Large Scale Alignment and Charge Transport Anisotropy of pBTTT Films Oriented by High Temperature Rubbing

A large-scale alignment method is used to orient the conjugated polymer poly(2,5-bis(3-dodecyl-2-yl)thieno[3,2-b]thiophene) (C12-pBTTT). This fast one-step alignment method does not use any alignment layer and does not necessarily require postalignment annealing of the films. It exploits the increased plasticity of the conjugated polymer films for 50 degrees C <= T <= 125 degrees C to obtain high in-plane orientations by mechanical rubbing of the films. As visualized by HR-TEM, the in-plane alignment of C12-pBTTT chains and size of the oriented domains increase with the temperature of the film during rubbing (T-r). The domains have a preferential face-on orientation; i.e., the pi-stacking direction is along the film normal and the chain direction parallel to the rubbing direction. Postrubbing annealing at T < 200 degrees C can further improve in-plane alignment whereas for T >= 200 degrees C, edge-on oriented C12-pBTTT crystals are formed. The anisotropy of hole transport for highly in-plane oriented face-on as well as edge-on oriented films was measured in OFET devices. Depending on the annealing conditions, this anisotropy of hole mobility varies in the range 7-70 with the highest mobilities along the rubbing direction and the highest anisotropies for the oriented face-on films.