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Fig. 17 a Chemical structures of PTB7 and PC
71
BM. b Absorption spectra of PTB7 and
PTB7:PC
71
BM blend films; PTB7 exhibited strong absorbance from 550 to 750 nm. c J-V curves
of films of the LBG polymer, fabricated using various solvent systems. d EQE spectra of devices
fabricated using two different solvent systems: CB containing 3 % DIO and DCB containing 3 %
DIO [
59
]
predicted theoretically that a PCE of approximately 11 % could be achieved if the
value of E
g
of the donor polymer were in the range 1.2-1.8 eV [
57
]. Many
promising LBG conjugated polymers have been reported [
58
-
61
]. For instance,
Yu's group prepared a series of LBG polymers based on alternating thieno
[3,4-b]thiophene and benzodithiophene units (Fig.
16
a) [
58
]. Figure
16
b summa-
rizes the properties of these materials, which exhibited onset light absorbencies of
762-786 nm and the relative values of E
g
of 1.58-1.63 eV. After these new
polymers were blended with PC
61
BM to serve as active layers for OPVs, a device
incorporating PTB4 exhibited the best performance, with a PCE of 5.9 %, as a
result of this polymer possessing the highest hole mobility (7.7 9 10
-4
cm
2
V
-1
s
-1
) among the tested polymers [
58
].
The Yu research team prepared a new PTB family member, the polymer PTB7,
that provided a device exhibiting a record high PCE of 7.4 % [
59
]. The structure of
PTB7 (Fig.
17
a) featured branched side chains on its ester and benzodithiophene
units to improve its solubility in organic solvents. The mobility in a film of this
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