An all-solution-processed transparent tin oxide (SnO2)-based humidity sensor was directly prepared on borosilicate glass (SnO2-G) and a flexible polyethylene terephthalate (SnO2-PET) substrate without using a template. The entire process included film deposition by a spin-spray process at 90 °C and subsequent hot water treatment (HWT) at 100 °C. The resistivity of the films dramatically decreased and had semiconductor characteristics after the HWT, even though the as-prepared SnO2-G and SnO2-PET samples were insulators. Based on the results, the variation of the resistivity could be attributed to the formation of a hydroxyl layer on the crystallized SnO2 surface. With the help of the HWT on the SnO2 films, the formation of tin hydroxyl derivatives provided mobile protons, which led to the variation of the electrical properties of SnO2 at ambient conditions with different humidities. The sensitivity of the SnO2-G-HWT and SnO2-PET-HWT at 95% relative humidity (RH) was 35.2 and 3.5 times higher, respectively, than that at 5% RH. Both the sensitivity of the SnO2-G-HWT and SnO2-PET-HWT samples showed a good uptrend corresponding to the increase of RH at 20 ± 1 °C, and the response/recovery time of SnO2-G-HWT and SnO2-PET-HWT was 51/38 s and 69/47 s in the % RH range of 30-70% at 20 ± 1 °C, respectively.
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