**1. Introduction**

Perovskites with the formula of AMX3 (where A stands for an organic group or inorganic cation with twelve neighboring, X is a halide anion, and M is a metal cation) have been evidenced to be exciting solar absorber materials [1]. Besides the light harvesting performance, these compounds have recently demonstrated several intriguing properties, such as high dielectric constant [2], ferroelecticity [3], photorestriction [4], resistive switching [5], and optical cooling [6]. However, the AMX3 perovskites suffer from the major obstacle of chemical and structural instability because they are sensitive to environmental factors. Previous work indicated that the perovskite lattice can interact with the polar water molecules due to the formation of strong hydrogen bonds between water molecules and the halide lattice [7]. This characteristic makes the properties of AMX3 heavily dependent on the environment moisture. For example, Alberto García-Fernández et al. [8] investigated the electric properties of CH3NH3PbI3 (MAPbI3) in wet and dry environments. The results showed that both capacitance and conductivity in wet condition were several orders of magnitude larger than those in dry condition. This humidity sensitive feature gives the materials tremendous promise as probes for sensing of humidity. Truly, outstanding humidity performances of large sensitivity, remarkable fast response/recovery time, small hysteresis loop, and good linearity were reported in humidity sensors based on Cs2PdBr6 [9], CH3NH3PbI3−*x*Cl*<sup>x</sup>* [10,11], Cs2BiAgBr6 [12], and CsPbBr3 [7].

MAPbI3, being an important member of the AMX3 family, has properties that strongly depend on environment humidity [8], indicating that it can be used as a humidity sensing material. Although humidity detection by MAPbI3 was attempt by Ilin and co-authors [13], details about its humidity performances have not been studied. Additionally, because of the low-cost solution-based method and cheaper CH3NH2, perovskite MAPbI3 is less expensive than most humidity sensing materials such as CsPbBr3 [14], Cs2PdBr6 [9], and Cs2BiAgBr6 [12]. Moreover, it was proved that illumination could lead to a giant dielectric

**Citation:** Zhao, X.; Sun, Y.; Liu, S.; Chen, G.; Chen, P.; Wang, J.; Cao, W.; Wang, C. Humidity Sensitivity Behavior of CH3NH3PbI3 Perovskite. *Nanomaterials* **2022**, *12*, 523. https:// doi.org/10.3390/nano12030523

Academic Editor: Sergey Ya. Istomin

Received: 14 December 2021 Accepted: 31 January 2022 Published: 2 February 2022

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constant of lead halide perovskite [2]. Hence, we herein present a systematical investigation on the humidity performances of an impedance-type humidity sensor based on MAPbI3.

In this work, MAPbI3 was fabricated by grinding CH3NH3I made from an ice bath with PbI2. The humidity sensing performance of the humidity sensor based on perovskite MAPbI3 were tested at room temperature in the range of 11–94% relative humidity. Our results show that the MAPbI3-based humidity sensor exhibits high humidity sensitivity performance. The possible mechanism of the humidity sensitivity and photoinduced changes were also discussed.
