Ternary Organic Solar Cells With 12.8% Efficiency Using Two Nonfullerene Acceptors With Complementary Absorptions

Abstract A new small‐molecule acceptor (2,9‐bis(2‐methylene‐(3(1,1‐dicyanomethylene)benz[f]indanone))7,12‐dihydro‐(4,4,10,10‐tetrakis(4‐hexylphenyl)‐5,11‐diocthylthieno[3′,2′:4,5]cyclopenta[1,2‐b]thieno[2″,3″:3′,4′]cyclopenta[1′,2′:4,5]thieno[2,3‐f][1]benzothiophene) (NNBDT) based on naphthyl‐fused indanone ending units is reported. This molecule shows a narrow optical bandgap of 1.43 eV and effective absorption in the range of 700–870 nm. The devices based on poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione))] (PBDB‐T):NNBDT yield a power conversion efficiency of 11.7% with a low energy loss of 0.55 eV and a high fill factor (FF) of 71.7%. Another acceptor (2,9‐bis(2‐methylene‐(3(1,1‐dicyanomethylene)benz[f]indanone))7,12‐dihydro‐4,4,7,7,12,12‐hexaoctyl‐4H‐cyclopenta[2″,1″:5,6;3″,4″:5′,6′]diindeno[1,2‐b:1′,2′‐b′]dithiophene (FDNCTF) is introduced as the third component to fabricate ternary devices. The two acceptors (NNBDT and FDNCTF) possess complementary absorption, same molecular orientation, and well‐miscible behavior. It is found that there exists a nonradiative energy transfer process from FDNCTF to NNBDT. The fullerene‐free ternary cells based on PBDB‐T:NNBDT:FDNCTF achieve a high efficiency of 12.8% with an improved short circuit current near 20 mA cm⁻² in contrast to the binary devices. The result represents the best performance for fullerene‐free ternary solar cells reported to date and highlights the potential of ternary solar cells.