| Solution-processible conjugated polymers and organic small molecules are on the verge of spawning an industry of cheap electronic and optoelectronic devices. To continue this rapid advance in product development, it is important to understand the fundamental structure/property relationships and the processing issues that govern the performance of this class of materials.; Two aspects of the structure/property relationship are studied: namely, the importance of side chain structure and the polydispersity in conjugation length. By varying the chemical structures of the alkoxy side-chains and their substitution pattern, we are looking into the effects of the side-chains on the properties of oxadiazole-contaning poly(p-phenylenevinylene) (PPVs) model compounds, monomers and polymers. Polymers with asymmetric substitution show very different external electroluminescence (EL) efficiencies (about one order lower) from the symmetrically substituted polymers. Taken together, this study provides insight to the balance between photoluminescence (PL) efficiency and carrier mobility in these polymers.; The conjugation of π-conjugated polymers is not infinite, but rather is interrupted by defects. We obtained polymers with different conjugation lengths (band gaps) through fractionation. It was found that incorporation of a small amount of the low band gap material into the high band gap matrix significantly degraded PLED device performance. Combined with the previous results from PPV monomer studies, these findings are significant in that they point to the importance of controlling the polydispersity in conjugation length for obtaining efficient PLEDs.; One important materials-processing issue is also addressed in this dissertation: the solvent effects on chain conformation and chain interactions. Oxadiazole-containing PPV (C8L) exhibits very different chain morphologies in solvents such as THF, 1,1,2,2-tetrachloroethane (TCE) and the mixture of the two. It is found that the polymer chains are more extended in TCE, which dissolves the side alkoxy chains better. The addition of THF into TCE disrupts the chain packing and enhances PL in film. The use of a mixed solvent system is seen to result in better device performance, probably due to a chain morphology, which better balances charge transport and EL. This study shows the possibility of optimizing device performance by engineering chain morphologies through solvent selection. |
