*2.2. Characterization of Materials*

The crystalline phase composition and structure of all powders were examined by X-ray diffraction (XRD, Rigaku Smartlab Beijing Co. Beijing, China) using Cu K α radiation. The morphology of MAPbI3 powders was analyzed by scanning electron microscope (SEM, Model S 4800, Hitachi Co, Tokyo, Japan). The possible ferroelectricity of MAPbI3 was investigated by measuring polarization-electric field (*P-E*) hysteresis loop using a TF analyzer (MultiFerroicII, Radiant technologies Inc, Albuquerque, New Mexico). In doing so, Au top electrodes were sputtered onto the surface of MAPbI3 film to form a sandwich structure of Au/MAPbI3/Au. The ultraviolet photoelectron spectroscopy (UPS) measurement was carried out using He-I radiation (21.2 eV) in vacuum (10−<sup>8</sup> mbar) to measure the highest filled energy level of valence band (VBM) of MAPbI3 film. The sample was biased at −10 V for measurement in the secondary electron cut off region.

### *2.3. Humidity Sensor Fabrication and Performances Measurements*

The MAPbI3-based humidity sensor was fabricated by aerosol deposition method on Al2O3 substrate covered with Au interdigitated electrodes. Figure 1 presents the photo images of the Al2O3 substrate and fabricated sensor. The sensor fabrication processes are as follows: firstly, a little amount of MAPbI3 powder was mixed with anhydrous ethanol, and ultrasound for 15 min to form a homogeneous paste. Then, the paste was uniformly sprayed onto a Al2O3 substrate using a 0.2 mm caliber spray pen (Sao Tome V130). Two copper wires were fixed on the electrodes with silver glue. Finally, the sensor was dried at 100 ◦C for 10 min and naturally cooled down to indoor temperature. The whole process of sample preparation was shown in Figure 2.

**Figure 1.** (**a**) Photo images of the Al2O3 substrate with Au interdigitated electrodes; and (**b**) the MAPbI3-based humidity sensor.

**Figure 2.** (**a**) Ice bath method; (**b**) grinding; (**c**) the structure of MAPbI3 powder; (**d**) aerosol deposition method; and (**e**) the polycrystalline layer structure of MAPbI3 thin film.

The different relative humidity (RH) environments were obtained by saturated salt solutions of LiCl, MgCl2, Mg (NO3)2, NaCl, KCl, and KNO3 in closed containers. The environments above these solutions can provided RH levels of 11, 33, 54, 75, 85, and 94%, respectively. The impedance of the sensor was measured by an impedance analyzer (Hioki 3532-50 LCR). The experimental setup was illustrated in Figure 3. The influence of illumination on the MAPbI3-based sensor was reflected by impedance variation with and without light provided by 10 small red LED lights.

**Figure 3.** Schematic of the humidity sensing experimental setup: (**a**) PC; (**b**) Hioki 3532-50 LCR; and (**c**) different humidity environments.
