Role of the Binder in Light Discharge Characteristics of ZnO Layers

In order to establish a correlation between the photoresponse of a zinc oxide coating and the nature of its binder, we measured the light induced discharge current. Since, in order to reduce the number of variables of the system, the solvent used for coating the layer should remain unchanged, the choice of polymer binders is considerably narrowed. This problem was solved by selecting a family of copolymers in which vinyl acetate, as the constant component, was copolymerized with one of the components: vinyl pyrrolidone, ethylene, vinyl laurate, vinyl chloride, vinyl chloride-maleic acid, and crotonic acid. With the exception of the latter, all polymers could be coated from a dichloroethane solution. The non-dye-sensitized layers were exposed to uv light. It was found that light discharge current is composed of two separate functions. Attempts to describe the complex curve by mathematical functions are discussed. Depending on nature and amount of the binder used, one discharge function predominates, thus creating either a rapid or a slow photoresponse. It is believed that the intrinsic photoresponse of zinc oxide is rapid, but is slowed down by using (a) increasing amounts of binder, (b) by the presence of electron acceptor functions on the polymer chain. The photoresponse of zinc oxide-polyvinyl acetate coatings is reduced to shorter times by replacing vinyl acetate segments by vinyl chloride or vinyl laurate portions. The response time is increased by the presence of crotonic and maleic acid. The photoresponse of slow layers may be accelerated by increasing the intensity of corona charging; rapid coatings are less affected by charging conditions. The consequences of these findings on electrophotographic image properties are discussed.