Flexible Transparent Planar Heater Comprising ZnO/Cu/Al2O3

In this study, we fabricated transparent heaters composed of an ultrathin Cu-layer heating element sandwiched between a ZnO underlayer and an Al2O3 overlayer. With the Cu layer thickness fixed at 8.5 nm, the thicknesses of the ZnO and Al2O3 layers were independently varied to reach the optimum antireflecting condition (maximum transmittance of 88.3% and average visible light transmittance of 79.8% were achieved). The sheet resistances for the ZnO/Cu/Al2O3 heaters can be varied by simply modulating the Cu layer thicknesses. In order to assess the flexibility of the transparent heaters, we constructed a ZnO/Cu/Al2O3 structure on flexible polyimide substrates, and the thermal, electrical, optical and mechanical characteristics were evaluated. Because of the planar heating element of the Cu layer, the thermal response was found to be extremely high, i.e., less than 10 s were required to reach 90% of the target temperatures. Once the target temperatures were reached, the heater temperatures were highly stable with no degradation of electrical and optical properties. Furthermore, the heating capability was maintained under severe mechanical deformation, e.g., at a bending radius of 4 mm. The structure also exhibited highly sustainable optoelectronic properties under repetitive mechanical deformation, confirming the potential for commercialization. Finally, we demonstrated that ZnO/Cu/Al2O3 rolled around a human finger exhibited highly uniform heating characteristics, rendering the heaters suitable for wearable, healthcare electronics.