Multifunctional coatings for photovoltaic devices: when one of the less studied components makes the difference

The scientific efforts in the field of innovative photovoltaic devices are strongly focused on the goal of obtaining high efficiencies. Synthesis of highly absorbing materials, engineering of interfaces, identification of electrons and holes transporters with high conductivity, replacement of expensive and toxic components with better ones are some of the current hot research topics. However, the resistance to meteorological phenomena, pollution and undesired UV radiation (present in the solar spectrum) cannot be easily tuned by working only on cell electrodes and electrolytes. To this purpose, great attention and expectations should be given to polymeric coatings that can be deposited on the external side of the cells. However, rather surprisingly, coatings have been developed almost exclusively for antireflective purposes. Indeed, despite the great academic and industrial efforts spent on cells components, the development of functional coatings for improving the outdoor stability and performance of third generation devices has been surprisingly practically neglected. Only in 2014 a couple of papers have shown the possibility to prepare smart coatings for solar cells. In this contribution, novel multifunctional coatings are proposed for different photovoltaic devices. In the first part of the work, photopolymerized coating systems were developed incorporating luminescent down-shifting, UV-screening and easy-cleaning functionalities, for use in Ru-free DSSC devices. In optimized systems, a 70% relative increase in PCE as compared with control uncoated devices was obtained. In addition, a long-term weathering study under real outdoor conditions showed that DSSCs incorporating the novel coating fully preserved their initial PCE, due to the combined action of the luminescent material (that acted as UV-screen) and the hydrophobic fluoropolymeric matrix (that acted as self-cleaning and water-repellent unit). In the second part of the work a series of novel stable and low cost organic molecules have been developed and used in a customized PU resins in order to obtain both tailored UV-screening and current improvement effect (down shifting without self absorption) in silicon solar cells. The general approach presented in this contribution to simultaneously improve performance and weatherability of photovoltaic devices may be readily extended to a large variety of sensitizer/luminophore combinations with appropriate spectral match as well as to the nascent world of perovskite solar cells, the instability to humidity and near-UV radiation of which represents one of the major problems currently under intense investigation