EXCITON FEATURES IN 0-DIMENSIONAL, 2-DIMENSIONAL, AND 3-DIMENSIONAL NETWORKS OF [PBI6]4- OCTAHEDRA

We have investigated reflection spectra at 4 K for single crystals of (CH3NH3)4 PbI6.2H2O which consists of isolated [PbI6]4- octahedra, (C10H21NH3)2PbI4 and (CH3NH3)PbI3, which consist 2- and 3-dimensional networks of [PbI6]4-, respectively. The formation energy of the lowest excition state increases from 1.633 eV to 3.4 eV with decreasing the dimensionality in the crystal structure, from the transfer energy between the adjacent octahedra is estimated to be about 0.5 eV. The on-site Coulomb energy in the octahedron is estimated to be 2.3 eV. We suggest that the large on-site Coulomb energy enhances the exciton binding energy, especially in the 2-dimensional system.