Abstract
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.
Highlights
Solution-processed organic small molecule solar cells (SMSCs) have a number of advantages over the more common polymer solar cells (PSCs), including well- defined chemical structure and monodisperse molecular weight, which allows easier purification and better reproducibility [1]
Long solvent vapour annealing (SVA) times such as 5 and 10 min show slight reductions in the Power conversion efficiency (PCE) probably due to high crystallinity of butyric acid methyl ester (BTR) phase as resolved by grazing-incidence X-ray diffraction (GI-XRD) measurements, which are discussed
We studied the photo-ageing effect of a highly efficient solution-processed SMSC system (BTR:PC71BM) as a function of SVA treatment time applied to the active layer under a dry nitrogen environment
Summary
Solution-processed SMSCs using BTR:PC71BM have attracted a lot of attention for its promising efficiency of 9.3%, and high fill factor (FF) of 77% with optimal active layer thickness of over 200 nm [13]. To achieve high efficiency for most of the solution-processed SMSC systems including BTR:PC71BM, SVA is a common method for the optimisation [1], which can increase/control the degree of crystallisation of the small molecule donor in the active layers [13]. We attempt to understand the degradation during the burn-in period of BTR:PC71BM devices with different levels of crystallinity of the active layers by controlling the SVA time, through studying the structural and optical properties of the photo-aged films. To minimise the stress factors, both the devices and films are degraded at room temperature under irradiation from visible light-emitting diodes (LEDs) with a constant flow of dry nitrogen for up to 192 h (8 days)
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