Humidity-dependent characteristics of thin film poly(3,4-ethylenedioxythiophene) field-effect transistor

π-Conjugated poly(3,4-ethylenedioxythiophene) (PEDOT) based field-effect transistors (FETs) were fabricated in this study. A thin PEDOT layer (thickness ≈500 nm) with the desired pattern was formed as an active and a gate electrode by a vapor polymerization of 3,4-ethylenedioxythiophene (EDOT) on the photolithographically patterned ferric p-toluenesulfonate (FTS) oxidant layer. Crosslinked poly(vinyl cinnamate) (PVCN) insulating layer was formed by spin-coating and UV crosslinking. The currents (Ids) of the PEDOT active channel decreased with increasing gate bias (Vg), implying p-type FET. The dc conductivities (σdc) and Ids of the PEDOT active channel were measured as a function of Vg under various relative humidities (RHs) ranging from 0% to 55%. The σdc and the Ids of the PEDOT channel rapidly decreased with increasing positive Vg under humid conditions, while those of the PEDOT channel showed no change with Vg in vacuum, i.e. 0% RH. The threshold gate bias, which is defined as the onset of the curve of Ids versus Vg, rapidly decreased with increase of RH. The moisture absorbed in the PEDOT active channel may assist the dedoping of the system by the screening and/or the relative separation of constituent ions from PEDOT chain upon applying a positive Vg. When the positive Vg was removed (Vg = 0), the σdc of the channel recovered slowly and a relatively long relaxation time was observed, which implies that the PEDOT active layer was slowly redoped. For the PEDOT based FET, we observed the moisture assisted dedoping and the redoping processes of the PEDOT channel upon applying and removal of positive Vg under the humid conditions.