Eutectic phase behavior induced by a simple additive contributes to efficient organic solar cells

Highlights: • Eutectic phase behavior between additive and acceptor contributes to tighter molecular stacking and more ordered arrangement. • DIB is tolerant to concentration range and work well in both polymer and all-small-molecule OSCs. • 17.36% and 17.72% efficiency with fill factor over 78% are achieved in PM6:Y6-based binary and ternary OSCs, respectively. Introducing a small amount of high boiling point solvent additive has been widely regarded as a feasible method to optimize the active layer morphology of organic solar cells (OSCs). However, current additives are initially developed for fullerene based OSCs and the development of additive engineering is lagging behind the development of non-fullerene acceptor based OSCs. Here, a simple and versatile solid additive, 1,4-diiodobenzene (DIB), is introduced to the non-fullerene OSCs. Due to the formation of a eutectic phase between the additive and the non-fullerene acceptor, a desired microstructure with tighter molecular stacking and more ordered molecular arrangement is achieved. As a result, DIB treated OSCs display significantly enhanced performance with a power conversion efficiency (PCE) of 17.72% for ternary device, 17.36% for binary device and 15.03% for thick-film (300 nm) device. Additional advantages of the DIB treatment include excellent device stability, toleration of a wide additive concentration range, and versatility in both polymer and small molecule OSCs. The results highlight the importance of additive engineering in high-performance OSCs and demonstrate the significance of supramolecular interactions.