posted on 2016-03-10, 18:40authored byTae Eui Kang, Joonhyeong Choi, Han-Hee Cho, Sung Cheol Yoon, Bumjoon J. Kim
The backbone composition of conjugated
copolymers is of great importance
in determining the conjugated structure and intermolecular assembly
and in manipulating their optical, electrochemical, and electronic
properties. However, limited attention has been directed at controlling
the backbone composition of donor–acceptor (D–A) type
low bandgap polymers. Herein, we developed a series of D–A
random copolymers (P(BDTT-r-DPP)) composed of different
compositions of electron-rich (D) thienyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT) and
electron-deficient (A) pyrrolo[3,4-c]pyrrole-1,4-dione
(DPP). The optical and electrical properties of D–A random
copolymers could be controlled by tuning the ratios of BDTT to DPP
(4:1, 2:1, 1:1, 1:2, and 1:4) in the polymer backbone; an increase
in BDTT resulted in increased absorption in the range of 400–600
nm and a lower-lying highest occupied molecular orbital energy level,
while a higher proportion of DPP induced stronger absorption in the
range of 700–900 nm. The P(BDTT-r-DPP) copolymer
with a D:A ratio of 2:1 produced the highest power conversion efficiency
(PCE) of 5.63% in the polymer solar cells (PSCs), which outperformed
the D–A alternating copolymer, P(BDTT-alt-DPP)
(1:1)-based PSCs (PCE = 5.03%), because of the improved light absorption
and open-circuit voltage. Thus, we highlight the importance of developing
random copolymers with controlled D:A compositions for optimizing
their optoelectronic properties and performances of PSCs. Also, we
compared the polymer packing structure and the electrical properties
between the P(BDTT-r-DPP) and P(BDTT-alt-DPP) copolymers and developed a quantitative understanding of the
effect of the D:A monomer sequence on the structural, electrical,
and photovoltaic properties of the D–A copolymers.