviors by temperature- and power density-dependent photoluminescence and optical transmission spectra. The single emission peak located at 3.1 eV and blueshift depending on decreasing the temperature is attributed to the radiative recombination of the free exciton at the cubic and the tetragonal phases, whereas the several peaks from the bound excitonic transition are just revealed under 120 K of the orthorhombic phase. We also determined the work function distribution and band structures with excitonic bound states via Kelvin probe force microscopy. The optoelectronic properties resulted in the excitonic behaviors can be a fundamental approach for the construction of perovskite-based optoelectronic and photonic applications.