High-Performance Flexible Humidity Sensors Based on MCl (M = Li, Na, K) Doped PVP/PVDF Self-Supporting Films for Boosted Real-Time Noncontact Moisture Monitoring

Nowadays, attractive progress has been made on flexible humidity sensors for multifunctional applications, yet most of them still suffer from intrinsic instability due to the device structure based on flexible substrates. Herein, high-performance flexible humidity sensors were constructed using alkali metal halide doped polyvinylpyrrolidone (PVP)/polyvinylidene fluoride (PVDF) freestanding films. These films are prepared via a versatile thermally induced phase separation procedure with a network-like porous structure. After doping with metal salts of MCl (M = Li, Na, K), the humidity sensing performance is highly improved, among which the LiCl@PVP/PVDF film is the optimized one. In the relative humidity range of 11–97% at room temperature, the LiCl@PVP/PVDF sensor exhibits high response (2.3 × 10 3 ), fast response (10 s), small hysteresis (0.41%), and excellent repeatability. Notably, the LiCl@PVP/PVDF film is featured as fully self-supporting without the assistance of any substrate, readily tailorable that can be cut into different shapes, as well as highly flexible that can be bent to certain angles, and meanwhile almost maintains its response without significant decrease. Furthermore, the excellent performance enables the LiCl@PVP/PVDF sensor to demonstrate great potential in real-time noncontact diaper and finger detection. This contribution provides a practical humidity sensing candidate for moisture monitoring and gives insights into the fields of humidity sensors and flexible electronics.