Semiconducting Polymer Matrix as Charge Transport Materials and its Application in Polymer Electronic Devices

Abstract

In the last decade, a number of intensive studies have been conducted to achieve efficient polymer light-emitting diodes (PLEDs). As a result of extensive multidisciplinary efforts, modern PLEDs offer substantial benefits over conventional cathode ray tubes (CRT) and liquid crystal displays (LCD). PLEDs display provides superior brightness and color purity, markedly lower power consumption, as well as full viewing angle without compromising image quality. Compared with small molecule organic LEDs (OLEDs), PLEDs use solution-based processes, which offer the potential for lower cost and roll-to roll processing on flexible substrates.An efficient PLED device typically consists of a stack of organic/polymeric thin layers, each of which performs a specific function aimed at improving the device performance or achieving the desired device functionality. In many cases, these layered structures are formed from the polymeric solution by spin-casting or printing with subsequent removal of the solvent carrier. However, solvent from the freshly deposited film frequently dissolve or partially dissolve the underlying layer, resulting in loss of the desired structure and corresponding device functionality. Undesirable changes in the morphology and interfaces of the polymer films are another detrimental effect associated with incompatible solvent and its removal.To make more robust hole transport layers (HTLs) and avoid solvent damage from subsequent emissive layer, the most common approach is to introduce polymerizable functional groups onto the base structure of the molecules with hole transporting (HT) property to form a cross-linkable HT molecules, which can form a cross-linked HTL upon treatment. However, such a type of polymerizable hole transport material is expensive and difficult to make, especially in large quantity.Herein we report a new approach to address this issue: Commercially available HT polymers are embedded into a crosslinked polymer network to “lock” uniformly distributed HT polymers inside the cross-linked polymer matrix. This approach proves to be more advantageous in terms of process simplicity and cost. Similarly, the same class of materials can potentially be employed in other polymer electronic devices, such as the organic photoconductor.An organic photoconductor commonly used in electrophotographic applications is a dual layer structure consisting of a thin (0.1um - 2 um) charge generation (CGL) bottom layer and a thick (about 20 um) charge transport (CTL) top layer. Light passes through the transparent CTL and strikes the CGL that generates free electrons and holes. Electrons are collected by the electrical ground of the photoreceptor and holes are driven towards to top of the CTL by an applied electrical field.CTL allows hole transport towards the surface, at which they are used to neutralize negative surface charges deposited during the pre-charging process. In essence, CTL consists of non-conductive organic material (usually polymer) with charge transport moieties embedded into it. We believe this semiconducting polymer matrix can act as charger transport materials for organic photoreceptor.