(603c) Understanding Charge Transport Behavior In Ionic Liquid Gated Semiconducting Polymers

For the past decades, the prospect of cheap, solution processable, and flexible electronic devices and circuits has driven organic electronics research.  Recently, low voltage flexible printed polymer transistor based circuits have been realized by utilizing ionic liquids in place of traditional gate dielectric materials.  Ionic liquid gating of polymer semiconductors functions via bulk electrochemical doping as opposed to the strictly interfacial field effect, which allows for low voltage high channel current transistors.  While promising, in order for this technology to reach its full potential, a fundamental study of the charge transport in ionic liquid gated polymer semiconductors is necessary.  Working with the bench mark polymer poly(3-hexylthiophene) (P3HT) gated with the ionic liquid, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (EMIFAP), an accurate electrochemical density of states was determined by cyclic voltammetry, which when coupled with in situ conductivity measurements allowed an accurate determination of charge carrier mobility.  Charge carrier mobility was found to be heavily dependent on charge carrier density, increasing to ~0.8 cm² V^-1 s^-1 with increasing charge density before falling back to zero, resulting in a finite potential window of electrical conductivity.  Variable temperature measurements were carried out over the whole window of finite conductivity, revealing a thermally activated charge transport mechanism, which became very weakly activated in the region of high carrier mobility.  The ionic liquid anion and cation were systematically varied to determine their individual effects on charge transport and chemical stability.  Additionally, poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) were investigated in a similar manner revealing the generality of this charge transport behavior amongst ionic liquid gated polymer semiconductors.