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Effects of solvent additive on inverted structure PCPDTBT:PC71BM bulk heterojunction organic solar cells

This work investigates the effects of the solvent additive, 1,8‐octanedithiol (1,8‐ODT), on solution‐processed inverted bulk heterojunction (BHJ) organic solar cells based on a thin film blend of poly[2,1,3‐benzothiadiazole‐4,7‐diyl[4,4‐bis(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b:3,4‐b′]dithiophene‐2,6‐diyl]] (PCPDTBT) and [6,6]‐phenyl C71 butyric acid methyl ester (PC71BM). In this work, varying concentrations of 1,8‐ODT were added into the polymer solution in order to optimise the device performance. The UV–Vis spectrometer measurements showed that adding 1,8‐ODT can improve the magnitude of light absorption in the blend film, as well as inducing an ∼40 nm red shift for long wavelengths. External quantum efficiency (EQE) measurements displayed improvements at all wavelengths for 1–4 vol% 1,8‐ODT concentration, when compared to the reference device. Current density–voltage (J–V) measurements confirmed this addition of photocurrent, as the short circuit current density (Jsc) increases by 23%, from 9.89 mA cm−2 for the reference to 12.14 mA cm−2 for the 1 vol% additive cell. The highest average device efficiency of 2.59% was obtained using a 1,8‐ODT concentration of 3 vol%. Further increasing additive concentration caused a reduction in efficiency. Additionally, the effect of drying the additive processed films in a vacuum was investigated. Drying in a vacuum had little effect on films processed with low additive concentration. However, the vacuum drying process led to a significant efficiency enhancement for high 1,8‐ODT concentrations. The efficiency of the 4 vol% 1,8‐ODT devices increased by 25% as a result of drying in a vacuum. This was due to the removal of excess residual additive from the active layer.

Authors:   Rui Lin, Matthew Wright, Ashraf Uddin
Journal:   physica status solidi (a)
Year:   2013
Pages:   n/a
DOI:   10.1002/pssa.201329195
Publication date:   29-May-2013
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