Assessment of the Factors Influencing Charge-Carrier Mobility Measurements in Organic Field-Effect Transistors

Evaluation of the charge mobilities is a critical link in the development of organic semiconductors and the associated devices. One of the most widely used measurements relies on an organic field-effect transistor (OFET) configuration. It is based on a relationship between charge-carrier mobility and current–voltage curves, which happens to often exclude factors that are extrinsic to the organic semiconductor but can in fact significantly affect the measured current. The consequence is that the actual meaning of the mobilities evaluated in this way remains ambiguous. As such, it is unclear how well OFET data represent the actual charge-transport properties of the organic semiconductors. Here, by employing a molecular-scale OFET device model that directly connects microscopic charge transfer to macroscopic current characteristics, the mobilities in an OFET configuration and their relationships to the bulk mobilities are investigated. Specifically, the role of disorder, nature of the semiconductor–dielectric interface, and presence of non-Ohmic contacts are addressed. By explicitly including these factors that are frequently impeding a clear evaluation of the experimental data, the study represents a major step into a robust molecular-scale description of OFET operation.