HOLE DIFFUSION IN TRIARYLAMINE POLYMER-FILMS IN A CONTACTING ELECTROLYTE - INITIAL COMPARISON WITH HOLE MOBILITIES

The first comparison of hole diffusion and mobility in a discrete hopping charge-transport polymer is presented. Charge transport (hole diffusion) in a triarylamine-containing polymer in contact with a liquid electrolyte is measured electrochemically by steady-state voltammetry and characterized by a hole diffusion coefficient D(h). D(h) rises with time spent in the oxidized state and is apparently due to electroanodic cross-linking of the polymer. That counterion transport is not rate limiting during the measurement of D(h) in the un-cross-linked polymer is rigorously demonstrated. Comparison of activation data suggests that hole hopping in the un-cross-linked and cross-linked polymers proceeds by different mechanistic pathways. Time of flight (TOF) drift mobilities-mu are measured in the un-cross-linked polymer and compared with D(h) via the Einstein equation over a range of temperatures. Predicted values of zero-field mobilities from D(h) and the Einstein equation agree qualitatively with experimental mobilities. The predicted mobility activation energy, however, is somewhat low relative to the experimental results. This is attributed to the differences in the physical state of the polymer in the two measurements. D(h) measurements are done in a liquid electrolyte while mobility measurements are done in the solid state. This leads to differences in the coupling of the electron-exchange step with microscopic polymer motions in the two techniques.