Photo-Induced Surface Charge Density Effects In Dye-Sensitized Zinc Oxide***

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Abstract
Measurements of surface potential and surface conductivity in zinc oxide thin films in contact with eosin dye show that two regimes of spectral sensitization are involved under vacuum conditions. The dye molecules at the interface trap conduction electrons in the dark at a site close in energy to the dye ground state leading to a Schottky barrier. The return of the trapped electrons from the excited dye is held responsible for “thin layer” sensitization. With “thick” dye layers an additional contribution arises from charge transfer between the photoconducting solid dye and the zinc oxide substrate, in a direction which tends to equilibrate the Fermi levels in the two media, The decay of the ”thick layer” sensitized photoeffect is identical to the decay of the field effect in undyed zinc oxide films and is controlled by zinc oxide surface states.

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