Optical Absorption in Band-Edge Region of (<b><i>C</i><i>H</i>₃<i>N</i><i>H</i>₃)₃<i>B</i><i>i</i>₂<i>I</i>₉</b>Single Crystals

Optical absorption spectra were measured around the band edge in ( CH 3 NH 3 ) 3 Bi 2 I 9 single crystals. In the very thin crystals, an intrinsic exciton band appears clearly at 2.51 eV at low temperatures, and the absorption peak remains even at room temperature reflecting a large binding energy of more than 300 meV. The exciton band does not show a discontinuous shift near the temperature where the methylammonium ions induce a structural phase transition. This fact indicates that the exciton 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 detailed analysis by the Urbach rule reveals that the exciton-phonon interaction in the present system is relatively strong.