Manipulating Doping of Organic Semiconductors by Reactive Oxygen for Field-Effect Transistors

Thin film organic field-effect transistors (OFETs) are usually featured with charge traps, limiting charge transport and leading to low mobility especially at low gate voltages. In this work, doping process of a model organic semiconductor 2,7-didodecyl[1]benzothieno[3,2-b][1]benzothiophene (C12-BTBT) has been manipulated, using low-cost reactive oxygen namely oxygen plasma or ozone. It is found that low-pressure (20 Pa) oxygen plasma can induce a low-moderate doping concentration, which is, however, sufficient to warrant field-effect mobilities over 5 cm² V⁻¹ s⁻¹ in a large gate voltage range and sharp subthreshold swings, without degradation of on/off ratio. Low-pressure oxygen plasma can also positively shift the threshold voltage of the device, thus largely reducing the working voltages. Oxygen plasma with higher pressure than 100 Pa induces higher doping concentration, more significantly shifting the threshold voltage and deteriorating on/off ratio due to substantial bulk electrical conductivity, which is similar to the treatment by UV-ozone at atmospheric pressure. As compared with the well-studied organic dopant F4-TCNQ, the doping process of reactive oxygen can be more easily in situ manipulated to reach an appropriate range of doping concentration, warranting higher performance and larger tunability for OFETs.