  A series of donor-acceptor-donor (D-A-D) structured small-molecule compounds,
with 3,3'-(ethane-1,2-diylidene)bis(indolin-2-one) (EBI) as a novel electron
acceptor building block coupled with various electron donor end-capping
moieties (thiophene, bithiophene and benzofuran), were synthesized and
characterized. When the fused-ring benzofuran is combined to EBI (EBI-BF), the
molecules displayed a perfectly planar conformation and afforded the best
charge tranport properties among these EBI compounds with a hole mobility of up
to 0.021 cm2 V-1 s-1. All EBI-based small molecules were used as donor material
along with a PC61BM acceptor for the fabrication of solution-processed
bulk-heterojunction (BHJ) solar cells. The best performing photovoltaic devices
are based on the EBI derivative using the bithiophene end-capping moiety
(EBI-2T) with a maximum power conversion efficiency (PCE) of 1.92%, owing to
the broad absorption spectra of EBI-2T and the appropriate morphology of the
BHJ. With the aim of establishing a correlation between the molecular structure
and the thin film morphology, differential scanning calorimetry, atomic force
microscopy and X-ray diffraction analysis were performed on neat and blend
films of each material.
