Eco‐Friendly Solvent‐Processed Fullerene‐Free Polymer Solar Cells with over 9.7% Efficiency and Long‐Term Performance Stability

A wide‐bandgap polymer, (poly(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo1,2‐b:4,5‐b′]dithiophene))‐alt‐(2,5‐(methyl thiophene carboxylate))]) (3MT‐Th), is synthesized to obtain a complementary broad range absorption when harmonized with 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno2,3‐d:2′,3′‐d′]‐s‐indaceno1,2‐b:5,6‐b′]dithiophene (ITIC). The synthesized regiorandom 3MT‐Th polymer shows good solubility in nonhalogenated solvents. A film of 3MT‐Th:ITIC can be employed for forming an active layer in a polymer solar cell (PSC), with the blend solution containing toluene with 0.25% diphenylether as a nonhalogenated additive. The corresponding PSC devices display a power conversion efficiency of 9.73%. Moreover, the 3MT‐Th‐based PSCs exhibit excellent shelf‐life time of over 1000 h and are operationally stable under continuous light illumination. Therefore, methyl thiophene‐3‐carboxylate in 3MT‐Th is a promising new accepting unit for constructing p‐type polymers used for high‐performance nonfullerene‐type PSCs.