Charging Transients of Organic Electrophotographic Photoreceptors

Abstract

This work presents results of investigation of charge carrier injection-extraction phenomena in single layer and double layer organic photoreceptors (OPC), consisting of various hole (HTM) and electron (ETM) transport materials and two types of phthalocyanines, during their dosed charging, especially in its first stage (early stage of charging). In the first stage of charging of double layer OPC, there is evidence of hole extraction from phthalocyanine crystallites in the charge photogeneration layer (CGL). In the case of the X-form metal-free phthalocyanine and ∼0.5 μm thick CGL, density of extracted holes reaches 1.0 × 1012 cm–2, and in the OPC with Y-form TiOPc, the extracted charge is less by an order of magnitude. The charge carrier reservoir in phthalocyanine crystallites is restored by exposure to red light or by keeping the OPC in uncharged state in the dark for several hours. Preliminary charging of double layer photoreceptors to a non-work potential (positive for normal construction and negative for IDL OPC) causes a decrease of work potential (negative for normal and positive for IDL OPC) in cyclic mode. During charging to non-work potential, electrons are injected from the CGL into the transport layer (CTL) and are trapped in deep levels at the CGL-CTL interface. Consequently, the critical electric field in the CGL, corresponding to onset of intense hole thermalfield generation, is reached at progressively lower work potentials. Nonlinear of charge-voltage characteristic for dosed charging of single layer OPC with the barrier sublayer or/and overcoat layer is strongly dependent on polarity of preliminary charging and photodischarge. During positive charging and photodischarge, holes accumulate on the barrier sublayer, and their density (5 × 1012 cm–2) can become several times larger than the density required for OPC charging to the maximum potential. During photoreceptor recharge by the opposite (negative) charge, holes drift from the barrier sublayer towards the OPC surface and partially accumulate on the inner surface of the protection layer. The same phenomena are observed in the case of OPC with overcoat barrier layer charge carriers accumulate at the barrier. Depending on the nature of hole and electron transport materials, holes partially accumulate near the OPC surface in the absence of the protection layer, too.