Controlling the Doping Mechanism in Poly(3-hexylthiophene) Thin-Film Transistors with Polymeric Ionic Liquid Dielectrics

Abstract

Two disparate modes of operation can occur in an electrolyte-gated transistors (EGTs) depending on the permeability of the semiconductor to ions that are opposite in charge to the induced carriers. High permeability to ions promotes volumetric doping whereas a low permeability promotes electric double layer (EDL) formation and field-effect doping at the semiconductor/dielectric interface. Here, we present a generalized method to control the mode of charge accumulation in an EGT with a constant semiconducting layer by gating with anhydrous polymeric ionic liquids (PILs) of opposite polarity. The polarity of the acrylate-based PILs was controlled by tethering ionic units of positive or negative charge to the backbone. In situ optical spectroscopy of EGTs with poly­(3-hexylthiophene), P3HT, reveals that selectively tethering either the anion or the cation dictates whether ions infiltrate P3HT. Gating with both dielectric materials results in similar trends in the dependence of the charge carrier mobility on carrier concentration, despite the difference in doping mechanisms. The EDL (interfacial) doping of the anion-tethered PIL leads to higher mobilities at low carrier concentrations in P3HT with lower onset voltages. Optical measurements during gating show that the anion-tethered PIL gate results in a more even distribution of carriers between ordered and less ordered domains, promoting the formation of a percolated network in the film.

Ion Pair Uptake in Ion Gel Devices Based on Organic Mixed Ionic–Electronic Conductors

“…The ion gel solutions were stirred at 50 °C for 1 h until dissolved. The PIL was synthesized as previously reported, [19] and stored in a N 2 -glovebox. PIL solutions were made by dissolving the PIL in acetonitrile.…”

“…We control ion uptake through the use of a PIL, where the imidazolium-based cation of the ionic liquid is chemically tethered to a polymer backbone, rendering the cation immobile while the counterion (TFSI − in this case) is mobile. [19,25] The resulting electrolyte inhibits ion pair uptake in the OMIEC film since the polyelectrolyte cannot enter the semiconducting polymer film. In contrast, the ion gel, which consists of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMIM:TFSI) mixed with PVDF-HFP, contains both mobile anions and cations (Figure 1a), and these electrostatically neutral ion pairs can permeate the OMIEC channel.…”

“…When operated as conventional OECTs, devices with the PIL electrolyte show considerable hysteresis compared to ion gel devices, despite the significantly slower scan rates used to compensate for the lower ionic conductivity of the PIL [19,1,58] (Figure 4a and Figure S6 and Table S2, Supporting Information). The presence of hysteresis indicates slow device speed as ion motion cannot keep up with the gate voltage sweep.…”

“…[17,18] Pairing polyelectrolytes and semiconductors of opposite polarity (e.g., a p-type channel with a polyanion dielectric) limits charging to the EDL formed at the channeldielectric interface, enhancing device speed since mobile ions need not permeate the bulk of the organic semiconductor, albeit at the expense of current density. [19] However, recent reports showing 20 ns switching in ion-gel-based neuromorphic devices, [6] as well as the high speed of internal ion-gated transistors [12,18] stand out as notable exceptions and suggest an incomplete understanding of the factors governing solidstate OMIEC device speed. Current consensus suggests that in p-type OMIECS positively charged holes on the conjugated chains are compensated by injection of negatively charged anions.…”

Ion Exchange Gels Allow Organic Electrochemical Transistor Operation with Hydrophobic Polymers in Aqueous Solution

“…A number of promising strategies have been pursued to enable ion insertion into the full volume of conjugated polymer active layers for improved OECT performance. Using non-aqueous gate dielectrics, such as ionic liquid-based ion gels [34][35][36] and polymeric ionic liquids, [37][38][39] has enabled volumetric electrochemical doping and OECT operation, yet hydrophobic polymers are generally incompatible with aqueous gate dielectrics, heretofore precluding their use in bioelectronics or biosensing applications. To enhance ion transport in conjugated polymers, many strategies, including developing new device designs 40 and new conjugated polymers, have been pursued.…”