Polymer-induced surface wrinkling and imine polymer-based doping of sol–gel zinc oxide in electrolyte-gated transistors

We report that thin-film morphology of sol–gel zinc oxide (ZnO) and their n-doping characteristics can be controlled using polymers, enabling high-performance n-type electrolyte-gated transistors (EGTs). The wrinkled surface of ZnO films was induced by dissolving an insulating polymer, for example, poly(4-vinyl phenol) (PVPh) and poly(2-hydroxyethyl methacrylate) (PHEMA), into the ZnO precursor solutions, followed by drying at 210 °C. The roughness peaked when the polymer composition was 2.5 wt%. The wavelength (λ) of the wrinkling structure was varied depending on the added polymer (0.49 μm for PVPh and 0.74 μm for PHEMA). For n-doping of the ZnO films, polyethylenimine (PEI) was deposited on the composite films, followed by high-temperature annealing at 500 °C. The constituent polymers (PVPh/PHEMA and PEI) were found decomposed after the heat treatment. The resulting n-doped ZnO films showed excellent electrical characteristics when used as a channel layer in EGTs based on a solid-state ion-gel. The device has a high electron mobility of 63.7 cm² V⁻¹ s⁻¹ when ZnO channel was made with 1.0% of PVPh in the precursor.

Thin films of sol–gel precursors of ZnO mixed with an insulating polymer form wrinkled surface during drying and become more susceptible to n-doping from a nitrogen-rich polymer by thermal annealing, enabling the mobility enhancement of ZnO in electrolyte-gated transistors.