Exciton-induced photocurrents in molecular crystals

Charge carrier production by singlet excitons striking an anthracene/aluminium interface is investigated. Two possibilities are considered: (i) non-radiative energy transfer creates hot electrons and holes in the metal, which can be injected into the crystal by analogy with the photoemission process; (ii) charge transfer, efficiently competing with energy transfer, takes place, in course of which the exciton donates its electron to an empty metal state, the hole remaining within the crystal. Experimental evidence for the second mechanism is presented based on (i) the magnitude of the photocarrier production efficiency, (ii) its polarity dependence and, essentially, (iii) its reduction upon inserting a bimolecular fatty-acid layer between metal and crystal.