Flexible Barrier Materials for Improving the Durability of Photovoltaic Devices

Abstract

The economic viability of photovoltaic (PV) devices is strongly dependent upon equipment cost and durability. During their usage, these devices are exposed to several atmospheric degradation agents and thus they need to be protected by coatings and encapsulants. However, even such coatings and encapsulants can degrade over time due to weather conditions, leading to potential efficiency loss and damage of the photovoltaic devices. Nowadays, the following main properties are basically required for solar cells coating materials to ensure PV devices durability: UV, oxygen and water barrier; thermal stability, transparency, anti-reflectance, anti-soiling, flexibility, affordable cost, electrical isolation. Therefore, in order to maintain a high efficiency during their lifetime solar cells require coating materials with several functions that are usually achieved with multilayer coatings, in which one or more layer have a specific functionality, such as gas and moisture barrier, liquid barrier and self-cleaning properties. However, a higher number of layers normally increases the cost and reduces the coating transparency and flexibility. A reduction of the number of layers would lower costs and also help to maintain a high transparency and flexibility. This study was focused to the development of novel flexible and transparent materials able to integrate into a single layer both liquid and gas barrier functionalities by means of simple and effective single step process carried out at room temperature, specifically applied to standard coating bilayers for PV cells. To this aim, a Self Assembly of Monolayers of alkylsilanes and fluoroalkylsilanes was chemisorbed on the silica surface of a PV standard coating bilayers for solar cells such as PET-SiOx and ETFE-SiOx. The so obtained nanocoated films showed high hydrophobic characteristics with average contact angle higher than 130° for the coated PET-SiOx substrate, and a significant improvement of the oxygen barrier properties, reducing the Oxygen Transmission Rate to 1/3 if compared to that of the uncoated film. Accelerate Ageing tests were performed in order to verify the chemical resistance of the nanocoated materials by simulating the degradation effect of both acidic and basic rains, damp heat, UV exposure. The measured contact angle values showed that after an initial slight reduction of the contact angle value, a constant hydrophobic value was maintained for the samples coated with the SAM of fluoroalkylsilanes, even after 1000 hours of very drastic test conditions... [edited by author]