Branched Alkoxy Side Chain Enables High-Performance Non-Fullerene Acceptors with High Open-Circuit Voltage and Highly Ordered Molecular Packing

The linear alkyl chains on the β-position of the thieno[3,2-b]thiophene units of Y6 play a critical role in determining the molecular properties of non-fullerene acceptors (NFAs) and device performance of the corresponding organic solar cells (OSCs). This linear chain can be substituted with either alkoxy or branched alkyl side chains, which are two common strategies to modify the property of Y6-type molecules. In this paper, we use a combination of these two strategies by using branched alkoxy side chains to modify Y6 and develop a new NFA named Y6-O2BO. Interestingly, this branched alkoxy substitution introduces different effects from previously used branched alkyl or linear alkoxy modifications. More ordered molecular packing and enhanced crystallinity are observed for Y6-O2BO-based blend films, which should be beneficial for charge carrier transportation. The Y6-O2BO-based device exhibits a much enhanced open-circuit voltage (VOC) compared to those based on linear alkoxy or branched alkyl chain substituted molecules. By using a mixture of Y6 and Y6-O2BO, the VOC of ternary devices can be linearly tuned between 0.84 and 0.96 V based on the ratio of these two acceptors. As a result, the optimal OSC yields an improved VOC of 0.88 V and a high FF of 0.79, leading to a maximum efficiency of 17.5%. This reveals the effectiveness of branched alkoxy chains in elevating VOC and further optimizing the performance of OSCs.