Effect of Branching Proportion of Outer Alkyl-Chains on Encapsulation Effect and Charge Transport of A-DA'D-A Type Small Molecule Acceptors.

Abstract

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.