Ionization potential, electron affinity and work function of photosensitive dyes adsorbed by zinc oxide have been determined by means of HMO calculations and of polarography, according to the procedure used in the previous work. Based on the above data, as well as with the results obtained during the course of the study on the adsorption of dyes by zinc oxide, discussions were made on the mechanism of spectral sensization of the photoconductivity of zinc oxide by considering it as an example of various spectral sensitzation of photoelectric effects of semi-conductors, and gave the explanation for the fact that the spectral sensitization of the photoconductivity of zinc oxide was not determined by the position of the electronic energy levels of dyes, but by the adsorbability of the dyes to zinc oxide.

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The surface potential of ZnO powder and ZnO-resin layers was measured as a function of ambient air pressure by the Kelvin method. The surface potential in the dark decrased with decrease of air pressure up to several torr by the desorption of water molecule adsorbed on the surface of ZnO powder. The surface potential of ZnO powder and ZnO-resin layers in the dark agreed with the surface potential of gold metal at a vacuum of 2×10^-4 torr. The surface photovoltage of ZnO powder layer increased linearly with the decrease of air pressure up to 10 torr. The dark decay rate of the surface photovoltage decreased with the decrease of air pressure. For ZnO-resin layer, the ambient air pressure dependence of the surface potential in the dark and under illumination was small compared with that of ZnO powder layer. However, the dark decay rate of the surface photovoltage of ZnO-silicone resin layer depends intensely on ambient air pressure. A visible light quenching phenomenon was observed depending on ambient air pressure for ZnO-silicone resin layer sensitized with rose bengal.

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The electrophotographic properties of zinc oxide can be much improved by adsorption of electron affinitive molecules. We have found that the dark- and photoconductivities of zinc oxide are greatly influenced if an electron acceptor is adsorbed in couple with a donor (or even with another sort of acceptor). In some cases, remarkable sensitization of photoconduction takes place. Samples employed here, however, have adsorbed oxygen on the surface as well as the two adsorbants. The oxygn itself acts as a strong acceptor, obscuring the interactions possibly existing between the two. Regarding this point, we have studied the effect of ammonia (donor), oxygen (acceptor), and their mixed gas on the photoconductive properties of zinc oxide. In this particular case, however, any evidence of donor-acceptor interactions on the zinc oxide surface has not been obtained.

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