High-Performance Bulk-Heterojunction Polymer Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 182

polymer was approximately 5.8 9 10 -4 cm 2 V -1 s -1 ; strong absorption occurred
from 550 to 750 nm (Fig. 17 b). To compensate for the absorption of PTB7, Yu
et al. employed PC 71 BM as the acceptor. The resulting absorption spectrum
(Fig. 17 b) revealed that the PTB7:PC 71 BM blend absorbed broadly from 300 nm
to approximately 800 nm. The authors further controlled the morphology of the
thin film by altering the solvent system used to dissolve the polymer blends
(Fig. 17 c). The device prepared when employing 3 % 1,8-diiodoctane (DIO) in
chlorobenzene (CB) as the solvent exhibited a significantly enhanced value of J sc
of 14.5 mA cm -2 , resulting in a high efficiency of 7.40 % [ 59 ].
6 Conclusion and Outlook
The research field of polymer solar cells continues to expand. Tremendous pro-
gress has been made recently on the design of new materials, the development of
novel device structures, and other aspects. In this chapter, we have reviewed
several important methods for enhancing device efficiency, including controlling
the morphologies of polymer blends, interfacial engineering of polymer-electrode
interfaces, and optical approaches toward increasing the light harvesting ability.
We have also briefly noted the development of new LBG polymers. In theory, a
single-junction BHJ device can achieve a maximum PCE of approximately 11 %
[ 57 ]. With a recently reported PCE of approximately 9 %, we believe that single-
junction OPVs with such high PCEs should appear in the near future. To achieve
even higher efficiency, Dennler et al. indicated that a PCE of approximately 15 %
might be possible through the development of multi-junction OPVs [ 62 ]. More
recently, Janssen and Nelson suggested that limits of 20-24 % might even be
reachable for single-junction OPVs based on a modified balance model; such high
PCEs are similar to the highest efficiencies obtained using crystalline Si tech-
nologies [ 63 ]. The materials described herein suggest that polymer solar cells have
great potential. With their additional advantageous features similar to those of
plastics, including light weight and high flexibility, we foresee the commerciali-
zation of practical, large-area OPV modules in the near future.
Acknowledgment We thank the National Science Council of Taiwan and the Ministry of
Education of Taiwan (through the ATU program) for financial support.
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