Tetrathienoanthracene-based copolymers for efficient solar cells

Article abstract of DOI:10.1021/ja1110915

A series of semiconducting copolymers (PTAT-x) containing extended π-conjugated tetrathienoanthracene units have been synthesized. It was shown that the extended conjugation system enhanced the π-π stacking in the polymer/PC₆₁BM blend films and facilitated the charge transport in heterojunction solar cell devices. After structural fine-tuning, the polymer with bulky 2-butyloctyl side chains (PTAT-3) exhibited a PCE of 5.6% when it was blended with PC₆₁BM.

Full text of DOI:10.1021/ja1110915

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Tetrathienoanthracene-Based Copolymers for Efficient Solar Cells
Feng He,† Wei Wang,† Wei Chen,‡ Tao Xu,† Seth B. Darling,‡ Joseph Strzalka,§ Yun Liu,^,z and Luping Yu*,†
†Department of Chemistry and The James Franck Institute, The University of Chicago, 929 E 57th Street, Chicago, Illinois 60637,
United States
‡Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States
§X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States
^NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
zDepartment of Chemical Engineering, University of Delaware, Newark, Delaware 19716, United States
S Supporting Information
b
Scheme 1. Structures of Polymers
ABSTRACT: A series of semiconducting copolymers
(PTAT-x) containing extended π-conjugated tetrathie-
noanthracene units have been synthesized. It was shown
that the extended conjugation system enhanced the π-π
stacking in the polymer/PC61BM blend films and facilitated
the charge transport in heterojunction solar cell devices.
After structural fine-tuning, the polymer with bulky 2-buty-
loctyl side chains (PTAT-3) exhibited a PCE of 5.6% when
it was blended with PC61BM.
olar energy, which can be converted into electricity using
photovoltaic devices, is the largest renewable energy source.
will enhance π-π stacking between the polymer chains; it may also
S
have a side effect of reduced solubility, which will be detrimental to
polymer processing. Several monomers with different side chains were
thus synthesized in order to obtain processable polymers. At the same
time, another PTB family polymer, PTB-8, with similar bulk side chains
as those on PTAT-3, with two 2-butyloctyloxy on the benzodithio-
phene moiety, was also synthesized for the purpose of comparison with
PTAT polymers.
Solar cells based on inorganic semiconductors can harvest solar
energy efficiently but with a high cost.1 Polymeric solar cells
(PSCs) based on the bulk heterojunction (BHJ) architecture are
thus pursued worldwide due to their distinctive potential for
fabrication on flexible substrates with large areas, lightweight, and
solution processability at a low cost.2 By blending electron-
donating semiconducting polymers and electron-withdrawing
fullerene derivatives together, a bicontinuous interpenetrating
network with a large donor-acceptor interfacial area will endow
the active layer with large photovoltaic effect.3 In the past decade,
extensive research effort by many groups around the world has
revealed promising features of PCSs.4 Solar energy conversion
efficiency larger than 8% has been achieved based on semicon-
ducting polymers developed in this lab.5
To explore strategies for further improving solar cell performance
and probing structure/property relationships, the effect of dimen-
sionality of conjugated systems on solar cell properties was inves-
tigated. The tetrathienoanthracene moiety with two-dimensional
(2D) extended π-conjugated structure was chosen as a building
block for new polymers,6 which favors stronger cofacial π-π
stacking. A series of new polymers (PTAT-x) were synthesized
with thieno[3,4-b]thiophene as comonomer. The results demon-
strate that tetrathienoanthracene is a promising building block for
the development of semiconducting polymers with high photo-
voltaic efficiency. Several interesting observations provide insights
for further design of photovoltaic polymers.
The synthesis of tetrathienoanthracene monomer is described
in the Supporting Information (SI). The bis-trimethyl-stannanyl
monomers were polymerized with 20-ethylhexyl-6-dibromo-3-
fluorothieno[3,4-b]thiophene-2-carboxylate by the Stille poly-
condensation reaction to yield the designed PTAT polymers. It
was found that polymer PTAT-1 with 2-ethylhexyl side chains in
the tetrathienoanthracene moiety showed poor solubility and
were barely processable in organic solvents; this polymer was not
investigated further. However, three other new polymers (PTAT
2-4) with more bulky side chains are soluble in common organic
solvents and exhibit number-average molecular weights (Mn) of
24.1, 18.1, and 18.2 kDa with polydispersity indices (PDI) of 1.6,
2.6 and 2.5, respectively. Since the side chains are bulky and long,
PTB-8 possesses a high Mn of 83.3 kDa with a PDI of 2.05 due to
the improved solubility in the polymerization medium.
UV/vis spectra (Figure 1a) of PTAT2-4 show absorption
peaks at 684, 664, and 665 nm with the absorption onset at 747,
737, and 752 nm, which correspond to optical band gaps around
The structures of the polymers studied here are shown in
Scheme 1. The extended π-conjugated system in tetrathienoanthracene
Received: December 9, 2010
Published: February 18, 2011
r
2011 American Chemical Society
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1.66, 1.69, and 1.65 eV, respectively. PTB-8 showed an absorp-
tion peak at 662 nm with an optical band gap at 1.71 eV. It had
also been found that the absorption of PTAT-3 in chloroform is
similar to that obtained in films, indicating minimal conforma-
tional change from chloroform solution to film. Cyclic voltam-
mograms of PTAT-3 (SI, Figure S5) showed reversible
reduction and oxidation behavior, from which a HOMO energy
level of -5.04 eV and a LUMO energy level of -3.28 eV are
deduced from the onset potentials, corresponding to a band gap
of 1.76 eV, which is quite close to the optical band gap (1.69 eV)
of the polymer films. The HOMO and LUMO levels and
electrochemical band gaps of other polymers are listed in Table 1.
All of these PTAT polymers possess low band gaps from 1.76 to
1.80 eV with the HOMO level from -5.04 to -5.12 eV and the
LUMO level from -3.28 to -3.32 eV. The band gap of PTB-8
was found to be slightly increased to 1.90 eV with a HOMO level
of -5.24 eV - the lowest among these polymers.
Photovoltaic properties of the polymers were investigated
by solar cell structures of ITO/PEDOT:PSS/polymer:
PC61BM(1:1, w/w)/Ca/Al. The active layers of polymer
composites were spin-coated from polymer/PC61BM solu-
tions prepared in chloroform (CF), chlorobenzene (CB), or
o-dichlorobenzene (DCB), with thickness controlled at
around 100 nm. The devices of PTAT-3 prepared from
different solvents exhibited similar open circuit voltage
(Voc) at 0.66 V, while there were significant differences in
the short circuit current density (Jsc) and fill factor (FF).
Compared with device from CF/diiodooctane (DIO) solvent,
the devices fabricated from CB and DCB showed moderate Jsc
and relatively low FF: 12.3 mA cm-1, 32% (CB/DIO), and
10.5 mA cm-1, 27% (DCB/DIO) with PCEs about 2.65%
(CB/DIO) and 1.91% (DCB/DIO), respectively. When the
cosolvent of CF/DIO was used, the Jsc of solar cell was
significantly enhanced to 15.0 mA cm-2, and a FF of 58%
was obtained. The combined effect led to a PCE up to 5.62%.
External quantum efficiency (EQE) data from the optimized
PTAT-3/PC61BM BHJ solar cells are presented in the SI
(Figure S6), and the device shows relatively high incident
photoconversion efficiency with EQE around 50-60% from
550 to 700 nm.
Solar cells prepared from the other PTAT-x polymers were
further studied, and representative characteristics are summar-
ized in Table 1. The PTAT-3 system showed the highest PCE
among these polymers. For the 2-ethyldedocyl-substituted poly-
mer, PTAT-4, the solubility was found to be poorer than that of
PTAT-3, leading to a lower Jsc (9.6 mA cm-1), FF (50%), and a
PCE value of only 2.98%. Another tetrathienoanthracene-based
polymer, PTAT-2, with the most bulky side chain, showed
excellent solubility even in CB and DCB. The devices from CB
solution exhibited relatively high Voc of 0.77 V, but low Jsc of 1.0
mA cm-2, resulting in a PCE of 0.37%. The low PCE can be
attributed to the steric effect of highly bulky 2-decyltetradecyl
side chains that will impede the cofacial π-π stacking in the
polymer film. All of these results indicate that the 2-butyloctyl
group is the best side chain of those studied for tetrathienoan-
thracene-based polymers to achieve the optimized solubility and
compatibility with fullerene derivatives.
In order to explore the unique features of PTAT-x polymers
with extended π-system, devices were fabricated using PTB-8
with similar bulky side chains. Figure 1b showed the J-V curves
of the solar cells based on PTB-8 and PTAT3-4. When CF/
DIO was used as cosolvent, corresponding devices of PTB-8/
PC61BM showed a PCE of 2.17% with a Voc of 0.79 V, Jsc of 5.1
mA cm-2, and FF of 54%. PTB-8 exhibits better solar cell
Figure 2. TEM images of polymer/PC61BM blend films. (a) PTAT-3/
PC61BM from CF/DIO (98/2, v/v); (b) PTAT-4/PC61BM from CF/
DIO (98/2, v/v); (c) PTB-8/PC61BM from CB/DIO (98/2, v/v), the
scale bar is 200 nm. (d) SANS profiles for as-spun thin films of PTAT-3/
PC61BM and PTB-8/PC61BM.
Figure 1. (a) UV/vis absorption spectra of polymers in film and in
solution. (b) Current-voltage characteristic of polymer/PC61BM (1:1,
w/w) solar cells under AM 1.5 condition (100 mW cm-2).
Table 1. Characteristic Properties of Polymer Solar Cells
polymer
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
HOMO (eV)
LUMO (eV)
Eg (eV)
PTAT-2a
PTAT-3
PTAT-4
PTB-8a
0.77
0.66
0.62
0.83
1.0
15.0
9.6
0.46
0.58
0.50
0.59
0.37
5.62
2.98
2.78
-5.12
-5.04
-5.08
-5.24
-3.32
-3.28
-3.31
-3.34
1.80
1.76
1.77
1.90
5.7
a Chlorobenzene/DIO (98/2, v/v), 800 rpm; others: chloroform/DIO (98/2, v/v), 2500 rpm.
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properties with a PCE of 2.79% (Voc = 0.83 V, Jsc = 5.7 mA cm-2
,
and FF = 59%) when using CB/DIO as cosolvent, which was still
only half of what we achieved with PTAT-3.
These results showed that solar cell properties strongly
depend on the electronic and solid-state structures. The signifi-
cant differences among the PTAT-x and PTB-8 are related not
only to electronic but also to morphological differences. The
purported ideal morphology is comprised of an interpenetrating
nanoscale network between donor and acceptor, which enables a
large interface area for exciton dissociation and continuous
percolating paths for hole and electron transport to the corre-
sponding electrodes. TEM images of PTAT-3 and PTAT-4
(Figure 2a,b) with different alkyl chains exhibited fine domains,
and no large phases can be found. In contrast, the film of PTB-8
from CB/DIO shows obvious domains of aggregation as shown
in Figure 2c, which suggests that benzodithiophene with 2-bu-
tyloctyoxy side chains would reduce the miscibility of PTB-8
with PC61BM and lead to the low PCE of solar cells via exciton
losses.
Since TEM studies generally do not provide definitive in-
formation on the 3D phase separation in bulk samples, small
angle neutron scattering (SANS) measurements were carried out
on PTAT-3 and PTB-8. The SANS profiles for as-spun thin films
of PTAT-3/PC61BM and PTB-8/PC61BM blend films at room
temperature are shown in Figure 2d. Using the empirical unified
exponential/power-law method developed by G. Beaucage,7 the
averaged domain sizes in PTAT-3/PC61BM and PTB-8/
PC61BM blend films could be achieved as 29 ( 5 and 168 (
119 nm, respectively. These scattering results are echoed with the
TEM data as shown above. It is known that large domains in BHJ
solar cells are detrimental to exciton migration and charge
separation. The relatively small domain size in the PTAT-3/
PC61BM film will enhance the donor-acceptor interfacial area
and enhance the solar cell properties.
Besides the miscibility with PC61BM, the dimensionality of
the comonomer will also influence the molecular packing
structures significantly. As shown in Figure 3a,b, grazing inci-
dence wide-angle X-ray scattering (GIWAXS) studies revealed
that the π-stacking of both PTAT-3/PC61BM and PTB-8/
PC61BM films are parallel to the substrate surface, namely in a
face-down orientation that is helpful to hole transport in the
polymer films between two electrodes. As shown in Figure 3d,
the in-plane linecuts of GIWAXS reveal a lamellar spacing of
about 23.7 Å for the PTAT-3/PC61BM blend film and 20.4 Å for
the PTB-8/PC61BM blend film.8 The difference of about 3.3 Å in
polymer lamella distance between PTAT-3 and PTB-8 is con-
sistent with the size difference in the comonomer because of the
different connection positions of the side chains. The out-of-
plane GIWAXS profiles (Figure 3c) show the π-π distance
between the backbones is 3.6 Å for the PTAT-3/PC61BM film
and 4.1 Å for the PTB-8/PC61BM film. Although the scattering
arising from the π-π stacking of polymer backbones in the
PTB-8/PC61BM blend film overlaps with one of the Bragg
diffractions of PC61BM, a broadened shoulder showed up around
1.5-1.55 Å-1, which corresponds to the π-π distance between
the backbones. This was further confirmed by GIWAXS results of
the neat PTB-8 homopolymer film (SI, Figure S7). A distinctive
scattering was observed in vertical (qz) of 1.50 Å-1, which
coincides with the blend film with a π-π stacking distance of
about 4.1 Å. Therefore, by using extended π-conjugated mono-
mer in PTAT-3, the π-π stacking distance is reduced by 0.5 Å
compared to the PTB-8 polymer with relatively smaller
Figure 3. Two-dimensional GIWAXS patterns of the blend films of
PTAT-3/PC61BM (1:1, w/w) (a) and PTB-8/PC61BM (1:1, w/w) (b).
(c) Out-of-plane linecuts of GIWAXS of PTAT-3/PC61BM and PTB-
8/PC61BM films. (d) In-plane linecuts of GIWAXS of PTAT-3/
PC61BM and PTB-8/PC61BM films. Note: GIWAXS profiles have been
shifted vertically for clarity.
benzodithiophene monomer, which is one of the major reasons
why the PCE of devices based on PTAT-3 is much higher than
that of PTB-8. This difference is also reflected in the hole
mobility, which has been measured by the method based on
the space charge limited current (SCLC) model.9 The hole
mobility is found to be around 1.69 ꢀ 10-4 cm2 V-1 s-1 for
PTAT-3 and 1.11 ꢀ 10-5 cm2 V-1 s-1 for PTB-8, respectively,
which coincides with the GIWAXS results. Higher hole mobility
will favor the charge transport in the film devices and, hence,
result in better performance.
In conclusion, a series of new semiconducting polymers with
extended π-conjugated unit, tetrathienoanthracene, were synthe-
sized. The polymer PTAT-3 with 2-butyloctyl alkyl side chains
exhibited power conversion efficiency about 5.6% with PC61BM
from CF/DIO cosolvent. The tetrathienoanthracene can rein-
force π-π stacking in the solid films, which enhanced the device
properties through more efficient charge transport. The results
indicate that with similar side chains, polymers with larger π
systems exhibit better solar cell properties in the systems studied
here. However, as the dimensionality of π systems in monomers
increases, the poor solubility of the resulting polymers will
compromise this advantage. A good solution is to balance the
two conflicting effects in search of further enhancement of solar
cell properties.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures,
b
synthesis of monomers and polymers, cyclic voltammograms,
EQE data, and GIWAXS of homopolymer films. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
Lupingyu@uchicago.edu
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’ ACKNOWLEDGMENT
We acknowledge financial support from NSF, AFOSR, NSF-
MRSEC (The University of Chicago). The works described here
were also partially supported by Solarmer Energy Inc. Use of the
Advanced Photon Source at Argonne National Laboratory and the
Center for Nanoscale Materials was supported by the U.S. Depart-
ment of Energy, Office of Science, Office of Basic Energy Sciences,
under Contract No. DE-AC02-06CH11357.
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