**4. Conclusions**

The successful humidity-sensing application of thin films of hydrophobically modified PVA copolymers, namely poly(vinylalcohol-*co*-vinylacetal)s (PVA–Ac), of acetal content in the range of 18%–28% is demonstrated. A noticeable decrease of hysteresis, increase of sensitivity, and widening of dynamic range for modified films as compared to the neat PVA films are observed. For optimization of sensor performance, post-deposition annealing at 60 and 180 ◦C is used and two different film thicknesses are used (80 and 200 nm). The best sensor characteristics are obtained for films modified with acetal content around 24% with thickness of 80 nm and post-deposition annealing temperature of 60 ◦C. For relative humidity higher than 70% an accuracy of 0.7% RH is achieved. Both post-deposition annealing at 180 ◦C and higher film thickness (200 nm) leads to deterioration of sensing operation of the films.

It was demonstrated that both types of substrate used, silicon substrate and glass with thin (30 nm) metal (Au–Pd) overlayer, are suitable for humidity sensing. The first one is preferable if color sensing of humidity is considered, while the second one enables transmittance measurements thus offering more technological convenience and higher accuracy/resolution of measurements. For further decrease of hysteresis, a doping of PVA–Ac (24%) with SiO2 particles (20%) is used. The thin film samples have full dynamic range and linear dependence of the measured signal in the entire humidity range. Humidity-sensitive films have thickness values around 80 nm that guarantees fast sensing.

**Author Contributions:** Conceptualization, T.B., D.C., and C.N.; methodology, T.B., D.C., and K.L.; software, T.B. and K.L.; validation, T.B., D.C., C.N., and K.L.; formal analysis, T.B., D.C., and K.L.; investigation, K.L. and S.B.; resources, D.C. and T.B.; data curation, K.L., D.C., and T.B.; writing—original draft preparation, T.B. and D.C.; writing—review and editing, T.B., D.C., and K.L.; visualization, T.B. and D.C.; supervision, T.B. and D.C.; project administration, T.B. and D.C.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Bulgarian National Science Fund, Grant No. DN08-15/14.12.2016.

**Acknowledgments:** K.L. and S.B. acknowledge the National Scientific Program for young scientists and postdoctoral fellows, funded by Bulgarian Ministry of Education and Science (PMC № 271/2019). This work was partially supported by the European Regional Development Fund within the Operational Programme "Science and Education for Smart Growth 2014–2020" under the Project CoE "National center of mechatronics and clean technologies" BG05M2OP001-1.001-0008-C01.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
