Light-emitting devices based on doped polymer thin films

Organic light-emitting devices (OLEDs) made of single-layer doped polymer thin films have been fabricated and studied. In the hole-transport and matrix polymer poly(N-vinylcarbazole) (PVK), different electron-transport and emissive agents are dispersed or blended to make polymer-based thin films capable of bipolar transport and light emission of various colors. Both the photoluminescence and electroluminescence properties are extensively studied. In photoluminescence, efficient transfer of energy can occur from the host to very dilute ({dollar}{bsol}sim{dollar}1 wt.%) amounts of emitting materials. Device characteristics are correlated with material properties to understand the operating mechanisms and to optimize the devices. In electroluminescence, excitons appear to be formed at doped emitting centers, rather than in the transport materials. The device performance is found to be a strong function of the composition of the blend thin films, depending on the luminescence efficiency of emitting centers in the host polymer and the relative hole and electron injection/transport abilities.; Various materials have been used as the anode or cathode contacts to the single-layer doped polymer devices. The device characteristics are very sensitive to the surface properties of the indium tin oxide (ITO) anode contact. OLEDs built on the cleaned as-grown ITO contacts are hole-limited because the ITO is less efficient for hole injection than low-work-function metal cathodes for electron injection. We have demonstrated that without degrading the bulk properties of the ITO, the chemical composition of ITO surface layers could be substantially modified by treatment in plasmas of different gases. With an oxygen plasma treatment, the device performance is greatly improved due to enhanced hole injection, while a hydrogen plasma treatment degrades devices. Ultra-violet photoemission spectroscopy (UPS) measurements indicate that oxygen plasma treatment increases the work function while hydrogen plasma treatment reduces the work function of the ITO surface.; Direct integration of OLEDs of different colors has proven difficult because of the weak resistance of organic materials to chemicals used (solvents, acid and water etc.) for patterning and processing the structures and materials. By sealing the organics from the harmful chemicals with carefully designed device structures and by using dry-etching techniques, a method has been developed to sequentially fabricate and therefore integrate devices of different colors onto a single substrate, all with performance similar to discrete OLEDs made on separate substrates. Taking advantage of the thin-film structures and the substrate versatility of both OLEDs and a-Si TFTs, we have also demonstrated the integration of both devices on a rugged, flexible and lightweight steel foil substrate. The TFT in the integration structure successfully provides enough current to drive the OLED.