Computer Simulation of Shallow Traps Created by Impurity Molecules in Anthracene Crystal

Abstract

Conductive properties of crystalline organic semiconductors depend to a great extent on the presence of traps. One of the possible sources of traps are impurity molecules incorporated into the crystal lattice. The equilibrium crystal structure around an impurity in anthracene and the corresponding depth of shallow traps are calculated using simulations with a polarizable force field, quantum chemistry, and perturbation theory. Two possible causes of the trap formation are considered: the first one is the electrostatic potential created by the permanent dipole of the impurity molecule, and the second one is the alteration of the polaron binding energy in the vicinity of a point defect. Changes in intermolecular couplings produced by the lattice distortion make an unexpectedly small contribution to the polaron energy. The dominant influence is exerted by the absence of the resonance level in the lattice site occupied by the impurity. This factor inhibits the formation of traps and partially compensates for ...