*3.3. Influence of Light on the Humidity Sensing Properties*

Previous work has indicated that red light affects the electric properties of MAPbI3 [26]. To investigate the influence of illumination on the humidity sensing properties of the MAPbI3-based sensor, 10 red LED lights were used as light source. The humidity sensor was placed in the environments of 11%, 54%, and 94%RH successively, and the impedance values were recorded during the three stages of darkness–lightness–darkness with the duration time of 300 s in each stage. The results were plotted in Figure 8. It is clearly shown that under a low humidity level of 11%RH, the impedance can be notably reduced by the light because the photogenerated carrier breaks the original carrier balance and increases the electron concentration, leading to the enhancement of electron conductivity. In the medium and high humidity levels, a similar situation of illumination-induced impedance

decrease to that of the low humidity level was observed when the lights were on. However, this impedance variation becomes much weaker than that in 11%RH. However, in the later stage of lighting, the electron concentration decreases due to electron–hole combination, so the impedance increases again. After the light source was switched off, considering that the non-equilibrium carriers does not disappear immediately, the impedance value increases at low and medium humidity levels due to the electron–hole interaction. However, impedance remarkably decreases at high humidity level, probably because the interaction between the light source and the continuous layer of water that forms on the surface of the sample.

**Figure 7.** Repeatability (**a**) and long-term stability (**b**) of the MAPbI3-based sensor.

**Figure 8.** Illumination-induced impedance variations of the MAPbI3-based sensor in the environment of (**a**) 11%, (**b**) 54%, and (**c**) 94%RH.
