The Deposition of Lanthanide Oxide Thin Films for Self-Cleaning Photovoltaics

Soiling across solar modules by dust and dirt particles remains a major obstacle for the use of solar photovoltaics. There is a critical need for self-cleaning solar utilities in order for these to replace the current unsustainable fossil fuel-based energy generation systems. To that effect, we present the design, synthesis and application of inorganic rare Earth oxide layers (Ce₂O₃, Er₂O₃) deposited by aerosol assisted chemical vapour deposition (AACVD) with intrinsic hydrophobicity. There are very few reports of lanthanide oxide complexes for hydrophobic coatings using AACVD within the literature. This prompted our investigation into such coatings. Water contact angles of up to 103° were established across the thin films deposited using these complexes, demonstrating their self-cleaning potential.<br/><br/>As a new approach to self-cleaning solar modules, we investigated the synthesis of volatile lanthanide -diketonate precursors. Notably, inorganic rare Earth (Ce, Y, Er, Lu) oxide layers have been shown to be durable, UV-stable, oxidant resistant with intrinsic hydrophobicity, with a water contact angle range of 110-125 °.^1,2 Likewise, -diketone ligand complexes have been proven to serve as effective and stable precursors. Compounds of the type [X(Y)_3,4] have been synthesised via adaption of a known synthetic route to [Ce(thd)₄] and [Er(thd)₃],^{3, 4} where X = lanthanide metal (Ce, Er) and Y = thd (dipivaloymethane), dbm (dibenzoylmethane).<br/><br/>AACVD of the synthesised complexes yielded lanthanide oxide films on fluorine tin doped (FTO) glass under a constant flow of nitrogen. Thermogravimetric analysis of these compounds indicated suitable deposition temperatures ranging from 400-600 °C. The most hydrophobic coatings, with contact angle of 103°, were obtained by the deposition of cerium oxide onto FTO at temperatures between 400 and 500 °C. These rare oxide thin films were intrinsically analysed using surface analysis and characterisation techniques. X-ray photoelectron spectroscopy confirmed the presence of a mixed phase system of Ce(III) and Ce(IV) across the surface. Further analysis of the deposited thin films was conducted using SEM and XRD to determine the surface morphologies of these hydrophobic coatings.<br/><br/>Further research on the synthesis of mixed ligand lanthanide systems has also been investigated for thin film deposition, holding further promise for the synthesis of self-cleaning photovoltaics.<br/><br/><b>References</b><br/>1. G. Azimi, R. Dhiman, H. M. Kwon, A. T. Paxson and K. K. Varanasi, <i>Nature Materials</i>, 2013, <b>12</b>, 315-320.<br/>2. Y. Zhao, <i>Materials</i>, 2012, <b>5</b>, 1413-1438.<br/>3. M. Becht, T. Gerfin and K. H. Dahmen, <i>Chemistry of Materials</i>, 1993, <b>5</b>, 137-144.<br/>4. K. J. Eisentraut and R. E. Sievers, <i>Journal of the American Chemical Society</i>, 1965, <b>87</b>, 5254-+.