Exciton Features in 0-, 2-, and 3-Dimensional Networks of PbI64-Octahedra

We have investigated reflection spectra at 4 K for single crystals of (CH 3 NH 3 ) 4 PbI 6 ·2H 2 O which consists of isolated PbI 6 4- octahedra, (C 10 H 21 NH 3 ) 2 PbI 4 and (CH 3 NH 3 )PbI 3 , which consist 2- and 3-dimensional networks of PbI 6 4- , respectively. The formation energy of the lowest exciton 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.