Revisiting Solvent Additives for the Fabrication of Polymer:Fullerene Solar Cells: Exploring a Series of Benzaldehydes

• Published in: Solar RRL Vol. 5; no. 9
• Main Authors: Sprau, Christian, Kattenbusch, Jens, Li, Yonghe, Müller, Erich, Gerthsen, Dagmar, Berger, Rüdiger, Michels, Jasper J., Colsmann, Alexander
• Format: Journal Article
• Language: English
• Published: 01.09.2021
• Subjects: bulk heterojunctions; morphologies; organic photovoltaics; phase separation; solvent additives
• ISSN: 2367-198X; 2367-198X
• DOI: 10.1002/solr.202100238

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. Structurally similar substituted benzaldehydes are investigated as solvent additives in PTB7:PC71BM bulk‐heterojunction solar cells. Only the lowest polymer solubility in the solvent additive suppresses liquid–liquid phase separation by polymer aggregation, reducing the size of the fullerene domains to a few ten nanometers for optimized device performance.