Molecular Design of Efficient Chlorine‐ and Carboxylate‐Functionalized Donor Polymers for Nonfullerene Organic Solar Cells Enabling Processing with Eco‐Friendly Solvent in Air

Realizing the commercial applications of nonfullerene organic solar cells (NFOSCs) require a balanced of power conversion efficiency (PCE), production cost, and stability. However, because most high‐performance NFOSC devices contain air‐sensitive donor polymers that have low solubility in eco‐friendly solvents, their fabrication requires halogenated solvents and an inert atmosphere. Herein, an air‐processed inverted NFOSC device is developed using a relatively cost‐effective chlorine‐ and carboxylate‐functionalized bithiophene‐based donor polymer, P(F‐BiT)‐COOBOCl(out). When blended with 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6,7‐difluoro)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐ d :2′,3′‐ d ′]‐ s ‐indaceno[1,2‐ b :5,6‐ b ′]dithiophene (IT‐4F), the resultant device yields a high PCE of 11.91%, with good shelf‐life stability and photostability under ambient conditions without encapsulation, and less performance degradation than most reported NFOSCs. Importantly, when the polymer blend is processed in air with an eco‐friendly solvent, 1,2,4‐trimethylbenzene, the resultant device exhibits a reasonably high PCE of 10.60% (certified PCE: 10.467%) without encapsulation, which is the highest value reported to date for NFOSCs fabricated under such conditions. The potential of this high‐performance and eco‐friendly processable polymer is further demonstrated in the excellent PCE of 14.22% of a device with a P(F‐BiT)‐COOBOCl(out):Y6‐BO‐4Cl blend prepared in o ‐xylene solvent. This study provides perspectives and opportunities for designing and developing efficient photoactive materials as a new strategy for the commercialization of NFOSCs.