A New Precursor Route to Semiconducting Zinc Oxide

We demonstrate a new precursor route toward 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 degrees C 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 that 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 vapors are released.