A new series of low-bandgap copolymers based on
electron-accepting thieno[3,4-b]pyrazine (TPZ) and different
electron-donating aza-heteroaromatic units, such as carbazole
(CZ), dithieno[3,2-b:20,30-d]pyrrole (TPR) and dithieno[3,2-b:20,30-
e]pyridine (TPY), have been synthesized by Suzuki or Stille
coupling polymerization. The resulting copolymers were characterized
by NMR, elemental analysis, gel permeation chromatography,
thermogravimetric analysis, and differential scanning
calorimetry. UV–vis absorption and cyclic voltammetry measurements
show that TPZ-based copolymer with TPR has the best
absorption due to the strongest intramolecular charge transfer
effect and smallest bandgap. The basic electronic structure of
D-A model compounds of these copolymers were also studied
by density functional theory (DFT) calculations. The conclusion
of calculation agreed also well with the experimental results.The polymer solar cells (PSCs) based on these copolymers were
fabricated with a typical structure of ITO/PEDOT:PSS/copolymer:
PC71BM/Ca/Al under the illumination of AM 1.5G, 100 mW
cm2. The performance results showed that TPZ-based copolymer
with TPR donor segments showed highest efficiency of
1.55% due to enhanced short-circuit current density. The present
results indicate that good electronic, optical, and photovoltaic
properties of TPZ-based copolymers can be achieved by just
fine-tuning the structures of aza-heteroaromatic donor segments
for their application in PSCs.