C O M M U N I C A T I O N S
while their LUMO energy levels are dictated by the acceptors on
the side chains.
Hence, a detailed hole mobility study of the polymers:PC71BM
blend films was conducted by using the space-charge-limited-current
(SCLC) method. The SCLC measurements have been widely used
to estimate the charge transporting ability of the PSC active layers.10
The blend films exhibited a hole mobility of 5.27 × 10-4 cm2 V-1
s-1 for PFDCN and 1.16 × 10-3 cm2 V-1 s-1 for PFPDT,
respectively, which is even higher than that of P3HT in a similar
device configuration.10 Considering their two-dimensional conju-
gated structure, PFDCN and PFPDT may have better isotropic
charge transport ability than linear polymers, which is beneficial
for PSC applications.
In conclusion, two new conjugated polymers PFDCN and PFPDT
have been designed and synthesized. Both of them exhibit excellent
photovoltaic properties with a PCE as high as 4.74%. This approach
provides great flexibility in fine-tuning the absorption spectra and
energy levels of the resultant polymers for achieving high device
performance.
The photovoltaic properties of PFDCN and PFPDT were studied
in PSCs using PC61BM or [6,6]-phenyl-C71-butyric acid methyl
ester (PC71BM) as the acceptor in a conventional device configu-
ration of ITO/PEDOT:PSS(40 nm)/polymer:PCBM(1:4, w/w 85
nm)/Ca(10 nm)/Al(100 nm) (ITO: indium tin oxide; PEDOT:PSS:
poly(styrene sulfonate)- doped poly(ethylene-dioxythiophene); PCBM:
PC61BM or PC71BM. A detailed device fabrication process is
described in the SI. The area of the device was determined by the
size of the cathode, which was 3.14 × 10-2 cm2. The PC71BM was
chosen as the acceptor because it has electronic properties similar
to those of PC61BM but a much stronger absorption in the visible
region with a broad peak from 440 to 530 nm,9 which can
complement the absorption valley of the polymers (Figure 1).
Acknowledgment. This work was partially supported by the
National Science Foundation’s NSF-STC program under Project
No. DMR-0120967, the DOE’s “Future Generation Photovoltaic
Devices and Process” program under Project No. DE-FC36-
08GO18024/A000, and the World Class University (WCU) program
through the National Research Foundation of Korea funded by the
Ministry of Education, Science and Technology (R31-10035).
A.K.Y.J. thanks the Boeing-Johnson Foundation for financial
support. S.K.H. and H.L.Y. thank the Intel Foundation PhD
Fellowship.
Figure 2. (a) J-V curves of PFDCN and PFPDT-based solar cells under
AM 1.5G illumination. (b) External quantum efficiency spectrum of PFDCN
and PFPDT-based solar cells.
Both of the polymers exhibit promising photovoltaic properties
with an average PCE (from eight devices for each polymer) of
4.47% and 4.19% for PFDCN and PFPDT, respectively, in
PC71BM-based devices. Figure 2a shows the current density-voltage
(J-V) curves of the best devices under the illumination of simulated
AM 1.5G conditions (100 mW/cm2). The detailed procedures for
characterization are listed in the SI. PCEs up to 4.74% were
observed for the PFDCN:PC71BM solar cells with an open circuit
voltage (VOC) of 0.99 V, a short circuit current density (JSC) of
9.62 mA/cm2, and a fill factor of 50.0%. The maximum PCE of
PFPDT:PC71BM solar cells also reaches 4.37% with a VOC of 0.99
V, a JSC of 9.61 mA/cm2, and a fill factor of 46.0%. For comparison,
the J-V curves and the external quantum efficiencies (EQEs) of
PC61BM devices are also shown in Figure 2. A significant drop in
JSC was observed in both PC61BM devices, indicating the importance
of the complementary absorption of PC71BM. Figure 2b shows the
EQE of the devices illuminated by monochromatic light. The results
from the EQE study clearly showed the photocurrent increments
of PC71BM devices because of the enhanced absorption from
PC71BM.
It is interesting that the PSC device performance based on
PFDCN and PFPDT is comparable to those of the state-of-the-art
works based on linear narrow band gap D-A conjugated polymers.2,5
Several factors contribute to their excellent photovoltaic properties.
First, both PFDCN- and PFPDT-based PSC devices exhibited a
much higher Voc (0.99 V) compared to most of the devices based
on the previously reported small band gap materials,2 due to their
relatively low HOMO level, which can be readily controlled by
adjusting the main chain donor. Second, their absorption spectra
can be easily tuned by controlling the acceptor strength of their
side chains or by using a more efficient π-bridge, without disturbing
their conjugated main chain structures. Originally, there are some
concerns that PFDCN and PFPDT may possess low hole mobility
because of their amorphous nature and bulky and rigid side chains.
Supporting Information Available: Experimental details of the
synthesis of the polymer, the fabrication and characterization of the
devices, measurements and instruments. This material is available free
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