Comparing electron- and hole transporting semiconductors in ion sensitive water- gated transistors

We present a systematic study comparing different solution- processed semiconductors in cation- sensitive water- gated thin film transistors (WGTFTs): A hole transporting semiconducting polymer (rrP3HT), and an electron- transporting precursor- route metal oxide (ZnO). To allow comparison, we used the same ionophore to sensitise the gate contact for both semiconductors. We find both organic hole transporter, and inorganic electron transporter, have their relative merits, and drawbacks, in ion- sensitive WGTFTs. Hole transporting rrP3HT WGTFTs show low hysteresis under water- gating and give super- Nernstian sensitivity. However, rrP3HT responds to ionic strength in water even when WGTFTs are not sensitised, compromising selectivity. Electron transporting ZnO WGTFTs show higher mobility, but also stronger hysteresis, and sub- Nernstian response. However, ZnO WGTFTs show little response to ionic strength when not sensitised. We rationalise the super- versus- sub- Nernstian sensitivities via a capacitative amplification/attenuation effect. Our study suggests that the optimum semiconductor material for ion- selective WGTFTs would be a precursor- route inorganic hole transporting semiconductor.