Benzobis(silolothiophene)-based low bandgap polymers for efficient polymer solar cells

Article abstract of DOI:10.1021/cm1020228

Two new low bandgap copolymers that contain a thiophene-phenylene-thiophene fused ring in which the linked carbon atoms are replaced by silicon atoms were designed and synthesized. A facile synthetic method was developed to synthesize the benzobis(silolothiophene) donor unit in high yield. 2-Bromothiophene was lithiated by LDA and subsequently quenched with trimethylsilyl chloride, yielding colorless oil. Further lithiation of 2 with LDA and then reaction with 2-isopropoxy-4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolane afforded 3 in 52% yields. After lithiation of 5 by n-BuLi, cyclization with dichlorodialkylsilane was performed to get the disilole 6 in 76% yields. Low band gap polymers PBSTBT and PBSTDTBT were obtained though a Stille reaction between monomer and 4,7-dibromo-2,1,3-benzothiadiazole or 4,7-bis(2-bromo-5-thienyl)-2,1,3- benzothiadiazole, using Pd(PPh₃)₄ as catalyst in toluene. The AFM images of the blended films containing both polymer and PC ₇₁BM showed rather smooth morphology with small phase segregation.

Full text of DOI:10.1021/cm1020228

Chem. Mater. 2011, 23, 765–767 765
DOI:10.1021/cm1020228
energy levels, good film-forming property, and high hole
mobility in polymer blend are important prerequisites.5
An extended rigid π-conjugation of the polymer back-
bone will facilitate intermolecular interaction between
polymer chains and increase charge mobility of the poly-
mers.6 Previously, several ladder-type copolymers have
been investigated for achieving efficient PSCs.7 Poly-
(thiophene-phenylene-thiophene)s, a new type of donor,
have demonstrated impressive hole mobilities up to 10-3 cm2
V-1 s-1.8 Cyclopentadithiophene-based polymers repre-
sent another type of material for highly efficient OPVs.9
When the carbon atoms in the 4-position of the cyclo-
pentadithiophene unit are replaced by silicon atoms,
enhanced interchain packing endows the polymer with a
PCE as high as 5.9%.10 Specifically, the introduction of
silicon atoms into the polymer’s backbone has been
proven to be able to bring several desirable characteristics
into polymers, such as lower HOMO and LUMO levels,
improved packing ability, and higher charge mobility.11
Considering the above issues, we have designed and
synthesized two new low bandgap copolymers that con-
tain a thiophene-phenylene-thiophene fused ring in
which the linked carbon atoms are replaced by silicon
atoms.12 A facile synthetic method was developed to syn-
thesize the benzobis(silolothiophene) donor unit in high
yield. The structures of the new copolymers are shown in
Scheme 1. The introduction of silicon atoms was expected
Benzobis(silolothiophene)-Based Low Bandgap
Polymers for Efficient Polymer Solar Cells†
Jie-Yu Wang, Steven K. Hau, Hin-Lap Yip,
Joshua A. Davies, Kung-Shih Chen, Yong Zhang,
Ying Sun, and Alex K.-Y. Jen*
Department of Materials Science and Engineering, University
of Washington, Seattle, Washington 98195
Received July 20, 2010
Revised Manuscript Received August 28, 2010
Polymer solar cells (PSCs) have attracted great atten-
tion for applications in renewable energy due to their
potential for low cost, light weight, and large-area pro-
cessability.1 To achieve high power conversion efficiency
(PCE), bulk heterojunction devices containing polymer
donors and fullerene-derived acceptors have been used
and represent the most efficient PSC structure.2 In the
past decade, many low band gap conjugated polymers
have been exploited to enhance light absorption.3 Among
them, benzodithiophene-based polymers have shown
high PCE of over 7%, which shows great potential for
commercialization.4
To achieve high performance PSC, conjugated polymers
with broad and strong absorption, suitable HOMO-LUMO
† Accepted as part of the “Special Issue on π-Functional Materials 2011”.
(1) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J.
Science 1995, 270, 1789. (b) Kim, J. Y.; Lee, K.; Coates, N. E.;
Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007,
317, 222. (c) Bradec, J.; Sariciftci, N. S.; Hummelen, J. C. Adv.
Funct. Mater. 2001, 11, 15. (d) Coakley, K. M.; McGehee, M. D.
(6) Xiao, S.; Stuart, A. C.; Liu, S.; Zhou, H.; You, W. Adv. Funct.
Mater. 2010, 20, 635.
(7) (a) Zheng, Q.; Jung, B. J.; Sun, J.; Katz, H. E. J. Am. Chem. Soc.
2010, 132, 5394. (b) Lu, J.; Liang, F.; Drolet, N.; Ding, J.; Tao, Y.;
Movileanu, R. Chem. Commun. 2008, 5315. (c) Tsai, J.-H.; Chueh,
C.-C.; Lai, M.-H.; Wang, C.-F.; Chen, W.-C.; Ko, B.-T.; Ting, C.
Macromolecules 2009, 42, 1897. (d) Wu, J.-S.; Cheng, Y.-J.;
Dubosc, M.; Hsieh, C.-H.; Chang, C.-Y.; Hsu, C.-S. Chem. Commun.
2010, 3259.
(8) (a) Chan, S.-H.; Chen, C.-P.; Chao, T.-C.; Ting, C.; Lin, C.-S.; Ko,
B.-T. Macromolecules 2008, 41, 5519. (b) Chen, C.-P.; Chan, S.-H.;
Chao, T.-C.; Ting, C.; Ko, B.-T. J. Am. Chem. Soc. 2008, 130,
12828. (c) Yu, C.-Y.; Chen, C.-P.; Chan, S.-H.; Hwang, G.-W.;
Ting, C. Chem. Mater. 2009, 21, 3262. (d) Chan, S.-H.; Hsiao,
Y.-S.; Hung, L.-I; Hwang, G.-W.; Chen, H.-L.; Ting, C.; Chen,
C.-P. Macromolecules 2010, 43, 3399.
€
Chem. Mater. 2004, 16, 4533. (e) Gunes, S.; Neugebauer, H.;
Sariciftci, N. S. Chem. Rev. 2007, 107, 1324.
ꢀ
(2) (a) Thompson, B. C.; Frechet, J. M. J. Angew. Chem., Int. Ed. 2008,
47, 58. (b) Chen, J. W.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709.
€
(c) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.;
Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789.
(3) (a) Kroon, R.; Lenes, M.; Hummelen, J. C.; Blom, P. W. M.; De
Boer, B. Polym. Rev. 2008, 48, 531. (b) Cheng, Y.-J.; Yang, S.-H.;
Hsu, C.-S. Chem. Rev. 2009, 109, 5868. (c) Huo, L.; Hou, J.; Zhang,
S.; Chen, H.-Y.; Yang, Y. Angew. Chem., Int. Ed. 2010, 49, 1500.
(d) Huang, F.; Chen, K.-S.; Yip, H.-L.; Hau, S. K.; Acton, O.;
Zhang, Y.; Luo, J.; Jen, A. K.-Y. J. Am. Chem. Soc. 2009, 131,
13886. (e) Zhang, Y.; Hau, S. K.; Yip, H.-L.; Sun, Y.; Acton, O.;
Jen, A. K.-Y. Chem. Mater. 2010, 22, 2696. (f) Zou, Y.; Najari, A.;
(9) (a) Mhlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.;
Gaudiana, R.; Brabec, C. Adv. Mater. 2006, 18, 2884. (b) Peet, J.;
Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.;
Bazan, G. C. Nat. Mater. 2007, 6, 497.
ꢀ
Berrouard, P.; Beaupre, S.; Aıch, B. R.; Tao, Y.; Leclerc, M. J. Am.
¨
Chem. Soc. 2010, 132, 5330. (g) Park, S. H.; Roy, A.; Beaupre, S.;
Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.;
Heeger, A. J. Nat. Photonics 2009, 3, 297. (h) Qin, R.; Li, W.; Li, C.;
Du, C.; Veit, C.; Schleiermacher, H.; Andersson, M.; Bo, Z.; Liu,
(10) (a) Hou, J.; Chen, H.-Y.; Zhang, S.; Li, G.; Yang, Y. J. Am. Chem.
Soc. 2008, 130, 16144. (b) Coffin, R. C.; Peet, J.; Rogers, J.; Bazan,
G. C. Nat. Chem. 2009, 1, 657. (c) Hoven, C. V.; Dang, X.-D.;
Coffin, R. C.; Peet, J.; Nguyen, T.-Q.; Bazan, G. C. Adv. Mater.
2010, 22, E63.
€
Z.; Inganas, O.; Wuerfel, U.; Zhang, F. J. Am. Chem. Soc. 2009,
(11) (a) Wang, E.; Wang, L.; Lan, L.; Luo, C.; Zhuang, W.; Peng, J.;
Cao, Y. Appl. Phys. Lett. 2008, 92, 033307. (b) Usta, H.; Lu, G.;
Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 9034.
(c) Lu, G.; Usta, H.; Risko, C.; Wang, L.; Facchetti, A.; Ratner,
M. A.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 7670. (d) Chen,
H.-Y.; Hou, J.; Hayden, A. E.; Yang, H.; Houk, K. N.; Yang, Y.
Adv. Mater. 2010, 22, 371. (e) Morana, M.; Azimi, H.; Dennler, G.;
Egelhaaf, H.-J.; Scharber, M.; Forberich, K.; Hauch, J.; Gaudiana,
R.; Waller, D.; Zhu, Z.; Hingerl, K.; van Bavel, S. S.; Loos, J.;
Brabec, C. J. Adv. Funct. Mater. 2010, 20, 1180.
131, 14612. (i) Bijleveld, J. C.; Zoombelt, A. P.; Mathijssen, S. G.;
Wienk, M. M.; Turbiez, M.; Leeuw, D.; Janssen, R. A. J. J. Am.
Chem. Soc. 2009, 131, 16616. (j) Baek, N. S.; Hau, S. K.; Yip, H.-L.;
Acton, O.; Chen, K.-S.; Jen, A. K.-Y. Chem. Mater. 2008, 20, 5734.
(4) (a) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.;
Yu, L.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649. (b) Liang, Y.;
Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv.
Mater. 2010, 22, E135.
ꢀ
(5) (a) Bredas, J.-L.; Norton, J.; Cornil, J.; Coropceanu, V. Acc. Chem.
€
Res. 2009, 42, 1691. (b) Inganas, O.; Zhang, F.; Tvingstedt, K.;
€
Andersson, L. M.; Hellstrom, S.; Andersson, M. R. Adv. Mater.
(12) Tierney, S.; Bailey, C.; Mitchell, W.; Blouin N. WO patent 2010/
031480 A1.
2010, 22, E100.
r
2010 American Chemical Society
Published on Web 09/07/2010
pubs.acs.org/cm

766 Chem. Mater., Vol. 23, No. 3, 2011
Wang et al.
Scheme 1. Structures of PBSTBT and PBSTDTBT
Scheme 2. Synthetic Route for Monomer 8a
Figure 1. UV-vis spectra of (a) PBSTBT and (b) PBSTDTBT in CHCl3
solution and film state.
Both PBSTBT and PBSTDTBT give rise to long-
wavelength absorption which is attributed to charge trans-
fer from the benzobis(silolothiophene) donor to the accep-
tor within the polymer chains. As shown in Figure 1a, the
absorption maxima of PBSTBT in dilute chloroform solu-
tion was observed at 620 nm. This main absorption peak
red shifts to 636 nm in cast film, indicating slight aggre-
gation formed in the solid state. The optical band gap,
obtained from the onset of the absorption spectrum in film,
was estimated to be 1.80 eV. Similarly, the red shift of the
maximum absorption from 590 nm in solution to 618 nm in
a film indicates PBSTTBT also form aggregates in solid
state, as shown in Figure 1b. A comparison of the poly-
mers reveals that PBSTDTBT shows much broader and
longer wavelength absorption than PBSTBT, which
should be due to the introduction of two more thiophene
units between the donor and the acceptor compared to
PBSTBT. The optical band gap of PBSTDTBT calcu-
lated is 1.70 eV, which is 0.10 eV lower than PBSTBT. The
HOMO and LUMO levelsof PBSTBT determined electro-
chemically by cyclic voltammetry (CV) were -5.32 and
-3.25 eV, respectively. The band gap obtained from CV is
2.07 eV which is slightly larger than that obtained from the
absorption spectrum. Only the HOMO level of PBSTDTBT
could be obtained from CV, which is -5.25 eV. With the
optical band gap as 1.70 eV, the LUMO level was esti-
mated to be -3.55 eV. In addition to optical and electro-
chemical methods, density functional theory (DFT) was
also used to predict the HOMO and LUMO energy levels
of the new polymers. The method used in this case, B3LYP/
6-31G*, has been found to be an accurate formalism for
predicting optical and structure properties of conjugated
polymers.13 HOMO and LUMO levels of PBSTBT were
predicted to be -4.77 and -2.65 eV, respectively, while
the corresponding levels for PBSTDTBT were -4.59 and
-2.72 eV, respectively (see the Supporting Information).
Although the DFT predicted energy levels are over-
estimated, some useful correlations can still be made
between experiment results and theory.12 For instance,
DFT accurately predicted a deeper lying HOMO level for
PBSTBT (-4.77 eV) relative to PBSTDTBT (-4.59 eV),
which is consistent with the trends in CV data (-5.32 eV
vs -5.25 eV, respectively). Moreover, DFT predicted lower
lying HOMO levels for both PBSTBT and PBSTDTBT,
with a difference of 0.10 and 0.08 eV, respectively, relative
to their analogues in which carbon replaces silicon. This
provides theoretical confirmation that the silole containing
a (i) LDA, THF, -78 °C, 1 h, then TMSCl, -78 °C to rt, overnight;
(ii) LDA, THF, -78 °C to rt, overnight, then 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane, -78 °C to rt, 6 h; (iii) 3, K2CO3,
Pd(PPh3)4, toluene/H2O, reflux, 2 d; (iv) n-BuLi, THF, -78 °C to rt,
1 h, then dichlorodioctylsilane, -78 °C to rt, overnight; (v) NBS, THF,
overnight; (vi) n-BuLi, THF, -78 °C to rt, 1 h, then trimethyltin
chloride, -78 °C to r.t., overnight.
to enhance hole mobility and lower the HOMO level of
the copolymers, which are important for obtaining high
PCE. The hole mobilities measured by using the field
effect transistor technique are 2.2 Â 10-3 and 1.1 Â 10-2
cm2 V-1 s-1 for PBSTBT and PBSTDTBT, respectively.
PSCs using these two polymers blended with [6,6]-phenyl-
C71-butyric acid methyl ester (PC71BM) as acceptor
showed PCE above 3.5%.
The synthetic route leading to the buliding blocks of
low band gap polymers is shown in Scheme 2. 2-Bromo-
thiophene was lithiated by LDA and subsequently
quenched with trimethylsilyl chloride, yielding a colorless
oil 2. Further lithiation of 2 with LDA and then reaction
with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
afforded 3 in 52% yield. 5 was obtained as a white solid in
40% yield through a Suzuki coupling reaction between
1,4-dibromo-2,5-diiodobenzene and 3. After lithiation of
5 by n-BuLi, cyclization with dichlorodialkylsilane was
performed to get the disilole 6 in 76% yield. The bromi-
nation of 6 with NBS and a successive reaction with
n-BuLi quenched with trimethyltin chloride afforded
8 as a green oil which was directly used without further
purification.
Low band gap polymers PBSTBT and PBSTDTBT
were obtained though a Stille reaction between monomer 8
and 4,7-dibromo-2,1,3-benzothiadiazole or 4,7-bis(2-bromo-
5-thienyl)-2,1,3-benzothiadiazole, using Pd(PPh3)4 as
catalyst in toluene. Both polymers possess good solubility
in common organic solvents such as chloroform and
dichlorobenzene, providing convenience for characteri-
zation and device processing. The number average mole-
cular weights (Mn) of PBSTBT and PBSTDTBT are
38.5 kDa and 22.4 kDa with polydispersity indices (PDI)
of 2.09 and 1.58, respectively. In addtion, the two polymers
showed high thermal stability with no obvious thermal
transition between 20 and 350 °C, which could be detected
by differential scanning calorimetry (DSC).
(13) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
^
Neagu-Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc,
M. J. Am. Chem. Soc. 2008, 130, 732.

Communication
Chem. Mater., Vol. 23, No. 3, 2011 767
Table 1. Performance of Photovoltaic Devices under the AM 1.5
Simulated Illumination (100 mW/cm2)
ratio
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
1:1a
1:2a
1:3a
1:1b
1:2b
1:3b
0.73 ((0.01)
0.71 ((0.01)
0.81 ((0.03)
0.81 ((0.02)
0.80 ((0.01)
0.80 ((0.01)
4.80 ((0.15)
8.0 ((0.49)
8.86 ((0.50)
5.66 ((0.37)
8.94 ((0.27)
8.80 ((0.21)
32.9 ((1.0)
36.4 ((2.6)
48.8 ((1.0)
35.8 ((1.2)
48.9 ((1.6)
51.6 ((1.5)
1.16 ((0.15)
2.07 ((0.28)
3.47 ((0.19)
1.69 ((0.14)
3.50 ((0.20)
3.64 ((0.14)
Figure 2. Output (a and c) and transfer (b and d) characteristics of FETs
based on PBSTBT and PBSTDTBT.
a PBSTBT:PC71BM. b PBSTDTBT:PC71BM.
donors have the potential to increase Voc. The deeper
HOMO levels of both PBSTBT and PBSTDTBT indicate
the advantage of introducing silicon atoms into the donor
part of polymer’s backbone, which is critical for increas-
ing the open circuit voltage (Voc). In this case, there is no
doubt that this family of silicon-containing low band gap
polymers has good potential for organic photovoltaics.
To understand if the incorporation of silicon atoms into the
fused rings can endow the copolymers with high hole mobility,
field effect transistors with a bottom-gate, top-contact config-
uration were fabricated using PBSTBT and PBSTDTBT as
the semiconductors. The detail of the device fabrication is
described in the Supporting Information. The hole mobilities
extracted from the saturation regime are 2.2 Â 10-3 and 1.1 Â
Figure 3. (a) J-V curves of polymer solar cells under illumination of AM
1.5G, 100 mW/cm2 at polymer:PC71BM ratio 1:3; (b) EQEs of polymer:
PC71BM 1:3 solar cells.
3.64%. A Jsc of 8.80 mA/cm2 with a Voc of 0.80 V and a
FF of 51.60% was achieved. This device showed a higher
photoconversion efficiency over the range of 600-700 nm.
The Jsc values calculated by integrating the EQE data
compared with an AM 1.5 reference spectrum were 9.00
and 9.10 mA/cm2 for PBSTBTand PBSTDTBT, respec-
tively, which are similar to those obtained from the J-V
measurements. The AFM images of the blended films
containing both polymer and PC71BM showed rather
smooth morphology with small phase segregation (see the
Supporting Information), which indicates good film-form-
ing properties of these new silole-containing copolymers.
In conclusion, we have developed a new type of benzobis-
(silolothiophene)-based low band gap copolymers using a
facile method in high yield. The introduction of silicon atoms
into the fused rings provides these new polymers with low
10-2 cm2 V-1 -1
based on the average taken from two batches
s
of five devices each for PBSTBT (Figure 2a,b) and PBST-
DTBT (Figure 2c,d), respectively. The on/off current ratios are
around 105 to 106. The highest hole mobilities of PBSTBT and
PBSTDTBT reached 2.5 Â 10-3 and 1.2 Â 10-2 cm2 V-1 s-1
,
respectively. Such high hole mobilities, compared to other
copolymers containing only large fused ring donors,8 maybe
arise from the silicon atoms that can enhance the intermole-
cular interaction and packing between polymer chains.11d
The photovoltaic properties of PBSTBT and PBSTDTBT
were studied in PSCs using a device structure of ITO/
PEDOT:PSS/polymer:PC71BM/Ca/Al with a device area
of 16 mm2. The device was annealed in an atmosphere of
dichlorobenzene for 10 min before the deposition of the
electrode. All the details of device fabrication and char-
acterization were demonstrated in the Supporting Infor-
mation. The effect of blending ratio between polymer and
PC71BM was systematically investigated as well. As shown
in Table 1, when implanted with three times PC71BM in
weight ratio, both PBSTBT and PBSTDTBT achieve the
highest PCE. Figure 3a shows the typical current density-
voltage (J-V) characteristics and the external quantum
efficiency (EQE) of solar cell devices based on PBSTBT
and PBSTDTBT with a 1:3 weight ratio of polymer to
PC71BM. Under the AM 1.5 simulated solar illumination
(100 mW/cm2), the device with a PBSTBT:PC71BM
weight ratio of 1:3 and an active-layer thickness around
80 nm demonstrated a short-circuit current density (Jsc)
of 8.86 mA/cm2, a Voc of 0.81 V, and a fill factor (FF) of
48.80%. The resulting PCE is 3.49%. The EQE curve of a
PBSTBT based device, as shown in Figure 3b, indicates
an efficient photoconversion efficiency from 400 to 700
nm, with EQE values of as high as 52%. For PBSTDTBT,
a device with a PBSTDTBT:PC71BM 1:3 weight ratio and
active-layer thickness around 90 nm shows a PCE up to
HOMO levels and high hole mobilities up to 0.01 cm2 V-1 -1
s .
Preliminary characterization of the bulk heterojuction solar
cell devices shows PCE of around 3.5% for both of PBSTBT
and PBSTDTBT. This work provides a new method for
synthesizing silole-containing heteroarenes, which show great
potential in the organic electronics applications, such as FETs
and PSCs. Further structural functionalization and device
optimazition based on PBSTBT and PBSTDTBT are ongoing.
Acknowledgment. The authors thank the support from the
National Science Foundation (DMR-0120967), the Depart-
ment of Energy (DE-FC3608GO18024/A000), the AFOSR
(FA9550-09-1-0426), the Office of Naval Research (N00014-
08-1-1129), and the World Class University (WCU) program
through the National Research Foundation of Korea under
the Ministry of Education, Science and Technology (R31-
10035). A.K.-Y. J. thanks the Boeing Foundation for supports.
Additionally the authors thank Professors B. Robinson, B.
Eichinger, and L. Johnson for providing the guidence and
framework necessary for calculations.
Supporting Information Available: Experimental details of the
synthesis of the polymers, the fabrication and characterization of
the devices, measurements, and instruments (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.