method were determined as three orders of magnitude lower than
the FET mobilities. The photovoltaic properties of the polymers
were investigated and the highest achieved PCEs for PIDTPyT
and PIDTDTPyT were 3.41% and 3.91%, respectively. The deep
LUMO level of the polymers combined with the low vertical
mobilities may not allow efficient charge dissociation and sepa-
ration, which may give rise to the limitation on photovoltaic
performance.
Acknowledgements
This work is supported by the National Science Foundation’s
NSF-STC program under Grant No. DMR-0120967, the
AFOSR (FA9550-09-1-0426), the Office of Naval Research
(N00014-11-1-0300), and the World Class University (WCU)
program through the National Research Foundation of Korea
under the Ministry of Education, Science and Technology (R31-
21410035). A.K.-Y.J. thanks the Boeing-Johnson Foundation
for financial support. Y. Sun thanks the State-Sponsored
Scholarship for Graduate Students from China Scholarship
Council. S. C. Chien thanks the National Science Council of
Taiwan (NSC98-2917-I-009-112) for supporting the Graduate
Students Study Abroad Program.
Fig. 7 Current density and voltage (J–V) curves of the hole-only devices
containing polymer and PC71BM with the ratio of 1 : 3. (V ¼ Vapplied
Vbi, Vbi ¼ 0.1 V).
ꢁ
separated free charges and the charge recombination rate may
potentially increase. The higher FF obtained from PIDTDTPyT
based devices might be correlated with its higher LUMO level
compared to the PIDTPyT-based device.
Although the results based on a field-effect transistor indicated
that both PIDTPyT and PIDTDTPyT feature superior lateral
hole carrier-transport abilities, the vertical hole carrier-transport
behaviours of the polymers blended with the fullerene play an
essential role in the photovoltaic devices performance.42 Herein,
hole-only devices based on ITO/PEDOT:PSS/BHJ/MoO3/Al
were fabricated to measure the vertical hole mobility. MoO3 was
used as an interfacial layer to suppress electron injection from Al
so that the hole-only current was measured.43 Fig. 7 shows the
current density and voltage curve (J–V) of the hole-only devices
containing polymer and PC71BM with the blending ratio of 1 : 3.
The space charge limited current (SCLC) model was employed to
investigate the vertical hole mobilities using the following equa-
tion, J ¼ 9303rmV2/8L3, where J is the current density (mA cmꢁ2),
303r is the permittivity of the polymer, m is the carrier mobility,
and L is the active layer thickness. The mobilities were extracted
by modelling the dark current in the SCLC region. The calcu-
lated vertical hole-transport mobilities of PIDTPyT and
PIDTDTPyT are 6.34 ꢃ 10ꢁ5 and 7.97 ꢃ 10ꢁ5 (cm2 Vꢁ1 sꢁ1),
respectively, which are lower than the FET mobilities by three
orders of magnitude. This discrepancy indicates that the vertical
hole-transporting ability was greatly inferior to the lateral
mobility, which might cause charge recombination issues,
thereby leading to lower FF and thinner optimized device
thickness.41
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4. Conclusion
To conclude, we have synthesized two new low bandgap
polymers (PIDTPyT and PIDTDTPyT) through the Stille
polymerization between the IDT donor and PyT as well as
thienyl-flanked PyT acceptor monomers. The stronger acceptor
character of the PyT unit broadened the absorption spectrum of
the polymers to longer wavelengths and resulted in deep energy
levels. The OFET results showed that the two polymers have
high lateral hole mobilities as high as 0.066 cm2 Vꢁ1 sꢁ1. However,
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This journal is ª The Royal Society of Chemistry 2011