Balanced distribution of donors and acceptors enabled by volatile isomerization additives for 20.81% efficiency layer-by-layer polymer solar cells

Abstract

The controlled interdiffusion and balanced distribution of donors and acceptors are the key to regulating the vertical phase separation morphology and achieving high-performance layer-by-layer polymer solar cells (LBL PSCs). In this work, two volatile isomerization additives 2-bromo-5-iodo-1,3-dimethylbenzene (2-BrIDB) and 5-bromo-2-iodo-1,3-dimethylbenzene (5-BrIDB) were developed to finely regulate the interdiffusion between D18 and L8-BO, enabling more balanced distribution of the donors and acceptors, and an improved vertical phase separation morphology. Two isomerization additives can effectively enhance the molecularly ordered stacking of L8-BO and regulate the interdiffusion between D18 and L8-BO, especially the liquid additive 2-BrIDB, which exhibited a stronger non-covalent interaction with L8-BO and partial dissolution of D18 films, prolonging the film-forming time, facilitating effective swelling of D18 and promoting intimate interdiffusion between D18 and L8-BO. As a result, the additive-treated films achieved the optimal vertical phase separation morphology with more balanced distribution of D18 and L8-BO and a more ideal D/A interpenetrating bicontinuous network structure, providing a higher power conversion efficiency (PCE) of 20.81% and 19.43% for 2-BrIDB and 5-BrIDB, respectively. Our work provided insights into achieving excellent vertical phase separation morphology and high-performance LBL PSCs by regulating the interdiffusion between donors and acceptors to balance the distribution through the use of volatile isomerization additives.