Charge-Transfer Exciton State and Energy Levels in Tetracene Crystal

Abstract

An experimental study of the double-quantum external photoelectric effect in tetracene reveals evidence of an exciton state of energy 2.9 eV different from the normal exciton. This mobile, bulk-generated state is considered to be a charge-transfer (CT) exciton and is most efficiently produced at an energy ≥Ec=3.7 eV. We associate Ec with the lower edge of a relatively broad upper conductivity band. Presumably, there are other conducting states Ec′ such that ECT<Ec′<Ec. The calculated value for the polarization energy of the free electron in the crystal, Pe=Ic—Ec=1.6 eV, where Ic is the threshold energy for external electron emission. This is the same as the polarization energy of the positive hole Ph=Ig—Ic, where Ig is the ionization energy of the tetracene molecule. The electron affinity of the isolated molecule must be less than 0.8 eV. The diffusion length of the charge-transfer exciton is less than 5000 Å.