A Bithiazole‐Substituted Donor for High‐Efficiency Thick Ternary Organic Solar Cells via Regulation of Crystallinity and Miscibility

Abstract Organic solar cells (OSCs) with thick active layers exhibit great potential for future roll‐to‐roll mass production. However, increasing the thickness of the active layer generally leads to unfavorable morphology, which decreases the device's performance. Therefore, it is a critical challenge to achieve OSCs with high efficiency and thick film simultaneously. Herein, a small molecular donor, ZW1, incorporating a bithiazole unit along with a thiophene group as a π ‐bridge is reported. ZW1 with high crystallinity is employed to fabricate D18:ZW1:Y6 ternary devices, which enhances the crystallization, optimizes the morphology, and suppresses bimolecular recombination. Additionally, ZW1 shows better miscibility with D18, resulting in the preferred vertical phase distribution. As a result, an outstanding power conversion efficiency (PCE) of 18.50% is realized in ternary OSCs with 120 nm active layer thickness. Importantly, the thick ternary OSCs attain a high PCE of 16.67% (thickness ≈300 nm), significantly higher than the corresponding binary devices (13.50%). The PCE of 16.67% is one of the highest values for thick‐film OSCs reported to date. This work demonstrates that the incorporation of highly crystalline small‐molecule donors into ternary OSCs, possessing good miscibility with host materials, presents an effective strategy for fabricating highly efficient thick OSCs.