OPTICAL-ABSORPTION IN BAND-EDGE REGION OF (CH3NH3)(3)BI2I9 SINGLE-CRYSTALS

Optical absorption spectra were measured around the band edge in (CH3N H3)(3)Bi2I9 single crystals. In the very thin crystals, an intrinsic e xciton band appears clearly at 2.51 eV at low temperatures, and the ab sorption peak remains even at room temperature reflecting a large bind ing energy of more than 300 meV. The exciton band does not show a disc ontinuous shift near the temperature where the methylammonium ions ind uce a structural phase transition. This fact indicates that the excito n states are independent of the methylammonium ions. The origin of the exciton band is examined on the basis of the annealing effect on the absorption bands in vacuum-evaporated films. The lower energy tail of the exciton band obeys the Urbach rule between 78 and 350 K. The detai led analysis by the Urbach rule reveals that the exciton-phonon intera ction in the present system is relatively strong.