Semiconducting polymer layer

Parker observed that the tum-on voltages for devices with different thicknesses of the semiconducting polymer layer are roughly the same and equal to the voltage required to reach the flat-band condition [22]. As shown in Fig. 4.9, however,... 

Fig. 4.9 Luminance vs voltage for ITO/MEH-PPV/Ca LEDs with semiconducting polymer layers of different thicknesses. Note that turn-on voltage is independent of thickness and equal to that needed to reach the flat-band condition (see Fig. 4.7(b)).

The origin of the EL emission is provided by the formation of excitons, or electron-hole pairs, which are formed at defect sites, i.e. donor/acceptor sites, in the material and at the interface of a heterojunction of two semiconducting polymer layers. Little is known at this time about the mobility of the electrons, holes, and excitons at the interface of heterojunctions. Limited exciton diffusion lengths in the materials and the interfacial nature of the photogeneration process could explain these exotic transport properties in terms of topological constraints. 

Liquid phase processable materials are important in order to fabricate devices using low-cost processes such as spin-coating, casting, or printing. Three methods have been used to fabricate polymer TFT devices from the liquid phase. In the first method, a semiconducting polymer layer is formed directly on the electrodes by electrochemical polymerization, and these electrodes are used subsequently as chain and source electrodes [5, 77-79]. The first polythiophene TFT was fabri-... 

The speed of p- and n-type doping and that of p-n junction formation depend on the ionic conductivity of the solid electrolyte. Because of the generally nonpolar characteristics of luminescent polymers like PPV, and the polar characteristics of solid electrolytes, the two components within the electroactive layer will phase separate. Thus, the speed of the electrochemical doping and the local densities of electrochemically generated p- and n-type carriers will depend on the diffusion of the counterions from the electrolyte into the luminescent semiconducting polymer. As a result, the response time and the characteristic performance of the LEC device will highly depend on the ionic conductivity of the solid electrolyte and the morphology and microstructure of the composite. 

Fig. 15 Simplified schematic representation of the electronic energy levels in a single-layer PLED. CB and VB are the conduction hand and valence hand, respectively, of the semiconducting polymer, which correspond to the ionization potential (IP) and electron affinity (EA) relative to vacuum level (EV). The work functions for anode (and cathode (

Water is known to cause electrical instability in a variety of electronic devices. For example, if water is absorbed on the surface of an unencapsulated amorphous silicon TFTs it induces a charge in the semiconducting film that acts as an additional channel layer for conduction, the back-channel effect [56]. The situation for semiconducting polymers is more complex, because water can be absorbed by the bulk of the film and interact directly with the accumulated carrier states. 

---