The modified electron transfer mechanism for spectral sensitization in silver halide photography has been reviewed and applied for spectral sensitization of the photoelectric effects in various photoconductors (ZnO, TiO₂, S, CdS, Se, and Polyvinylcarbazole).

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Various spectral dye-sensitization effects of PVK were investigated as follows.
(1) Sensitization by cyanine dyes and styryl dyes, especially correlation of photosensitivity and dye-structure.
(2) Sensitization by 3, 3-dicarbazolyl methane dyes, especially excellent sensitizing effect and dye structural factor
(3) Photochemical sensitization by organic polyhalides, especially behaviors of photochemical treatment.
(4) Prinicipal factors of dye-sensitization by spectral absorptive characterstic observations and electronic energy-level calculation.
From these results, we found fhe photosensitivity of dye-sensitized PVK plate E_1/2 reached to 4~5 lx sec and proposed one dye-sensifization model.

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The studies by the author were summarized on the interactions between sensitizing dye and zinc oxide in electrophotography.
Parallelism between adsorption and sensitization was observed in dye/ZnO system, as was often the case with silver halide photography, too. A great emphasis, however, was layed upon the matter in the former system, because of the low adsorptive activity of the photoconductor. Thus a polar, particularly an acidic group should be introduced into the dye when it was desired to design a “good” sensitizer molecule in zinc oxide electrophotography.
Infrared spectroscopic analysis suggested the probability of chemical interaction between zinc oxide and acidic dye, i.e., formation of the zinc salt. Atomic absorption spectrometry also supported the suggestion, when zinc ion was determined in the extract of dyed zinc oxide. Acidic groups, e. g., caboxyl, phenolic hydroxyl, et al., attached to dye molecules might, consequently, play an important role in combining the dye skeletons to zinc oxide crystals by the ionic binding forces. One of the reasons for the usefulness of xanthene dyes in the electrophotographic engineering could be considered the presence of such “reactive” functional groups in them.
Fluorescence of dyes was completely quenched, when they were adsorbed by zinc oxide. Iodination of a dye molecule was attempted with a view to accelerating the inter-system crossing (S*→T*), but no more effective sensitizer was obtained. These two facts suggested that the first excited singlet state of dye molecule might take part in the spectral sensitization of photoconduction of zinc oxide.

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It has been known that phthalic anhydride or other substances can greatly improve the photoconductivity of ZnO. This sensitization for the intrinsic absorption region is called “chemical sensitization”. The mechanism of chemically sensitization was studied with ZnO that adsorbed phthalic anhydride. Significance of shallow electron traps on the surface was demonstrated through experiments of photo-induced injection of electrons from the surface to the bulk, for unsensitized and variously sensitized ZnO samples. Due to chemical interactions between phthalic anhydride and chemisorbed oxygen, phthalic anhydride molecules seemingly are transformed into phthalate ions, which work as a catalysis for the photodesorption of oxygen from the ZnO surface. Phthalate ions are considered to form shallow and non-volatile electron traps at the surface. Most of observations could be interpreted by this mechanism for chemical sensitization.

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Dye-sensitized zinc oxide has two spectral photoconductive responses, one is intrinsic of zinc oxide (Ⅰ), and the other is due to dye-sensitization (Ⅱ). In order to speculate dye-sensitization mechanism, photoconductive properties of dye-sensitized zinc oxide irradiated by two beams of light in wavelength of I and II are observed.
The properties are completely altered with thickness of the sample. The samples having thin layer show that photocurrent by two beams is larger than the sum of independent photocurrent by each beam. This phenomenon is observed with photodesorption of oxygen by beam of wavelength region I.
When the samples having thick layer show that photocurrent by beam of wavelength region I is quenched by beam of wavelength region II. This phenomenon is speculated to be due to the hole injection from dye to the surface of zinc oxide. The ratio of quenching current to dye-sensitized photocurrent is small in zinc oxide sensitized by sensitive dyes.

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Dey-sensitized zinc oxide has two kinds of spectral photoconductive response. One is intrinsic of zinc oxide (Ⅰ) and the other is due to the dye-sensitization (Ⅱ).
In order to speculate dye-sensitization mechanism, properties of thermally stimulated currents (TSC) of zinc oxide sensitized by xanthene dyes are observed.
From the result of experiments, pure zinc oxide is found to have two kinds of trap levels, and their depths from conduction band are calculated to be 0.42 e V and 0.52 e V. When zinc oxide is sensitized by xanthene dyes, an additional trap level is observed and its depth is varied from 0.56 e V to 0.59 e V depending on dyes added.
When the low temperature excitation at the TSC measurement is by the light of wavelength region I, TSC curve shows peaks corresponding to the zinc oxide trap levels, and rarely show peaks corresponding to dye's traps, except in the case of rose bengal dyesensitized ZnO.
While the excitation is by the light of wavelength region II, TSC shows a peak corresponding to dye's traps, and peaks corresponding to zinc oxide's traps.
The facts that the electrons from excited dye fill the traps of zinc oxide and of dye, and that the electrons from zinc oxide fill the zinc oxide intrinsic traps only, indicate that, (1) the positions of two kinds of traps are different and perhaps one lies in the dye and the other in the zinc oxide and (2) electron excited level of dye is located higher relative to the conduction band of zinc oxide.
The relative relationship between the energy levels indicates the possibility of electron transfer from dye to zinc oxide during the photoconduction in the sensitized region.

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