EFFECTS OF ORDER AND DISORDER ON FIELD-EFFECT MOBILITIES MEASURED ON CONJUGATED POLYMER THIN-FILM TRANSISTORS

The morphology of conjugated polymer thin films has been controlled by exploiting the processes giving rise to solvatochromism in the initial polymer solutions from which the polymer films were deposited by spin coating. The material used was the substituted polythiophene, 3[2(S2-methylbutoxy)ethyl]-polythiophene. Starting with polymer dissolved in ''good'' solvent, various quantities of ''bad'' solvent when added lead to the reorganization of the solubilizing side chains attached to the conjugated polymer backbone. Ordering of these side chains increases with addition of bad solvent decreasing the flexibility of the backbone, yielding more rigid rodlike polymer chains. This in turn causes aggregation and finally precipitation. It is found that this molecular order can be transferred from solution to solid film during the spinning process, ascertained from optical spectroscopy. With these films acting as the active layer in a field-effect transistor structure, the mobility of the carriers injected into the films can be measured. It is found that as the molecular order and aggregation increases, carrier mobility decreases from approximately 10(-5) cm-2 V-1 S-1 to less-than-or-equal-to 10(-8) cm2 V-1 s-1 in films displaying the highest degree of molecular order. This is ascribed to the increased interchain separation in the ordered systems along with effects due to macroscopic aggregate grain boundaries.