Temperature-dependent optical properties of hybrid organic-inorganic perovskite single crystals (CH 3 NH 3 PbI 3 and CH 3 NH 3 PbBr 3 )

We investigated temperature-dependent optical properties of methylammonium lead iodide (MAPbIalt;subagt;3alt;/subagt;: CHalt;subagt;3alt;/subagt;NHalt;subagt;3alt;/subagt;PbIalt;subagt;3alt;/subagt;) and bromide (MAPbBralt;subagt;3alt;/subagt;: CHalt;subagt;3alt;/subagt;NHalt;subagt;3alt;/subagt;PbBralt;subagt;3alt;/subagt;) single crystals using optical spectroscopy and photoluminescence (PL) techniques. We obtained the optical conductivity from measured reflectance spectra of the single crystals at various temperatures using a Kramers-Kronig (KK) relation and measured their PL spectra at selected temperatures. We observed structural phase transitions and a mixed-structure phase from the infrared-active phonon modes in the measured optical data. In the mixed phase we observed unsystematic temperature-dependent bandgap behavior in both optical and PL spectra. The unsystematic bandgap behavior can be understood by taking into account both the contributions of thermal effect and structural phase to the size of bandgap in the temperature region of the mixed structural phase or the structural phase transition. Interestingly we also observed quantitative differences in those optical and PL properties between thin film and single crystal samples. We expect that our results will supply helpful information for further development of hybrid organic-inorganic perovskite solar cells.