Organic photoconductors for electrophotographic printing

Abstract

Organic photoconductors, introduced into the copier industry in 1970, are overtaking inorganic materials in today's market, as a result of their cost, flexibility and sensitivity at GaAs laser wavelengths. Organic photoconductors for use in EP are frequently multilayered structures, with the functions of charge generation and charge transport separately optimized. The charge generation material is a dye aggregate or pigment, coated in a thin layer on a conducting ground plane. The transport layer consists of a molecularly doped polymer of order 20 μm in thickness. The dopant molecule is an electron donor and acts as a hopping site for the motion of charge through the layer. The photoconductor must exhibit efficient generation of electron hole pairs, high mobility of charge carriers through the transport layer, high dark resistivity and stable electrical and optical properties. It must possess suitable mechanical properties, such as resistance to delamination and wear. This paper focuses on studies aimed at understanding and optimizing the mobility of holes through the transport layer. The concentration and temperature dependence of the mobility are consistent with a picture of thermally activated hopping. Charge trapping is shown to play a crucial role in the shape of time-of-flight current transients, and also affects functional parameters such as residual voltage and cycle-to-cycle stability.