Molecular-Orbital Studies of Charge-Carrier Transport in Orthorhombic Sulfur. II. Electronic States of the Crystal

In order to interpret the difference of four orders of magnitude in the electron and the hole drift mobilities in orthorhombic sulfur, the intermolecular electronic-interaction energies for an excess electron and an excess hole are computed with the molecular orbitals obtained in the preceding paper. The excess-carrier energy bands are obtained by summing these energy integrals according to the symmetry characteristics of the crystal structure. Contrary to what Spear and co-workers have expected, the electronic contributions to the widths of an excess electron band and an excess hole band are found to be of the same order of magnitude and hence do not account for the large difference in the mobilities. The changes in the electronic charge distributions when an electron is added to or removed from a neutral molecule are used to estimate the relative values of molecular deformations and the polaron binding energies Eb. It is found that Eb associated with an excess electron is almost an order of magnitude larger than that associated with an excess hole. Holstein and Siebrand's theory is then used to show that this difference in electron-molecular vibration couplings is the major effect leading to the large difference in the electron and the hole mobilities. The same argument is used to explain the large difference in the charge-carrier mobilities of metal-free phthalocyanine and copper phthalocyanine. The excitation-transfer matrix elements are formulated in terms of molecular-orbital coefficients. The matrix elements for one of the excited states are found to be always almost zero due to the molecular symmetry. This is interpreted as the "localized excited state" which is responsible for the nonphotoconducting absorption observed in orthorhombic sulfur and vitreous selenium.