A Terminally Tetrafluorinated Nonfullerene Acceptor for Well-Performing Alloy Ternary Solar Cells

Fabricating ternary solar cells is a pivotal strategy to improve the power conversion efficiencies (PCEs) of organic photovoltaic devices. However, it is still a challenge to simultaneously improve the performance parameters of ternary devices. Therefore, the third ingredient in ternary blends should be precisely designed or selected. Herein, a new medium-bandgap small-molecule acceptor, namely, 3,9-bis(2-methylene-(3-(1-(3,5-dimethylphenyl)-1cyanomethylene)indanone))-5,5,11,11-tetrakis-(4-hexylphenyl)dithieno[2,3-d:2',3'-d']-sindaceno[1,2-b:5,6-b']dithiophene (ITIF), is synthesized by end-capping with a new fluorinated, asymmetric terminal group, (Z)-2-(3,5-difluorophenyl)-2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene) acetonitrile. Replacing the CN substituent with the asymmetric 3,5-difluorophenyl substituent obviously up-shifts the lowest unoccupied molecular orbital (LUMO) level of ITIF to -3.78 eV, enlarges the bandgap to 1.82 eV, and improves the absorption coefficient to approximate to 50% higher than that of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis-(4-hexylphenyl)dithieno[2,3-d:2',3'-d']-sindaceno[1,2-b:5,6-b']dithiophene (ITIC). Due to the similar structures, ITIF and ITIC can combine as an alloyed acceptor, which makes it convenient to tune the morphology and optical and electrochemical properties of ternary blends. The enhanced absorption coefficient of ITIF and the rapid fluorescence resonance energy transfer from ITIF to ITIC remarkably improve the absorption of the ternary blend film, hence compensating for the external quantum efficiency (EQE) curves. When ITIF is introduced into ternary solar cells 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):ITIF:ITIC blends, the PCEs of the ternary devices are increased from 9.2% to 10.5%, and the short-circuit currents, open-circuit voltages, and fill factors are simultaneously improved.