Role of Singlet and Triplet Excitons in Extrinsic Photocurrent Production in the Anthracene–Gold System

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Abstract
Extrinsic hole photocurrents in a gold coated anthracene crystal are measured as a function of the absorption depth of the exciting light. Charge pairs are formed at the gold‐anthracene interface as a result of exciton–gold interactions. It is found that both singlet and triplet excitons are involved as precursors to charge formation. The contributions of singlets and triplets to the observed photocurrent quantum yields are determined for 394 nm excitation light (a and b polarized) and 340 nm excitation light (a and b polarized). Since reabsorption is known to have a significant effect on the distribution of excitons and on the effective singlet lifetime, a steady state numerical solution for the reabsorption problem is offered for this system. The inclusion of the reabsorption effect allows the calculation of a “reabsorption free” singlet diffusion length (316 Å) and an evaluation of the electron transfer efficiency ratio (ηT/ηS ≈ 3.5) where ηS is the probability of electron injection per singlet exciton quenched by the surface and ηT is the corresponding probability for a triplet exciton. The magnitude of the ratio ηT/ηS is interpreted in terms of singlet energy transfer to the gold layer.