Overcoming Disordered Preaggregation in Liquid State for Highly Efficient Organic Solar Cells Printed from Nonhalogenated Solvents

The current power conversion efficiencies of laboratory‐sized organic solar cells (OSCs), based on the spin‐coating process with halogenated solvents, have exceeded 19%. Environmentally friendly printing is needed to bridge the gap between laboratory and industrialization by being compatible with roll‐to‐roll large‐area production. Here, the molecular design rules are revealed for enhancing the green printing potential of the state‐of‐the‐art photovoltaic martial systems by investigating the detailed structure formation dynamic and the key determining factors. By comparing two model systems based on D18:Y6 and D18:BTP‐eC9, it is found that disordered preaggregation in liquid state can result in over‐sized domains with reduced crystallinity and disordered molecular orientation, which significantly limits device performance. By systematically tuning the length of the inner alkyl side chains with multiple Y‐series materials, the authors demonstrate that molecular side‐chain engineering can effectively supress the detrimental disordered preaggregation in liquid state during environmentally friendly printing process, leading to enhanced crystallization with preferential faceon molecular orientation, more efficient exciton dissociation and charge carrier transport, and finally high upscaling potential. The work provides deeper insights into molecular engineering and structure formation dynamics toward environmentally friendly production of OSCs. Disordered preaggregation in liquid state during environmentally friendly printing of organic solar cells significantly limits device performance based on nonfullerene acceptors. Molecular side‐chain engineering can effectively tune this disordered preaggregation in liquid state, leading to enhanced crystallization with preferential face on molecular orientation, more efficient exciton dissociation and charge carrier transport, and finally good upscaling potential.