Regulation of Crystallinity and Vertical Phase Separation Enables High‐Efficiency Thick Organic Solar Cells

Abstract Thick film organic solar cells have great potential for preparing large‐area devices in the future. However, unsuitable vertical donor/acceptor distribution, poor crystallinity of materials, and voltage losses remain unclear, limiting the performance of thick film devices. Here, a target ternary strategy by introducing a nematic liquid crystalline small molecule donor BTR‐Cl into the PM6:Y6 host system is used to reduce the performance degradation caused by the increase of active layer thickness. By incorporating 10% BTR‐Cl into the blend film, the voltage loss is minimized in both thin and thick ternary devices. Moreover, the power conversion efficiency of the ternary device is maintained at 15.28% compared to 12.94% of binary devices with an active layer thickness of 300 nm. It is benefited from the optimized crystallization behavior and more suitable vertical phase distribution driven by differences in miscibility between components. While ensuring sufficient phase separation, the charge transport is improved, it also prompts the balance of charge transport and reduces the non‐radiative recombination. This work demonstrates that the use of the third component to enhance the crystallization of the active layer and improve the vertical phase distribution is an effective method to build thick films for future large‐scale printing techniques.