Spectral Sensitization, Supersensitization, and the Mechanism(s) of Dye-Sensitized Photoconductivity in AgBr Single Crystals

Abstract

Spectral sensitization of photoconductivity in single crystals of AgBr in the 590–600-nm region has been accomplished using a substituted thiacarbocyanine dye. Relative quantum efficiencies (RQE) in this region, as determined by photoconductivity measurements, are shown to increase as dye-layer coverage decreases. The RQE of the larger dye aggregates is degraded by the presence of iodide on the AgBr surface, resulting in a shift of the RQE maximum to the short aggregate absorption region, particularly that of the dimer and trimer. The same red-absorbing dye was supersensitized with a green-absorbing dye and action spectra and efficiencies of carrier generation compared with the previous experiments. Efficiency maxima are now found in the 590–600-nm region, in agreement with the iodide-treated system. The photo-responses and consequent efficiencies, in these systems, decay with time. The form of the decay curve has been investigated with emphasis on the relative behavior of various absorption regions within the spectrally sensitized band. The RQE of larger aggregates is shown to decay more rapidly and more severely. Dye-sensitized photoconductivity occurs through a mechanism of either electron injection or energy transfer. An estimate of the maximum depth through which energy transfer can take place, if this is the appropriate mechanism, is 40–90 Å. We feel that electron injection is a more likely mechanism and the result of this calculation raises some question of the applicability of the energy-transfer hypothesis to this system.