Use of gas cluster ion source depth profiling to study the oxidation of fullerene thin films by XPS

The analysis of organic materials such as phenyl-C61-butyric acid methyl ester (PC61BM) by depth profiling is typically fraught with difficulty due to the fragile nature of the sample. In this work we utilise a gas cluster ion source for the controlled depth profiling of organic materials that would historically have been too fragile to analyse and obtain quantitative compositional data through the whole thickness of the film. In particular we examine the oxygen diffusion and photo-oxidation kinetics of one of the most commonly used electron acceptor materials for many organic optoelectronic applications, namely PC61BM, in both neat films and in blends with polystyrene. Exposure to AM1.5G light and air under ambient conditions, results in a higher level of surface oxidation of blended PC61BM:polystyrene than is observed for either pure control film. Gas cluster ion source depth profiling further confirms that this oxidation is strongest at the extreme surface, but that over time elevated oxygen levels associated with oxidised organic species are observed to penetrate through the whole blended film. The results presented herein provide further insights on the environmental stability of fullerene based organic optoelectronic devices

Photo-stability study of a solution-processed small molecule solar cell system: correlation between molecular conformation and degradation

<p>Solution-processed organic small molecule solar cells (SMSCs) have achieved efficiency over 11%. However, very few studies have focused on their stability under illumination and the origin of the degradation during the so-called burn-in period. Here, we studied the burn-in period of a solution-processed SMSC using benzodithiophene terthiophene rhodamine:[6,6]-phenyl C₇₁ butyric acid methyl ester (BTR:PC₇₁BM) with increasing solvent vapour annealing time applied to the active layer, controlling the crystallisation of the BTR phase. We find that the burn-in behaviour is strongly correlated to the crystallinity of BTR. To look at the possible degradation mechanisms, we studied the fresh and photo-aged blend films with grazing incidence X-ray diffraction, UV–vis absorbance, Raman spectroscopy and photoluminescence (PL) spectroscopy. Although the crystallinity of BTR affects the performance drop during the burn-in period, the degradation is found not to originate from the crystallinity changes of the BTR phase, but correlates with changes in molecular conformation – rotation of the thiophene side chains, as resolved by Raman spectroscopy which could be correlated to slight photobleaching and changes in PL spectra.</p

Toward Improved Environmental Stability of Polymer:Fullerene and Polymer:Nonfullerene Organic Solar Cells: A Common Energetic Origin of Light- and Oxygen-Induced Degradation

With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation

The role of fullerenes in the environmental stability of polymer:fullerene solar cells

Environmental stability is a common challenge for the commercialisation of low cost, encapsulation-free organic opto-electronic devices. Understanding the role of materials degradation is the key to address this challenge, but most such studies have been limited to conjugated polymers. Here we quantitatively study the role of the common fullerene derivative PCBM in limiting the stability of benchmark organic solar cells, showing that a minor fraction (<1%) of photo-oxidised PCBM, induced by short exposure to either solar or ambient laboratory lighting conditions in air, consistent with typical processing and operating conditions, is sufficient to compromise device performance severely. We identify the effects of photo-oxidation of PCBM on its chemical structure, and connect this to specific changes in its electronic structure, which significantly alter the electron transport and recombination kinetics. The effect of photo-oxidation on device current–voltage characteristics, electron mobility and density of states could all be explained with the same model of photoinduced defects acting as trap states. Our results demonstrate that the photochemical instability of PCBM and chemically similar fullerenes remains a barrier for the commercialisation of organic opto-electronic devices