Diodes Made from Soluble Semiconducting Polymers: Electrical Transport and Electroluminescence.

The combination of electronic, optical, and mechanical properties available in soluble semiconducting polymers makes possible a wide array of applications. Semiconductor devices made from conjugated polymers include the building blocks of the electronics industry: diodes and transistors. Moreover, clever synthetic chemistry allows design of semiconducting polymers with not only a wide range of electronic properties but also a versatile range of mechanical abilities. The mechanical world of polymers permits the flexibility of plastic wrap rather than the fragile nature of inorganic semiconductor crystals. Similarly, the processing techniques used on polymers readily lend themselves to economies of scale. This dissertation demonstrates that soluble semiconducting polymers can form highly rectifying diodes that emit light with a variety of colors and high efficiency. Poly(alkyl-thiophenes) allowed the first preparation of metal-polymer diodes from soluble semiconducting polymers. In comparison to techniques based on non-soluble polymers, solution processing offers greater flexibility in device assembly and, simultaneously, produces devices with superior rectification characteristics. The temperature dependence of current vs. voltage characteristics reveals behavior more consistent with tunneling theory than the thermally activated mechanisms expected in metal-semiconductor junctions. The diodes emit light under suitable bias conditions, but the light is quite dim. Diodes made with poly(2-methoxy,5-(2^ ' ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) demonstrated the first emission of visible light emission from diodes made with soluble semiconducting polymers. The results show that light emitting diodes can be fabricated by casting the polymer film from solution with no subsequent processing or heat treatment required. Use of low work -function electrodes leads to diodes with rectification ratios greater than 10^6 and quantum efficiencies of 0.01 photons per electron. Again, the electrical characteristics indicate that tunneling dominates charge transport and provides the means for light emission.