Influence of encapsulation materials on the optical properties and conversion efficiency of heat-sealed flexible polymer solar cells

Encapsulation with suitable techniques and materials is a critical issue in Polymer Solar Cell (PSC) fabrication because it has to prevent oxygen and water diffusion into the devices, responsible for their degradation. In this work heat sealing with several commercial materials (among them polyvinyl butyral, first used in Organic Photovoltaics) is proposed as an encapsulation method and the influence of such technique on the optical and electrical performances of flexible PSCs is investigated. Optical analysis was carried out on semitransparent test structures exposed outdoor and the bleaching of active layer was delayed from a few days to a month by using the proper combination of sealants. At the same time, electrical analysis was performed on devices; power conversion efficiency (PCE), measured before and after the encapsulation process, showed that sealing at temperatures higher than 100 °C strongly damages the electrical parameters. Moreover, it was found out that an interlayer was needed to prevent damaging interaction between device and encapsulant. As a result of the above developments, PSCs retained their properties after the encapsulation step and durability was slightly improved. The use of interlayer materials with Water Vapor Transmission Rate (WVTR) as low as 10− 3g/m2day might allow to achieve an effective protection with a viable and scalable process.