Ternary Nonfullerene Polymer Solar Cells with 12.16% Efficiency by Introducing One Acceptor with Cascading Energy Level and Complementary Absorption

A novel small‐molecule acceptor, (2,2′‐((5E,5′E)‐5,5′‐((5,5′‐(4,4,9,9‐tetrakis(5‐hexylthiophen‐2‐yl)‐4,9‐dihydro‐s‐indaceno1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(4‐(2‐ethylhexyl)thiophene‐5,2‐diyl))bis(methanylylidene)) bis(3‐hexyl‐4‐oxothiazolidine‐5,2‐diylidene))dimalononitrile (ITCN), end‐capped with electron‐deficient 2‐(3‐hexyl‐4‐oxothiazolidin‐2‐ylidene)malononitrile groups, is designed, synthesized, and used as the third component in fullerene‐free ternary polymer solar cells (PSCs). The cascaded energy‐level structure enabled by the newly designed acceptor is beneficial to the carrier transport and separation. Meanwhile, the three materials show a complementary absorption in the visible region, resulting in efficient light harvesting. Hence, the PBDB‐T:ITCN:IT‐M ternary PSCs possess a high short‐circuit current density (J_sc) under an optimal weight ratio of donors and acceptors. Moreover, the open‐circuit voltage (V_oc) of the ternary PSCs is enhanced with an increase of the third acceptor ITCN content, which is attributed to the higher lowest unoccupied molecular orbital energy level of ITCN than that of IT‐M, thus exhibits a higher V_oc in PBDB‐T:ITCN binary system. Ultimately, the ternary PSCs achieve a power conversion efficiency of 12.16%, which is higher than the PBDB‐T:ITM‐based PSCs (10.89%) and PBDB‐T:ITCN‐based ones (2.21%). This work provides an effective strategy to improve the photovoltaic performance of PSCs.