Three fluorene-based copolymers carrying reactive side groups (oxetanes) were synthesised and characterised. Their optical properties are essentially controlled by the introduction of either fluorene or lower energy gap (benzothiadiazole or terthiophene) comonomers, randomly distributed along the polymer chain. We show that, in the presence of a small amount of a photoacid dispersed in the polymer films, these are converted into insoluble polymeric networks upon UV irradiation and heating. Absorption and emission spectra of the polymeric networks are similar to those of the starting polymers, showing that the cross-linking, due to the polymerisation of the oxetane groups, does not interfere. This ability to induce their insolubility upon UV irradiation is used to fabricate multi-layer light-emitting diodes. Using a transmission electron microscope grid as a shadow mask we show that micrometer-size patterns can be created.
Efficient blue Polymer Light-Emitting Diodes (PLEDs) were fabricated by evaporating thin LiF layers between Al or Ca cathodes. Electroabsorption measurements of the built-in potential across the diodes show that devices fabricated with LiF/Ca/Al cathodes exhibit the smallest average barrier height and operating voltage (compared to both Ca and LiF/Al currently amongst the most efficient electron injectors). The turn-on bias is essentially equivalent to the built-in potential (~2.7 V), indicating an effective minimisation of the barrier to electron injection. Results are also compared with devices incorporating CsF layers and are correlated with the electroluminescent characteristics of the LEDs. A very strong dependence (~ exponential) between the built-in potential and the current and luminance at a fixed electric field (0.5MV/cm) is observed and is explained with the reduction of the cathodic barrier height brought about by the different cathode multilayers.
Further routes have been developed for the synthesis of a 1,4-bishalomethylbenzene derivatives for Gilch dehydrohalogenation polycondensation. Poly(2,3-dibutoxy- 1,4-phenylenevinylene) is a protoypical conjugated polymer which is thought to derive its high PL solid state fluorescence efficiency from the sterically twisted backbone and devices carrying this polymer have been further evaluated. Distyrylbenzene derivative carrying the structural feature of a 2,3-dibutoxy substitution pattern on the central ring have been prepared. One in particular has been copolymerized with a 9,9-dialkyl-fluorene-2,7- diboronate ester. The resulting conjugated polymer shows a good green emission maximum in an electroluminescent device.
The synthesis of poly(1,4-phenylene vinylene)s (PPVs) containing a 2,3-dialkoxy substitution pattern has been developed. Poly[2,3-bis(2-ethylhexyloxy)-1,4-phenylene vinylene] (BEH-PPV) 4 was prepared by Gilch polycondensation, and its optical properties were compared with the recently discussed poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) 1. The precursors for the Gilch method have traditionally been prepared by methods which have certain disadvantages. These can be overcome by the use of directed metallation reactions which are illustrated in the synthesis of some poly(2,5-disilyl-substituted 1,4-arylene vinylene) derivatives.
Highly luminescent poly(arylene vinylene)s can be prepared by a range of polycondensation methods. In this paper we report the synthesis of useful monomers and their application in the Gilch dehydrohalogenation and Wittig polycondensation methods to prepare highly luminescent poly(1,4-phenylene vinylene) (PPV) homo- and copolymers for use in light emitting devices.
The high luminescence efficiencies and significant blue shift of the 2,3-disubstituted poly(1,4-phenylene vinylene) polymer 4 have prompted further investigation, and in this paper the synthesis and characteristics of the homopolymer 9 and copolymers 10 and 12 are described. Semiempirical calculations and single x-ray crystallography offer further insight into the explanation of the properties of this class of polymers. A promising organic semiconductor 15 has been prepared and used as the active layer in a thin film transistor. This material exhibited excellent device characteristics, including a field effect mobility of 0.02- 0.05 cm2 V-1s-1 and a high On/Off ratio.
We report an experimental and theoretical study of the effects of interference in polymeric light-emitting diodes (LEDs). These effects are due to the complex optical structures of the devices, which include many layers of materials with different refractive indices, and are of considerable importance since they affect spectral distribution and intensity of the absorption and emission in a significant way. By way of comparison, they can also provide a flexible, non-invasive optical probe of the electroluminescent processes. In this paper we analyze single-layer diodes with indium-tin oxide (ITO) and Al electrodes, where poly (p-phenylene vinylene) (PPV) is the luminescent polymer. We find that photo-induced excitation of the radiative species produce different spectral shapes depending on the excitation energy which we can describe in terms of interference phenomena. The theoretical analysis is conducted by means of multilayer stack theory and transfer matrix calculations, and takes into account additional quenching effects due to In contaminations from the ITO electrode. The theoretical results are in good agreement with the experiment.
We report the fabrication of efficient green light-emitting diodes using a side-chain polymer based on a high-electron affinity (EA) naphtalimide moiety (PNI). The chromophore is attached to a polymethacrylate backbone through a spacer, and emits in the green with high efficiency. In single-layer light-emitting diodes (LEDs), we find that the electroluminescence (EL) efficiency is not limited by Al cathodes as for poly(p-phenylene-vinylene), PPV, and we attribute this to the increased EA. We report maximum internal efficiencies of about 1.7 percent for Ca and 0.9 percent for Al in double-layer devices where PPV serves as both Hole-injector and emitter. Compared to some oxadiazole based electron injection/transport layer, PNI gives higher efficiencies at high currents, and longer lifetimes. Tuning of emission in the red is possible by dye-doping the PNI and causing the emission to happen in this layer. We discuss the properties of the different device configurations with a view to the electronic structure of the materials and in particular to the influence of the thickness of the individual layers on efficiency and driving conditions.