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

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'] dithiophene4,8-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[1,2b: 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 PBDBT: 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.