Soiling behavior of flexible ethylene tetrafluoroethylene (ETFE) frontsheets deployed in outdoor environment for photovoltaic applications

This work presents the soiling behavior of 12-month outdoor-deployed Ethylene Tetrafluoroethylene (ETFE) front-sheets, evaluating four surface types—hydrophilic-smooth (HL-S), hydrophilic-matt (HL-M), hydrophobic-smooth (HB-S), and hydrophobic-matt (HB-M)—alongside a reference glass. The extent of soiling was evaluated based on the change in AM1.5 weighted average transmittance of soiled and as-cut samples. Soiling was nearly 1%–2% higher for ETFE than for glass. Interestingly, hydrophilic surfaces outperformed hydrophobic counterparts, and matt surfaces exhibited lower soiling than smooth surfaces for the initial 8 months. In later months, all ETFE samples exhibited similar soiling. HB-S and HB-M showed a consistent decrease in water contact angles (WCA) and exhibited comparable values to those of HL-S and HL-M after 12 months. XPS analysis revealed contamination by carbonized layers on all ETFE surfaces after outdoor exposure, which may be responsible for the similar WCA after 12 months. Two types of soiling are identified: oxide-based sand grains with sizes < 20 µm and amorphous carbon-based particles with sizes up to 100 µm. The coverage of contaminants was significantly higher for ETFE than for glass, particularly for the carbon-based particles. Artificial soil deposition and rain-soaking experiments suggested that larger soil grains were washed away efficiently, while smaller particles remained on the surface. Moreover, only glass showed complete recovery in transmittance after rain soaking, whereas all ETFE surfaces exhibited a 0.5%–1% loss. This suggests that soiling on ETFE surfaces can be more severe than on glass, especially due to organic contaminants, even at sites with considerable precipitation.