Improving Order and Mobility in MEH-PPV Films by Reducing Polydispersity

The effect of polydispersity on morphology and charge transport in drop cast films of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) was investigated using grazing incidence X-ray diffraction and time-of-flight respectively. Morphologically, reducing polydispersity by removing short chain segments promoted the capability of crystallization. This resulted in higher hole mobility and non-dispersive transport down to lower temperatures for the lower polydispersity sample. The slope for the Poole-Frenkel relationship at 298 K was increased, and its change with temperature decreased, indicating reduced spatial inhomogeneity. Analysis using Bassler's Gaussian disorder model (GDM), found that the value for energy disorder ($\sigma \sim $53meV for both films) and infinite temperature zero field mobility ($\mu $o$\sim $3 x10$^{-6}$ cm$^{2}$/Vs) were similar for both films. However, a good fit for hopping site separation and spatial disorder was only possible for the lower polydispersity device, suggesting that the lower polydispersity films have less mesoscopic inhomogeneity.