Effect of vacancies on the charge-transfer energy in the anthracene crystal

Abstract

In order to explore whether vacancies could trap charge-transfer (CT) states, the polarization and charge-quadrupole energies of CT configurations near vacancies in anthracene are calculated. Polarization and charge–quadrupole energies of single charges and of CT configurations in perfect crystals and of single charges near vacancies are recalculated treating anthracene as 14 submolecules at the heavy atoms, rather than three at the centers of the rings as in previous work. This improves agreement with experiment. A vacancy adjacent to either of the two lowest-energy CT configurations reduces the dielectric screening of the Coulomb stabilization, thereby stabilizing the configuration by typically 20–50 meV. However, for some higher-energy CT configurations a vacancy on or near the CT axis can actually increase the screening and destabilize the configuration by up to 40 meV. A vacancy also changes the charge–quadrupole energy by as much as ±250 meV, so that this effect dominates (as for single charges), leading to traps as deep as 300 meV for the two lowest-energy CT configurations. Such traps could reduce the efficiency of charge-carrier photogeneration by enhancing geminate recombination of CT configurations.