Field effect measurements in doped conjugated polymer films: Assessment of charge carrier mobilities

Conductivity and field-effect mobility measurements using metal-insulator-semiconductor field-effect-transistor devices and acceptor density measurements using metal-insulator- semiconductor (MIS) diodes are presented. The measurements were made on thin polymer films of the organic semiconductor, poly(β′-dodecyloxy-α,α′,-α′,α″terthienyl), which were doped to different conductivities using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as an oxidizing agent. It is found that both the field-effect mobility and the conductivity of these films increases superlinearly upon doping while the transistor amplification, the on/off ratio, decreases. Acceptor densities as obtained from MIS diode measurements are in close agreement with the bulk charge density as calculated from the DDQ content. However, the product of this bulk charge density, field-effect mobility, and the elementary charge e is a factor of 100 larger than the polymer conductivity. This indicates that the average mobility for charge carriers in the bulk is considerably lower than the field-effect mobility. It is considered that the bulk charge carriers are trapped by the Coulomb interaction with their parent charge compensating counter-ions, whereas charges in accumulation have no associated counter-ions and, therefore, are more mobile.