Transport properties of highly aligned polymer light-emitting diodes

We investigate hole transport in polymer light-emitting diodes in which the emissive layer is made of liquid-crystalline polymer chains aligned perpendicular to the direction of transport. Calculations of the current as a function of time via a random-walk model show excellent qualitative agreement with experiments conducted on electroluminescent polyfluorene demonstrating nondispersive hole transport. The current exhibits a constant plateau as the charge carriers move with a time-independent drift velocity, followed by a long tail when they reach the collecting electrode. Variation of the parameters within the model allows the investigation of the transition from nondispersive to dispersive transport in highly aligned polymers. It turns out that large interchain hopping is required for nondispersive hole transport and that structural disorder obstructs the propagation of holes through the polymer film.