Investigating Traps in Organic Field-Effect Transistor through Field Dependent Mobility

Organic semiconductors can be processed in ambient conditions, a low-cost manufacturing process compatible with substrates such as paper, plastic, or fabric. Unfortunately, the electrical properties displayed by thin-film devices based on these materials do not match the performance requirements of consumer applications. While several compounds exhibit excellent performance in single crystal form, efficient charge carrier transport is mitigated in thin film devices by the presence of electronic traps. Here we report on the study of traps in organic semiconductors by performing electric field-dependent mobility measurements in organic field-effect transistors (OFETs). We were able to tune the mobility five orders of magnitude, from 0.001 to 10 cm² V^-1 s^-1 through varying device geometry and the choice of dielectric layers. We correlated the mobility value with its field dependence, quantified using the Poole-Frenkel model, and found that the field dependence is reduced with increasing mobility and that mobility is independent of the applied field when the trap-free regime is reached. We show that the devices with highest mobility and field-independent operation display band-like transport, while devices with low mobility display activated transport and strong field dependence.