Transparent planar layer copper heaters for wearable electronics

Abstract To keep up with the progress in wearable electronics, it is imperative to create flexible transparent heaters with low sheet resistance and high transmittance. This allows for low-voltage operation. In this study, we fabricated highly flexible ultrathin transparent heaters composed of Cu layers with a thickness in the range of 4–40 nm sandwiched between ZnO layers. The sheet resistance (0.8–165 Ω/sq) and optical transparency (average visible light transmittance of 29.3–90.0%) can be considerably modulated by varying the Cu layer thickness. The fabricated transparent heaters exhibited superior heating capability over reported network-type transparent heaters, i.e., extremely fast thermal response (90% of target temperature reached within 5 s), excellent thermal uniformity (less than 2.5% thermal variation at a heater temperature of 100 °C), and outstanding heater durability (no degradation with a continuous heating test at 80 °C for 10,000 min). Based on the area-dependent thermoelectrical parameters, it was shown that large heaters could reach 50 °C with a portable rechargeable lithium-ion battery with 12 V DC output. The excellent flexibility in the bent and unbent state, when subjected to various bending tests, confirmed the high suitability of the ultrathin transparent Cu heaters for wearable electronics, such as biomedical devices.