Water gated thin film-transistors : ion sensing and modes of operation

This thesis investigates the ion sensing properties, and modes of operation, of water gated thin film transistors (WGTFTs). For ion sensing, suitable sensitisers (‘ionophores’) are integrated into the (WGTFT) architecture that lead to a membrane or interface potential in response to changing concentrations of the ionophore’s ‘primary’ ion, leading to a shift in transistor threshold voltage when a WGTFT is gated using a medium containing such ions. Quantitatively potential and threshold shift follow a modification of the Nernst equation. Hence measurement of TFT characteristics transduces ion concentration in the aqueous gate medium into a concentration-dependent WGTFT threshold shift ΔVT(c), similar as in the traditional ISFET (ion-selective field effect transistor). Here I report investigations on two aspects of ion-selective WGTFTs: (i) Previous studies on WGTFT ion sensors followed the design of ISFETs and conventional potentiometric ion sensors in so far that they introduced the ionophore within a permeable membrane that is distinct from the transducer. We have demonstrated a simplified ion- selective WGTFT architecture that requires no separate ion- selective membrane. Instead, we added a calcium-selective calix[4]arene ionophore directly to spin casting solutions of the organic semiconductor rrP3HT. We find that resulting WGTFTs cast from ionophore-doped rrP3HT solutions selectively respond to calcium cations dissolved in the gating water with similar characteristics as previous ion- selective WGTFTs. The ionophore-doped rrP3HT simultaneously acts as a semiconductor, and ion- sensitive layer, without the need for a separate ion- selective membrane. (ii) We have systematically compared the use of hole- vs electron transporting semiconductors in ion-selective WGTFTs. When using the same ion- selective membrane and otherwise identical device architecture to gate either a p-type (solution processed rrP3HT) or an n-type (spray pyrolysed ZnO) semiconductor, we find a systematic difference in WGTFT response characteristics: The hole transporter leads to super- Nernstian response, while the electron transporter shows sub- Nernstian response. We explain this by a capacitive amplification (or attenuation) mechanism based on the ratio of cationic to anionic electric double layer capacitances. A further study reported in this thesis is on the mode-of-operation in WGTFTs. When the common organic semiconductor ‘PBTTT’ is used in water- gated thin film transistors, it has so far been found to operate only in field effect (interfacial gating) mode. Electrochemical transistor (volumetric gating) mode, which is actuated by the penetration of waterborne ions into the bulk of the semiconducting film, enables significantly higher transistor currents. However, this has until now been prevented by the hydrophobicity of PBTTT, and could only be observed for derivatives of PBTTT with hydrophilic side chains. We report here for the first time the operation of PBTTT water-gated transistors in electrochemical mode, despite PBTTT’s hydrophobicity. This is enabled by a specific choice of the waterborne ion, which apart from its ionic character somewhat resemble typical PBTTT solvents and therefore can penetrate PBTTT bulk.