Causes of carrier trapping asymmetry in aromatic hydrocarbon crystals

Mechanisms by which the same physical defect could produce different trap depths for electrons and holes are investigated semiquantitatively for a model comprising one molecule and one ion. Charge-quadrupole interactions must be included in addition to the polarization energy. The unsymmetrical molecular environment gives rise to a reaction field which polarizes the anion and cation by different amounts, causing electron traps to be possibly 5% deeper than hole traps. The extent of lattice relaxation is governed by the different repulsive potentials of the anion and cation, which are estimated from INDO calculations on naphthalene. Shallow hole traps could be twice as deep as electron traps, with smaller relative changes for deeper traps. These results are compatible with experimental information on trapping asymmetry.