Increased open circuit voltage in fluorinated benzothiadiazole-based alternating conjugated polymers

Article abstract of DOI:10.1039/c1cc14586j

Small band-gap conjugated polymers based on monofluoro- and difluoro-substituted benzothiadiazole were developed. Highly efficient polymer solar cells (PCE as high as 5.40%) could be achieved for devices made from these polymers.

Full text of DOI:10.1039/c1cc14586j

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COMMUNICATION
Increased open circuit voltage in fluorinated benzothiadiazole-based
alternating conjugated polymersw
Yong Zhang,a Shang-Chieh Chien,ab Kung-Shih Chen,a Hin-Lap Yip,a Ying Sun,a
Joshua A. Davies,a Fang-Chung Chenb and Alex K.-Y. Jen*ac
Received 27th July 2011, Accepted 2nd September 2011
DOI: 10.1039/c1cc14586j
Small band-gap conjugated polymers based on monofluoro- and
difluoro-substituted benzothiadiazole were developed. Highly
efficient polymer solar cells (PCE as high as 5.40%) could be
achieved for devices made from these polymers.
It is well known that the introduction of an electron-
withdrawing group into the polymer backbone can lower its
energy levels.4 The change of HOMO and LUMO energy
levels is dependent on where the position of the electron-
withdrawing group is on the D–A polymer. Among numerous
electron-withdrawing groups, the use of fluorine has been
proven to be effective in lowering the HOMO energy level
and resulting in higher Voc and improved performance in
fluorinated poly(benzodithiophene-thienothiophene)s.4a To
investigate the effect of F atom on other acceptors, herein
we have introduced it onto the benzothiadiazole (BT) unit to
synthesize the monofluoro- and difluoro-substituted BT units,
which were then copolymerized with the indacenodithiophene
(IDT) donor to obtain two new conjugated polymers (PIDT–
FBT and PIDT–DFBT). Fig. 1 shows the chemical structures
of the polymers.
Polymer solar cells (PSCs) have been widely investigated
recently due to their potential for light-weight, low-cost, and
large-scale roll-to-roll processing.1 Power conversion efficiencies
(PCEs) of more than 7% have been reached based on bulk
hetero-junction (BHJ) PSCs, in which a blended film of an
electron-rich conjugated polymer and an electron-deficient
fullerene acts as the active BHJ layer.2
Open circuit voltage (Voc) is an important device parameter
in determining the overall performance of a PSC. In general,
Voc is proportional to the difference between the highest
occupied molecular orbital (HOMO) of a polymer and the
lowest unoccupied molecular orbital (LUMO) of fullerene,
although some device engineering factors, such as the type of
cathode material and exciton non-radiative recombination,
may also affect the Voc of a BHJ PSC.3 A Voc can be increased
either by increasing the LUMO of fullerene or decreasing the
HOMO of the polymer. There are several reports regarding
the development of fullerene derivatives with higher LUMO
levels than the commonly used PCBM to achieve higher Voc.
However, low band-gap polymers tend to have low-lying
HOMO, which are commonly accompanied by low-lying
LUMO. Hence, the difference between the LUMO levels of
a polymer and fullerene frequently results in inefficient charge
separation, leading to low Jsc. On the other hand, a higher
LUMO in a low band-gap polymer also accompanies with a
higher HOMO giving a lower Voc. Therefore, it is a challenge
to fine tune the band-gap and energy levels of a polymer to
have high Voc and Jsc in BHJ PSCs.
The synthetic route of monofluoro- (FBT) and difluoro-BT
(DFBT) is shown in Scheme 1. Starting from 2,5-dibromo-3-
fluorobenzene and 2,5-dibromo-3,4-difluorobenzene, compounds
1 and 5 toward FBT and DFBT, respectively, were synthesized
in three steps as shown in the ESI.w The selective nitration of 1
and 5 in the mixture of HNO3 and conc. H2SO4 afforded 2 and
6 with 485% yield. Next, the trifluoroacetyl was de-protected
in 2 and 6, by refluxing in aqueous H2SO4 to give 3 and 7. The
NO2 groups were further reduced by SnCl2 to result in
diamines 4 and 8, respectively. Finally, the ring closure of
diamines 4 and 8 with SOCl2 in pyridine generated the
monofluoro- and difluoro-substituted BT monomers, FBT and
DFBT, respectively, in 480% yield. The distannyl IDT 9 was
synthesized by following the same method reported previously.5
The polymers PIDT–FBT and PIDT–DFBT (Scheme 1)
were synthesized via the Stille polycondensation of 9 with
FBT or DFBT, respectively, using toluene as solvent and
Pd2(dba)3/P(o-tol)3 as catalyst. Purification of PIDT–FBT
and PIDT–DFBT was conducted by Soxhlet extraction of
a Department of Materials Science and Engineering, University of
Washington, Seattle, Washington 98195-2120, USA.
E-mail: ajen@u.washington.edu; Fax: +1 206 543 2600;
Tel: +1 206 543 2626
b Department of Photonics & Display Institute, National Chiao Tung
University, Hsinchu 30010, Taiwan
c WCU Center, Korea University, Jochiwan, Chungnam, 339-700,
South Korea
w Electronic supplementary information (ESI) available: The synthesis
of monomers and polymers, the device fabrication and characterization.
See DOI: 10.1039/c1cc14586j
Fig. 1 The chemical structures of PIDT–BT, PIDT–FBT and
PIDT–DFBT. (PIDT–FBT is a regiorandom polymer.)
c
11026 Chem. Commun., 2011, 47, 11026–11028
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LUMO levels at about the same strength. The absorption
spectra of PIDT–DFBT in both chloroform and thin film
(Fig. 2a and b), however, showed blue-shift peaks compared
with those of PIDT–FBT. The ICT peaks of PIDT–DFBT are
636 nm and 639 nm, respectively, in solution and in film, which
blue-shifted B10 nm compared to that of PIDT–FBT. The
onset of PIDT–DFBT in film was 698 nm, corresponding to a
band-gap of 1.78 eV. The larger band-gap compared to that of
PIDT–FBT is believed to be the stronger effect of two F atoms
on the HOMO energy level than for the PIDT–FBT polymer,
therefore, having a further lower HOMO and a similar LUMO
compared to that of PIDT–FBT.
The HOMO and LUMO energy levels of PIDT–FBT and
PIDT–DFBT were determined by cyclic voltammetry (CV)
with Pt as a counter electrode and Ag/Ag+ as a reference
electrode in acetonitrile with 0.1 M tetrabutylammonium
hexafluorophosphate at a scan rate of 100 mV sÀ1. The CV
curves of PIDT–FBT and PIDT–DFBT in thin films were
shown in Fig. 3. PIDT–FBT and PIDT–DFBT showed the
similar oxidation and reduction behaviours with good rever-
sible peaks. The HOMO and LUMO energy levels of
PIDT–FBT were determined to be À5.38 eV and À3.64 eV,
respectively, with ferrocene as reference. By comparing with
the HOMO (À5.23 eV) of PIDT–BT,6a the HOMO of
PIDT–FBT is 0.15 eV deeper through the introduction of a
F atom onto the BT unit. The LUMO energy level of
PIDT–FBT is À3.64 eV, which is 0.12 eV deeper than that
of PIDT–BT (À3.52 eV). The similar decrease of the HOMO
and LUMO energy levels in PIDT–FBT compared with that
of PIDT–BT echoed the similarity of band-gap observed in
absorption. Similarly, the HOMO and LUMO energy levels of
PIDT–DFBT were calculated to be À5.48 eV and À3.67 eV,
respectively. Again, further decreases in both HOMO and
LUMO energy levels were found, but with a larger decrease
of the HOMO value than that of the LUMO, thus, leading to
the larger band-gap. From the results, it can be concluded that
the F substitution effectively lowered the HOMO levels of
PIDT–FBT and PIDT–DFBT, which may have a positive
effect on Voc in PSC. In addition, the differences (40.5 eV)
of LUMO levels between polymers and PCBM (À4.20 eV) are
still adequate for efficient charge transfer, even for the lower
LUMO in PIDT–DFBT.
Scheme 1 Synthetic routes of monomers and polymers.
the crude product with acetone and hexane to remove oligomers
and residual catalyst. It is noted that PIDT–FBT is a regio-
random polymer because of the asymmetrical FBT unit.
PIDT–BT was synthesized as previously reported with a
number-average molecular weight (Mn) of 28 kDa.6 All polymers
possess good solubility in organic solvents, such as chloroform,
chlorobenzene, and dichlorobenzene, which was benefiting
from the tetrahexylbenzene groups on the IDT unit. The
molecular weights of PIDT–FBT and PIDT–DFBT were
measured using GPC with THF as an eluent. The Mn of
PIDT–FBT and PIDT–DFBT are determined to be 32.9 kDa
and 61.4 kDa with the polydispersity indices (PDI) of 3.70 and
3.04, respectively. The DSC measurements showed that there
were no thermal transitions found between 20 and 300 1C for
both polymers.
Fig. 2 shows the UV-Vis absorption spectra of PIDT–FBT
and PIDT–DFBT in chloroform solutions and in thin films.
The UV-Vis spectrum of PIDT–BT is also shown for comparison.
All polymers showed two similar characteristic peaks resulting
from similar polymer main chains. The band at the longer
wavelength was attributed to the intramolecular charge transfer
(ICT) from IDT to BT unit. In chloroform solution, as shown
in Fig. 2a, PIDT–FBT shows almost the same absorption
peaks (413 nm and 644 nm) and onset (715 nm) as those of
PIDT–BT. Similarly, the absorption spectra of PIDT–FBT
and PIDT–BT in thin film also possess the same peaks and
onset. The main peak and absorption onset of PIDT–FBT and
PIDT–BT in thin films are 649 nm and 720 nm, respectively,
with an optical band-gap of 1.72 eV. The results showed that
the introduction of monofluoro-substituent onto PIDT–FBT
showed a little effect on the polymer optical properties. The
same band-gap of PIDT–FBT and PIDT–BT also indicates
that the F atom in PIDT–FBT lowered both HOMO and
The photovoltaic properties of the polymers were investigated
in the device structure of ITO/PEDOT:PSS/polymer:PC71BM
(1 : 3 wt)/Ca/Al. The detailed device fabrication is described in
the ESI.w The PIDT–BT device was also fabricated under the
Fig. 2 UV-Vis spectra of polymers in chloroform solution (a) and
Fig. 3 CV curves of polymers PIDT–FBT and PIDT–DFBT (the CV
thin film states (b).
curve of ferrocene is shown as a blue line).
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11026–11028 11027

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transporting property of the BHJ layer. The similar absorption
and hole mobilities of PIDT–FBT and PIDT–BT combined
with similar morphology, as revealed by atomic force microscopy
(AFM), resulted in similar Jsc and FF. Thus, the improved
performance of the PIDT–FBT device should come mainly
from the increased Voc. In the PIDT–DFBT device, the low
FF may be due to the significantly decreased hole-mobility.
Similarly, the comparable PCE of the PIDT–DFBT device is
mainly due to the very high Voc of 0.92 V.
Fig. 4 J–V (a) and EQE (b) curves of polymer/PC71BM devices.
In conclusion, fluoro-substituted PIDT–FBT and
PIDT–DFBT were synthesized and used as the donor polymer
in PSCs. PIDT–FBT and PIDT–DFBT showed deep HOMO
energy levels of À5.38 eV and À5.48 eV, respectively, after
introducing the electron-withdrawing fluorine atoms onto the
BT unit. The PCEs of PIDT–FBT and PIDT–DFBT devices
reached 5.40% and 5.10%, with high Vocs of 0.86 V and 0.92 V,
respectively, compared with the PCE of 5.02% and Voc of
0.81 V in the PIDT–BT device. The high Voc and improved
device performance make PIDT–FBT and PIDT–DFBT
promising candidates for high-performance PSCs.
same conditions for comparison. Fig. 4a displays the J–V
curves of the devices that were measured under 100 mW cmÀ2
illumination (AM 1.5G). The PIDT–FBT/PC71BM device
showed a Voc of 0.86 V, a Jsc of 11.23 mA cmÀ2 and a FF
of 56%, leading to an overall PCE of 5.40%. Under the same
conditions, however, the PIDT–BT device gave a PCE of
5.02%, with a Voc of 0.81 V, a Jsc of 11.23 mA cmÀ2, and a
FF of 55%. An increase of 0.05 V in the Voc of the PIDT–FBT
device was found while comparing to that of the PIDT–BT
device. It is interesting that both devices showed very similar
Jsc and FF, therefore, the increased PCE of the PIDT–FBT
device over the PIDT–BT device was mainly attributed to the
increase of Voc, benefiting from the deeper HOMO energy
level of PIDT–FBT. As indicated above, PIDT–DFBT has
the deepest HOMO energy level, it is expected that a further
increase of Voc would be achieved. As expected, the Voc of the
PIDT–DFBT/PC71BM device reached 0.92 V, 0.12 V enhance-
ments compared to that of the PIDT–BT device. Combining a
The authors thank the support from NSF (DMR-0120967),
DOE (DEFC3608GO18024/A000), AFOSR (FA9550-09-1-0426),
ONR (N00014-11-1-0300), and the World Class University
program through National Research Foundation of Korea
(R31-21410035). A. K. Y. J. thanks the Boeing Foundation for
support. S. C. C. thanks the National Science Council of
Taiwan (NSC98-2917-I-009-112) for support.
J
sc of 10.87 mA cmÀ2 and a FF of 51%, the overall PCE of the
Notes and references
PIDT–DFBT device was 5.10%, which is slightly higher than
the PIDT–BT device. The comparable performance was also
attributed to the significant enhancement of Voc which was offset
by the loss in Jsc and FF in comparison with the PIDT–BT
device. The results clearly indicate that increasing the Voc in
PSC by fine tuning the HOMO energy level of the polymer is
an effective way to enhance the overall device performance.
The external quantum efficiency (EQE) spectra of the
devices were studied to further verify the photoresponse. As
shown in Fig. 4b, all devices showed an efficient photoresponse
in the range of 340 nm to 750 nm. The highest EQE value of
the devices reached B70% in PIDT–BT and PIDT–FBT
devices, and B60% in the PIDT–DFBT device. A flat EQE
value of more than 40% between 350 nm to 700 nm indicates
the balanced contribution from both polymer and PC71BM.
The almost overlapped EQE curves of PIDT–FBT and
PIDT–BT devices are very consistent with the similarities in
the measured Jsc and FF in both devices. In the PIDT–DFBT
device, the low EQE value between 350 nm to 550 nm coming
from the PC71BM may be one of the main factors for the
measured low Jsc.
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The hole mobility measured by the space-charge-limited-
current (SCLC) method for the polymer/PC71BM BHJ film
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and PIDT–DFBT were calculated to be 4.69 Â 10À2, 3.38 Â 10À2
,
and 2.88 Â 10À3 cm2 vÀ1
s
À1, respectively. It is known that
both Jsc and FF are partially dependent on the charge
c
11028 Chem. Commun., 2011, 47, 11026–11028
This journal is The Royal Society of Chemistry 2011