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Abstract
The power conversion efficiencies of organic solar cells delicately depend on the morphology of the light‐harvesting bulk heterojunctions (BHJ). Upon deposition from solution, the formation of tailored bicontinuous networks of polymers and fullerenes is often achieved using combinations of solvents and solvent additives. Common wisdom infers that best solar cell performances are achieved when the solvent additives exhibit excellent fullerene solubility. Herein, this concept is revisited based on the investigation of a series of structurally similar, substituted benzaldehydes. It is concluded that the solvent additives do not only have to feature the commonly accepted good fullerene solubility, but must also exhibit lowest polymer solubility to suppress liquid–liquid demixing and hence achieve best solar cell performance. Thus, this study adds an important item to the list of selection criteria of solvent additives toward the production of polymer:fullerene solar cells with optimized power conversion efficiencies. The microscopic picture of the resulting domain configurations within the light‐harvesting layers is developed around comprehensive multiscale investigations of the BHJ morphology, using atomic force microscopy, scanning transmission electron microscopy, and nano‐infrared microscopy. The latter is operated in two complementary modes, one of which is more bulk sensitive, whereas the other mode is surface sensitive.
Highlights
The power conversion efficiencies of organic solar cells delicately depend on the decade, tremendous efforts on both the morphology of the light-harvesting bulk heterojunctions (BHJ)
It is concluded that the solvent additives do have to (BHJs) in organic solar cells comprise feature the commonly accepted good fullerene solubility, but must exhibit lowest polymer solubility to suppress liquid–liquid demixing and achieve best solar cell performance
This improvement originated from a minor increase of the fill factor (FF) to 45% and of the JSC to 7.8 mA cmÀ2, but the performance still remained significantly below the power conversion efficiency (PCE) 1⁄4 7% that can be expected for optimized PTB7: PC71BM solar cells
Summary
To investigate the effect of the series of benzaldehyde solvent additives on the device performance, we fabricated organic solar cells with an inverted indium tin oxide (ITO)/ZnO/PTB7: PC71BM/MoOx/Ag device architecture. Upon addition of DIO to the main solvent o-xylene, the best PTB7:PC71BM solar cell PCE 1⁄4 6.6% was achieved at a concentration of 3%. The shape of the J–V curves, and in particular, the changes observed in JSC and FF, hint at differences in the charge carrier generation and extraction, depending on the additives and their amounts used during the deposition of the BHJ. We discuss the most relevant loss mechanisms, i.e., field-dependent generation of free charge carriers and enhanced bimolecular recombination, in the Supporting Information, analyzing the J–V curves in reverse direction (Figure S2, Supporting Information) At this point, the lesson learned is that SA and AA are excellent solvent additives for best performing solar cells with FF > 65% and JSC > 13 mA cmÀ2, leading to PCEs of about 7%, even outperforming the DIO reference. Neither BA nor TA significantly improved the solar cell performance over the deposition from neat o-xylene
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