Innovative printable self-cleaning for covering photovoltaic cells

In the present study, polymeric films based on Sulfonated Polyethersulfone (SPES) and Ionic Liquids (I.Ls.) were successfully prepared and tested as novel self-cleaning materials, with likely future application as printable films for covering solar cells. In fact, self-cleaning materials offer a very advantageous solution for photovoltaic cells covers. Both service life and efficiency of a solar cell can be enhanced by preventing accumulation of dust and filth on cell covers, thus, by reducing the loss of incident light by absorption or scattering. In this field, polymeric materials such as SPES stand out thanks to the possibility of modulating their hydrophilic/hydrophobic properties. The use of I.Ls. combined with SPES opens new possibilities of tailoring hydrophobic materials into suitable solar cell covers. SPES, a completely amorphous polymer, can be an excellent substitute for heavy and fragile glasses; in fact, it’s characterized by excellent UV and thermal resistance (e.g. glass transition temperature up to 230°C and low thermal expansion coefficient), optical properties (refractive index: n=1.63 at λ=589.3 nm), stability to oxidation and hydrolysis, and high mechanical properties. The step from lab-scale processing to industrial manufacturing implies up-scaling of the single-device size, with practical limitations in producing modules, in particular by deposition techniques. The biggest challenges in scaling-up from laboratory to full production are both selecting the right tools and processes and improving yield and quality while keeping low manufacturing costs. In this context, easy processability of SPES due to its viscosity and surface energy makes it a good material for the production of films through InkJet printer technology, a very simple and economic deposition technique. I.Ls., a class of molten salts with high thermal stability, can be suitably modified in either their cationic or anionic part to exhibit specific physical-chemical properties. For instance, using long alkyl chains as apolar cationic groups results in more efficient ion packing of the I.Ls. and, in turn, increased hydrophobicity. In this work, SPESs with different concentrations of sulfonic acid moiety, expressed as degree of sulfonation, were successfully synthesized using a sulfonated co-monomer. The wetting properties of SPES could be modulated by introducing different cationic apolar groups through a novel ionic exchange reaction between the K+ cation of the sulfonic moiety of SPES and the cation of an I.Ls.. The hydrophobic properties of SPESs treated with I.Ls. were found to improve with the increasing degree of sulfonation of SPES, i.e. with the number of K+ ions available for substitution by the hydrophobic I.Ls. cations. Both conventional and innovative characterization techniques were used: laboratory Wide Angle X-Ray Scattering (WAXS) and synchrotron radiation allowed, respectively, for a direct and an indirect study of the SPES structure and dimensional stability, in view of a future use for printing. The thermal properties of these materials were characterized by Differential Scanning Calorimetry (DSC); the spatial arrangement of sulfonic groups modified with I.Ls., the wetting properties and the morphology of SPES films were studied by static contact angle measurement and Scanning Electron Microscopy (SEM).