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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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