Optical properties due to electronic transitions in two-dimensional semiconductors (CnH2n+1NH3)2PbI4

Optical spectra in the visible and uv regions are investigated in layer-type perovskite compounds (${\mathrm{C}}_{\mathit{n}}$${\mathrm{H}}_{2\mathit{n}+1}$${\mathrm{NH}}_{3}$)${\mathrm{PbI}}_{4}$ with n=4, 6, 8, 9, 10, and 12. The spacing between the ${\mathrm{PbI}}_{4}$ layers changes from 15.17 \AA{} for n=4 to 24.51 \AA{} for n=12. In spite of these different spacings, the optical spectra are almost the same for these compounds, which means that the interaction between the layers is weak. The lowest exciton is located at 2.56 eV at 1.6 K, and its oscillator strength and binding energy are 0.7 per formula unit and 320 meV, respectively. These values are very large compared with those in a three-dimensional analog ${\mathrm{PbI}}_{2}$. The large oscillator strength and binding energy can be explained by the small dielectric constant of the alkylammonium ``barrier layer,'' which strengthens the Coulomb interaction between an electron and a hole.