A study of conjugated polymers and their applications in light-emitting diodes
The initiation of research into conjugated polymer electroluminescent devices in Durham is reported. The apparatus required for the fabrication and characterisation of polymer light-emitting diodes (LEDs) is outlined, and the essential assumptions and calculations required to determine their efficiency, brightness and colour are summarised. Optical and electrical characterisation of a range of polymers is reported, including polypyridine (PPY) and a range derivatives, poly(p-phenylene vinylene) (PPV) and poly(2-methoxy, 5-(2'ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV). The optical properties of PPY in solution, film and dilute solid state are characterised. The conjugation length of PPY is investigated by deliberate disruption of the conjugation by the inclusion meta-links in the otherwise para-linked polymer. The effect of altering the chemical structure of PPY is investigated by the study of some PPY derivatives including random and regular copolymers. The consequences of using precursor polymers, especially the effects of conversion on indium tin oxide coated substrates are investigated using absorption and luminescence spectroscopy and capacitance-voltage analysis. Electroluminescence from a number of conjugated polymers is reported and the efficiency, spectral output and device characteristics are presented. The formation of Schottky barriers in some polymer devices is investigated using quasi-static capacitance-voltage measurements. Improvements in the quantum and power efficiency of polymer LEDs have been achieved using two hole-transporting polymers, poly(vinyl carbazole) and the doped, conducting form of polyaniline. A substantial improvement in quantum efficiency has also been demonstrated when PPY is used as an electron-transporting layer in PPV and MEH-PPV based light-emitting diodes. The variation in quantum efficiency and emission spectrum with the ratio of the thickness of the two polymer layers is reported and analysed.