Highly stable humidity sensor based on lead-free Cs3Bi2Br9 perovskite for breath monitoring

Perovskite materials as humidity sensors for breath monitoring have attracted tremendous attention in recent years due to their cost effectiveness, easy fabrication process, and controllable and adjustable performance, showing great potential in portable wearable devices. However, the long-term performance of the perovskite-based humidity sensor is far from satisfied for the issue of the intrinsic instability of the related perovskites, especially working at a high humidity range. In this work, we demonstrate a superior resistive-type Cs3Bi2Br9 humidity sensor for the first time, which exhibits a reliable performance for over 35 days under a 90% relative humidity (RH). Ultrafast response (5.56 s) and recovery time (6.237 s) are achieved for the proposed sensors with outstanding repeatability and selectivity in a wide range of humidity detection. Owing to its good stability and superfast response, the Cs3Bi2Br9 perovskite-based humidity sensor is integrated with a commercially available mask for real-time monitoring of the changes in the respiratory rate of an adult under different physiological states, providing a facile and reliable path for diagnostic breath analysis.