Electrochemical gating of a hydrophobic organic semiconductor with aqueous media

We show that even the highly hydrophobic semiconducting polymer poly [2,5-bis(3-tetradecylthiophen-2-yl) thieno[3,2-b]thiophene] (PBTTT) can operate in organic electrochemical transistor (OECT) rather than field effect (OFET) mode when gated with aqueous media. The required bulk penetration of anions into the semiconducting film within the electrochemical window of water is here enabled by the choice of anion, namely, picric acid (PA). OECT mode operation in PBTTT films or its analogues had previously been seen only when gated with solid electrolytes with a larger electrochemical window, or for PBTTT analogues with hydrophilic sidechains. We assign the ability of PA anions to penetrate bulk PBTTT from their similarity to PBTTT solvents (chlorinated benzenes), in the sense that they both display an electron-deficient π electron system. In control experiments, we confirm that OECT mode is not observed when gating PBTTT with another organic acid (acetic acid) that does not display such π electron system, and that the gating mechanism indeed is electrochemical doping rather than a charge transfer mechanism. OECT mode is observed only at rather high PA concentrations in an aqueous gate medium (50 mM or more), but when it is, it leads to conductivities of ≈80 S/cm, slightly larger than for a PBTTT analogue with hydrophilic side chains gated with chloride anions.