Tuning energy levels of low bandgap semi-random two acceptor copolymers

Article abstract of DOI:10.1021/ma400531v

A series of low bandgap semi-random copolymers incorporating various ratios of two acceptor units - thienothiadiazole and benzothiadiazole - were synthesized by Pd-catalyzed Stille coupling. The polymer films exhibited broad and intense absorption spectra, covering the spectral range from 350 nm up to 1240 nm. The optical bandgaps and HOMO levels of the polymers were calculated from ultraviolet-visible spectroscopy and cyclic voltammetry measurements, respectively. By changing the ratio of the two acceptor monomers, the HOMO levels of the polymers were tuned from -4.42 to -5.28 eV and the optical bandgaps were varied from 1.00 to 1.14 eV. The results indicate our approach could be applied to the design and preparation of conjugated polymers with specifically desired energy levels and bandgaps for photovoltaic applications.

Full text of DOI:10.1021/ma400531v

Article
pubs.acs.org/Macromolecules
Tuning Energy Levels of Low Bandgap Semi-Random Two Acceptor
Copolymers
Jinjun Zhou,† Sibai Xie,† Emily F. Amond,† and Matthew L. Becker*,†,‡
†Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States
‡Center for Biomaterials in Medicine, Austen Bioinnovation Institute in Akron, Akron, Ohio 44308, United States
S
* Supporting Information
ABSTRACT: A series of low bandgap semi-random copolymers
incorporating various ratios of two acceptor unitsthienothiadiazole
and benzothiadiazolewere synthesized by Pd-catalyzed Stille
coupling. The polymer films exhibited broad and intense absorption
spectra, covering the spectral range from 350 nm up to 1240 nm. The
optical bandgaps and HOMO levels of the polymers were calculated
from ultraviolet−visible spectroscopy and cyclic voltammetry measure-
ments, respectively. By changing the ratio of the two acceptor
monomers, the HOMO levels of the polymers were tuned from −4.42
to −5.28 eV and the optical bandgaps were varied from 1.00 to 1.14
eV. The results indicate our approach could be applied to the design
and preparation of conjugated polymers with specifically desired
energy levels and bandgaps for photovoltaic applications.
regions.5,7,9 The D−A copolymer is also employed to rationally
1. INTRODUCTION
tune the HOMO and LUMO levels, crystallinity, and charge
Conjugated polymers (CPs) are promising materials for
mobility of the CPs by changing the chemical structures of the
applications in sensors, field-effective transistors, organic light-
donor and acceptor units. Recently, a series of D−A random
emitting diodes, and organic photovoltaics (OPVs) due to their
copolymers incorporating donor or acceptor units randomly
cost-effective solution processability.1 Devices based on CPs
distributed in the backbone have been reported.4,10 These
can be easily fabricated into lightweight, large-area, and flexible
polymers showed compositional dependent energy levels,
panels.2 The power conversion efficiency (PCE) of OPVs has
solubility, charge mobility, and crystallinityall useful in
photovoltaic applications. Copolymers incorporating two
acceptor units are of particular interest: the properties could
be manipulated by changing different acceptors structure and
composition. In this contribution, we report a series of
semirandom copolymers containing different ratios of two
strong acceptorsbenzothiadiazole and thienothiadiazole
which have been utilized to prepare D−A low bandgap
CPs.9c,d,11 By systematically changing the composition of the
two acceptors, each with different electron-accepting abilities,
the energy levels and bandgaps can be tuned stoichiometrically,
thus offering a new class of CPs with bandgaps as low as 1.0 eV.
reached 12% recently;3 devices with PCE that reach 15%
efficiencies are expected to compete with silicon-based solar
cells in the next decade.1a,4 New CPs that collect as much
sunlight as possible are needed to meet the fundamental
requirement for the photovoltaic energy conversion.5
The efficiency of a polymer solar cell is proportional to the
short-circuit current density (Jsc), the open-circuit voltage (Voc),
and the fill factor (FF). These variables require optimization to
reach maximum efficiencies.4,6 The Jsc is related to the product
of the breadth and intensity with which polymers absorb the
solar spectrum, where broader and more intense absorption
theoretically leads to larger Jsc.4,7 Optimization requires the
design and synthesis of low bandgap CPs extending the
absorption further into red and near-infrared (NIR) spectral
regions. An effective device requires CPs to have a low highest
occupied molecular orbital (HOMO) to help increase the Voc
and high lowest unoccupied molecular orbital (LUMO) for
efficient charge dissociation in the CP/PCBM bulk hetero-
juction device.5 It is clear that achieving specific HOMO and
LUMO energy levels with precision and controlling the
bandgaps are essential for effective material development.5,8
Introducing donor (D) and acceptor (A) monomers together
in the polymer backbone is a promising strategy to obtain CPs
with low bandgaps that absorb light in the red to NIR spectral
2. RESULTS AND DISCUSSION
Polymer Synthesis. Copolymerization of 5,7-bis(5-bromo-
4-decanyl-2-thienyl)thieno[3,4-b]thiadiazole (TTD), 4,7-bis[5-
bromo-4-(2-ethyhexyl)-2-thienyl]-2,1,3-benzothiadiazole
(TBT), and 2,5-bis(tri-n-butyltin)thiophene in chlorobenzene
at 120 °C with Pd(PPh3)4 as the catalyst yielded the desired
polymers, as illustrated in Scheme 1. All the polymers show
Received: March 12, 2013
Revised: April 4, 2013
Published: April 22, 2013
© 2013 American Chemical Society
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Scheme 1. Synthesis and Structures of the Copolymers with Different Compositions of TTD and TBT
good solubility at room temperature in organic solvents such as
chloroform, chlorobenzene, and o-dichlorobenzene (ODCB).
The comonomer feed ratios are 85:15, 70:30, 50:50, 30:70, and
15:85, and the corresponding polymers are named as
PTTDTBT8515, PTTDTBT7030, PTTDTBT5050,
PTTDTBT3070, and PTTDTBT1585, respectively. Polymers
containing only one acceptor unit were also synthesized for
comparison, named as PTTD and PTBT. Table 1 summarizes
Table 1. Polymerization Results and Thermal Properties of
Polymers
a
b
d
Mn
Mw
Td
ratio in the
e
c
polymer
PTTD
(kDa)
(kDa)
PDI
(°C)
polymer
1.6
6.4
4.0
16.3
12.1
14.2
17.0
24.3
18.6
2.4
2.5
3.7
3.8
3.4
2.6
305
269
314
350
369
360
417
Figure 1. UV−vis absorption spectra of polymers in thin films.
PTTDTBT8515
PTTDTBT7030
PTTDTBT5050
PTTDTBT3070
PTTDTBT1585
PTBT
92:8
3.2
79:21
56:44
32:68
13:87
3.7
exhibited localized π−π* transitions at 350−480 nm, while the
absorption band resulted from ICT was between 480 and 800
nm. It has been shown that 4,6-di(2-thienyl)thieno[3,4-
c][1,2,5]thiadiazole has a coplanar structure which generally
leads to better π-electron delocalization in conjugated polymers
and hence lower bandgap between the HOMO and the
LUMO.12 As illustrated in Figure 1, PTTD has absorption that
extends to the NIR region with an optical bandgap as low as
0.98 eV, while PTBT has an optical bandgap of 1.84 eV. The
optical bandgap was calculated from the onset wavelength of
absorption by Egopt = 1240 nm/λonset, where Egopt is the optical
bandgap and λonset is the onset wavelength of absorption. It is
clear that D−A polymer with a structure containing TTD as the
acceptor units has a longer conjugation length and better π-
electron delocalization and hence a lower bandgap than that
incorporating TBT as the acceptor units.
By introducing these two acceptor units in one D−A
conjugated polymer and systematically changing the ratios of
the two components from 85:15 to 15:85, we were able to tune
the absorption properties and bandgaps of the resulting
copolymers. As shown in Figure 1, all of the polymers contain
two absorption bands that correspond to the two different ICT
states originated from the two individual acceptor units. The
absorption band at 440−800 nm was attributed to ICT of the
TBT containing D−A structure, while the absorption band
around 800−1240 nm comes from the ICT of TTD containing
D−A structure. The relative intensity of the absorption band
increases as the content of the corresponding acceptor unit
5.0
9.3
13.2
1.4
a
b
Number-average molecular weight. Weight-average molecular
weight. Mw/Mn. Decomposition temperature (5% weight loss)
determined by thermal gravimetric analysis (TGA) under N2.
TTD:TBT in the polymer estimated from absorption spectra.
c
d
e
the polymerization results including molecular weight, molec-
ular weight distribution (PDI), thermal stability, and estimated
compositions of the copolymers. The polymers containing two
acceptor units have relatively low molecular weights (Mn); this
could probably be increased further by optimization of
polymerization conditions such as monomer concentration in
solution, reaction time, catalyst, and temperature.9h The
polymers have good thermal stability with an increasing
decomposition temperature (5% weight loss) when the TBT
content is increasing.
Optical Properties. The UV−vis−NIR absorption spectra
of the copolymers in thin film formats are shown in Figure 1.
PTTD, in which only thienothiadiazole was incorporated as the
acceptor unit, displayed two absorption bands: the first one at
358−620 nm, which was assigned to localized π−π* transitions,
and the second broader band at 620−1266 nm, in the long
wavelength region, which corresponds to intramolecular charge
transfer (ICT) between D−A−D charge transfer states.9d
Incorporating benzothiadiazole as the only acceptor unit, PTBT
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increases. As two different acceptor units with different
electron-accepting abilities are incorporated in the polymer,
by systematically changing the composition of the acceptor
units, the relative planarity and π-electron delocalization of the
resulting polymer can be tuned. As a result, the bandgaps and
the energy levels can be engineered. Figure 1 shows that the
absorption onset wavelength of the CPs changed from 1237 to
1091 nm as the ratio of TTD to TBT changed from 85:15 to
15:85. This variation resulted in a bandgap change from 1.00 to
1.14 eV (Table 2). A lower bandgap is obtained when the TTD
Table 2. Optical and Electrochemical Properties of Polymers
a
b
c
d
e
Eox,onset
(V)
EHOMO
(eV)
ELUMO
(eV)
λonset
Egopt
polymers
PTTD
(nm)
(eV)
Figure 2. Oxidation scans of cyclic voltammograms of polymer thin
films in 0.1 M Bu4NClO4 solution in acetonitrile at a scan rate of 100
mV/s.
0.05
−0.02
−0.02
0.37
−4.45
−4.42
−4.42
−4.77
−5.05
−5.28
−5.27
−3.47
−3.42
−3.39
−3.69
−3.93
−4.14
−3.72
1266
1237
1199
1145
1106
1091
800
0.98
1.00
1.03
1.08
1.12
1.14
1.55
PTTDTBT8515
PTTDTBT7030
PTTDTBT5050
PTTDTBT3070
PTTDTBT1585
PTBT
3. CONCLUSION
0.65
0.88
In conclusion, we have synthesized a family of low bandgap D−
A copolymers containing two different acceptor units (TTD
and TBT) with well-defined compositions. The copolymer with
a specific composition (TTD to TBT ratios between 70:30 and
50:50) has relatively strong absorption covering the entire
visible to NIR region. As illustrated above, systematically
changing the ratio of the two acceptors affords the ability to
designate the HOMO/LUMO levels and bandgaps of the
copolymers. The integration of these polymers into organic
photovoltaic devices and the application of this approach to
other D−A systems are currently underway.
0.87
a
b
Onset potential of oxidation scan (vs Ag/AgCl). EHOMO = −(4.4 +
c
d
Eox,onset) (eV). ELUMO = EHOMO + Egopt
. Onset of absoption of thin
e
films. Egopt = 1240 nm/λonset
.
acceptor unit has a higher mole fraction in the polymer, since
increased TTD leads to a structure with higher planarity and π-
electron delocalization.
As has been reported, a main weakness of the D−A
copolymers is the frequently observed red-shifting of the
absorption profile to the long wavelength region instead of a
true broadening across both the visible and NIR regions. This
red-shift could hinder the desired increase in Jsc and ultimately
the efficiency.10d,13 Interestingly, the copolymers reported here
have relatively strong absorption in the entire visible to NIR
region when the TTD to TBT ratios are between 70:30 and
50:50 (Figure 1). The absorption profile comes from two
complementary ICT absorption bands generated by the two
different acceptor units in the copolymers. Furthermore, the
specific compositions results in a balanced absorption of those
bands. This unique feature of absorption may lead to better
performance of the polymers in photovoltaic applications.
Electrochemical Properties. The electrochemical proper-
ties of the copolymers were investigated by using cyclic
voltammetry (CV) to determine the energy levels of the
HOMO and the LUMO. Tetrabutylamonium perchlorate
(Bu4NClO4) in acetonitrile (0.1 M) was used as the electrolyte
system. The HOMO level of each polymer was calculated from
the onset of electrochemical oxidation (Figure 2). The Ag/
AgCl reference electrode was calibrated by ferrocene. By
assuming the energy level of ferrocene/ferrocenium (Fc/Fc+)
to be −4.8 eV below the vacuum level, the HOMO level of
polymer was determined by EHOMO = −(4.4 + Eox,onset) (eV).14
4. EXPERIMENTAL SECTION
Materials and Methods. All reagents purchased from Sigma-
Aldrich or Fisher were used without further purification, unless
otherwise noted. 5,7-Bis(4-decanyl-2-thienyl)thieno[3,4-b]thiadiazole
(1) and 4,7-bis[4-(2-ethyhexyl)-2-thienyl]-2,1,3-benzothiadiazole (2)
were synthesized by the reported procedures.9d,11d,e Detailed synthesis
and purification procedures for the chemicals and characterization
details are given in the Supporting Information.
Synthesis of 5,7-Bis(5-bromo-4-decanyl-2-thienyl)thieno-
[3,4-b]thiadiazole (TTD). 1 (0.645 g, 1.1 mmol) was dissolved in
pyridine (20 mL) under argon, and N-bromosuccinimide (NBS)
(0.411 g, 2.34 mmol) was added in several portions during 30 min in
the absence of light. The reaction mixture was stirred for another 30
min, pyridine was evaporated in vacuo, and a dark blue solid (0.69 g,
84%) was obtained with column chromatography on silica gel in
hexane/dichloromethane (11:1 by volume). 1H NMR (300 MHz,
CDCl3): δH (ppm) = 0.89 (t, J = 6.73 Hz, 6H), 1.24−1.44 (m, 28H),
1.54−1.71 (m, 4H), 2.57 (t, J = 7.61 Hz, 4H), 7.17 (s, 2H). 13C NMR
(125 MHz, CDCl3): δC (ppm) = 156.04, 143.42, 134.33, 124.65,
111.58, 109.79, 31.92, 29.66, 29.63, 29.60, 29.54, 29.42, 29.35, 29.28,
22.70, 14.14. ESI-MS: Mcalcd = 742.08, Mfound = 741.9.
Synthesis of 4,7-Bis[5-Bromo-4-(2-ethyhexyl)-2-thienyl]-
2,1,3-benzothiadiazole (TBT). 2 (1.43 g, 2.73 mmol) was dissolved
in anhydrous THF (40 mL) under argon, and N-bromosuccinimide
(NBS) (1.02 g, 5.74 mmol) was added in several portions at 0 °C for
30 min in the absence of light. Stirred for another 30 min, the reaction
mixture was warmed up to room temperature and stirred overnight.
Saturated ammonium chloride solution (80 mL) was added and
stirred, extracted with ethyl acetate (3 × 100 mL), washed with H2O
(3 × 300 mL) and brine (300 mL), and dried over Na2SO4. A brown
solid (1.71 g, 92%) was obtained with column chromatography on
silica gel in hexane. 1H NMR (500 MHz, CDCl3): δH (ppm) = 0.89−
0.98 (m, 12H), 1.29−1.44 (m, 16H), 1.72 (dt, J = 12.29, 6.21 Hz, 2H),
2.58 (d, J = 7.34 Hz, 4H), 7.70 (s, 2H), 7.73 (s, 2H). 13C NMR (125
MHz, CDCl3): δC (ppm) = 152.19, 142.22, 138.30, 128.58, 125.25,
The LUMO levels were estimated as ELUMO = EHOMO + Egopt
,
where Egopt is the optical bandgap (Table 2).1c The HOMO
level positions were −4.42, −4.42, −4.77, −5.05, and −5.28 eV
for the polymers as the ratio of TTD to TBT changed from
85:15 to 15:85 (Table 2). The HOMO level decreased (860
meV range) and the Egopt increased (140 meV range) almost
monotonically with increasing TBT content in the copolymers.
These results further demonstrated the ability of tuning the
energy levels and bandgaps of this kind of copolymer.
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124.75, 112.25, 40.01, 33.92, 32.53, 28.80, 25.75, 23.08, 14.13, 10.87.
ESI-MS: Mcalcd = 680.06, Mfound = 680.1.
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Representative Procedure for Stille Coupling Polymer-
ization: Polymerization of TTD and TBT in Mole Ratio of 1:1
To Afford Polymer PTTDTBT5050. A 50 mL Schlenk tube was
heated under reduced pressure and then allowed to cool to room
temperature under argon. In this tube, TTD (122.5 mg, 0.165 mmol),
TBT (112.6 mg, 0.165 mmol), and 2,5-bis(tri-n-butyltin)thiophene
(218.1 mg, 0.329 mmol) were dissolved in dry chlorobenzene (7 mL)
and degassed (argon) for at least 30 min. A catalytic amount of
Pd(PPh3)4 (15.2 mg, 0.0132 mmol) was then added to the reaction
mixture under argon and degassed and filled with argon three times.
The reaction mixture was stirred vigorously at 120 °C for 24 h under
argon. After it was cooled to room temperature, the polymerization
mixture was poured and stirred into 200 mL of methanol and 5 mL of
hydrochloric acid solution for 5 h. The polymer precipitated out as a
dark solid and filtered using a filter paper. The polymer was purified by
Soxhlet extraction with methanol (24 h) and hexane (24 h). The final
polymers were dried under vacuum for at least 24 h and then subjected
for the required analysis, yielding 183 mg (87%) black solid. 1H NMR
(500 MHz, CDCl3): δH (ppm) = 0.92 (s, br, 18H), 1.11−1.52 (m,
44H), 1.58−1.84 (m, 6H), 2.50−2.91 (m, 8H), 7.01−8.03 (m, 10H).
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: becker@uakron.edu.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge support from the Akron
Functional Materials Center. Ms. Emily F. Amond from Saint
Vincent’s College was supported by an NSF REU summer
fellowship (DMR 1004747).
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