A Poh benzo[1,2-6:4,5-b′]dithiophene derivative with deep HOMO level and its application in high-performance polymer solar cells

Article abstract of DOI:10.1002/anie.200906934

Chemical Equation Presented Sun catcher: The new benzo[l,2-b:4,5-b′]dithiophene-containing polymer PBDTTBT exhibits a power conversion efficiency of up to 5.66%. The high open-circuit voltage (0.92 V) originates from the low HOMO level of the polymer and the high internal and external quantum efficiencies over a wide spectral range. PBDTTBT is a promising donor material for application in polymer solar cells.

Full text of DOI:10.1002/anie.200906934

Communications
DOI: 10.1002/anie.200906934
Solar Cells
A Polybenzo[1,2-b:4,5-b’]dithiophene Derivative with Deep HOMO
Level and Its Application in High-Performance Polymer Solar Cells**
Lijun Huo, Jianhui Hou,* Shaoqing Zhang, Hsiang-Yu Chen, and Yang Yang*
Although there is a consensus on the development of solar
energy as a green energy resource, the importance of
development of the polymer solar cell (PSC) has only been
realized in recent years owing to its advantages of low cost
and flexibility in large-area applications.[1] Bulk heterojunc-
tion (BHJ) type polymer solar cells now play a leading role in
realizing high power conversion efficiency (PCE). The
development of new polymer materials to further improve
the efficiencies of BHJ-type PSCs will accelerate their
commercial application. At present, P3HT-PCBM-based
PSCs (P3HT= poly(3-hexylthiophene), PCBM = methano-
fullerene (6,6)-phenyl-C61-butyric acid methyl ester) have
exhibited high efficiencies of 4–5%. However, the narrow
absorption spectrum of P3HT in 300–650 nm is one of the
main hindrances to the further improvement of the efficien-
cies of P3HT-based devices. To overcome this problem, some
low-band-gap polymer materials as donors have been synthe-
sized successfully and applied to photovoltaic devices. Large
short-circuit current density (Jsc) and higher efficiencies have
been demonstrated using this strategy.[2] In spite of the high Jsc
achievable with low-band-gap polymers, the device perfor-
mance suffers owing to the low open-circuit voltage (Voc) of
0.5–0.7 V. According to published models and experimental
results, the Voc of a PSC is closely related to the offset between
energy levels of the highest occupied molecular orbital
(HOMO) of the donor and lowest unoccupied molecular
orbital (LUMO) of the acceptor.[3] Thus, to enhance the open-
circuit voltage of these devices, some building blocks with
lower-lying HOMOs should be introduced in donor molecules
to lower their HOMO levels.
reported to show lower HOMO levels and correspondingly
exhibited higher open-circuit voltages. However, the wider
band gaps of these polymers resulted in lower values of Jsc and
consequently lower efficiencies. To enhance the efficiency,
there is a need for a low-band-gap polymers that can have a
high Voc. Few polymers with high open-circuit voltages (above
0.8 V) and consequently high PCEs (above 5%) have been
reported to date, except for two kinds of polymers, poly[(2,7-
silafluorene)-alt-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)]
(PSiFDBT) and poly[N-9’’-heptadecanyl-2,7-carbazole-alt-
5,5-(4’,7’-di-2-thienyl-benzothiadiazole) (PCDTBT), both of
which contain a fluorene unit in the polymer main chain.[4,5]
Considering the similarity in the molecular structures of
PSiFDBTand PCDTBTwith analogous fluorene units in both
of their main chains, it is necessary to explore new molecular
structures with favorable characteristics for higher open-
circuit voltages and conversion efficiencies.
Recently, some benzo[1,2-b:4,5-b’]dithiophene-contain-
ing polymers were applied to field-effect transistor (FET)
and PSC devices (Table 1).[2d,e,6,7] A high mobility of 0.15–
Table 1: Photovoltaic data of the high-performance PSCs based on
several donor materials with deep HOMO levels.
Polymer
Voc [V]
Jsc
FF [%]
PCE [%]
Ref.
[mAcmÀ2
]
PBDTTT-CF[a]
PSiFDBT
PCDTBT
PBDTTBT
0.76
0.90
0.88
0.92
15.2
9.55
10.60
10.70
66.9
50.7
66.0
57.5
7.73[b]
5.40[c]
6.10[b]
5.66[b]
[2e]
[4]
[5]
this work
[a] PBDTTT-CF=poly[4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b’]dithio-
phene-2,6-diyl-alt-(4-octanoyl-5-fluoro-thieno[3,4-b]thiophene-2-carboxyl-
ate)-2,6-diyl]. [b] PC71BM was used as the acceptor. [c] PC61BM was used
as the acceptor.
Polyfluorene and its derivatives are a well-known class of
wide-band-gap organic semiconducting materials. Many
donor polymers containing fluorene on the main chain were
0.25 cm2 VÀ1 sÀ1 was achieved owing to the relatively large and
planar molecular structure of the thiophene derivative, which
would help promote cofacial p–p stacking, thus benefiting
charge transport. Given its symmetrical planar structure and
higher mobility, the incorporation of a low-band-gap unit into
the benzo[1,2-b:4,5-b’]dithiophene unit could potentially
result in a red-shifted absorption. A more appropriate
energy level could be obtained by fine tuning of molecular
structures for photovoltaic applications.
[*] Dr. L. Huo, Dr. J. Hou, Prof. Dr. Y. Yang
Department of Materials Science and Engineering
University of California, Los Angeles
Los Angeles, CA 90095 (USA)
Fax: (+1)310-206-7353
E-mail: yangy@ucla.edu
Dr. J. Hou, S. Zhang, Dr. H. Chen
Solarmer Energy Inc., El Monte, CA 91731 (USA)
Fax: (+1)626-456-8082
E-mail: jianhuih@solarmer.com
[**] The authors would like to acknowledge the financial support from
AFOSR (Grant No. FA9550-07-1-0264). J.H. designed the polymer
structures and synthesis routes. L.H. and S.Z. performed the
synthesis of polymers. H.Y.C. performed the polymer solar cell
fabrication experiments and measurements. Y.Y. and J.H. concep-
tualized and directed the research project. All authors discussed the
results and commented on the manuscript.
Herein we report the benzo[1,2-b:4,5-b’]dithiophene
derivative PBDTTBT (Scheme 1), which exhibits high Voc
and PCE values. It would thus be a promising candidate for
high-efficiency PSCs. The synthesis of the polymer is
described in Scheme 2 (see also the Experimental Section).
First, to ensure good solubility, four octyl chains were
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Scheme 1. Molecular structure of PBDTTBT.
symmetrically added to benzo[1,2-b:4,5-b’]dithiophene by the
linkage of thienylene to give 2. 4,7-Bis(2-thienyl)-2,1,3-
benzothiadiazole)-5,5’-diyl (3) was copolymerized with
benzo[1,2-b:4,5-b’]dithiophene to lower the band gap. The
new polymer poly{4,8-bis(2,5-dioctyl-2-thienyl)-benzo[1,2-
b:4,5-b’]dithiophene-alt-[4,7-bis(2-thienyl)-2,1,3-benzothia-
diazole)-5,5’-diyl]} (PBDTTBT) was prepared by Suzuki
coupling in good yield. PBDTTBT is soluble in common
organic solvents, and the number average molecular weight
(Mn) is 27.4 kDa, with a polydispersity index (PDI) of 1.8.
Thermogravimetric analysis showed an excellent thermal
stability with a decomposition temperature of 3378C without
the protection of an inert atmosphere.
Figure 1. a) Absorption spectra of PBDTTBT as a film and in chloro-
form solution. b) Cyclic voltammograms of PBDTTBT films on a
platinum electrode in 0.1 molLÀ1 Bu4NPF6, CH3CN solution.
As shown in Figure 1a, PBDTTBT shows three absorp-
tion bands in the range 300–700 nm both in chloroform and as
a solid film. The main absorption peak of PBDTTBT is
and one irreversible n-doping (Ered = À1.36 V vs. Fc/Fc+)
process (Figure 1b). The HOMO (À5.31 eV) and
LUMO (À3.44 eV) of PBDTTBT could be calculated
from these electrochemical results. Compared to the
HOMO of P3HT (À5.0 eV), that of PBDTTBT is 0.3 eV
lower and is analogous to those of poly[(9,9-dialkyl-
fluorene)-2,7-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzo-
thiadiazole)-5,5’-diyl] (PFDTBT), PSiFDBT, and
PCDTBT (À5.5 eV, À5.39 eV, and À5.5 eV, respec-
tively), which means a higher Voc could be expected,
because Voc is linearly correlated with the difference of
the HOMO of the donor and the LUMO of the
acceptor.[3a] Furthermore, the electrochemical band
gap of PBDTTBT (Egec = 1.87 eV) is well matched
with its optical band gap (Egopt = 1.75 eV) within the
experimental error.
To further explore the photovoltaic properties of
PBDTTBT, solar cell devices were fabricated. Figure 2a
presents a typical J–V curve of the device. PCEs up to
5.66% were obtained from the PBDTTBT-based device,
with a Voc of 0.92 V, a Jsc of 10.7 mAcmÀ2, and an fill
factor (FF) of 57.5%, which is one of the highest PCEs
for single-active-layer PSCs. The higher Voc agrees well
with the low-lying HOMO level calculated from electro-
chemical results. It is worth noting that the high performance
of PBDTTBT solar cells was obtained without the need for
extra treatments such as annealing or additive addition. There
should be more space to improve the performances with
development of new processing techniques. The internal
quantum efficiency (IQE) and external quantum efficiency
(EQE) of the device are shown in Figure 2b. It is apparent
that the device exhibits a very high IQE above 80% with a
broad response from 300 to 700 nm, which validates the
Scheme 2. Synthesis route of the polymer PBDTTBT. R=n-C8H17. a) n-Butyl-
lithium, À788C, 2 h, then n-C8H17Br, 14 h; b) n-butyllithium, THF, 08C, then
508C, 2 h; benzo[1,2-b:4,5-b’]dithiophen-4,8-dione, 508C, 1 h, then SnCl2, HCl;
c) n-butyllithium, THF, ambient temperature, 2 h; then borolane, ambient
temperature, 6 h; d) [Pd(PPh3)4], toluene, reflux, 18 h.
located at approximately 581 nm in solution. However, this
peak is red-shifted to 596 nm in solid films and showed a
stronger shoulder at 627 nm, which could be attributed to a
more aggregated configuration in the solid state. The
absorption edges of 695 and 708 nm in solution and the
solid film correspond to optical band gaps (Egopt) of 1.78 and
1.75 eV for PBDTTBT, respectively.
Cyclic voltammetry (CV) was used to measure oxidation
and reduction potentials. PBDTTBT presents one reversible
p-doping (Eox = 0.51 V vs. ferrocene/ferrocenium (Fc/Fc+))
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Ag electrode as working electrode, counter electrode, and reference
electrode, respectively, in a 0.1 molLÀ1 tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) acetonitrile solution; a trace amount of
ferrocene was used as reference material.
Device fabrication: Polymer solar cell devices with the structure
ITO/PEDOT-PSS/polymer:PC70BM(1:2, w/w)/Ca/Al were fabricated
as follows: After spin-coating a 30 nm layer of poly(3,4-ethylene
dioxythiophene):poly(styrenesulfonate) onto a precleaned indium tin
oxide (ITO) coated glass substrate, the polymer/PCBM blend
solution was spin-coated. Typical concentration of the polymer/
PC70BM (1:2, w/w) blend solution for spin-coating the active layer was
10 mgmLÀ1, and dichlorobenzene was used as solvent. To optimize
device performance, different donor/acceptor (D/A) weight ratios
(1:1 to 1:4) were used during the device fabrication process. However,
it was found that the 1:2 D/A ratio gave the best device performance.
The thickness of the active layer was approximately 80 nm.
1: Under the protection of argon, n-butyllithium (2.88m, 28.1 mL)
was added dropwise to 3-octylthiophene (14.45 g, 73.7 mmol) in THF
(150 mL) at À788C over about 2 h, then the mixture was stirred for
1 h at À788C. The cooling bath was removed and the mixture was
stirred for another 1 h at room temperature. 1-Bromooctylane
(21.23 g, 110 mmol) was injected in one portion at room temperature;
then the solution was stirred overnight. After 14 h the reaction was
stopped and water (100 mL) was added, and the mixture was
extracted with diethyl ether twice. After removing the solvent, the
residual oil was distilled in high vacuum to obtain 1 as a colorless
liquid (13.07 g, yield 57.4%). 1H NMR (CDCl3, 400 MHz): d = 6.69 (s,
1H), 6.63 (s, 1H), 2.81 (t, 2H), 2.54 (t, 2H), 1.70 (m, 4H), 1.35 (m,
20H), 0.91 ppm (t, 6H). 13C NMR (CDCl3, 100 MHz): d = 145.59,
142.90, 125.43, 117.08, 31,92, 31.74, 30.47, 29.76, 29.45, 29.32, 22.72,
14.13 ppm.
Figure 2. a) J–V curve of a PBDTTBT/PC70BM-based solar cell device
under illumination of AM1.5G, 100 mWcmÀ2. b) External quantum
efficiency (EQE) and internal quantum efficiency (IQE) of a PBDTTBT/
PC70BM-based solar cell device.
2: Under the protection of argon, n-butyllithium (2.88m, 7.64 mL)
was added dropwise to 1 (6.20 g, 20 mmol) in THF (16 mL) at 08C;
then the mixture was warmed up to 508C and stirred for 2 h.
Subsequently, 4,8-dehydrobenzo[l,2-b:4,5-b’]dithiophene-4,8-dione
(1.1 g, 5 mmol) was added, and the mixture was stirred for 1 h at
508C. After cooling down to ambient temperature, SnCl2·2H2O
(9.0 g, 40 mmol) in 10% HCl (16 mL) was added, and the mixture was
stirred for an additional 1.5 h and poured into ice water. The mixture
was extracted by diethyl ether twice, and the combined organic phase
was concentrated to obtain raw 2. Further purification was carried out
by column chromatography using petroleum ether as eluent to obtain
pure 2 as a light yellow liquid (1.76 g, yield 44.1%). 1H NMR (CDCl3,
400 MHz): d = 7.39 (d, 2H), 7.24 (d, 2H), 6.80 (s, 2H), 2.86 (t, 4H),
2.39 (t, 4H), 1.75 (m, 8H), 1.36 (br, 40H), 0.92–0.81 ppm (m, 12H).
13C NMR (CDCl3, 100 MHz): d = 145.79, 141.24, 141.05, 140.09,
137.72, 130.27, 127.36, 125.72, 123.76, 31.84, 31.50, 30.37, 29.38,
29.21, 29.11, 22.66, 14.12 ppm.
efficient photon conversion properties of this system. Such a
broad response above 50% EQE has seldom been reported.
Such high EQE values are the main contribution behind the
high Jsc. Additionally, when PC61BM was used instead of
PC71BM, the Jsc of the device decreased owing to the narrower
absorption band of PC61BM than PC71BM.
In conclusion, we have reported a new benzo[1,2-b:4,5-
b’]dithiophene-containing polymer PBDTTBT that exhibited
a PCE of up to 5.66% under AM1.5G 100 mWcmÀ2
illumination. The higher Voc of 0.92 V from PBDTTBT-
based device originates from the lower HOMO level of the
polymer and the higher IQE (above 80%) and the EQE
response value beyond 50% in a wide spectral range are the
main contributions to high Jsc. These results indicate that
PBDTTBT is a promising polymer donor material for
application in polymer solar cells.
3: Under the protection of argon, n-butyllithium (2.88m, 2.48 mL)
was added dropwise to 2 (2.60 g, 3.24 mmol) in THF (35 mL) at room
temperature and stirred for 2 h at 508C. Then 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (1.5 g, 8.1 mmol) was added in one
portion at room temperature. After 6 h, the reaction was stopped and
water (120 mL) was added, and then the mixture was extracted by
diethyl ether twice. After removing the solvent, the residue was
purified by column chromatography with petroleum ether/ethyl
acetate (10:1) as eluent to obtain pure 3 as a yellow, sticky liquid
Experimental Section
Instruments and measurements: The absorption spectra were
obtained from a Varian Cary 50 UV/Vis spectrophotometer. The
current–voltage (J–V) curves were obtained by a Keithley 2400
source-measure unit. The photocurrent was measured under illumi-
nation using a solar simulator [ThermoOriel 150W solar simulator
(AM1.5G)], and the light intensity was calibrated with a Newport
818T-10 thermopile detector. To ensure accuracy, the simulated light-
source and device area were calibrated by the method in our previous
work.[9] The EQE measurements of the encapsulated devices were
performed in air [PV Measurements Inc., Model QEX7]. The
internal quantum efficiency (IQE) of the device was calibrated
according to the our previous work.[10] Cyclic voltammetry was
conducted with a Pt disk coated with the polymer film, a Pt wire, and a
1
(2.61 g, yield 76.2%). H NMR (CDCl3, 400 MHz): d = 7.79 (s, 2H),
6.75 (s, 2H), 2.82 (m, 4H), 2.36 (m, 4H), 1.75 (m, 8H), 1.34–1.27 (br,
64H), 0.91–0.81 ppm (m, 12H). 13C NMR (CDCl3, 100 MHz): d =
145.79, 143.77, 141.40, 141.20, 139.53, 133.39, 129.94, 125.60, 124.56,
84.42, 31.83, 31.53, 30.34, 29.39, 29.26, 29.14, 24.73, 22.66, 14.14 ppm.
4,7-Di(2-bromothien-5-yl)-2,1,3-benzothiadiazole and 4,8-dehy-
drobenzo[l,2-b:4,5-b’]dithiophene-4,8-dione were synthesized accord-
ing to the reported method.[7,8]
Polymerization to PBDTTBT: 4,7-Di(2-bromothien-5-yl)-2,1,3-
benzothiadiazole (160 mg, 0.35 mmol) and 3 (369 mg, 0.35 mmol)
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were dissolved in 15 mL toluene with sodium carbonate (1m, 3 mL).
The mixture was purged with argon for 10 min, then [Pd(PPh3)4]
(15 mg) was added. After being purged with argon for 20 min, the
reaction was heated to 1108C for 18 h under argon atmosphere. Then,
the mixture was cooled to room temperature, and the polymer was
precipitated by addition of 100 mL methanol and filtered through a
Soxhlet thimble, which was then subjected to Soxhlet extraction with
methanol, hexane, and chloroform. The polymer recovered from
chloroform was purified by preparative gel permeation chromatog-
raphy. Then, the product was dried under vacuum for 1 day to recover
the target polymer PBDTTBT as a dark solid.(yield 34%, Mn =
27.4 K, PDI = 1.8). 1H NMR (CDCl3, 400 MHz): d = 7.99–6.81 (m,
10H), 2.91 (br, 4H), 2.39 (br, 4H), 1.80 (m, 8H), 1.48–1.26 (m, 40H),
1.07–0.75 ppm (m, 12H). Calcd for C64H78N2S7: C 69.89, H 7.15,
N 2.55; found: C 68.97, H 7.12, N 2.51.
Zhang, G. Li, Y. Yang, J. Am. Chem. Soc. 2008, 130, 16144;
d) J. H. Hou, H. Y. Chen, S. Q. Zhang, R. I. Chen, Y. Yang, Y.
Wu, and G. Li, J. Am. Chem. Soc. 2009, 131, 15586; e) H. Y.
Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang, Y. Yang,
L. P. Yu, Y. Wu, G. Li, Nat. Photonics 2009, 3, 649; f) L. J. Huo,
T. L. Chen, Y. Zhou, J. H. Hou, H. Y. Chen, Y. Yang, Y. F. Li,
Macromolecules 2009, 42, 4377; g) L. J. Huo, H. Y. Chen, J. H.
Hou, T. L. Chen, Y. Yang, Chem. Commun. 2009, 5570.
[3] a) M. C. Scharber, D. Mꢀhlbacher, M. Koppe, P. Denk, C.
Waldauf, A. J. Heeger, C. J. Brabec, Adv. Mater. 2006, 18, 789;
b) G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mater. 2009,
21, 1323.
[4] E. G. Wang, L. Wang, L. F. Lan, C. Luo, W. L. Zhuang, J. B.
Peng, Y. Cao, Appl. Phys. Lett. 2008, 92, 033307.
[5] a) N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair, R.
Plesu, M. BelletÞte, G. Durocher, Y. Tao, M. Leclerc, J. Am.
Chem. Soc. 2008, 130, 732; b) S. H. Park, A. Roy, S. Beaupre, S.
Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, A. J.
Heeger, Nat. Photonics 2009, 3, 297.
Received: December 9, 2009
Published online: January 25, 2010
[6] a) H. Pan, Y. Li, Y. Wu, P. Liu, B. S. Ong, S. Zhu, G. Xu, J. Am.
Chem. Soc. 2007, 129, 4112; b) H. Pan, Y. Wu, Y. Li, P. Liu, B. S.
Ong, S. Zhu, G. Xu, Adv. Funct. Mater. 2007, 17, 3574.
[7] a) J. Hou, M. Park, S. Zhang, L. Chen, J. Li, Y. Yang, Macro-
molecules 2008, 41, 6012; b) L. J. Huo, J. H. Hou, H. Y. Chen,
S. Q. Zhang, Y. Jiang, T. L. Chen, Y. Yang, Macromolecules 2009,
42, 6564.
[8] Q. Hou, Y. Xu, W. Yang, M. Yuan, J. B. Peng, Y. Cao, J. Mater.
Chem. 2002, 12, 2887.
[9] V. Shrotriya, G. Li, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Adv.
Funct. Mater. 2006, 16, 2016.
Keywords: electronic structure · photochemistry · polymers ·
solar cells
.
[1] a) G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science
1995, 270, 1789; b) J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-
Q. Nguyen, M. Dante, A. J. Heeger, Science 2007, 317, 222; c) J.
Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater. 2001,
11, 15.
[2] a) D. Mꢀhlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller,
R. Gaudiana, C. Brabec, Adv. Mater. 2006, 18, 2884; b) J. Peet,
J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, G. C.
Bazan, Nat. Mater. 2007, 6, 497; c) J. H. Hou, H. Y. Chen, S. Q.
[10] M. Chen, J. Hou, Z. Hong, G. Yang, S. Sista, L. Chen, Y. Yang,
Adv. Mater. 2009, 21, 4238.
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