Electroluminescence in Ion-Gel Gated Conjugated Polymer Field-Effect Transistors

We report electroluminescence from ion-gel gated, field-effect transistors based on the conjugated polymer, poly(9,9′-dioctylfluorene-co-benzothiadiazole) gated by an 1-ethyl-3-methylimidazoliumbis (trifluoromethylsulfonyl) imide/poly (styrene-block-ethylene oxide-block-styrene) ion gel, and investigate the mechanism for light emission. The devices emit light from near the electron-injecting drain electrode when the drain source voltage exceeds approximately the energy gap of the polymer ( V ds > E g /e ). Charge accumulation spectroscopy is used to demonstrate the significant penetration of the negative TFSI – ions into the F8BT assisted by the application of negative gate voltages, where they lead to significant p-type doping of the bulk of the F8BT film. In contrast, no evidence for diffusion of positive ions with positive gate voltages is observed, and this is consistent with the location of the recombination zone in the proximity of electron injecting electrode and the absence of a comparable electron current at positive gate voltages. We conclude that in the light-emission regime the devices operate more akin to a hole-current dominated light-emitting electrochemical cell than a transistor.