Photostability of plasma polymerized γ-terpinene thin films for encapsulation of OPV

Optically transparent, smooth, defect-free, chemically inert and with good adhesion to a variety of substrates, plasma polymers from plant-derived secondary metabolites have been identified as promising encapsulating materials for organic electronics and photovoltaics. Here, we demonstrate that an encapsulating layer of plasma polymerized gamma-terpinene reduces degradation-related loss in conversion efficiency in PCPDTBT:PC70BM solar cells under ambient operating conditions. The stability of gamma-terpinene films was then investigated under extreme UV irradiation conditions as a function of deposition power. When exposed to ambient air, prolonged exposure to UV-A and UV-B light led to notable ageing of the polymer. Photooxidation was identified as the main mechanism of degradation, confirmed by significantly slower ageing when oxygen was restricted through the use of a quartz cover. Under unnatural high-energy UV-C irradiation, significant photochemical degradation and oxidation occurred even in an oxygen-poor environment.