Three Isomeric Non-Fullerene Acceptors Comprising a Mono-Brominated End-Group for Efficient Organic Solar Cells

Non-fullerene acceptors (NFAs) carrying a 1,1-dicyanomethylene-3-indanone
(IC) end-group are the most powerful ones to boost the power conversion
efficiency of organic solar cells (OSCs). However, the well-known
Knoevenagel condensation of the mono-halogenated IC end-group will
result in an NFA isomeric effect, a chemical issue that needs to be
addressed. Herein, facile preparations and separations of three well-defined
mono-brominated isomers BTzIC-2Br-δ, BTzIC-2Br-γ, and
BTzIC-2Br-δγ via column chromatography with a well-chosen
mixing solvent were demonstrated for Knoevenagel condensation, and
their structures were verified by NMR spectra and matrix-assisted
laser desorption ionization-time of flight mass spectrometry (MALDI-TOF)
mass spectra. It is the first time that an asymmetric isomer BTzIC-2Br-δγ
is reported, and the regioisomeric effect on optoelectronic properties
can be investigated based on all three isomers. Moreover, the single-crystal
structure was successfully achieved for the symmetric molecule BTzIC-2Br-γ.
With benzodithiophene (BDT)-free PFBT4T-T20 as an easily accessible
and low-cost polymer donor, the three isomers could show differentiated
device performances, with a power conversion efficiency order of BTzIC-2Br-γ
(16.00%) > BTzIC-2Br-δγ (15.81%) > BTzIC-2Br-δ
(15.29%).
The best efficiency of 16.00% achieved with BTzIC-2Br-γ is among
the highest ones for binary OSCs based on the low-cost BDT-free donors.
The facile and complete synthesis of isomeric NFAs with mono-halogenated
IC end-groups would promote the elucidation of the structure–property
relationship.