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

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.