Synthesis and photovoltaic properties of low band gap polymers containing benzo[1,2- b:4,5- c ′]dithiophene-4,8-dione

Article abstract of DOI:10.1021/ma202578y

Two low-bandgap polymers P1 and P2 were synthesized by copolymerization of benzo[1,2-b:4,5-c']-dithiophene-4,8-dione (BDTD) with benzo-[1,2-b:4,5-b'] dithiophene (BDT) and dithieno[3,2- b:2',3'-d]silole (DTS), respectively. Thermogravimetric analyses of P1 and P2 were found and the temperature of 5% weight-loss was selected as the onset point of decomposition. Both the polymers are found to exhibit low-lying HOMO and good light-absorption properties. The absorption spectra of the polymers in chloroform and films are found to be of the range of 500-650 nm. The optimized weight ratios for P1 and P2 both are found to be 1:1.5. The AFM images show the morphology change for the polymer/PC61BM composite films and the rms roughness for the film is found to decrease from 6.94 to 0.47 nm. P2 device exhibits higher external quantum efficiency (EQE) than that of P1, which is due to stronger sunlight absorption of P2 film.

Full text of DOI:10.1021/ma202578y

Note
pubs.acs.org/Macromolecules
Synthesis and Photovoltaic Properties of Low Band Gap Polymers
Containing Benzo[1,2-b:4,5-c′]dithiophene-4,8-dione
Jiamin Cao,†,‡ Wei Zhang,‡ Zuo Xiao,‡ Lingyan Liao,‡ Weiguo Zhu,*,† Qiqun Zuo,§ and Liming Ding*,‡
†Key Lab of Environment-Friendly Chemistry and Application of the Ministry of Education, College of Chemistry, Xiangtan
University, Xiangtan 411105, China
‡National Center for Nanoscience and Technology, No. 11 Beiyitiao, Zhongguancun, Beijing 100190, China
§Jiahong Optoelectronics, Suzhou 215151, China
S
* Supporting Information
s the energy crisis is becoming a serious problem all over
the world, cheap production of electricity from solar
in PSCs. Solar cells based on P2/PC61BM blend afforded a
PCE of 4.33% with an impressively high Voc, 0.93 V.
A
energy has attracted more and more attention. Among all the
photovoltaic devices, polymer bulk heterojunction solar cells
(PSCs) have shown many advantages, such as low fabrication
cost, excellent mechanical flexibility and lightweight. The active
layer of PSCs is based on the blend of a conjugated polymer as
the electron donor and a fullerene derivative as the electron
acceptor.1 Up to now, poly(3-hexylthiophene) (P3HT) is one
of the best commercialized donor materials. P3HT/PC61BM-
based PSCs have achieved power conversion efficiencies (PCE)
up to ∼4−5%.2 However, P3HT only absorbs part of the solar
emission (300−600 nm) and its high-lying HOMO energy level
(−4.76 eV)3 limits the open circuit voltage (Voc) of the solar
cells. Therefore, design and synthesis of new conjugated
polymers with deep HOMO levels for high Voc and low
bandgaps for broad absorption, as well as high mobilities and
good solubilities, are essential for obtaining high performance
PSCs.4
A recent strategy to satisfy these requirements above is to
design conjugated copolymer containing alternating donor and
acceptor units in the polymer backbone. Yu et al. reported the
D−A polymers based on poly(benzo-[1,2-b:4,5-b′]-
dithiophene)-alt-(thieno[3,4-b]thiophene) (PBDTTT) with
efficiency up to 7.73%.5 However, the synthesis of the
monomer thieno[3,4-b]thiophene in PBDTTT is very tedious
and with low yield. Li et al. reported a new strong electron-
withdrawing unit naphtho[2,3-c]thiophene-4,9-dione (NTDO),
which was synthesized in short route and with high yield. The
copolymer with NTDO as acceptor unit has relatively lower
HOMO (−5.52 eV) and gives a high PCE of 5.21%.6 Inspired
by Li’s work, we designed a new acceptor unit benzo[1,2-b:4,5-c′]-
dithiophene-4,8-dione (BDTD) by replacing the benzene
ring of NTDO with thiophene ring because the fused
thiophene ring systems are well-known to stabilize the
quinoidal structure and can efficiently reduce the bandgap
(Scheme 1).7 Furthermore, BDTD unit possesses the similar
structure as thieno[3,4-b]thiophene but more electron
deficient. In this work, we synthesized two low-bandgap
polymers P1 and P2 by copolymerization of BDTD with
benzo-[1,2-b:4,5-b′]dithiophene (BDT) and dithieno[3,2-
b:2′,3′-d]silole (DTS), respectively. Both P1 and P2 exhibit
low-lying HOMO and good light-absorption properties.
We further demonstrated that P1 and P2 are good donor materials
The synthetic routes for monomers and polymers were
illustrated in Scheme 1. Thiophene-3,4-dicarboxylic was
brominated by bromine in acetic acid to give 2,5-dibromothio-
phene-3,4-dicarboxylic acid in 50% yield. Treating 2,5-
dibromothiophene-3,4-dicarboxylic acid with oxalyl chloride
afforded the intermediate 2,5-dibromothiophene-3,4-dicarbonyl
dichloride. The 2,5-dibromothiophene-3,4-dicarbonyl dichlor-
ide was then treated with 2-hexylthiophene in presence of AlCl3
to obtain monomer BDTD in 12% yield. Monomers BDT and
DTS were prepared according to the literature methods.8,9 P1
and P2 were both synthesized by the Stille reaction using
Pd(PPh3)4 as the catalyst. The crude products were precipitated
in methanol. The impurities in the products were successively
removed by methanol and hexane in a Soxhlet extractor 24 h
each. Finally, the polymers were extracted with chloroform and
reprecipitated in methanol and dried under vacuum. P1 and P2
have good solubilities in common organic solvents such as
chloroform and toluene. The molecular weights of P1 and P2
were determined by gel permeation chromatography (GPC)
against polystyrene standards in THF eluent, and the data are
listed in Table 1.
Thermogravimetric analyses of P1 and P2 are shown in
Figure 1. The temperature of 5% weight-loss was selected as the
onset point of decomposition (Td). The Td values of P1 and P2
are 329 and 346 °C, respectively, indicating that they are
thermally stable for organic photovoltaic applications.
The absorption spectra of the polymers in chloroform and
films are shown in Figure 2. The optical properties of P1 and
P2 are summarized in Table 2. Both polymers show strong
absorption in the range of 500−650 nm. The absorption
maxima of the polymers in solution are located at 551 and
586 nm for P1 and P2, respectively. P2 shows stronger absorp-
tion at the long wavelength region than P1, while P1 exhibits a
broader spectrum, especially in the 400−500 nm region. In
solid films, both P1 and P2 show red shifts at the absorption
maxima (576 nm for P1 and 626 nm for P2), indicating the
enhanced interchain π−π stacking in the solid state. The optical
band gaps were calculated to be 1.78 eV for P1 and 1.63 eV for P2.
Received: November 28, 2011
Revised: January 11, 2012
Published: January 23, 2012
© 2012 American Chemical Society
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Note
Scheme 1. Synthesis of P1 and P2
Table 1. Molecular Weights and Thermal Properties of
Polymers
calculated according to the following equation: HOMO or
LUMO = −(Eox or Ered + 4.8) eV. As a result, the HOMO
energy levels of P1 and P2 are −5.49 and −5.33 eV,
respectively, which are significantly deeper than that of P3HT
(−4.76 eV) and close to ideal HOMO energy level (−5.4 eV).10
Deep HOMO should render these polymers good stability
against oxidization. The LUMO energy levels of P1 and P2
are −3.53 and −3.47 eV, respectively, and the electrochemical
bandgaps of P1 and P2 are 1.96 and 1.86 eV, respectively.
The electrochemical bandgaps of the polymers are ∼0.2 eV
higher than the optical bandgaps, which might result from the
interface barrier between the polymer film and the electrode
surface.11
a
a
a
b
polymers
Mn
Mw
PDI
Td (°C)
P1
P2
11 490
7037
39 382
13 384
3.43
1.90
329
346
a
Mn, Mw, and PDI were determined by GPC using polystyrene
b
standards with THF as eluent. The temperature at 5% weight-loss
under nitrogen.
The difference of P1 and P2 in optical properties reflects the
different electron donating strength of their donor units (BDT
and DTS).
The electrochemical properties of polymers were investigated
by cyclic voltammetry (CV) method. Figure 3 shows the cyclic
voltammograms of P1 and P2 films on glassy-carbon electrode
in CH3CN solution containing 0.1 mol/L Bu4NPF6. The CV
data are summarized in Table 3. The onset oxidation potentials
(Eox) of P1 and P2 are 0.69 and 0.53 V, respectively, while the
onset reduction potentials (Ered) of P1 and P2 are −1.27 and
−1.33 V, respectively. The energy level of polymers were
Solar cells with a configuration of ITO/PEDOT:PSS/
polymer:PC61BM/Ca/Al were fabricated and tested under
simulated AM1.5G irradiation (86 mW/cm2). The weight
ratios of polymer to PC61BM for P1 and P2 cells were
optimized from 1:1 to 1:2. J−V curves for P1 and P2 are shown
in Figure 4. The corresponding open-circuit voltage (Voc),
short-circuit current density (Jsc), fill factor (FF), and power
conversion efficiency (PCE) of the cells are listed in Table 4.
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Note
Table 2. Optical Properties of P1 and P2
λmax (nm)
solution
λonset (nm)
a
film
film
Egopt (eV)
P1
P2
551
586
576
626
697
762
1.78
1.63
a
Calculated from the absorption band edge of the films, Eg
1240/λonset
=
.
Figure 1. TGA plots of P1 and P2 with a heating rate of 10 °C/min
under nitrogen.
Figure 3. Cyclic voltammograms of P1 and P2 films in CH3CN
containing 0.1 mol/L Bu4NPF6.
Table 3. Electrochemical Properties of P1 and P2
a
on
on
Eox (V) Ered (V) HOMO (eV) LUMO (eV) Egec (eV)
P1
P2
0.69
0.53
−1.27
−1.33
−5.49
−5.33
−3.53
−3.47
1.96
1.86
a
Calculated from Eg = e(Eox − Eredon).
on
Figure 4. J−V curves for the polymer solar cells based on P1 and P2
under the illumination of AM1.5G, 86 mW/cm2.
Figure 2. Normalized UV−vis absorption spectra of P1 and P2: (a) in
Voc of 0.87 V, a Jsc of 4.75 mA/cm2 and a FF of 45%. The
higher performance of P2 results from its higher Jsc and FF,
which is probably due to its better absorption at the long
wavelength region. The good hole mobility of silole moiety may
account for the high Jsc.12 The AFM images (tapping mode)
(see Figure S1, Supporting Information) show the morphology
change for the polymer/PC61BM composite films. The rms
roughness for P1/PC61BM film decreased from 6.94 to 0.47 nm,
chloroform; (b) as thin film.
Cells based on P1/PC61BM and P2/PC61BM both show high
Voc (∼0.9 V), due to the quite low HOMO levels of P1 and P2.
The optimized weight ratios (polymer/PC61BM) for P1 and P2
both are 1:1.5. As shown in Table 4, P2 exhibits the best PCE
of 4.33% with a Voc of 0.93 V, a Jsc of 6.66 mA/cm2 and a FF of
60% in comparison with the best PCE of 2.18% for P1 with a
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Note
Table 4. Photovoltaic Properties of the Solar Cells Based on
P1 and P2
ACKNOWLEDGMENTS
■
This work was supported by the “100 Talents Program” of the
Chinese Academy of Sciences. Funding from Suzhou Jiahong
Optoelectronics and the Ministry of Science and Technology of
China are greatly appreciated. We thank Professor Jinsong Zhu
for gifting a MBRAUN glovebox and Professor Zhong Zhang
for providing lab space. We also thank Professor Jianhui Hou
for his kind assistance in EQE measurements.
polymer polymer:PC61BM Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
P1
1:1
0.91
0.87
0.89
0.95
0.93
0.92
4.03
4.75
5.23
6.64
6.66
6.56
42
45
37
55
60
59
1.79
2.18
1.99
4.02
4.33
4.04
1:1.5
1:2
P2
1:1
1:1.5
1:2
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Figure 5 shows external quantum efficiency (EQE) curves for
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Figure 5. EQE curves for the devices based on P1 and P2.
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In summary, we have synthesized two new low bandgap
D−A copolymers with benzo[1,2-b:4,5-c′]dithiophene-4,8-
dione as the acceptor unit. The polymers possess good light-
absorption properties, thermal stability, and deep HOMO levels
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S
* Supporting Information
Synthetic procedures, NMR spectra, and AFM images. This material
is available free of charge via the Internet at http://pubs.acs.org/.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: (L.D.) opv.china@yahoo.com; (W.Z.) zhuwg18@126.com.
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