Two-dimensionally extended π-conjugation of donor-acceptor copolymers via oligothienyl side chains for efficient polymer solar cells

Article abstract of DOI:10.1021/acs.macromol.5b00056

A series of two-dimensional conjugated polymers containing π-conjugated oligothienyl side chains, namely PBDT2FBT-T1, PBDT2FBT-T2, PBDT2FBT-T3, and PBDT2FBT-T4, was designed and synthesized to investigate the effect of two-dimensionally extended π-conjugation on the polymer solar cell (PSC) performance. The oligothienyl units introduced into the side chains significantly affect the optoelectronic properties of the parent polymers as well as the performances of the resulting solar cell devices by altering the molecular arrangement and packing, crystalline behavior, and microstructure of the polymer:PC₇₁BM blend films. The crystallinity and blend morphology of the polymers can be systematically controlled by tuning the π-conjugation length of side chains; PBDT2FBT-T3 exhibited the most extended UV/vis light absorption band and the highest charge mobility, leading to a high short-circuit current density up to 12.5 mA cm^-2 in the relevant PSCs. The PBDT2FBT-T3:PC₇₁BM-based PSC exhibited the best power conversion efficiency of 6.48% among this series of polymers prepared without the use of processing additives or post-treatments. These results provide a new possibility and valuable insight into the development of efficient medium-bandgap polymers for use in organic solar cells.

Full text of DOI:10.1021/acs.macromol.5b00056

Article
pubs.acs.org/Macromolecules
Two-Dimensionally Extended π‑Conjugation of Donor−Acceptor
Copolymers via Oligothienyl Side Chains for Efficient Polymer Solar
Cells
Jaewon Lee,† Joo-Hyun Kim,† Byungho Moon, Heung Gyu Kim, Min Kim, Jisoo Shin, Hyeongjin Hwang,
and Kilwon Cho*
Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea
S
* Supporting Information
ABSTRACT: A series of two-dimensional conjugated poly-
mers containing π-conjugated oligothienyl side chains, namely
PBDT2FBT-T1, PBDT2FBT-T2, PBDT2FBT-T3, and
PBDT2FBT-T4, was designed and synthesized to investigate
the effect of two-dimensionally extended π-conjugation on the
polymer solar cell (PSC) performance. The oligothienyl units
introduced into the side chains significantly affect the
optoelectronic properties of the parent polymers as well as
the performances of the resulting solar cell devices by altering the molecular arrangement and packing, crystalline behavior, and
microstructure of the polymer:PC71BM blend films. The crystallinity and blend morphology of the polymers can be
systematically controlled by tuning the π-conjugation length of side chains; PBDT2FBT-T3 exhibited the most extended UV/vis
light absorption band and the highest charge mobility, leading to a high short-circuit current density up to 12.5 mA cm−2 in the
relevant PSCs. The PBDT2FBT-T3:PC71BM-based PSC exhibited the best power conversion efficiency of 6.48% among this
series of polymers prepared without the use of processing additives or post-treatments. These results provide a new possibility
and valuable insight into the development of efficient medium-bandgap polymers for use in organic solar cells.
alkyl−thieno[3,2-b]thienyl, and alkyl−thienylenevinylenethien-
yl groups.16−23 The π-conjugated side chains efficiently
increased the electron density and interchain aggregation
among conjugated polymers, thereby enhancing light absorp-
tion and charge transport properties of the related PSCs.
Moreover, the BDT-based 2D conjugated polymers showed a
weak electron-donating property because the thienyl plane was
twisted with respect to the BDT plane, providing a low-lying
HOMO level that improved the open-circuit voltage (Voc).
Recently, there are a few reports on attempts of introducing
oligothienyl side chains to further extend the 2D conjugation
lengths of conjugated copolymer systems.17−27 The design
strategy of extended 2D conjugation described above was used
to improve the light absorption properties of donor polymers
with only small effects on the molecular energy levels; however,
these studies did not achieve improved device performances,
probably due to the presence of oversized conjugated side
chains that can disrupt backbone coplanarity, thereby
decreasing the conjugation length and chain packing in the
polymer system.28 Thus, it remains a challenge to develop new
molecular design strategies to evolve toward optimizing the
physical, chemical, and optoelectronic properties of 2D
1. INTRODUCTION
Over the past decade, polymer solar cells (PSCs) have attracted
significant attention due to their predominant advantages of
low-cost, lightweight, large-area, flexible, and environmentally
friendly energy-converting devices.1−3 Tremendous efforts have
been devoted toward improving the power conversion
efficiencies (PCEs) of bulk-heterojunction (BHJ) solar cells
by developing new photoactive polymers. The PCEs of single-
junction PSCs now exceed 10.0%.4−8 So far, much work has
been done to rationalize the energy levels and bandgaps of
donor polymers in an effort to maximize photovoltaic
efficiency.9 Donor−acceptor (D−A) type alternating con-
jugated polymers have recently been identified as highly
promising donor materials for use in future PSCs because the
energy levels and bandgaps are tunable via intramolecular
charge transfer (ICT) effects.10
The molecular design of two-dimensional (2D) conjugated
polymers presents a promising strategy for improving the PCEs
of PSCs based on the formation of extended π-conjugated side
chains.11−15 Recently, several research groups demonstrated
that 2D conjugated polymers with 2-alkylthienyl groups
exhibited broader absorption bands, higher hole mobilities,
and better photovoltaic properties compared to their alkoxy
substituted analogues. For example, benzo[1,2-b:4,5-b′]-
dithiophene (BDT)-based units have largely been utilized to
produce 2D conjugated polymers with a variety of side chains
including alkyl−phenyl, alkyl−thienyl, alkyl−selenophenyl,
Received: January 9, 2015
Revised: February 17, 2015
© XXXX American Chemical Society
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Figure 1. Molecular design of extended 2D-conjugated polymers.
a
Scheme 1. (a) Synthesis of Oligothienyl Side Chains, T1, T2, T3, and T4, (b) Synthesis of Electron Donating Monomers,
b
c
BDT-Tn , and (c) Synthesis of Electron Withdrawing Monomer, DT2FB T
a
b
Key: (i) n-Butyl lithium, NaCl, THF, 55 °C, 6 h; (ii), (iii), and (iv) Pd(PPh3)4, toluene/DMF, 105 °C, 48 h. Key: (v) n-butyl lithium, SnCl2·
c
2H2O, THF, 55 °C, 6 h; (vi) n-Butyl lithium, trimethyltin chloride, THF, −75 °C, overnight. Key: (vii) Pd(PPh3)4, (4-(2-ethylhexyl)thiophen-2-
yl)trimethylstannane, toluene, 105 °C, 48 h; (viii) NBS, THF, room temperature, overnight.
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Figure 2. (a) Synthetic procedure of PBDT2FBT-Tn 2D-conjugated polymers. (b) TGA plots of PBDT2FBT-Tn polymers with a heating rate of 10
°C min−1 under an inert atmosphere. (c) Cyclic voltammograms of PBDT2FBT-Tn polymers in the CH3CN solution at a scan rate of 50 mV s−1.
Table 1. Molecular Weights, Thermal Properties, and Solubility of PBDT2FBT-Tn
a
a
a
b
c
polymer
Mn (kDa)
Mw (kDa)
PDI
Td (°C)
solubility (g L−1
)
PBDT2FBT-T1
PBDT2FBT-T2
PBDT2FBT-T3
PBDT2FBT-T4
28.1
42.8
48.2
33.8
49.3
48.6
1.75
1.13
2.11
3.24
409
422
432
414
34.3
20.1
22.6
28.7
102.1
109.5
a
b
Determined by GPC using polystyrene standards and chlorobenzene (CB) as an eluent. 5% weight loss temperatures measured by TGA under
c
nitrogen atmosphere. The concentration of the saturated solution in 1,2-dichlorobenzene (DCB) at 25 °C according to the Beer−Lambert law.
conjugated polymers that could be useful in high-efficiency
PSCs.
2. RESULTS AND DISCUSSION
2.1. Design, Synthesis and Characterization. The
detailed synthetic procedures and characterization data of
monomers are described in Scheme 1 and the Experimental
Section. A weak electron-rich BDT unit was employed as the
comonomer to construct D−A medium-bandgap copolymers
bearing a strongly electron-deficient 5,6-difluorobenzo[c]-
[1,2,5]thiadiazole (2FBT) unit through the “push-pull”
interactions. The four different side chains (alkylated thienyl-,
bithienyl-, terthienyl-, and quaterthienyl-) were attached to the
central benzene ring of the BDT unit which has a flexibility of
attaching different substituents. The large twisting motions of
the thienyl plane relative to the BDT plane weakened the
electron-donating properties of the 2D conjugated BDT-T unit
by altering the backbone coplanarity in comparison with the
BDT unit prepared with alkoxy side chains for use in
photovoltaic polymers, thus providing a lower HOMO
level.29 Because maximum value of the Voc is determined by
the energy difference between the HOMO level of the donor
and LUMO level of the acceptor, a deep HOMO level of donor
polymer is desirable to yield a high Voc.30 Moreover, the use of
fluorine atoms in the acceptor unit of a D−A copolymer has
been shown to decrease the HOMO level. The smaller size of
the fluorine atom as compared to other electron withdrawing
On the basis of these considerations, here we report a
synthetic strategy for preparing 2D conjugated polymers by
introducing π-conjugated thienyl side chains, i.e., alkyl−
oligothienyl groups, into benzodithiophene−difluorobenzothia-
diazole (BDT-2FBT) copolymers (Figure 1). A series of four
medium-bandgap polymers was synthesized by incorporating
alkylated thienyl-, bithienyl-, terthienyl-, or quaterthienyl side
chains (namely PBDT2FBT-T1, -T2, -T3, and -T4). This series
was used to systematically investigate the effects of extended π-
conjugation on the key parameters that determine the PCE.
The optical, morphological, and photovoltaic properties of the
PSCs were evaluated based on the synthesized conjugated
polymers to demonstrate that the simple introduction of π-
conjugated units into the side chains could effectively improve
the PCE of a PSC. The tailored polymer with the optimal side
chains, PBDT2FBT-T3, exhibited the highest short-circuit
current density (Jsc) among the polymers tested, 12.3 mA cm−2,
and a maximized PCE of 6.48%, without introducing any
processing additives or post-treatments.
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Table 2. Optical Properties and Molecular Energy Levels of PBDT2FBT-Tn
HOMO/LUMO (eV)
λonset film
b
cyclic
voltammetry
Egcalc
(eV)
EgEC
(eV)
Egopt
c
a
b
polymer
λmax soln. (nm)
λmax film (nm)
(nm)
A0−2/A0−1
DFT
(eV)
PBDT2FBT-T1
PBDT2FBT-T2
PBDT2FBT-T3
PBDT2FBT-T4
343, 407, 548
317, 391, 587
410, 608
370, 442, 592
406, 604
706
717
726
737
0.66
0.67
0.94
1.63
−5.02/−2.73
−4.94/−2.76
−4.87/−2.77
−4.84/−2.76
−5.47/−3.58
−5.37/−3.61
−5.31/−3.62
−5.30/−3.63
2.29
2.18
2.10
2.08
1.89
1.76
1.69
1.75
1.73
1.70
1.68
418, 613
425, 608
440, 618
1.67
c
a
b
Measured in dilute chlorobenzene solution at a concentration of 0.05 g L−1. Spin-cast from 10 mg mL−1 chlorobenzene solution. Estimated from
the onset of the UV−vis spectra measured from thin films.
Figure 3. (a) Normalized UV−vis absorption spectra of PBDT2FBT-Tn polymers in DCB at a concentration of 0.125 mg mL−1 at 25 °C. (b) UV−
vis absorption coefficient of the PBDT2FBT-Tn polymers in spun cast films prepared by DCB. The reversed triangles indicate absorption band of
A0−0 (navy), A0−1 (opened navy), and A0−2 (opened red). For all polymers, we selected the maximum intensity in the low-wavelength region (300−
550 nm) as A0−2 for simple comparison even there exist another faint peaks. Absorption spectra of PBDT2FBT-Tn polymers and BDT-Tn
monomers in DCB solution: (c) T1, (d) T2, (e) T3, and (f) T4.
groups did not introduce much steric hindrance into the
polymer backbone. The synergetic effects of BDT and the
fluorine substituent contributed to the potential utility of
PBDT-2FBT-based copolymer systems for use in BHJ-PSCs.31
The general procedure used to synthesize the four polymers
is shown in Figure 2a. The target polymers were prepared
through a Stille cross-coupling reaction between bis-
(trimethyltin) BDT and the dibrominated DT2FBT under
nearly the same conditions. All reactions gave a yield of 60−
80%. The crude products were isolated using chloroform after
Soxhlet extraction with methanol, acetone, and n-hexane. All
four resulting copolymers exhibit good solubility in common
solvents such as chloroform (CF), chlorobenzene (CB), and
1,2-dichlorobenzene (DCB), but their solubilities varied
considerably at room temperature (Table 1). The number-
average molecular weights (Mn) of the resulting polymers were
28.1, 42.8, 48.2, and 33.8 kDa, with polydispersity index (PDI)
of 1.75, 1.13, 2.11, and 3.40, respectively.
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Figure 4. (a) Molecular orbital distribution of HOMO and LUMO for energy-minimized structure (B3LYP/6-31G*) of the model compounds at
the bottom and top, respectively. The yellow, sky, and blue symbols represent sulfur, fluorine, and nitrogen atoms, respectively. (b) The LUMO
energy levels are estimated by adding (c) the absorption onset to (d) the HOMO energy levels measured by cyclic voltammograms in thin films.
2.2. Thermal and Electrochemical Properties. Ther-
mogravimetric analysis (TGA) was carried out to evaluate the
thermal stabilities of the polymers under a nitrogen
atmosphere. The four polymers with thienyl-, bithienyl-,
terthienyl-, and quaterthienyl side chains displayed a decom-
position temperature (defined as the 5% weight loss temper-
ature, Td) of 409, 422, 432, and 414 °C, respectively (Figure
2b). Since our results showed no significant differences between
the Td values of the four polymers, we concluded that the size
and topology of the oligothienyl side chains attached to the
BDT moiety did not play a critical role in determining the
polymer thermal stability.
The actual HOMO and LUMO energy levels of the polymers
were measured from the cyclic voltammograms as the onset
potentials of the first oxidation and the reduction peaks,
respectively, as shown in Figure 2c and summarized in Table 2.
PBDT2FBT-T1 exhibited a deep-lying HOMO level of −5.47
eV, suggesting that the polymer would be stable against
oxidization, leading to a high device stability; however, the
HOMO levels of these four polymers gradually increased as the
number of thienyl units increased in their side chains because
the extended conjugation length reduced the bandgap of the
2D conjugated polymers. The LUMO energy levels of the
polymers were estimated from the HOMO energy levels and
In diluted CB solution, the optical properties may be
regarded as the properties of single chain species undergoing
rotation in a solvent with minimal intermolecular π−π
interactions because the polymer chains are isolated by the
solvent molecules.24 As shown in parts c−f of Figure 3, the
high-energy bands of the PBDT2FBT-Tn polymers corre-
sponded with those of the BDT-Tn monomers, suggesting that
the differences in the high-energy absorptions of PBDT2FBT-
Tn resulted from the π-conjugation along the oligothienyl side
chains. In addition, the intensities of these absorption bands
(the evolution of the A0−2/A0−1 ratio) gradually increased as the
number of thienyl units present in the side chains increased.
This result clearly indicates that the light-harvesting properties
of the parent conjugated polymers could be improved by
introducing chromophoric side chains to broaden its absorption
range and maximize overlap with the solar emission spectrum.
In particular, low absorption of one-dimensional D−A
conjugated polymer systems in the UV region, which appears
in the shape of a biased dual band, can be improved using a 2D
conjugation molecular design strategy.
The low-energy absorption band of conjugated polymers in
the visible region is assigned mainly to the ICT transition
characteristic of the conjugated main chain. As shown in Figure
3b, these 2D conjugated polymers displayed a gradual red-shift
in the absorption band maxima (λmax) and band edges in
polymer thin films as the number of thienyl units increased in
the side chains, for a given conjugated backbone. These red-
shifts indicated that π-conjugation length of conjugated
polymers could be extended by introducing 2D conjugated
system.
the optical band gaps in the solid state according to the
32
following equation: ELUMO = Egopt + EHOMO
.
2.3. Optical Properties. UV−vis absorption spectra of
PBDT2FBT-Tn in dilute chlorobenzene (CB) solutions and
spin-coated films are shown in Figure 3, parts a and b,
respectively. The detailed absorption data, including the
absorption maxima and band edges in solution and film, are
summarized in Table 2. The absorption spectra of these
copolymers were characterized by two spectral features, a high-
All spectra obtained from these four polymers displayed a
pronounced absorption shoulders (vibronic peaks) due to a
strong interchain π−π* transition (A0−0), indicating that the
polymers strongly tended toward a structurally organized and
orderly packed structure in the thin film state.35,36 The intensity
of the vibronic peak (the evolution of the A0−0/A0−1 ratio)
gradually increased from PBDT2FBT-T1 to PBDT2FBT-T3.
energy band attributed to the localized π−π* transition (A0−2
)
and a lower-energy band ascribed to the intracharge transfer
(ICT) transition (A0−1) similar to other copolymers comprising
D−A units.33,34
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Figure 5. (a) 2D GIXD images and (b) 1D patterns of out-of-plane direction for neat polymer thin films. (c) Schematic illustration of intramolecular
steric hindrance for PBDT2FBT-Tn polymers.
For PBDT2FBT-T4, however, the abrupt decrease in the
vibronic peak intensity may have arisen from the oversized
conjugated side chains, which reduced the coplanarity of the
backbone, thereby decreasing the intermolecular chain packing
of the polymer system.28
explain why the ICT absorption band is decreasing significantly
for PBDT2FBT-T4 (Figure 3).
The values of HOMO energy levels of the model compounds
were observed to increase with the number of thienyl units
present (Table 2). Extending the π-conjugation length along
the side chains decreased the bandgap of the polymer systems,
thereby improving the absorption properties, in accordance
with the results obtained using UV−vis spectroscopy (Figure
3).
2.4. Theoretical Calculations. Density functional theory
(DFT) calculations using the B3LYP/6-31G* model37−40 were
performed on single repeat units (BDT2FBT-T1, BDT2FBT-
T2, BDT2FBT-T3, and BDT2FBT-T4) of the four polymers,
using methyl-trimmed alkyl chains for simplicity, to explore the
electronic and conformational structures of the polymers in
depth (Figure 4). The theoretically predicted energy levels and
energy gaps of the model structures are summarized in Table 2.
We plotted the frontier orbitals of the HOMO and LUMO
of the single repeat units to elucidate the changes in the optical
and electrochemical properties of the polymers given different
side chain architectures. Our calculations indicated that the
LUMOs of these four model compounds were essentially
localized on a single fragment and displayed similar antibonding
characteristics at the bridging atoms of each conjugated
segment. By contrast, the HOMOs were distributed along the
2D conjugated system, including the oligothienyl side chains,
suggesting that the π-electrons were delocalized further upon
introducing the 2D conjugated system. Interestingly, the
HOMOs are less and less delocalized along the main backbone
even further delocalized on the 2D conjugated system with the
increase of the thienyl unit. The overlap between the HOMO
and the LUMO on the 2FBT unit is also decreasing. This could
2.5. Crystalline Structures of Neat Polymer Films.
Grazing incident X-ray diffraction (GIXD) patterns were
measured to understand the molecular packing and the
crystalline structures of the polymers in the neat films. Note
that a diffraction peak of PBDT2FBT-T1 was very weak in the
neat polymer films (Figure 5, parts a and b), implying an
amorphous polymeric property. The bulky and branched alkyl
side chain (2-butyloctyl) appended to the monothienyl unit
(T1) on the BDT unit has a strong chance to induce an
intramolecular steric hindrance with adjacent alkyl side chain
(2-ethylhexyl) on the π-conjugated bridge thienyl unit (Figure
5c). However, the intramolecular steric hindrance in this series
of 2D conjugated system could be decreased as the number of
thienyl units increased because the numerically extended
thienyl units put some distance between the bulky 2-butyloctyl
and 2-ethylhexyl alkyl chains. The topological aspects of side
chains in this 2D conjugated system led to significant
differences in the molecular packing and crystalline property
in the thin films. PBDT2FBT-T3 showed most distinct
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diffraction patterns, implying a higher crystalline property. For
PBDT2FBT-T4, however, an abrupt decrease of crystalline
property was observed probably due to the oversized
conjugated side chains to induce a large intermolecular steric
hindrance with neighboring polymers in the films, which is
accordance with a decreased intensity of the ICT absorption
band.
The interchain dlamellar spacings of PBDT2FBT-T2, -T3, and
-T4 were 24.6, 27.7, and 30.2 Å, respectively. Note that the
much smaller dlamellar spacings in the PBDT2FBT-Tn polymer
chains, as observed in the GIXD results, compared to the 2-fold
intrinsic lengths of the side chains calculated based on the DFT
results suggested that interdigitated packing structures were
constructed between the stacked lamella, as illustrated in Figure
6. It is presumed that the presence of repetitive oligothienyl
units along the side chains permitted the formation of an
interdigitated packing structure. This interdigitated lamellar
packing structure extended the linearity of the backbone,
facilitated the formation of π-stacking structures between the
polymer backbones, and promoted the formation of highly
ordered laterally extended microstructures composed of π-
stacked lamella.41
2.6. Photovoltaic Performance. BHJ-PSC devices with
the configuration ITO/MoO3/polymer:PC71BM/LiF/Al were
fabricated to investigate and compare the photovoltaic
properties of these four 2D conjugated polymers. The devices
were fabricated using PBDT2FBT-Tn as donor materials and
PC71BM as an acceptor material. The solvent DCB was chosen
for processing because it readily dissolves PBDT2FBT-Tn and
has a high boiling point, which is advantageous for developing
nanoscale morphologies in the photoactive layers.42−44 The
solubilities of the four polymers are listed in Table 1. We
fabricated photovoltaic devices using various donor and
acceptor weight ratios (1:0.7, 1:0.8, 1:0.9, 1:1, 1:1.2, 1:1.5,
and 1:2) and a wide range of photoactive layer thicknesses to
optimize the PCEs (Tables S1 and S2, Supporting
Information). The photovoltaic performances depended
significantly on the thienyl side chains of the polymers and
the PBDT2FBT-T3:PC71BM-based PSCs reached a maximum
PCE of 6.48%, with a Voc of 0.86 V, Jsc of 12.3 mA cm−2, and fill
factor (FF) of 61.4%. The trends in the photovoltaic
parameters for the optimal PCE results of four polymer-based
OPV device will be discussed below.
The Voc values of each optimized device decreased gradually
from 0.98 to 0.84 V as the number of thienyl units in the side
chains increased because the HOMO levels increased
monotonically, as confirmed by the CV and DFT results. Voc
is mainly determined by the difference between the HOMO
level of the donor polymer and the LUMO level of the
PC71BM;30 therefore, the increase of the HOMO levels in this
2D conjugated system, from −5.47 to −5.30 eV, largely
accounted for the decrease in Voc. Although the anisotropic π-
conjugation expansion of this 2D conjugated system increased
the HOMO level, the solar cell based on PBDT2FBT-T4,
which showed the highest HOMO level of −5.30 eV in this
series of polymers, still exhibited a high Voc value of 0.84 V. The
synergic effects between the fluorine atoms, which acted as
strong electron-withdrawing substituents, and the BDT units,
which acted as weak donors, produced deep-lying HOMO
levels in the PBDT2FBT-Tn series.
Unlike the trends in Voc, the Jsc values increased from 9.62 to
12.3 mA cm−2 with the increase of the thienyl unit from T1 to
T3, as confirmed by the external quantum efficiencies (ηEQE
)
and estimated Jsc values obtained from the ηEQE results (Figure
7b and Table 3); however, the Jsc of PBDT2FBT-T4 was
reduced abruptly to 6.35 mA cm−2. The difference between the
ηEQE values of polymers mainly resulted from the light
absorption efficiency (ηA) and the internal quantum efficiency
(ηIQE) with the sequential steps involved in exciton diffusion
(ηED), exciton dissociation through charge transfer (ηCT), and
the collection of free charge carriers as a result of charge
transport (ηCC) in the corresponding devices: ηEQE = ηAηIQE
=
ηAηEDηCTηCC.45,46 The PSC device performances were analyzed
by evaluating ηA, ηIQE, and ηEQE (Figure 7b and Figure 8).45−50
The values of ηA for the photovoltaic devices prepared using
the PBDT2FBT-Tn:PC71BM blend films featured a distinct
increase in both the short wavelength and long wavelength
regions with the increase of the thienyl unit up to T3. This
trend resulted from the enhanced absorption of the π−π*
transition (blue absorption band) and the ICT transition (red
absorption band) (Figure 8a). The PC71BM also contributed to
the overall absorption of the blend film in the short wavelength
region because the corresponding films were based on the
Figure 6. (a) Schematic diagram of two-dimensionally stacked packing
structure of PBDT2FBT-T3 polymer molecule with side chain
interdigitation. 2-Ethylhexyl alkyl chains on the backbone were
trimmed for simplicity. (b) Plots of the dlamellar and dsided‑chain value
vs the number of thienyl unit in the side chain. dlamellar, Lsc, and Lsc2 are
lamellar d-spacing, length of side chain, and 2-fold length of side chain,
respectively.
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Figure 8. (a) Light absorption (ηA) and (b) internal quantum
efficiencies (ηIQE) of the PBDT2FBT-Tn:PC71BM based polymer solar
cells.
Figure 7. (a) J−V characteristics of PBDT2FBT-Tn:PC71BM solar cell
prepared by DCB measured under illumination of AM 1.5G, 100 mW
cm−2 and (b) EQE curves of the corresponding polymer solar cells.
mixture of conjugated polymers and fullerene.51 The values of
ηA were evaluated by recording the reflectance spectra of the
devices (Rd) and the electrodes (Re) (see Figure S21 in the
Supporting Information): ηA = 100(1 − Rd/Re).49,50 The
photoactive layer based on PBDT2FBT-T3 displayed the most
extended absorption behaviors among the four polymer-based
blend films tested here and the enhanced light absorption
positively contributed to photocurrent generation.
The ηIQE values were enhanced over a more extensive range
in the PBDT2FBT-T2 and PBDT2FBT-T3 devices compared
with the values obtained from the other two devices. These ηIQE
values reached 96.7% (PBDT2FBT-T2) and 99.8%
(PBDT2FBT-T3) at 480 nm (Figure 8b). The ηIQE depends
on an exciton dissociation by charge transfer followed by charge
transport to the electrodes. These processes were evaluated by
measuring the photoluminescence (PL) quenching efficiency
(Figure S22) and the charge carrier mobility (Figure S23 and
Figure S24), respectively. The PL quenching efficiencies of the
PBDT2FBT-Tn:PC71BM blend films were found to exceed
96% in all photoactive layers, indicating that the excitons were
well-dissociated into free charge carriers, regardless of the side
chains.52,53 The enhanced ηIQE in the device based on
PBDT2FBT-T3 appeared to be affected mainly by the
transport of charge carriers.
The effects of the various oligothienyl side chains on charge
carrier transport were explored by measuring the space-charge-
limited-current (SCLC)54,55 and the field-effect transistor
(FET) mobilities.56 We fabricated SCLC devices with hole-
only and electron-only structures, and FETs were prepared
with bottom-gate and top-contact configurations. The dark
current densities (JD) in the SCLC devices with hole-only and
electron-only structures were multiplied by the cube of the film
thickness (L) to account for variations in the active layer
thickness (Figure S23). The drain currents of the FETs with
different gate biases are shown in Figure S24. The charge
carrier mobilities varied with the difference in the side chain
structures (Table S5). The hole and electron mobilities of the
PBDT2FBT-T3 photoactive layer devices were 2.31 × 10−4
cm2 V−1 s−1 and 2.04 × 10−3 cm2 V−1 s−1, respectively, and the
ratio of the electron mobility to the hole mobility reached 8.83.
The estimated FET mobility of PBDT2FBT-T3 displayed a
value as high as 2.48 × 10−4 cm2 V−1 s−1. Both the SCLC and
Table 3. Photovoltaic Properties of PSCs Based on PBDT2FBT-Tn:PC71BM
a
b
c
d
polymer
ratio
thickness (nm)
Voc (V)
Jsc (Jsc,cal. ) (mA cm−2
)
FF (%)
PCEmax (PCEave) (%)
PBDT2FBT-T1
PBDT2FBT-T2
PBDT2FBT-T3
PBDT2FBT-T4
1:1.2
1:0.8
1:0.8
1:1
91
118
140
95
0.98
0.90
0.86
0.84
9.62 (9.3)
10.6 (10.6)
12.3 (12.5)
6.35 (6.0)
59.7
54.7
61.4
40.5
5.46 (5.36)
5.22 (5.11)
6.48 (6.45)
2.17 (2.15)
a
b
c
Polymer:PC71BM blend ratio (wt %). Thickness of active layer was measured by surface profiler (Alpha-step 500). Calculated Jsc from the
d
integrated area of the EQE spectrum. Performance metrics are average numbers from eight devices of each type under the illumination condition of
AM 1.5G, 100 mW cm−2. DCB was used as a processing solvent for all polymers.
H
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Figure 9. (a−d) AFM height images (1.0 μm × 1.0 μm) and (e−h) TEM images of PBDT2FBT-Tn:PC71BM blend films prepared by DCB.
FET devices based on PBDT2FBT-T3 showed higher mobility
values compared to those of devices based on PBDT2FBT-T1,
-T2, and -T4. The enhanced intermolecular chain packing and
isotropically extended π-conjugation in PBDT2FBT-T3
appeared to promote carrier transport in the corresponding
photoactive layers, resulting in a higher hole mobility.
Bimolecular recombination between separated charges and
space-limited charges may be significantly reduced in the
PBDT2FBT-T3-based device. Therefore, the PBDT2FBT-T3-
based device generated a more photocurrent than the devices
prepared using other polymers due to the high internal
quantum efficiency based on the facilitated carrier transport and
enhanced the absorption efficiency.
2.7. Morphology of Photoactive Layers. The nanoscale
morphologies of the PBDT2FBT-Tn:PC71BM blend films were
visualized using tapping-mode atomic force microscopy (TM-
AFM) (Figure 9a−d) and transmission electron microscopy
(TEM) (Figure 9e−h). The PBDT2FBT-T3:PC71BM blend
films showed a most rough surface, with an RMS roughness of
1.12 nm. The enhanced crystalline property of polymers
induced the increase in the surface roughness from T1 to T3 by
promoting the formation of aggregated domains. The TEM
images also revealed the presence of the enhanced fibrillar
morphology in a similar aspect with the AFM result. The
formation of polymer fibrillar structures could provide
percolation pathways for the efficient charge carrier transports
in the PBDT2FBT-T3:PC71BM-based photoactive layers.56−59
The molecular orientation, intermolecular distance, and
crystallinity in the blend films based on PBDT2FBT-Tn
polymers were investigated by measuring the GIXD patterns
(Figure 10). The two-dimensional GIXD patterns of the
polymer thin films showed pronounced diffraction peaks in the
out-of-plane direction except for PBDT2FBT-T1, indicating
that edge-on crystal orientations were preferred in the blend
films.60,61 The blend films prepared using PBDT2FBT-T2,
PBDT2FBT-T3, or PBDT2FBT-T4 displayed (100) lamellar
stacking peaks along the out-of-plane direction at q-values of
0.255, 0.227, and 0.208 Å−1, respectively, which corresponded
to the d-spacing values of 24.6, 27.7, and 30.2 Å, respectively.
The PBDT2FBT-T1 blend film did not display distinct crystal
peaks like as the neat films. Long-range packing order is
quantitatively measured by determining correlation length (LC)
of the polymers in the optimized active layers using the
Scherrer eq (2πK/fwhm), which is defined as the length over
which a crystalline structure is preserved.62 The coherence
lengths of the blend films prepared using PBDT2FBT-T2,
PBDT2FBT-T3, and PBDT2FBT-T4 were calculated to be 9.8,
12.7, and 8.8 nm, respectively. The higher degree of crystalline
ordering for the PBDT2FBT-T3:PC71BM films demonstrated
convincingly that optimal extension of π-conjugated side chains
in the 2D conjugated system can effectively enhance the
crystalline property of parent polymer.
3. CONCLUSIONS
In summary, a 2D conjugation system composed of D−A
medium-bandgap polymers was prepared via a side chain
engineering approach with the aim of improving the PCEs of
PSCs prepared using these polymers. The optimum size of the
chromophoric side chains in the conjugated polymers was
crucial for balancing light absorption and carrier transport.
PBDT2FBT-T3 showed the highest PCE, 6.5%, among the
conjugated polymers tested, with an extended absorption
spectrum and high carrier transport. Our study provides new
insights into the molecular design and synthesis of highly
efficient PSCs.
4. EXPERIMENTAL SECTION
1
4.1. Characterization. All monomers were characterized by H
NMR (400 MHz) and 13C NMR (100 MHz) on a Bruker AVANCE
400 spectrometer in chloroform-d solutions at room temperature. All
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were dried with anhydrous MgSO4 and evaporated. The crude product
was purified by vacuum distillation to give pure 1 as a pale yellow oil
1
(10.8 g, yield 72%). H NMR (CDCl3, 400 MHz), δ (ppm): 7.10 (d,
1H), 6.92 (t, 1H), 6.75 (d, 1H), 2.75 (d, 2H), 1.61 (m, 1H), 1.4−1.15
(m, 16H), 0.94−0.80 (m, 6H). MALDI−MS: calcd, m/z = 252.5;
found, 252.4 [M+].
5-(2-Butyloctyl)-2,2′-bithiophene (3). 2-(2-Butyloctyl)thiophene-5-
bromide (5.0 g, 15.0 mmol), 2-(trimethylstannyl)thiophene (3.7 g,
15.0 mml), and tetrakis(triphenylphosphine)palladium(0) (350 mg,
2.0 mol %) were added to a three neck flask under a nitrogen
atmosphere followed by the addition of toluene (50 mL) and DMF (5
mL). The reaction mixture was stirred at 105 °C for 48 h and then
cooled to room temperature. The crude product was extracted with
CH2Cl2 (100 mL × 3) and purified by column chromatography with
n-hexane:dichloromethane = 10:1 (v:v) as eluent. The solvent was
evaporated and the product was recrystallized from 2-propanol as pale
1
orange solid (3.18 g, yield 63%). H NMR (CDCl3, 400 MHz), δ
(ppm): 7.15 (d, 1H), 7.10 (d, 1H), 7.00 (d, 1H), 6.98 (t, 1H), 6.65 (d,
1H), 2.07 (d, 2H), 1.62 (m, 1H), 1.33−1.18 (m, 16H), 0.94−0.80 (m,
6H). MALDI−MS: calcd, m/z = 334.2; found, 334.2 [M+].
5-(2-Butyloctyl)-2,2′;5′,2″-terthiophene (5). 2-(2-Butyloctyl)-
thiophene-5-bromide (4.0 g, 12.0 mmol), 5-(trimethylstannyl)-2,2′-
bithiophene (3.98 g, 12.0 mmol), and tetrakis(triphenylphosphine)-
palladium(0) (280 mg, 2.0 mol %) were added to a three neck flask
under a nitrogen atmosphere followed by the addition of toluene (50
mL) and DMF (5 mL). The reaction mixture was stirred at 105 °C for
48 h and then cooled to room temperature. The crude product was
extracted with CH2Cl2 (100 mL × 3) and purified by column
chromatography with n-hexane:dichloromethane = 8:2 (v:v) as eluent.
The solvent was evaporated and the product was recrystallized from 2-
1
propanol as orange solid (3.37 g, yield 67%). H NMR (CDCl3, 400
MHz), δ (ppm): 7.19 (d, 1H), 7.15 (d, 1H), 7.06 (d, 1H), 7.02 (d,
1H), 7.00 (d, 1H), 6.98 (t, 1H), 6.66 (d, 1H), 2.72 (d, 2H), 1.63 (m,
1H), 1.36−1.13 (m, 16H), 0.94−0.83 (m, 6H). MALDI−MS: calcd,
m/z = 416.2; found, 416.8 [M+].
5-(2-Butyloctyl)-2,2′;5′,2″;5″,2‴-quaterthiophene (7). 2-(2-
Butyloctyl)thiophene (3.0 g, 9.1 mmol), 5-(trimethylstannyl)-
2,2′;5′,2″-terthiophene (3.72 g, 9.1 mol), and tetrakis-
(triphenylphosphine)palladium(0) (210 mg, 2.0 mol %) were added
to a three neck flask under a nitrogen atmosphere followed by the
addition of toluene (50 mL) and DMF (5 mL). The reaction mixture
was stirred at 105 °C for 48 h and then cooled to room temperature.
The crude product was extracted with CH2Cl2 (100 mL × 3) and
purified by column chromatography with n-hexane:dichloromethane =
7:3 (v:v) as eluent. The solvent was evaporated and the product was
recrystallized from 2-propanol as pale orange solid (3.20 g, yield 71%).
1H NMR (CDCl3, 400 MHz), δ (ppm): 7.24 (d, 1H), 7.21 (d, 1H),
Figure 10. (a) 2D GIXD images and (b) 1D patterns of out-of-plane
direction for PBDT2FBT-Tn:PC71BM blend films prepared by DCB.
polymers were characterized by 1H NMR (500 MHz) on Bruker DRX
500 spectrometer in 1,2-dichlorobenzene-d4 at 383 K due to low
solubility of PBDT2FBT-T2 and -T3. Number-average (Mn) and
weight-average (Mw) molecular weights were determined with gel
permeation chromatography (GPC, Shimadzu) in chlorobezene at 80
°C, using two PL mixed B columns in series, and calibrated against
narrow polydispersity polystyrene standards. UV−vis spectra were
recorded on a Varian CARY-5000 UV−vis spectrophotometer. For the
measurements of thin films, polymers were spun coated onto
precleaned glass substrates from chlorobenzene solutions (10 mg
mL−1). TGA plots were measured with a TA Instruments, Inc., TGA
2050 under a nitrogen atmosphere at heating and cooling rates of 10
°C·min−1. AFM and TEM images were obtained with a MultiMode 8
Scanning Probe Microscope (VEECO Instruments Inc.) and a
Hitachi-7600 system by using an accelerating voltage of 100 kV,
respectively. GIXD measurements were performed using the 3C and
9A (2D) beamlines at the Pohang Accelerator Laboratory (PAL).
4.2. Synthesis of Monomers. 2-(2-Butyloctyl)thiophene (1). To
a solution of thiophene (5.0 g, 59.4 mmol) in THF (10 mL) in an N2
purged flaskwas added n-BuLi (1.6 M in THF, 40.8 mL, 1.1 equiv)
dropwise at −78 °C. The solution was stirred for 90 min at 55 °C
before 2-butyloctyl bromdie (14.3 g, 57.6 mmol) was dropwise added.
After 120 min, the reaction mixture was cooled to room temperature,
and then aqueous NaCl solution (2.0 M in 20 mL) was added. The
solution was stirred for another 60 min before was poured into water
and extracted with CH2Cl2 (100 mL × 3). The combined extracts
7.08 (d, 1H), 7.06 (d, 1H), 7.01 (d, 1H), 7.00 (d, 1H), 7.01 (d, 1H),
6.99 (t, 1H), 6.66 (d, 1H), 2.72 (d, 2H), 1.63 (m, 1H), 1.38−1.18 (m,
16H), 0.92−0.83 (m, 6H). MALDI−MS: calcd, m/z = 498.2; found,
498.2 [M+].
General Synthetic Procedure of BDT-Tn Monomers. Substitution
of Thienyl Side Groups (8). Under nitrogen atmosphere, n-BuLi (1.6
M in THF, 1.1 equiv) was added dropwise to a solution of T1 (or T2,
T3, T4) in dry THF at 0 °C. The mixture was then warmed to 50 °C
and stirred for 1 h. Subsequently, 4,8-dihydrobenzo[1,2-b:4,5-b′]-
dithiophen-4,8-dione was added to the reaction mixture, which was
then stirred for another 1 h at 50 °C. Cooling the mixture down to
ambient temperature, SnCl2·2H2O (4.5 g, 20 mmol) in 8 mL HCl
(10%) was added, and the mixture was stirred for another 4 h. The
mixture was subsequently poured into ice water and extracted with
CH2Cl2 (100 mL × 3). The combined extracts were dried with
anhydrous MgSO4 and evaporated. The crude product was purified by
column chromatography on silica gel to give pure BDT-Tn.
4,8-Bis(5-(2-butyloctyl)thiophene-2-yl)benzo[1,2-b;4,5-b′]-
dithiophene. The crude product was purified by column chromatog-
raphy on silica gel eluting with dichloromethane:hexane = 1:10 (v:v)
1
to give pure product as a pale yellow liquid. (yield 61%). H NMR
(CDCl3, 400 MHz), δ (ppm): 7.64 (d, 2H), 7.45 (d, 2H), 7.29 (d,
2H), 6.88 (d, 2H), 2.85 (d, 4H), 1.72 (m, 2H), 1.48−1.25 (br, 32H),
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0.94−0.88 (m, 12H). 13CNMR (CDCl3, 100 MHz), δ (ppm): 145.86,
139.14, 137.34, 136.65, 127.8, 127.58, 125.53, 124.31, 123.56, 40.16,
34.82, 33.51, 33.18, 31.95, 29.62, 29.05, 26.78, 23.21, 23.18, 22.85,
14.31. MALDI−MS: calcd, m/z = 690.3; found, 691.2 [M+].
6.66; S, 16.29. Found: C, 56.87; H, 6.59; S, 16.20. MALDI−MS: calcd,
m/z = 1180.2; found, 1181.1 [M+].
2,6-Bis(trimethyltin)-4,8-bis(5″-(2-butyloctyl)-[2,2′;5′,2″-terthio-
phene]-5-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-T3). A dark yellow
1
solid (yield 67%). H NMR (CDCl3, 400 MHz), δ (ppm): 7.73 (s,
4,8-Bis(5′-(2-butyloctyl)-[2,2′-bithiophen]-5-yl)benzo[1,2-b:4,5-
b′]dithiophene. The crude product was purified by column
chromatography on silica gel eluting with dichloromethane:hexane =
2H), 7.43 (d, 2H), 7.32 (d, 2H), 7.18 (d, 2H), 7.06 (d, 2H), 7.03 (d,
2H), 6.69 (d, 2H), 2.76 (d, 4H), 1.65 (m, 2H), 1.38−1.22 (br, 32H),
0.95−0.86 (m, 12H), 0.43 (s, 18H). 13CNMR (CDCl3, 100 MHz), δ
(ppm): 144.47, 143.56, 143.48, 139.30, 138.21, 137.57, 137.51, 135.40,
134.84, 131.01, 129.07, 126.11, 124.69, 123.72, 123.64, 123.55, 121.98,
40.12, 34.74, 33.35, 33.21, 32.06, 29.78, 29.02, 26.75, 23.16, 22.84,
22.69, 14.27, −8.13. Anal. Calcd for C64H82S8Sn2: C, 57.14; H, 6.14; S,
19.07. Found: C, 57.06; H, 6.11; S, 19.01. MALDI−MS: calcd, m/z =
1344.2; found, 1345.1 [M+].
2,6-Bis(trimethyltin)-4,8-bis(5‴-(2-butyloctyl)-[2,2′;5′,2″;5″,2‴-
quaterthiophene]-5-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-T4). A
brown solid (yield 59%). 1H NMR (CDCl3, 400 MHz), δ (ppm): 7.72
(s, 2H), 7.44 (d, 2H), 7.34 (d, 2H), 7.19 (d, 2H), 7.11 (d, 2H), 7.10
(d, 2H), 7.03 (d, 2H), 7.00 (d, 2H), 6.68 (d, 2H), 2.75 (d, 4H), 1.64
(m, 2H), 1.37−1.21 (br, 32H), 0.94−0.85 (m, 12H), 0.42 (s, 18H).
13CNMR (CDCl3, 100 MHz), δ (ppm): 144.47, 143.67, 142.87,
1
1:10 (v:v) to give pure product as a yellow solid. (yield 64%). H
NMR (CDCl3, 400 MHz), δ (ppm): 7.69 (d, 2H), 7.46 (d, 2H), 7.39
(d, 2H), 7.24 (d, 2H), 7.09 (d, 2H), 6.70 (d, 2H), 2.77 (d, 4H), 1.66
(m, 2H), 1.38−1.20 (br, 32H), 0.95−0.86 (m, 12H). 13CNMR
(CDCl3, 100 MHz), δ (ppm): 144.79, 139.19, 138.43, 137.85, 137.19,
137.18, 135.12, 128.97, 128.62, 126.41, 125.14, 124.37, 123.12, 40.15,
34.72, 33.34, 33.17, 31.98, 29.77, 28.99, 26.75, 25.37, 23.18, 22.83,
14.26. MALDI−MS: calcd, m/z = 854.3; found, 855.2 [M+].
4,8-Bis(5″-(2-butyloctyl)-[2,2′;5′,2″-terthiophene]-5-yl)benzo[1,2-
b:4,5-b′]dithiophene. The crude product was purified by column
chromatography on silica gel eluting with dichloromethane:hexane =
2:8 (v:v) to give pure product as a yellow ocher solid. (yield 61%). 1H
NMR (CDCl3, 400 MHz), δ (ppm): 7.71 (d, 2H), 7.52 (d, 2H), 7.41
(d, 2H), 7.29 (d, 2H), 7.16 (d, 2H), 7.05 (d, 2H), 7.03 (d, 2H), 6.69
(d, 2H), 2.76 (d, 4H), 1.65 (m, 2H), 1.38−1.20 (br, 32H), 0.95−0.86
(m, 12H). 13CNMR (CDCl3, 100 MHz), δ (ppm): 144.53, 139.25,
138.54, 138.23, 137.89, 137.51, 136.74, 135.15, 134.78, 129.17, 128.09,
126.12, 124.75, 123.73, 123.60, 123.54, 123.41, 40.12, 34.74, 33.36,
33.25, 33.17, 32.04, 29.78, 29.02, 26.75, 23.16, 22.84, 14.30. MALDI−
MS: calcd, m/z = 1018.3; found, 1019.3 [M+].
137.18, 136.56, 135.26, 134.76, 134.45, 128.04, 126.09, 124.52, 124.37,
124.17, 124.02, 123.84, 123.71, 123.64, 123.59, 123.52, 123.14, 121.89,
40.10, 34.73, 33.17, 33.03, 32.04, 29.78, 29.00, 26.74, 23.15, 23.01,
22.83, 14.26, −8.10. Anal. Calcd for C72H86S10Sn2: C 57.29, H 5.74, S
21.21; found: C 57.21, H 5.68, S 21.16. MALDI−MS: calcd m/z =
1508.2; found 1509.1 [M+].
5,6-Difluoro-4,7-bis(4-(2-ethylhexyl)-2-thienyl)-2,1,3-benzothia-
diazole (9). Into a 250 mL flame-dried 3-neck round-bottom flask with
a condenser were added 5,6-difluoro-4,7-diiodobenzo[c][1,2,5]-
thiadiazole (2.1 g, 5 mmol), (4-(2-ethylhexyl)thiophen-2-yl)-
trimethylstannane (5.2 g, 11 mmol), Pd(PPh3)4 (120 mg), and dry
toluene (80 mL) under nitrogen protection. The reaction mixture was
stirred at 105 °C for 48 h and then cooled to room temperature. The
crude product was extracted with CH2Cl2 (100 mL × 3) and purified
by column chromatography with hexane/dichloromethane (95:5) as
eluent. The solvent was evaporated and the product was recrystallized
from 2-propanol as orange solid (2.12 g, yield 73%) 1H NMR (CDCl3,
400 MHz), δ (ppm): 8.09 (s, 2H), 7.19 (s, 2H), 2.65 (d, 4H), 1.65 (m,
2H), 1.25−1.40 (m, 16H), 0.80−0.94 (m, 12H). 13C NMR (CDCl3,
100 MHz), δ (ppm): 151.05, 150.85, 148.81, 148.77, 148.47, 148.27,
142.23, 132.70, 131.03, 124.81, 111.57, 111.48, 40.45, 34.53, 32.56,
29.16, 25.72, 23.06, 14.11, 10.88.
5,6-Difluoro-4,7-bis(5-bromo-4-(2-ethylhexyl)-2-thienyl)-2,1,3-
benzothiadiazole (DT2FBT). Compound 7 (1.0 g, 1.91 mmol) and N-
bromosuccinimide (NBS) (0.74 g, 4.16 mmol) were added into THF
under stirring. The reaction mixture was stirred at a room temperature
overnight and then washed with brine and dried over MgSO4. The
solvent was removed under a reduced pressure and an orange solid
was obtained by column chromatography using hexane/dichloro-
methane (9:1) as the eluent. Needle-like crystal was obtained by
recrystallization from iso-propanol (1.00 g, yield 77%). 1H NMR
(CDCl3, 400 MHz), δ (ppm): δ 7.93 (s, 2H), 2.61 (d, 4H), 1.72 (m,
2H), 1.25−1.41 (m, 16H), 0.79−0.96 (m, 12H). 13C NMR (CDCl3,
100 MHz), δ (ppm): 151.19, 150.99, 148.60, 148.54, 147.93, 141.92,
132.49, 132.44, 131.21, 115.25, 115.25, 111.11, 40.14, 33.90, 32.64,
28.92, 25.87, 23.22, 14.28, 11.01. 19F NMR (CDCl3, 100 MHz), δ
(ppm): −127.18 (s, 2F). Anal. Calcd: C30H36Br2F2N2S3: C, 50.14; H,
5.05; S, 13.39. Found: C, 50.62; H, 5.07; S, 13.69.
4,8-Bis(5‴-(2-butyloctyl)-[2,2′;5′,2″;5″,2‴-quaterthiophene]-5-yl)-
benzo[1,2-b:4,5-b′]dithiophene. The crude product was purified by
column chromatography on silica gel eluting with dichloromethane:-
hexane = 3:7 (v:v) to give pure product as a dark yellow solid. (yield
1
68%). H NMR (CDCl3, 400 MHz), δ (ppm): 7.72 (d, 2H), 7.53 (d,
2H), 7.43 (d, 2H), 7.35 (d, 2H), 7.18 (d, 2H), 7.11 (d, 2H), 7.09 (d,
2H), 7.03 (d, 2H), 7.00 (d, 2H), 6.68 (d, 2H), 2.75 (d, 4H), 1.64 (m,
2H), 1.38−1.20 (br, 32H), 0.94−0.85 (m, 12H). 13CNMR (CDCl3,
100 MHz), δ (ppm): 144.47, 137.18, 136.21, 135.31, 135.26, 134.76,
134.53, 128.04, 126.09, 125.01, 124.98, 124.37, 124.17, 123.84, 123.67,
123.52, 123.24, 123.15, 122.93, 122.62, 122.54, 40.10, 34.73, 33.34,
33.16, 33.03, 32.04, 29.78, 29.00, 26.74, 23.15, 22.83, 14.26. MALDI−
MS: calcd, m/z = 1182.3; found, 1183.2 [M+].
Stannylation of 8. Compound 8 was dissolved in dry THF in a
nitrogen purged flask. The solution was cooled to −78 °C before n-
BuLi (1.6 M in THF, 2.3 equiv) was added dropwise. After the
reaction was stirred at −78 °C for 90 min, trimethyltin chloride (1 M
in hexane, 2.6 equiv) was added in one portion. The solution was
allowed to warm to room temperature slowly and stirred overnight.
After the reaction, the solution was diluted with dichloromethane (200
mL), washed with water (150 mL × 3), dried over anhydrous MgSO4,
and evaporated. The crude product was purified by recrystallization in
2-propanol to give pure distannylated BDT-Tn.
2,6-Bis(trimethyltin)-4,8-bis(5-(2-butyloctyl)thiophene-2-yl)-
benzo[1,2-b;4,5-b′]dithiophene (BDT-T1). A pale yellow viscous oil
1
(yield 55%). H NMR (CDCl3, 400 MHz), δ (ppm): 7.68 (s, 2H),
7.31 (d, 2H), 6.90 (d, 2H), 2.86 (d, 4H), 1.74 (m, 2H), 1.45−1.25 (br,
32H), 0.95−0.86 (m, 12H), 0.39 (s, 18H). 13CNMR (CDCl3, 100
MHz), δ (ppm): 145.52, 143.45, 142.38, 138.14, 137.47, 131.30,
127.69, 125.45, 122.57, 40.19, 34.89, 33.64, 33.23, 31.97, 29.85, 29.10,
26.86, 25.43, 23.20, 22.84, 14.29, −8.22. Anal. Calcd for C48H74S4Sn2:
C, 56.70; H, 7.34; S, 12.61. Found: C, 56.64; H, 7.33; S, 12.50.
MALDI−MS: calcd, m/z = 1016.27; found, 1017.1 [M+].
4.3. Synthesis of Polymers. Compounds BDT-Tn monomers
(0.5 mmol), DT2FBT monomer (0.5 mmol), Pd2(dba)3 (8.5 mg), and
P(o-tol)3 (12.5 mg) were added to a flame-dried and nitrogen-filled 3-
neck flask (50 mL). Three nitrogen purge/vacuum cycles under
vigorous stirring were done before adding degassed toluene/N,N-
dimethylformamide (DMF) (10:1, v/v). After stirring at 110 °C for 48
h, trimethylthienyltin (0.15 equiv) was added to the reaction flask and
the reaction was kept at 110 °C for an additional 3 h. 2-
Bromothiophene (0.5 equiv) was then added to the reaction flask,
and the temperature was kept at 110 °C for an additional 5 h to
complete the end-capping reaction. The mixture was cooled to room
2,6-Bis(trimethyltin)-4,8-bis(5′-(2-butyloctyl)-[2,2′-bithiophen]-5-
yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-T2). A yellow solid (yield
1
72%). H NMR (CDCl3, 400 MHz), δ (ppm): 7.71 (s, 2H), 7.40 (d,
2H), 7.25 (d, 2H), 7.10 (d, 2H), 6.71 (d, 2H), 2.77 (d, 4H), 1.66 (m,
2H), 1.38−1.22 (br, 32H), 0.95−0.86 (m, 12H), 0.41 (s, 18H).
13CNMR (CDCl3, 100 MHz), δ (ppm): 144.45, 143.61, 143.09,
138.93, 138.65, 137.58, 134.97, 131.07, 128.89, 126.10, 123.71, 123.30,
122.07, 40.17, 34.77, 33.37, 33.21, 32.06, 29.81, 29.03, 26.77, 25.38,
23.19, 22.84, 14.28, −8.13. Anal. Calcd for C56H78S6Sn2: C, 56.95; H,
K
DOI: 10.1021/acs.macromol.5b00056
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Macromolecules
Article
temperatures and added dropwise to methanol (300 mL) to obtain a
precipitate. After stirring for several hours, the resultant polymer was
collected by filtration, dried and extracted successively with methanol,
acetone, and hexane by using a Soxhlet extraction apparatus to remove
oligomers and catalyst residue. The remaining solid was extracted with
chloroform or chlorobenzene and then precipitated in methanol. The
precipitate was filtered and dried in vacuum at 40 °C overnight.
PBDT2FBT-T1. Dark blue solid (321 mg, 63%). GPC: Mn = 28.1 kg
mol−1, Mw = 49.3 kg mol−1, PDI = 1.75. 1H NMR (DCB-d4, 500 MHz,
383 K) δ (ppm): 8.50 (s, 2H), 8.20 (s, 2H), 7.69 (s, 2H), 7.18 (s, 2H),
3.21 (m, 4H), 3.12 (m, 4H), 2.14 (m, 2H), 2.04 (m, 2H), 1.7−1.5
(bm, 48H), 1.15 (m, 12H), 1.07 (m, 12H). Anal. Calcd:
C72H94F2N2S7: C, 69.18; H, 7.58; N, 2.24; S, 17.96. Found: C,
69.15; H, 7.55; N, 2.17; S, 17.89.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterizations. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*(K.C.) E-mail: kwcho@postech.ac.kr.
■
Author Contributions
†J.L. and J.-H.K. contributed equally to this work.
Notes
The authors declare no competing financial interest.
PBDT2FBT-T2. Dark brown solid with a metallic luster (428 mg,
75%). GPC: Mn = 42.8 kg mol−1, Mw = 48.6 kg mol−1, PDI = 1.13. 1H
NMR (DCB-d4, 500 MHz, 383 K) δ (ppm): 8.48 (s, 2H), 8.22 (s,
2H), 7.73 (s, 2H), 7.46 (s, 2H), 7.29 (s, 2H), 6.90 (s, 2H), 3.21 (m,
4H), 2.99 (m, 4H), 2.14 (m, 2H), 1.94 (m, 2H), 1.75−1.5 (bm, 48H),
1.15−1.06 (bm, 24H). Anal. Calcd: C80H98F2N2O2S9: C, 67.94; H,
6.98; N, 1.98; S, 20.41. Found: C, 67.91; H, 6.95; N, 1.91; S, 20.56.
PBDT2FBT-T3. Dark blue solid (321 mg, 63%). GPC: Mn = 48.2 kg
ACKNOWLEDGMENTS
■
This work was supported by a grant (Code No. 2011-0031628)
from the Center for Advanced Soft Electronics under the
Global Frontier Research Program of the Ministry of Science,
ICT and Future Planning, Korea. The authors thank the
Pohang Accelerator Laboratory for providing the synchrotron
radiation sources at 3C and 9A beamlines used in this study.
1
mol−1, Mw = 102.1 kg mol−1, PDI = 2.11. H NMR (DCB-d4, 500
MHz, 383 K) δ (ppm): 8.48 (s, 2H), 8.23 (s, 2H), 7.77 (s, 2H), 7.50
(s, 2H), 7.32 (s, 2H), 7.19 (s, 4H), 6.88 (s, 2H), 3.22 (m, 4H), 2.95
(m, 4H), 2.15 (m, 2H), 1.91 (m, 2H), 1.74−1.49 (bm, 48H), 1.17−
1.07 (bm, 24H). Anal. Calcd: C88H102F2N2S11: C, 66.96; H, 6.51; N,
1.77; S, 22.35. Found: C, 66.87; H, 6.48; N, 1.76; S, 22.32.
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DOI: 10.1021/acs.macromol.5b00056
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