Innate cation sensitivity in a semiconducting polymer

Water-gated organic thin film transistors (OTFTs) using the hole transporting semiconducting polymer, poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), show an innate response of their threshold voltage to the addition of divalent metal cations to the gating water, without deliberately introducing an ion-sensitive component. A similar threshold response is shown for several divalent cations, but is absent for monovalent cations. Response is absent for transistors using the inorganic semiconductor ZnO, or the similar organic semiconductor poly(3-hexylthiophene) (rrP3HT), instead of PBTTT. We assign innate cation sensitivity to residues of the organometallic Pd(0) complex used as catalyst in PBTTT synthesis which bears strong resemblance to typical metal chelating agents. Organometallic Pd(0) residues are absent from ZnO, and also from rrP3HT which is polymerised with a different type of catalyst. However, when Pd(0) complex is deliberately added to rrP3HT casting solutions, resulting OTFTs also display threshold response to a divalent cation.