Abstract
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 γ-terpinene reduces degradation-related loss in conversion efficiency in PCPDTBT:PC70BM solar cells under ambient operating conditions. The stability of γ-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.
Highlights
Organic electronic devices promise mechanical flexibility and low-cost fabrication
This study investigates the effect of direct encapsulation of organic photovoltaics, PCPDTBT: phenyl C70 butyric acid methyl ester (PC70BM) solar cell, by γ-terpinene plasma polymer using radio frequency (RF) plasma polymerization
Known as 1-isopropyl-4-methyl-1,4-cyclohexadiene, γ-terpinene belongs to a group of monocyclic terpenes and is isolated directly from Melaleuca alternifolia essential oil by distillation[19]
Summary
Organic electronic devices promise mechanical flexibility and low-cost fabrication. when operating under ambient conditions, organic materials are unstable and require physical encapsulation to protect devices from degradation due to water vapor and oxygen permeation[1]. The use of glass lid encapsulating layers greatly limits the use of these devices in flexible applications, whereas currently-available thin film strategies either require the deposition of multi-layer structures (e.g. Barix barrier layers based on alternating dyads of organic and inorganic films) or rely on slow-throughput atomic layer deposition (ALD)[2,3]. Many common natural and synthetic polymers are subject to UV degradation, with UV rays cleaving tertiary carbon-hydrogen bonds in the chain structures to form free radicals, which are in turn subject to further oxidation by oxygen present in ambient air This can lead to discoloration, cracking, delamination, with the degree of disintegration linked to the duration and intensity of UV exposure, as well as the presence of heat and oxidizing agents. In this study samples fabricated at 10, 25, 50 and 75 W RF power levels are compared to investigate the effect of deposition power on the stability of the polymer films under different irradiation conditions
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