Effect of Branching Proportion of Outer Alkyl‐Chains on Encapsulation Effect and Charge Transport of A‐DA′D‐A Type Small Molecule Acceptors

The rapid development of organic solar cells (OSCs) is closely linked to advances in active layer photovoltaic materials, particularly A-DA'D-A type small molecule acceptors (SMAs). Despite extensive studies on SMAs, certain critical factors governing their molecular properties remain insufficiently explored. Here, we identify a previously overlooked alkyl-chain encapsulation effect in SMAs, where the conjugated backbone is partially encapsulated by surrounding alkyl-chains, thereby hindering intermolecular π-π interactions and fragmenting conductive networks. To mitigate this effect, we propose a strategy of altering the molecular stacking mode through precise control of the branching proportion of the outer alkyl-chains. Systematic investigations reveal that non-branched outer alkyl-chains lead to a pronounced encapsulation effect, whereas high branching proportion enlarges intermolecular distances, impairs electronic coupling, and induces blue-shifted absorption. In contrast, a moderate branching proportion effectively suppresses the encapsulation effect without significantly compromising intermolecular electronic coupling, enabling more coherent conductive networks and enhanced electron mobility. Consequently, the optimized SMA BT15-F delivers a power conversion efficiency of 20.53% in binary OSCs, ranking among the highest values reported for binary OSCs. This work establishes suppression of the alkyl-chain encapsulation effect as a key consideration in SMA molecular design and highlights the importance of alkyl-chain structure-property relationships for high-performance OSCs.