Interaction of triplet excitons with trapped and free holes in crystalline anthracene. Exciton quenching, current enhancement and magnetic field effects

Abstract

The effect of interaction between free triplet excitons and free or (mainly) trapped holes is studied in crystalline anthracene, both on the triplet lifetime and the detrapping of holes. The rate constant of the triplet-hole interaction is R = (5.5 ± 2) 10^-12 cm3 s-1 for trapped holes and R' = (2 ± 1) 10^-9 cm3 s-1 for free holes; these values are comparable to those expected for diffusion limited processes, by triplet and hole diffusivity respectively. The efficiency of detrapping by triplets is large ≳ 0.1. The effect of a magnetic field on the photocurrent is investigated, as a function of intensity and orientation of the field. A model for the triplet-doublet interaction similar to that for the triplet-triplet interaction, where triplet-doublet pairs formation is considered, is used to calculate the rate constants for pair formation (4 × 10^-11 cm3 s-1) separation (2 x 10^8 s-1) and reaction (3 x 10^7 s-1). The values of the parameters D and E of the triplet exciton may be determined too. The study of the variation of the effect with the applied voltage shows evidence of the importance of free holes.