Single ion conducting, polymerized ionic liquid triblock copolymer films: high capacitance electrolyte gates for n-type transistors

There has been impressive progress in the fabrication and characterization of p-type organic electrolyte-gated transistors (EGTs). Unfortunately, despite the importance of n-type organic transistors for complementary circuits, fewer investigations have focused on developing electrolytes as gate dielectrics for n-type organic semiconductors. Here, we present a novel single ion conductor, a polymerized ionic liquid (PIL) triblock copolymer (PS-PIL-PS) composed of styrene (PS) and 1-[(2-acryloyloxy)ethyl]-3-butylimidazolium bis(trifluoromethylsulfonyl)imide (PIL), that conducts only the TFSI anion. This triblock copolymer acts as a gate dielectric to allow low-voltage n-type organic EGT operation. Impedance characterization of PS-PIL-PS reveals that there are three polarization regions: (1) dipolar relaxation, (2) ion migration, and (3) electric double layer (EDL) formation. These polarization regions are controlled by film thickness, and rapid EDL formation can be obtained in thinner polyelectrolyte films. In particular, a 500 nm-thick polyelectrolyte film exhibits a large capacitance of ∼1 μF/cm(2) at 10 kHz. Employing this single ion conducting PIL triblock copolymer as the gate insulator, we achieved low voltage operation (<1 V supply) of poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)) n-type organic EGTs (electron mobility of ∼0.008 cm(2)/(V·s) and ON/OFF current ratio of ∼2 × 10(3)) by preventing electrochemical doping. Furthermore, the recognition that the performance of n-type organic EGTs is diminished by 3D electrochemical doping suggests that it may be necessary to have a unipolar electrolyte to gate n-type organic semiconductors. Finally, we highlight that the use of PIL block copolymer electrolytes as gate insulators opens unique opportunities to explore the role of ion penetration in n-type organic EGTs by tuning the extent of electrochemical doping.