Generation of Charge Carriers in Organic Molecular Crystals through Multiple Exciton Interactions

Abstract

In this paper we have examined theoretically three possible mechanisms of photoconduction in organic molecular crystals: namely, the production of charge carriers as the result of (i) the interaction of two singlet excitons, (ii) the interaction of two triplet-state excitons, and, (iii) the interaction of a singlet-state exciton with a triplet-state exciton. Depending upon illumination conditions and crystal purity we find that any one of these three may be the dominant mechanism for the generation of free carriers by a multiple exciton process. In general, however, the singlet—triplet mechanism is shown to result in higher steady-state carrier concentrations than either the double—singlet mechanism [S. Choi and S. A. Rice, J. Chem. Phys. 38, 366 (1963)] or the double—triplet mechanism. The Franck—Condon factors have been formally included in the calculation of the generation rates and it is clear that they could greatly reduce the magnitude of the rates calculated using purely electronic wavefunctions. In attempting to relate the theoretical predictions with the experimental measurements, the effect of triplet—triplet annihilation has been included. The effect of impurities which could act as energy traps has been considered. It is concluded that trapping of the excitation energy at impurity sites adversely affects the generation of carriers, and when present will lead to photocurrents which vary exponentially with temperature. Such a temperature dependence would be predicted for any carrier generation mechanism which requires the transport of electronic excitation energy through the crystal.