Field-effect doping phenomenon in an organic semiconductor layer deposited by the Langmuir–Blodgett technique

The gate-voltage-dependence of the drain current–voltage characteristics have been measured for organic field-effect transistors (OFETs) based on thick and thin Langmuir–Blodgett (LB) films of octadecyl tetracyano quino dimethane. The obtained results have indicated that the field-effect doping phenomena in OFETs with thick and thin LB films are basically the same with each other. A theoretical approach based on a simple four-layer model has also be taken to obtain basic understanding of the field-effect doping phenomenon in OFETs. Through this approach, it has been suggested that when the organic semiconductor layer is extremely thin, the total amount of the doped charge, Q, deviates from the prediction of the well-known threshold relationship, Q ∝ (VG − Vth), where VG and Vth are the gate voltage and its threshold value, respectively, but the charges are still localized in the interface region like in the case of OFETs with thick semiconductor layers.