Innovative printable self-cleaning material for covering photovoltaic cells

The use of polymeric materials, such as sulfonated Polyethersulfones (SPESs), carrying sulfonic acid groups directly on the polymeric chain, can be very advantageous in fields when hydrophilic/hydrophobic properties can be modulated, i.e. photovoltaic applications; moreover, the use of Ionic Liquids (I.Ls.) combined with SPESs can be a way to create tailor-made hydrophobic materials for solar cells covering. In particular SPES, a totally amorphous polymer, is characterized by excellent thermal and UV resistance, optical features, chemical stability, oxidation resistance, as well as by good mechanical properties and easy processability, i.e. for the production of films or layers [1]. I.Ls., a class of molten salts, have excellent thermal stability and their physical-chemical properties can be modulated changing the nature of the cation or anion. Modulating cationic apolar groups can dramatically influence the tendency of I.L. towards efficient ion packing and, in turn, its hydrophobic features, e.g. the longer the alkyl chains, the more hydrophobic the salt [2]. In this work, SPESs with different concentrations of sulfonic acid moiety, expressed as degree of sulfonation (DS), 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 DS of sulfonation of SPES, i.e. with the number of K+ ions available for substitution by the hydrophobic I.Ls. cations, obtaining contact angles up to 120° (θ) and so self-cleaning surfaces. 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 [3]. Several techniques were used to characterize the films obtained: Wide Angle X-Ray Scattering allowed for an indirect study of the SPES structure and dimensional stability. The thermal properties were characterized by Differential Scanning Calorimetry; the wettability and the morphology were studied by static contact angle measurement and Scanning Electron Microscopy. Keywords: Sulfonated Polyethersulfone; Ionic Liquids; self-cleaning material; printable material. * Corresponding author: valentina.sabatini@unimi.it References [1] C. Klaysom, B.P. Ladewig, J. Mater. Sci. 48 (2011) 368 [2] P. Bai, X. Cao, J. Biomater. Sci. Polym. 21 (2010) 1559 [3] K. Bica, K. Gaertner, J. Org. Chem. 31 (2008) 3235