A Durable Superhydrophilic Self-Cleaning Coating Based on TiO2–SiO2-PAA Nanocomposite for Photovoltaic Applications: Long-term Outdoor Study

Self-cleaning coatings and/or surfaces have attracted great attention for photovoltaic (PV) panel and building window glass applications. In this work, we have developed TiO2-SiO2-PAA (polyacrylic acid) nanocomposite superhydrophilic coating by spraying and brushing deposition. Scanning electron microscope (SEM), UV-Vis spectra, water contact angle test, photocatalytic activity and waterfall impact test were used to characterize the self-cleaning coating. We found that the introduced PAA in the TiO2-SiO2 nanocomposite can improve the superhydrophilicity, durability and antireflection of the coating. Long-term outdoor tests, ranging from one year to four years, were conducted for the variation of both the average transmission in visible region (400-700 nm) of the coating in glass, PET (polyethylene terephthalate) and PV power generation. The relative self-cleaning efficacy (RSCE) shows different variations over time in the PET and glass substrate. The RSCE of the coating deposited on PET is only about 2 % in the first 150-day period and increases to similar to 15 % after that time, which then stabilizes at one year mark. However, for the coating on glass substrate, we found that the average RSCE can be maintained at 4 % within 3 years, then it varies to about 2 % in the fourth year. This is in agreement with our data of the increase (2.8 %) of power generation test in PV panel. Finally, we found that the duration of the coating deposited by spraying was longer than that of deposited by brushing, due to the thinner thickness of the coating deposited by brushing. However, the coating deposited on a commercially available anti-reflection (AR) coating glass surface only becomes effective a month after its application. Our results indicate that the coating has strong potential in applications of PV panel maintenance.