Twisted but conjugated: Building blocks for low bandgap polymers

Article abstract of DOI:10.1002/anie.201400674

Here we report a novel twisted monomer based on a distorted C=C double bond for low bandgap conjugated copolymers. This new building block provides several unique characteristics when compared to classical planar systems such as high solubility, electron accepting ability, and isomeric tunability. The resulting copolymers exhibit broad absorption spanning both visible and near-infrared regions leading to promising solar cell performance. A simplified synthetic approach provided novel building blocks for conjugated polymers featuring a non-planar repeat unit and a distorted double bond. The resulting copolymers are characterized by a broad absorption profile and show promising results in solar cells.

Full text of DOI:10.1002/anie.201400674

Angewandte
Chemie
DOI: 10.1002/anie.201400674
Organic Solar Cells
Twisted but Conjugated: Building Blocks for Low Bandgap Polymers**
Chien-Yang Chiu, Hengbin Wang, Fulvio G. Brunetti, Fred Wudl, and Craig J. Hawker*
Abstract: Here we report a novel twisted monomer based on
P3HT (poly(3-hexylthiophen-2,5-diyl)) as the donor material,
solar cells with 1.7% power conversion efficiency (PCE) and
an open circuit voltage (Voc) of 1.20 V were obtained.[28] The
potential of 99’BF as a small molecule, n-type material
strongly suggested that the concept of non-planar building
blocks could be applied to the development of a broad
monomer platform for conjugated polymers. Here we report
the synthesis, properties, and polymerization of a new class of
building blocks based on 8,8’-biindeno[2,1-b]thiophenylidene
(BTP, 1a, in Figure 1), for application as low bandgap
polymers in BHJ solar cells.
=
a distorted C C double bond for low bandgap conjugated
copolymers. This new building block provides several unique
characteristics when compared to classical planar systems such
as high solubility, electron accepting ability, and isomeric
tunability. The resulting copolymers exhibit broad absorption
spanning both visible and near-infrared regions leading to
promising solar cell performance.
I
n the last decade, significant attention has been focused on
organic solar cells (OSCs) for cost effective manufacturing of
photovoltaics on large area and flexible substrates.[1,2] Tradi-
tionally,
a
bulk-heterojunction (BHJ) architecture is
employed, with a p-type conjugated polymer in conjunction
with a fullerene-based small molecule as the n-type mate-
rial.[3–9] Over the last decades, various building blocks have
been developed for preparation of conjugated polymers with
the most recent activity being directed towards the prepara-
tion of low bandgap (Eg < 1.5 eV) systems.[10–20] In these
systems, the conjugated polymers are derived from flat,
planar aromatic monomers with large solubilizing groups
attached to enhance processibility. In breaking with this
restricted view of building block design for conjugated
polymers, the development of novel monomers that are
non-planar and have interesting electronic properties are
highly desirable. Examples of crowded alkenes were initially
reported by Feringa[24] as molecular switches and a similar
concept also offers the opportunity to prepare new structures
with improved properties and performance in polymer-based
solar cells and organic electronics.[21–26]
Figure 1. Chemical structure of (E)-8,8’-biindeno[2,1-b]thiophenylidene
(trans-BTP, 1a) and the formation of its radical anion after the addition
of one electron.
An attractive feature of the bis(thiophene) analog, BTP,
when compared to the 99’BF system is the ability to use well
established chemistry for the introduction of functional
groups to the thiophene rings. As shown in Scheme 1,
synthesis of BTP derivatives proved to be a facile process
from readily available starting materials. Initial Suzuki–
Miyaura coupling of methyl 2-bromobenzoate, 2a, and 3-
thiopheneboronic acid is followed by ester hydrolysis and
Friedel–Crafts ring closure to afford the ketone 5a as a bright
yellow solid. Treatment of the aryl ketone with Lawessonꢀs
To address this challenge we report a novel, twisted
=
monomer family based on a distorted C C double bond. The
inspiration for this work is a report describing electron-
accepting materials based on 9,9’-bifluorenylidene (99’BF).[27]
From these studies, 99’BF-based materials were found to
readily accept electrons due to the relief of steric strain and
the fulfillment of the 14-p-electron rule, making them
candidates to replace PCBM (phenyl-C61-butyric acid
methyl ester) as the acceptor in organic solar cells. With
[*] Dr. C.-Y. Chiu, Dr. F. G. Brunetti, Prof. F. Wudl, Prof. C. J. Hawker
Materials Research Laboratory, University of California
Santa Barbara, CA 93106 (USA)
E-mail: hawker@mrl.ucsb.edu
Dr. H. Wang
Mitsubishi Chemical USA, Inc., Chesapeake, VA 23320 (USA)
[**] We thank the National Science Foundation (MRSEC program, DMR
1121053) and the Mitsubishi Chemical Company (Mitsubishi
Chemical Center for Advanced Materials) for financial support and
Dr. Guang Wu for performing single-crystal X-ray diffraction
measurements.
Scheme 1. Synthesis of BTP and its derivatives (1a–1c). a) Na2CO3,
[Pd(PPh3)2Cl2], THF/H2O; b) NaOH, H2O/EtOH; c) oxalyl chloride,
CH2Cl2 and then AlCl3; d) Lawesson’s reagent, toluene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400674.
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reagent yields the parent BTP derivative, 1a (R = H), in
excellent yield as a mixture of cis- and trans-isomers. A
significant advantage of this synthetic strategy is the modu-
larity which allows a wide range of substituted 2-bromoben-
zoates and thiophene derivatives to be used. To illustrate this
feature, the methyl and trifluoromethyl derivatives, 1b and 1c
respectively, were also prepared in high yields (Scheme 1).
An intriguing facet of the dimerization reaction with
Lawessonꢀs reagent is the generation of two regioisomers, one
with thiophenes on opposite sides (trans-BTP) and the other
with thiophenes on the same side (cis-BTP). Experimentally,
the ratio of trans to cis isomer was found to be ca. 2 to 1;
presumably the more abundant trans isomer is the thermody-
namic product since steric repulsion between the proximal C-
Hꢀs of the two benzene rings is avoided. Derivatives 1b and
1c were also shown to be a mixture of cis- and trans-isomers,
albeit with slight different ratios, 1.6:1 for 1b and 2.6:1 for 1c.
The purified isomers could be isolated by recrystallization
(trans) or flash chromatography (cis) with the isomers
Figure 3. a) Chemical structure of 1a: b) side view of trans-BTP mole-
cules, 1a, in the crystal structure with a splay angle about 208 between
proximal benzene and thiophene rings. The intermolecular distance is
about 3.5 ꢀ. b) BTP molecules arrange in monoclinic fashion with
a close intermolecular sulfur–sulfur distance about 3.4 ꢀ.
1
showing distinct sets of H and 13C NMR signals, indicating
different electronic environment resulting from the dissim-
ilarity in chemical structures (Figure 2).
Information). The first reduction wave could be attributed to
the formation of the radical anion with the second corre-
sponding to formation of the dianion.[29] Using ferrocene as
the internal standard, the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO) for 1a were calculated to be À5.55 eV and
CV
À3.45 eV (cf. PC61BM À3.8 eV) with a bandgap Eg
=
2.10 eV. Further investigation using CV reveals lower bandg-
aps for the substituted BTP derivatives; 1b has a bandgap of
2.06 eV while introduction of trifluoromethyl groups further
decreases the bandgap of 1c to 1.95 eV (Figure 4a). A similar
trend is observed in the UV/Vis absorption spectra of 1a–1c
with all derivatives showing a broad absorption extending to
Figure 2. 1H NMR spectra of BTP, 1a, showing two distinct isomers,
a) purified trans isomer and b) purified cis isomer.
Final confirmation of isomer structure was obtained by
single-crystal X-ray diffraction studies of the trans-BTP
derivative (Figure 3). From single-crystal X-ray diffraction
analysis, the trans isomer has a twisted structure, presumably
resulting from the congestion between proximal aromatic
rings with the splay angle between the thiophene and benzene
rings being ca. 208 (Figure 3a). Within a monoclinic lattice,
these nonplanar molecules stack themselves in a fully eclipsed
fashion with an intermolecular distance of about 3.5 ꢁ and
a short sulfur–sulfur distance of about 3.4 ꢁ (Figure 3b).
Significantly, this twisted, nonplanar structure leads to BTP
derivatives having good to excellent solubility in common
solvents, such as acetonitrile, ethyl acetate, tetrahydrofuran
and dichloromethane.
Having developed a facile synthetic approach to twisted,
bis(thiophene) derivatives, 1a–c, the electronic and physical
properties of this novel family of building blocks was
examined. The ability to accept two electrons, presumably
favored by release of steric strain from proximal aromatic
rings and development of a 14-p electron system was studied
by cyclic voltammetry (CV) measurement which shows two
clear reversible reduction waves (Figure S1 in the Supporting
Figure 4. a) UV/Vis absorption spectra of 1a–1c. b) HOMO, LUMO
and bandgap of 1a–1c.
2
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600 nm with optical bandgaps estimated from the onset of
absorption matching well with those determined from CV
measurement (Figure 4b). Significantly, it was found that the
purified trans- and cis- isomers, for each BTP derivative show
essentially the same absorption and electrochemical profile
which demonstrates the utility of BTP as a building block
platform for the preparation of low bandgap conjugated
polymers.
Synthetically, a key advantage of incorporating thiophene
units into the BTP structure is the facile functionalization of
the 2-position of the thiophene rings. In analogy with standard
thiophene chemistry, dibromination and subsequent stanny-
lation of 1a proved to be essentially quantitative reactions
which allow monomers suitable for metal-catalyzed polymer-
ization to be easily prepared (Scheme 2). It is worth noting
Scheme 3. Synthesis of BTP-based conjugated polymers by Stille
coupling. a) [Pd2(dba)3], P(o-tol)3, chlorobenzene; b) [Pd2(dba)3],
P(o-tol)3, chlorobenzene, microwave reaction.
Scheme 2. Synthesis of brominated trans-BTP (tBTP-Br) and stanny-
lated trans-BTP (tBTP-Sn). a) NBS, DCM; b) nBuLi, hexane and then
SnMe3Cl, THF.
that when the pure cis-isomer, or the mixture of isomers
(trans:cis ꢀ 2:1), was subjected to bromination using N-
bromosuccinimide (NBS), significant isomerization was
observed with the brominated product being obtained with
a trans:cis ratio of ca. 7:1 with pure cis- or trans-derivatives
being isolated by recrystallization (Figure S3). This observa-
tion is consistent with radical-catalyzed isomerization of the
strained double bond during bromination of the thiophene
rings.[30]
Low bandgap conjugated polymers were then prepared
from both the trans- and cis-BTP derivatives by copolymer-
ization with the appropriately functionalized diketopyrrolo-
pyrrole (DPP) and benzodithiophene (BDT) monomers by
Stille coupling (Scheme 3). While polymer growth was slower
and reached substantially lower molecular weights for the cis-
isomer when compared to the trans-isomer, high molecular
weight materials were prepared in all cases (Table 1). From
a structural viewpoint, the reduced reactivity of the cis-isomer
is understandable given the “kinked” nature of the resulting
repeat unit. Optical properties for the BTP-based copolymers
were then investigated by UV/Vis spectroscopy in both
solution and as spin-coated films (Figure 5 and Table 1).
Figure 5. UV/Vis absorption of trans-PBTPBDT and trans-PBTPDPP in
solutions and as-cast films.
Significantly, the introduction of the BTP unit gives rise to
broad absorption covering the visible and near-IR region with
the BTP-DPP copolymer (PtBTPDPP) displaying thin film
absorption above 1100 nm. The optical bandgap estimated
from the onset of absorption was determined to be 1.46 eV for
PtBTPBDT and 1.21 eV for PtBTPDPP.
Photovoltaic performance was investigated with a conven-
tional bulk-heterojunction device configuration of ITO/
PEDOT:PSS/polymer:PCBM/Ca/Al and is summarized in
Table 2. Although PtBTPDPP has a broad, strong
near-infrared absorption which is required for har-
vesting the lower energy portion of solar spectrum,
Table 1: GPC and photophysical properties of both BTP-based copolymers.
the highest power conversion efficiency with
opt
Polymer
Mn
Mw
PDI
lmax [nm]
lonset [nm]
Eg
PC61BM was only 0.9% (Table 2). We infer that
a low lying LUMO energy level for the copolymer
impedes efficient charge carrier separation due to
[kDa] [kDa]
Solution Film Solution Film [eV, film]
PtBTPDPP 27.0
PtBTPBDT 70.1
PcBTPBDT 27.0
49.5
133
70.0
1.83 881
1.89 735
2.58 735
890
742
735
982
835
835
1020 1.21
845
837
1.46
1.48
a
LUMO mismatch with PC61BM (LUMO =
À3.8 eV). In order to improve charge separation by
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Table 2: Averaged solar cell device performance of tBTP-based copoly-
mers.
sub-micrometer domains without DIO to a fine nanoscale
morphology with DIO (Figure S4). In addition, the external
quantum efficiency (EQE) curve in Figure 6b shows that
PtBTPBDT has substantial contribution in photo current
generation between 500 nm to 850 nm. Further increase in
performance was observed on changing the acceptor to
PC71BM with a non-optimized averaged power conversion
efficiency of 3.8% which corresponds to an increase in Jsc
from 7.9 mAcmÀ2 to 10.2 mAcmÀ2 (Figure 6a).
Polymer PCBM
DIO
Ratio
(v/v%)
Thickness Voc Jsc
[nm]
ff
h
[V] [mAcmÀ2
]
[%]
PtBTPBDT PC61BM
PtBTPBDT PC61BM 1.5
PtBTPBDT PC71BM 2.5
PtBTPDPP PC61BM
PtBTPDPP PC61BM
0
75
66
89
58
68
0.66 4.84
0.67 7.93
0.67 10.2
0.59 0.91
0.60 2.47
0.50 1.6
0.56 3.0
0.56 3.8
0.48 0.26
0.61 0.91
0
2
For these materials based on a distorted double bond,
a fundamentally important control is the direct comparison
with the corresponding regioisomer prepared using the cis-
BTP isomer and BDT. In the UV/Vis spectroscopy, the peak
wavelength is identical in both solution and solid state while
the onset wavelength is slightly higher in film than that in
solution. In solar cells, the performance was significantly
reduced when compared to the trans derivative with the best
device characteristics having h = 1.2%, Voc = 0.61 V, Jsc =
3.6 mAcmÀ2 and ff = 0.54. It is proposed that this clear
difference in performance is the result of a major change in
polymer conformation on going from the linear, trans-BTP
unit to the kinked, cis-BTP units, significantly reducing
intermolecular charge transport. This effect might also reflect
on the photophysical properties based on UV/Vis absorption
measurements. A difference is also observed in the thin film
morphology with the overall result being a low Jsc and
associated low efficiency (Figure S5).[31]
increasing the energy difference between LUMO levels, the
DPP units was replaced with benzodithiophene (BDT) based
monomers and copolymerization with 1a, gave the desired
PtBTPBDT systems. Using CV measurements to estimate the
HOMO with LUMO calculated from the optical bandgap, it is
determined that PtBTPDPP has HOMO = À5.07 eV and
LUMO = À3.86 eV and PtBTPBDT gives HOMO =
À5.09 eV and LUMO = À3.63 eV. As a result, solar cells
based on the trans-isomer of PBTPBDT gave an initial power
conversion efficiency of 1.6%; with the addition of diiodo-
octane (DIO) as a process additive resulting in a significant
improvement in performance with h = 3.0%, Voc = 0.67 V,
Jsc = 7.9 mAcmÀ2 and ff = 0.56 (Figure 6a). This improvement
correlates with changes in the film morphology from large
In conclusion, we have synthesized and characterized
a new class of isomeric building blocks for low bandgap
=
conjugated polymers based on a distorted C C double bond.
A major advantage of these cis- and trans-BTP derivatives is
their ease of synthesis and modular structure which allows
a wide range of BTP-based materials to be prepared for
organic electronics applications. While non-planar, the
twisted nature of these repeat units still allows for conjugation
along the polymer backbones with broad visible and near-
infrared absorbance and inherent high solubility. Direct
comparison of materials prepared from the cis- and trans-
isomers reveals superior performance in organic solar cell
devices for the trans-based systems with preliminary results
showing a power conversion efficiency of 3.8%. These results
demonstrate the design of new monomers that deviate from
the accepted view of conjugated polymer building blocks and
offer the opportunity to prepare new structures with
improved properties and performance for the broad field of
polymer-based solar cells and organic electronic.
Received: January 21, 2014
Published online: && &&, &&&&
Keywords: building blocks · electron acceptors · low bandgap ·
.
polymers · solar cells
Figure 6. a) Current–voltage curves of PBTPBDT polymer solar cell
devices with and without DIO additive under AM 1.5G illumination.
The device structure is ITO/PEDOT:PSS/trans-PBTPBDT):PC61BM (1:2
weight ratio)/Ca/Al. Polymer solution was prepared in chlorobenzene.
b) EQE curve for Figure 6a, devices prepared from mixtures of
chlorobenzene and 0.0–2.5% (v/v) DIO.
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Communications
Communications
Organic Solar Cells
A simplified synthetic approach provided
novel building blocks for conjugated
polymers featuring a non-planar repeat
unit and a distorted double bond. The
resulting copolymers are characterized by
a broad absorption profile and show
promising results in solar cells.
C.-Y. Chiu, H. Wang, F. G. Brunetti,
F. Wudl, C. J. Hawker*
&&&& — &&&&
Twisted but Conjugated: Building Blocks
for Low Bandgap Polymers
6
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!