We have developed means for processing conjugated polymers to allow fabrication of a variety of semiconductor device structures. These polymers include polyacetylene prepared via the Durham precursor route and various poly(arylene vinylenes) prepared via sulphonium precursor polymers. We have shown that these devices exhibit novel electro-optical properties which depend for their operation on the non-linear character of the electronic excitations of the polymer chain. We discuss here the use of poly(phenylene vinylene) and derivatives as the active layers in light-emitting diodes.