On the Origin of Trapping Centres in Organic Molecular Crystals

Traps for current carriers in molecular crystals may be due either to imperfections of the crystal structure (structural traps) or to chemical impurities introduced into the lattice (chemical traps). In the present paper simple hypotheses are presented explaining the formation of traps of the two groups. Perturbations of the crystal structure are not localized in one unit cell, but may extend over several farther cell. The local values of the polarization energy of the crystal lattice will change near to the centre of the defect, thus the “perturbed” molecules can act as traps for currentcarriers.Energetic distributions of traps as calculated under such conditions are foundto be in a good agreement with those determined experimentally. Trapping states may be also formed by impurity molecules having either greater electron affinity or smaller ionization energy than the host ones. The depth of the chemical traps estimated from the relationships given in the paper has been compared with the experimental data.