A new precursor route to semiconducting Zinc Oxide

We demonstrate a new precursor route towards solution- processed films of the II-VI semiconductor Zinc oxide (ZnO). Spray pyrolysis of aqueous solutions of the Zinc salt Zinc chloride (ZnCl2) onto a substrate heated to at least 250 oC gives films that are insoluble in water, display an absorption edge at 365 nm, and when electrically gated display thin film transistor characteristics similar to ZnO films derived via the established Zinc Acetate (ZnAc) precursor route; we therefore identify it as ZnO. Control experiments attempting spray pyrolysis of aqueous Zinc sulfate solutions, and delayed pyrolysis of cold- sprayed and dried ZnCl2 films, do not lead to semiconducting films. Formation of ZnO from an aqueous Zinc salt requires the simultaneous presence of Zinc ions, Chloride ions, and water, at the time of pyrolysis. We therefore suggest the actual ZnO precursor is the ZnClxH2O(4−x) species that forms when ZnCl2 dissolves in water [The Journal of Chemical Physics 39, 3436 (1963)]. The reported process is easy to upscale for large area ZnO coatings, e.g. on window panes for thermal control, as no organic solvent vapours are released

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