Heat-Resistant, Flexible Piezoelectric Sheet Sensors Based on Solution-Processed Zinc Oxide Films for In-Vehicle Driver Monitoring Applications

Heat-resistant, low-cost, and large-area flexible piezoelectric sheet sensors that detect the postures of drivers are required for practical driver monitoring systems for vehicles. However, typically used polymer-based piezoelectric materials have low heat resistance. Here, we present a simple heat-resistant piezoelectric sheet sensor based on solution-processed zinc oxide films and discuss its sensing performance. Piezoelectric lithium-doped zinc oxide (Zn1–xLixO) films are directly prepared on polyimide substrates by a facile solution process, producing relatively large-area sheet sensors with a simple structure and high electrical resistance. The solution-processed Zn1–xLixO films have a c-axis-oriented wurtzite structure and exhibit an inherent piezoelectric response probably owing to its preferential orientation along the O-polar grains in the self-organized wurtzite films during the formation process. The piezoelectric response of the sheet sensors is unaffected by a heat-resistance test conducted at 393 K. When the sheet sensors are attached to chairs, they are sufficiently sensitive to detect the vital signs (such as respiration and pulse rates), body movements, and countermovements of a seated subject in a laboratory setting. This work demonstrates the feasibility of an unobtrusive driver monitoring system in which heat-resistant piezoelectric sheet sensors are embedded in the seats of future smart vehicles.