Enhanced Photovoltaic Performance of Ternary Polymer Solar Cells by Incorporation of a Narrow-Bandgap Nonfullerene Acceptor

We developed a novel nonfullerene electron acceptor, IffBR, that consists of electron-rich indaceno[1,2-b:5,6-b']dithiophene as the central unit and an electron-deficient 5,6-difluorobenzo[c][1,2,5]thiadiazole unit flanked with rhodanine as the peripheral group. IffBR exhibits peak UV-vis absorbance at 658 nm, which is complementary with the absorption profiles of the wide-bandgap conjugated polymers poly[4,8-bis(4,5-dihexylthiophen-2-yl)benzo[1,2-b:4,5-b']-dithiophene-alt-2-(2-butyloctyl)-5,6-difluoro-4,7-di-(thiophen-2-yl)-2H-benzo [d] [1,2,3]triazole] (PBTA-BO) and the fullerene acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). The ternary device constructed with PBTA-BO/PC71BM/IffBR as the light-absorption layer exhibited significantly better photovoltaic performance than those obtained from devices based on a bulk-heterojunction layer comprised of binary components. This improvement was attributed to the broadened absorbance, formation of cascade charge-transfer pathways, reduced nongeminate recombination, enhanced charge extraction, and more favorable morphologies of the bulk-heterojunction films. The optimized ternary device exhibited a power conversion efficiency of 9.06%, which is significantly higher than those of binary devices based on either PBTA-BO/IffBR (6.24%) or PBTA-BO/PC71BM (4.73%). These results indicate that IffBR is an outstanding electron acceptor, suitable for the fabrication of nonfullerene or multicomponent-blend polymer solar cells.