Molecular orientation anisotropy and hole transport properties of diluted semiconducting films of poly(p-phenylenevinylene) derivative

The electrical and optical properties of certain semiconducting polymers are improved in blend films with inert host materials. The improvements have been attributed to the dilution effect of electron traps and interchain species in semiconductor materials. In this paper, we report on anisotropy in the blend films of poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and its strong dependence on an inert host material, such as polystyrene (PS) or polyvinylcarbazole (PVK). We found that the orientation of the MEH-PPV backbone in the blend film changed from horizontal to random on dilution with PS. Moreover, the in-plane hole conductivity was enhanced in the MEH-PPV:PVK blend film, although the out-of-plane conductivity reduced owing to excessive dilution. Mott–Schottky analysis of the capacitance–voltage characteristics revealed opposite trends for dilution with PS and PVK; i.e., PS increased the accumulated charge density in the bulk of the blend film, whereas PVK reduced it. These results demonstrate that isotropy was induced in the PS blend films, whereas anisotropy was induced in the PVK blend films. Anisotropy is an important factor for understanding the hole transport characteristics in the diluted films.