Characterization of microscopic electrical defects in xerographic photoreceptors

Microscopic electrical defects in photoreceptors can be a source of serious copy quality degradation, particularly in laser printers operating in the discharge area development mode. In order to study these defects we have constructed an instrument which can measure variations of electrical photoreceptor properties with a resolution of a few tens of μm. The instrument is based on a shielded stylus which is scanned in contact with the photoreceptor surface using two computer controlled stepping motors. At every stylus position the bias is applied to the photoreceptor ground plane leading to charge flow in the stylus which is measured using a coulombmeter. Anomalously large charge flow signifies the occurrence of an electrical defect. The computer generated maps of photoreceptor electrical properties show excellent reproducibility and correlate with defects detected by xerographic development. Measurements of current-voltage characteristics of individual defects exhibit the space charge limited current behavior, indicating that they represent injection spots from the ground plane of the photoreceptor. An upper limit for the defect size can be ascertained from the magnitude of the injection current.