Influence of alkyl chain length on charge transport in symmetrically substituted poly(2,5-dialkoxy-p-phenylenevinylene) polymers

We report on the hole transport characteristics, as measured by time of flight, of a family of symmetrically substituted dialkoxy poly(p-phenylenevinylene) polymers with different side-chain length. As side-chain length is decreased, the magnitude of the hole mobility mu(h) increases while the field dependence of mu(h) becomes more positive and the temperature dependence of mu(h) becomes stronger. For the shortest side-chain derivative studied, mu(h) exceeds 10(-4) cm(2) V(-1) s(-1) at electric fields greater than 10(5) V cm(-1). The trend in magnitude of mu(h) with side-chain length is consistent with the expected increase in electronic wave-function overlap as interchain separation decreases, while the trends in electric-field and temperature dependences of mu(h) are consistent with increasing site energy disorder. We show that the electrostatic contribution to the site energy difference for pairs of oligomers follows the observed trend as a function of interchain separation, although the pairwise contribution is too small to explain the data quantitatively. Nonresonant Raman spectroscopy is used to characterize the microstructure of our films. We construct spatial maps of the Raman ratio I(1280)/I(1581) and confirm an expected decrease in average film density with side-chain extension. The structural heterogeneity in the maps is analyzed but no clear correlation is observed with transport properties, suggesting that the structural variations relevant for charge transport occur on a length scale finer than the resolution of similar to 1 mu m.