Field Dependent Losses of Electrons and Holes by Bimolecular Volume Recombination in the Excitation Layer of Anthracene Single Crystals Studied by Drift Current Pulses

Charge carriers of both signs are produced in the bulk of anthracene single crystals by strongly absorbed light of λ < 300 nm. The overlap time of the electron and hole spatial distributions in this case depends on the relative drift velocity in an applied electric field E separating these distributions and on the penetration depth of the light. During this time recombination losses occur. The experimental drift current pulses obtained with λ = 280 nm light are explained quite well by a theory worked out for this model. A bimolecular recombination coefficient γ between 10−5 and 10−6 cm3/s is found, in agreement with results by other methods, depending somewhat on crystal quality and purity. For λ = 250 nm light, which is absorbed stronger than λ = 280 nm by a factor of about 100 and where the charge carriers therefore should be produced in a much narrower region, strange results are obtained (γ ∼ E2), possibly caused by flat traps or by diffusion.