Chemistry Reference
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optical band gaps of 1.50-1.70 eV. The highly electron-rich character of dialkoxybithiophene in these
polymers, however, destabilizes their HMO and significantly affects the photovoltaic efficiency with
power conversion efficiencies below 1.5%. On the other hand, cyclopentadithiophene copolymers
achieved a better power conversation efficiency greater than 3%.
Sharma and coworkers [
278
] reported synthesis of two low-band gap copolymers. One consists of
alternating dihexyloxyphenylene and
-[4-(diphenylamino)phenyl methylene]-4-nitrobenzene aceto-
nitrile. The other one consists of alternating dihexyloxyphenylene and
a
a
0
-[(1,4-phenylene)
a
,
dimethylidyne]bis(-4-nitrobenzene acetonitrile):
C
6
H
13
NO
2
H
13
C
6
n
C
6
H
13
NC
N
H
13
C
6
NC
CN
O
2
N
NO
2
polymer 2
polymer 1
They reported that these copolymers showed broad absorption curves with long-wavelength
absorption maximum around 620 nm and optical band of 1.68 and 1.64 eV for both polymers. Both
polymers were studied for photovoltaic response in bulk heterojunction solar cells. They observed an
overall power conversion efficiency of 3.15 and 2.60% for the cast polymers. Further improvement
led up to 4.06 and 3.35% for the devices based on thermally annealed materials.
Wei and coworkers [
279
] used Stille polycondensation to prepare a series of low-band gap
copolymers, by conjugating the electron-accepting pyrido[3,4-
b
]pyrazine moieties with electron
rich benzo[1,2-
]di thiophene or cyclopentadithiophene units. All resulting polymers exhibited
excellent thermal stability and sufficient energy offsets for efficient charge transfer and dissociation.
The band gaps of the polymers could be tuned in the range 1.46-1.60 eV by using the two different
donors, which have different electron-donating abilities. The three-component copolymers,
incorporating the thiophene and bithiophene segments, respectively, absorbed broadly, covering
the solar spectrum from 350 to 800 nm. The best device performance resulted in power conversion
efficiency of 3.15%.The polymeric materials were illustrated as follows:
b
:3,4-
b
N
OC
8
H
17
C
8
H
17
H
17
C
8
Donor
Donor
a
S
n
m
S
N
N
S
S
S
OC
8
H
17
C
8
H
17
O
OC
8
H
17
donor
donor
Liu et al. [
280
] synthesis and evaluation of n-conjugated copolymers were based on a soluble
electro active benzo[1,2-
b
:4,5-
b
]difuran chromophore. The comonomer units consisted of thiophene/
benzo [
c
][1,2,5] thiadiazole/9-phenylcarbazole. These copolymers cover broad absorption ranges
