Isopropanol: A Green Solvent Additive for 20% Efficiency Organic Solar Cells with Excellent Generality and Improved Eco‐Compatibility

Abstract Solvent additive engineering has been regarded as the most common and effective approach to finely tune the bulk heterojunction (BHJ) morphology of organic solar cells (OSCs). However, most conventional solvent additives are environmentally unfriendly and difficult to completely remove due to their toxicity and high boiling points. Herein, isopropanol (IPA) is first reported as a green solvent additive to significantly improve the photovoltaic performance of OSCs. Notably, the addition of IPA can facilitate the formation of highly ordered molecular stacking and suppress charge recombination in OSCs through precisely modulating molecular aggregation behaviors during film formation. As a result, the IPA processed PM6:BTP‐eC9‐based OSC yields an impressive power conversion efficiency (PCE) of 20.0%, which is much higher than 18.2% for that without additives. Furthermore, the all‐green‐solvent processed OSCs based on PM6:BTP‐eC9‐4ClO with o‐xylene as the main solvent and IPA as an additive achieve a high PCE of 19.4%. With the combined high device efficiency, excellent generality and improved eco‐compatibility, the morphology control strategy with IPA as an additive provides a new promising route toward commercial OSC technology.