Synthesis and photovoltaic effect in dithieno[2,3-d:2′,3′- d′]benzo[1,2-b:4,5-b′]dithiophene-based conjugated polymers

Article abstract of DOI:10.1002/adma.201204238

Conjugated polymers based on a heteroacene, 3,7-dialkyl-dithieno[2,3-d: 2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DBD), are synthesized. These polymers show broad UV-vis absorption with energy bandgaps below 1.7 eV. PTDBD2, showing good miscibility in a polymer/phenyl-C71-butyric acid methyl ester (PC₇₁BM) blend film, achieves a power conversion efficiency (PCE) of 7.6%. The results indicate that copolymers containing DBD are promising candidates for high-performance organic solar cells. Copyright

Full text of DOI:10.1002/adma.201204238

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Synthesis and Photovoltaic Effect in Dithieno[2,3-d:2′,3′-d′]
Benzo[1,2-b:4,5-b′]dithiophene-Based Conjugated Polymers
Hae Jung Son, Luyao Lu, Wei Chen, Tao Xu, Tianyue Zheng, Bridget Carsten,
Joseph Strzalka, Seth B. Darling, Lin X. Chen, and Luping Yu*
The recent surge of enthusiasm in bulk-heterojunction (BHJ)
organic photovoltaics (OPVs) is driven by their potential for
fabricating flexible and light-weight solar cells via facile, low-
cost solution processing techniques.[1] New materials are
crucial in order for OPVs to mature fully from research and
development into cost effective products. The power conver-
sion efficiency (PCE) of large-area OPV solar cells is still infe-
rior to the corresponding inorganic devices and should be
continuously improved through major advances in new mater-
ials and enhancing our understanding of structure-property
relationships.[2–6]
of the charge-separated state that leads to free charge carriers
and the charge-transfer state that is a strongly bound hole and
electron pair in isolated polymer chains is linearly propor-
tional to the power conversion efficiency (PCE) of the device,
which is related to the Δμeg values.[8] In a BHJ film, as shown
in Figure S1 in the Supporting Information, upon photoexcita-
tion, a large Δμeg(D) in the polymer leads to a large displace-
ment in the electron-density distribution in the lowest occu-
pied molecular orbital (LUMO) away from that in the highest
occupied molecular orbital (HOMO). Consequently, a polar-
ized exciton is produced with the positive charge concentrated
on the benzo[1,2-b:4,5-b′]dithiophene (BDT) moiety (red + in
Figure S1) and the negative charge on the electron-deficient
ester group on thieno[3,4-b]thiophene (TT) moiety from which
the electron density will transfer to PCBM (curved red arrow
in Figure S1). The intrinsic local electron-affinity gradient due
to the displacement and the distance between the negative
and positive charge densities reduces Coulombic interactions
between the hole and electron inside the initial charge-transfer
complex, making further charge separation/charge-carrier trans-
port easier and resulting in an enhancement of the solar-cell
efficiency. Although exciton delocalization can extend beyond
a repeating unit, as seen from a series of calculations,[9] corre-
lations between the local dipole change of the monomer with
the device PCE have been observed in several copolymers with
charge-transfer character.[8,9] Theoretical bases of this phenome-
nological correlation are being investigated. The observed linear
correlation between the local dipole change and the PCE sug-
gests two ways to enhance the device PCE in developing new
donor polymers: 1) increasing the local dipole moment further
to produce even more polar excitons, which will further reduce
the exciton binding energy and the probability of forming tightly
bound radical pairs; and 2) expanding the conjugation system in
the rigid BDT unit to delocalize the hole, thus lowering the local
charge density and hence the Coulombic interactions between
the hole and the electron in the donor-acceptor interface.
Our recent work[7] showed that when the bandgap, the
energy levels, and the morphology were all optimized, a large
local dipole-moment change between the ground and excited
states, Δμeg, played a crucial role in the performance of OPV
BHJ cells. To our surprise, the initial relative population ratio
Dr. H. J. Son,[+] L. Lu, T. Xu, T. Zheng, B. Carsten,
Prof. L. P. Yu
Department of Chemistry and James Franck Institute
The University of Chicago
Chicago, IL 60615, USA
E-mail: lupingyu@uchicago.edu
Dr. W. Chen
Materials Science Division
Argonne National Laboratory
Argonne, Illinois, 60439, USA
Dr. W. Chen
Institute for Molecular Engineering
The University of Chicago
Chicago, Illinois, 60637, USA
Dr. J. Strzalka
X-ray Science Division
Argonne National Laboratory
Argonne, IL 60439, USA
Dr. S. B. Darling
Center for Nanoscale Materials
Argonne National Laboratory
9700 South Cass Avenue, Argonne, IL 60439, USA
Based on these design ideas, we proceeded to develop poly-
mers with a more-extended conjugation in the donor monomer
units. Large acenes, including heteroacenes, are believed to pro-
vide many advantageous physical properties for high-perform-
ance organic solar-cell devices.[10] They have shown enhanced
charge-carrier mobilities[10c] and are theoretically predicted to
show narrower energy bandgaps relative to smaller acenes and
polycyclic aromatic compounds with an identical number of
aromatic rings.[10b,d] This is due to an increased effective con-
jugation length and concomitant decrease in reorganization
energy in these species, resulting in facile exciton separation
into free charge carriers.[10,11] In this paper, we describe the
Dr. S. B. Darling
Institute for Molecular Engineering
The University of Chicago
Chicago, IL 60637, USA
Prof. L. X. Chen
Department of Chemistry
and ANSER Center, Northwestern University
2145 Sheridan Road, Evanston, IL 60208, USA
[+] Present address: Photo-electronic Research Center, Korea Institute
of Science and Technology (KIST), Hwarangno 14-gil 5, Seongbuk-gu,
Seoul 136-791, Republic of Korea
DOI: 10.1002/adma.201204238
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O
O
O
O
O
O
S
F
S
S
F
F
*
*
S
*
S
S
S
S
S
S
S
S
S
S
S
S
S
n
S
n
n
PTDBD3
PTDBD1
PTDBD2
Figure 1. Structures of PTDBD1–PTDBD3.
synthesis and photovoltaic effects of new polymers incorpo-
rating a heteroacene, dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]
dithiophene (DBD), as the building block of the polymers. The
heteroacene was successfully synthesized and incorporated into
conjugated polymers, which exhibited an energy bandgap below
1.7 eV (Figure 1). After optimization of the polymer’s solubility
and morphological compatibility with [6,6]-phenyl-C71 butyric
acid methyl ester (PC71BM), the polymer BHJ solar-cell devices
achieved a PCE of 7.6%.
The synthetic route to the DBD monomer is shown in
Scheme 1. The synthesis of compounds 8a–b and 12 is
described in the Supporting Information. The DBD monomers
are copolymerized[12] with alkyl 4,6-dibromo-3-fluorothieno[3,4-
b]thiophene-2-carboxylate (FTT). The initial polymer PTDBD1
showed low solubility and was difficult to process in organic sol-
vents; this polymer was not investigated further. To improve the
solubility of the polymer, we tuned the polymer’s structure by
incorporating bulkier alkyl side chains either into the FTT, DBD
Scheme 1. Synthetic routes to the DBD monomer and PTDBD polymers: a) Pd(PPh3)4, K2CO3 (or NaOH), toluene/ethanol/H2O, 110 °C, 24 h;
b) CF3SO3H, P2O5, 3 d; c) pyridine, reflux, 12 h; d) Br2, CHCl3, 6 h; e) n-BuLi, THF, −78 °C, 25 min; f) Me3SnCl, −78 °C, 12 h; g) Pd(PPh3)4, toluene/
DMF, 120 °C, 12 h.
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5
0
1.0
(a)
PTDBD2
PTDBD3
PTDBD2
PTDBD3
0.8
0.6
0.4
0.2
0.0
-5
-10
-15
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
Voltage (V)
400
500
600
700
800
Wavelength (nm)
5
(b)
PTDBD2
PTDBD3
Figure 2. Absorption spectra of the pristine polymer films.
0
units, or both of these units. The resulting polymers, PTDBD2
and PTDBD3, show similar weight-averaged molecular weights
of 108.6 and 105.5 kDa, with polydispersity indices of 2.84, and
2.32, respectively. Thermogravimetric analyses (TGA) indicated
that both polymers were stable up to about 250 °C (Supporting
Information, Figure S2). No clear glass transition was observed
from 25 to 240 °C in its DSC curves of the second heating and
cooling runs (5 °C min−1 ) (Supporting Information, Figure S3).
PTDBD2 and PTDBD3 exhibited absorption from 450 nm to
800 nm in the polymer films, but the absorption of PTDBD3
was broader and more featureless (Figure 2). The optical-energy
bandgaps of the polymers estimated from the onsets (λonset) of
the absorption spectra were ≈1.67–1.68 eV. From cyclic voltam-
metry (CV) measurements in thin films,[13] the HOMO energy
level of the PTDBD3 (−5.30 eV) was slightly lower than that of
the PTDBD2 (−5.24 eV) (Supporting Information, Figure S4).
PTDBD2 showed a higher charge-carrier mobility of 1.69 ×
10−4 cm2 V−1 s−1 compared with 0.71 × 10−4 cm2 V−1 s−1 for
PTDBD3, as determined by space-charge-limited current (SCLC)
measurements for pristine polymer films. This is attributable
to the large non-conductive bis(2-ethylhexyl)methyl group in
the FTT of PTDBD3, which may disturb efficient charge-carrier
transport in the polymer film, although blending with fuller-
enes typically alters the polymer-stacking morphology.
-5
-10
-0.2 0.0 0.2 0.4 0.6 0.8 1.0
Voltage (V)
Figure 3. a,b) Current–voltage plots of solar cells composed of polymer/
PC71BM (a) and polymer/PC61BM (b).
to 13.0 mA cm−2 , with an open-circuit voltage (Voc) of 0.89 V
and a fill factor (FF) of 65.3%. The Jsc value (12.66 mA cm−2 )
calculated by integrating the EQE data with the AM 1.5G spec-
trum is in rather good agreement with the directly measured
Jsc value (PCE estimated from the calculated Jsc: 7.4%). It is
notable that the Voc of the PTDBD2/PC71BM solar cell (0.89 V)
is remarkably improved from 0.74 V of a poly[4.8-bis[(2-ethyl-
hexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] (PTB7) device
despite their similar HOMO energy levels (HOMO of PTB7:
−5.15 eV), but accompanied by a reduction in Jsc.[14] This phe-
nomenon is consistent with that which is predicted by theory
and observed in other work, caused by the change in side alkyl
chains.[5–6,15–24]
To characterize the photovoltaic properties for PTDBD2 and
PTDBD3, solar-cell devices were prepared with the structure of
indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(s
tyrenesulfonate) (PEDOT:PSS)/polymer:PCBM/Ca/Al. The opti-
mized polymer/PC71BM ratios of PTDBD2 and PTDBD3 used
to form the active layers of the polymer solar cells were 1:1.2
and 1:1.3, respectively. The same ratios were used for polymer/
[6,6]-phenyl C61 butyric acid methyl ester (PC61BM) solar cells.
The polymer active layers were spin-coated from solutions in
chlorobenzene (CB)/3% (v/v) 1,8-diiodooctane (DIO) additive.
The J–V curves are shown in Figure 3, and the device perform-
ances are summarized in Table 1. It was found that the solar
cells based on PTDBD2 exhibited a PCE of 7.6% when PC71BM
was used as the acceptor material. The external quantum effi-
ciency (EQE) spectrum in Figure S5 in the Supporting Informa-
tion shows that the EQE of the PTDBD2/PC71BM device was
close to 60% in the 400 nm to 700 nm range. In the J–V curve
of the device, the short-circuit current density (Jsc) reaches up
The solar-cell device based on PTDBD3/PC71BM, prepared
using the same fabrication conditions except for the PTDBD3/
PCBM ratio, also exhibited a high Voc of 0.88 V. However,
PTDBD3 achieved a lower Jsc of 10.7 mA cm−2 , which, in com-
bination with its FF value of 52.1%, results in PCE values of
4.9%. When the device was fabricated using PC61BM, PTDBD2
T a b le 1 . Performance of polymer solar cells of PTDBD2 and PTDBD3.
Polymer/PCBM (w/w)
Jsc
Voc
[V]
FF
[%]
PCE
[%]
[mA cm−2
]
PTDBD2/PC71BM (1:1.2)
PTDBD3/PC71BM (1:1.3)
PTDBD2/PC61BM (1:1.2)
PTDBD3/PC61BM (1:1.3)
13.0
10.7
9.7
0.89
0.88
0.87
0.87
65.3
52.1
63.0
59.4
7.6
4.9
5.3
3.9
7.5
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0.45 Å−1 , corresponding to the periodic lamellae of polymer
backbones, and out-of-plane scattering at around q ≈ 1.6 Å−1 ,
corresponding to the π-stacking of the polymer backbones
(Figure 4a,d). For PTDBD2, the lamellar spacing is 15.4 Å (qy =
0.408 Å−1 ) and the π-stacking distance is 4.0 Å (qz = 1.55 Å−1 ).
For PTDBD3, its lamellar spacing is 17.3 Å (qy = 0.364 Å−1 ) and
its π-stacking distance is 3.9 Å (qz = 1.62 Å−1 ) (Supporting Infor-
mation, Figure S4). These polymers exhibit a similar π−π dis-
tance. The fact that π-stacking scattering is only observed out of
plane suggests that the PTDBD2 and PTDBD3 polymer chains
preferentially stack out of the film plane, though with a disper-
sion of orientations.
Secondly, upon blending with PC71BM, both PTDBD2 and
PTDBD3 retain their structural characteristics (Figure 4b,e),
but the full width at half maximum (FWHM) of the peak asso-
ciated with the backbone lamellae narrows from 0.132 Å−1 to
0.110 Å−1 for PTDBD2 and from 0.061 Å−1 to 0.042 Å−1 for
PTDBD3, respectively (Figure 4c,f). This indicates a higher
degree of crystallinity in both polymers as a result of mixing
with PC71BM,[5,16–20] which is beneficial for charge transport in
the OPV devices. It indicates that PC71BM plays the role of a
template to enhance the crystallinity. A detailed investigation of
this effect will be the topic for a separate publication.
also showed better solar-cell performances with higher Jsc
and FF values. The improved photocurrents of PTDBD2 from
PTDBD3 are, to a certain degree, attributed to its higher charge-
carrier mobility.
Transmission electron microscopy (TEM) studies indicated
that the enhanced device performance of PTDBD2 over that
of PTDBD3 is related to changes in the morphology of the
polymer/PCBM blend films resulting from different alkyl side
chains in the FTT unit of the polymer. PTDBD3 fi lms blended
with PC71BM or PC61BM exhibited larger and darker features
with ≈100 nm dimensions, indicative of aggregated PCBM
clusters, whereas the blend films of PTDBD2 showed uniform
and fine features roughly 10–15 nm in size (Supporting Infor-
mation, Figure S6). Bulky side chains introduced to a conju-
gated polymer tend to reduce the miscibility of the polymer with
PCBM molecules, leading to a large phase separation between
the polymer and the PCBM molecules. Smaller domains in
devices allow for a higher polymer-PCBM interfacial area and,
hence, more-efficient exciton migration to the interface and the
generation of more charge carriers, resulting in higher photo-
currents in the PTDBD2 solar cells. The nanoscale morphology
and molecular ordering in the blend films were investigated by
using grazing-incidence wide-angle X-ray scattering (GIWAXS)
and some unusual features were revealed and need discus-
sion. First of all, the 2D GIWAXS patterns of neat PTDBD2
and PTDBD3 polymers exhibit ring-like scattering at q ≈ 0.35–
Thirdly, besides the diffuse scattering from either the pol-
ymer or the PC71BM, a couple of sharp scattering spots attrib-
uted to well-defined, pure PC71BM crystals are detected in the
Figure 4. a–f) 2D GIWAXS patterns of neat PTDBD2 (a) and PTDBD3 (d) polymers and their blends with PC71BM (b,e) on the Si substrate with a layer
of PEDOT:PSS, as well as the corresponding background-subtracted qy linecuts (c,f) of the GIWAXS patterns. Here, the solid lines represent the best
fitting of the peaks using the pseudo-Voigt type-1 peak function. Note: the line-cuts have been shifted vertically for clarity.
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were distilled over sodium and benzophenone. Alkyl 4,6-dibromo-3-
fluorothieno[3,4-b]thiophene-2-carboxylates were prepared according
to the procedures reported in the literature.[18] 1H NMR and 13C NMR
spectra were recorded at either 400 or 500 MHz using a Bruker DRX-400
or DRX-500 spectrometers respectively. Chemical shifts are reported in
parts per million (ppm) from low to high frequency and referenced to the
residual solvent resonance. Standard abbreviations indicating multiplicity
are used as follows: s = singlet, d = doublet, t = triplet, m = multiplet,
q = quartet, and b = broad. Matrix-assisted laser desorption ionization
time-of-flight (MALDI-TOF) mass spectra were recorded using a Bruker
Reflex II-TOF mass spectrometer using a 337 nm nitrogen laser with
tetracyanoquinodimethane (TCNQ) as the matrix. Electron ionization
(EI) mass spectrometry was performed using a Varian Saturn 2000 GC/
MS mass spectrometer. The molecular weights and distributions of the
polymers were determined by gel-permeation chromatography (GPC)
with a Waters Associates liquid chromatography instrument equipped
with a Waters 510 HPLC pump, a Waters 410 differential refractometer,
and a Waters 486 tunable absorbance detector. Tetrahydrofuran (THF)
was used as the eluent and polystyrene as the standard. The optical
2D GIWAXS patterns of the polymer/PC71BM blended films,
demonstrating that there are PC71BM crystals interspersed in
the polymer/PC71BM blended phase. These scattering spots
were rarely observed in the polymer/PC61BM blends (Sup-
porting Information, Figure S8). These interspersed PC71BM
crystals can drain photoexcited electrons out of the polymer/
PC71BM blended phase and transport them to the electrode,
thereby promoting charge separation and transport.[17]
Fourthly, the full-width half maximum (FWHM) linewidth
of the polymer backbone peak of the PTDBD2/PC71BM film
is more than twice that of the PTDBD3/PC71BM film, sug-
gesting that the crystallinity of PTDBD2 is much lower. This
result is consistent with transmission electron microscopy
(TEM) observations and likely caused by the more-symmetric
side chains in the PTDBD3. However, this high crystallinity
seems to be in contradiction to its lower mobility. The discrep-
ancy can be resolved if we realize that charge transport has to
go through not only stacked π−π channels, but also crystalline
domains separated by the alkyl chain layers. Otherwise, they
will be trapped in the crystalline domain. The lamellar spacing
will play an important role as the barrier for charge transport
between the crystalline domains increases when the lamellar
spacing increases. PTDBD3 has a much larger lamellar spacing
than PTDBD2 (17.3 Å vs. 15.4 Å) and thus has a high barrier to
charge transport. This is an example that a balance in crystal-
linity and side-chain structure is needed to optimize the solar-
cell efficiency.
Taken together, we can argue that the PTDBD2 polymer
exhibits a suitable balance in charge separation and transport,
solubility, crystallinity, and miscibility with PCBM for high
solar-cell performance. The PCBM-induced crystallinity and
the interspersed well-defined, pure PCBM crystals are an added
benefit for the superior OPV performance of the PTDBD2
polymer when blended with PC71BM. Large alkyl chains asso-
ciated with higher crystallinity in PTDBD3 lead to diminished
solar-cell performances.
In conclusion, a series of new semiconducting polymers
containing DBD derivatives were synthesized based on the idea
of expanding the π-conjugation system to lower positive charge
density and exciton binding energy. Optimal structures of side
alkyl chains in the polymers improved the compatibility of the
polymer chains with PCBM molecules, improving the mor-
phology of the polymer/PCBM blend film with concomitant
enhancement of the crystallinity of the polymer chains in the
blend film. Consequently, the solar-cell efficiency in PTDBD2
was enhanced up to 7.6%, although this polymer exhibited
lower crystallinity than PTDBD3. This preliminary result indi-
cates that the copolymers containing DBD are promising can-
didates for high-performance organic solar cells and deserve to
be studied further. Exploration of conjugated polymers incorpo-
rating large heteroacenes with structural and size diversities is
underway.
absorption spectra were acquired using
a Shimadzu UV-2401PC
spectrophotometer. Thin films of the polymers were spin-coated from
their solutions in chlorobenzene, and the film absorption spectra were
measured.
Synthesis of Polymers: PTDBD2 : 2-Butyloctyl 4,6-dibromothieno[3,4-b]
thiophene-2-carboxylate (32.7 mg, 0.062 mmol) was weighed into
a
10 mL round-bottom flask. 17b (56.3 mg, 0.062 mmol) and
tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (3 mg) were
added. The flask was subjected to five successive cycles of vacuum
followed by refilling with argon. Then, anhydrous dimethylformamide
(DMF) (0.4 mL) and anhydrous toluene (1.6 mL) were added via a
syringe. The polymerization was carried out at 120 °C for 12 h under
nitrogen protection. The raw product was precipitated into methanol
and collected by filtration. The polymer was purified by Soxhlet extraction
with hexane, methanol, chloroform, and chlorobenzene. The resulting
solid from the chlorobenzene fraction was obtained after drying in vacuo
overnight (30 mg, 50%).
1H NMR (500 MHz, CDCl2CDCl2, δ): 0.50–2.10 (61H, br), 2.50–2.90
(4H, br), 4.0–4.50 (2H, br), 7.20–7.70 (2H, br). GPC: Mw = 1.086 ×
105 g mol−1 ; PDI = 2.82. Elemental analysis: calcd. for C53H69S-6O2F:
C 67.08, H 7.27, S 20.28; found: C 65.71, H 7.87, S 19.64.
PTDBD3: PTDBD3 was synthesized according to a similar procedure
as that for PTDBD2, with respective monomers. The 1H NMR
spectroscopy and gel-permeation chromatography (GPC) data of the
polymers are listed below.
1H NMR (500 MHz, CDCl2CDCl2, δ): 0.40–2.20 (72H, br), 2.50–2.90
(4H, br), 4.0–4.40 (1H, br), 7.20–7.70 (2H, br). GPC: Mw = 1.055 ×
105 g mol−1 ; PDI = 2.34.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
We acknowledge support from NSF (DMR-1004195), AFOSR (FA9550-
12-1-0061), NSF-MRSEC (the University of Chicago, DMR-0820054),
and a gift from Zhejiang Pharma. Partial support for L.Y. and L.C. by
the Energy Frontier Research Center funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences under Award
Number DE-SC0001059 is acknowledged. L.Y., W.C., and S.B.D. thank
the University of Chicago-Argonne Strategic Collaborative Initiative Seed
Grant program for partial support. W.C. also gratefully acknowledges
financial support from Argonne Director’s Postdoctoral Fellowship. This
work was performed, in part, at the Center for Nanoscale Materials, a
Experimental Section
Materials and Methods: Unless stated otherwise, all of the chemicals
were purchased from Aldrich and used without further purification. All
of the reactions were carried out under a nitrogen atmosphere unless
otherwise noted. Tetrahydrofuran (THF) and diethyl ether (Et2O)
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