A solution-processed tin dioxide film applicable as a transparent and flexible humidity sensor

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.