Synthesis, molecular and photovoltaic properties of donor-acceptor conjugated polymers incorporating a new heptacylic indacenodithieno[3,2-b] thiophene arene

Article abstract of DOI:10.1021/ma3019552

We have developed a new multifused indacenodithieno[3,2-b]thiophene arene (IDTT) unit where the central phenylene is covalently fastened with the two outer thieno[3,2-b]thiophene (TT) rings, forming two cyclopentadiene rings embedded in a heptacyclic structure. This rigid and coplanar IDTT building block was copolymerized with electron-deficient acceptors, 4,7-dibromo-2,1,3- benzothiadiazole (BT), 4,7-dibromo-5,6-difluoro-2,1,3-benzothiadiazole (FBT) and 1,3-dibromo-thieno[3,4-c]pyrrole-4,6-dione (TPD) via Stille polymerization, respectively. Because the higher content of the thienothiophene moieties in the fully coplanar IDTT structure facilitates π-electron delocalization, these new polymers show much improved light-harvesting abilities and enhanced charge mobilities compared to PDITTBT copolymer using hexacyclic diindenothieno[3,2-b] thiophene (DITT) as the donor moieties. The device using PIDTTBT:PC ₇₁BM (1:4, w/w) exhibited a decent power conversion efficiency (PCE) of 3.8%. Meanwhile, the solar cell using PIDTTFBT:PC₇₁BM (1:4 in wt %) blend exhibited a greater V_oc value of 0.9 V and a larger J _sc of 10.08 mA/cm², improving the PCE to 4.2%. The enhanced V_oc is attributed to the lower-lying of HOMO energy level of PIDTTFBT as a result of fluorine withdrawing effect on the BT unit. A highest PCE of 4.3% was achieved for the device incorporating PIDTTTPD:PC₇₁BM (1:4 in wt %) blend.

Full text of DOI:10.1021/ma3019552

Article
pubs.acs.org/Macromolecules
Synthesis, Molecular and Photovoltaic Properties of Donor−Acceptor
Conjugated Polymers Incorporating a New Heptacylic
Indacenodithieno[3,2‑b]thiophene Arene
Huan-Hsuan Chang, Che-En Tsai, Yu-Ying Lai, De-Yang Chiou, So-Lin Hsu, Chain-Shu Hsu,
and Yen-Ju Cheng*
Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsin-Chu, 30010 Taiwan
S
* Supporting Information
ABSTRACT: We have developed a new multifused
indacenodithieno[3,2-b]thiophene arene (IDTT) unit where
the central phenylene is covalently fastened with the two outer
thieno[3,2-b]thiophene (TT) rings, forming two cyclo-
pentadiene rings embedded in a heptacyclic structure. This
rigid and coplanar IDTT building block was copolymerized
with electron-deficient acceptors, 4,7-dibromo-2,1,3-benzothia-
diazole (BT), 4,7-dibromo-5,6-difluoro-2,1,3-benzothiadiazole
(FBT) and 1,3-dibromo-thieno[3,4-c]pyrrole-4,6-dione
(TPD) via Stille polymerization, respectively. Because the
higher content of the thienothiophene moieties in the fully coplanar IDTT structure facilitates π-electron delocalization, these
new polymers show much improved light-harvesting abilities and enhanced charge mobilities compared to PDITTBT copolymer
using hexacyclic diindenothieno[3,2-b]thiophene (DITT) as the donor moieties. The device using PIDTTBT:PC71BM (1:4, w/
w) exhibited a decent power conversion efficiency (PCE) of 3.8%. Meanwhile, the solar cell using PIDTTFBT:PC71BM (1:4 in
wt %) blend exhibited a greater Voc value of 0.9 V and a larger Jsc of 10.08 mA/cm2, improving the PCE to 4.2%. The enhanced
Voc is attributed to the lower-lying of HOMO energy level of PIDTTFBT as a result of fluorine withdrawing effect on the BT
unit. A highest PCE of 4.3% was achieved for the device incorporating PIDTTTPD:PC71BM (1:4 in wt %) blend.
their Voc values are generally limited to ca. 0.6 V as a result of
the high-lying HOMO levels.4 Thieno[3,2-b]thiophene (TT)
unit emerges as a superior thiophene-based building block to
achieve high mobility p-type semiconductors.5 This compact
structure actually possesses higher aromatic stabilization energy
than a thiophene, which can potentially lower the HOMO level
for higher Voc.6 Moreover, the C2h symmetry and coplanar
geometry may promote more ordered packing and stronger
interchain interactions to obtain exceptional hole mobility,
which is beneficial for Jsc.7 Introducing the alkyl chains into the
two β-positions of thieno[3,2-b]thiophene unit is usually
necessary to improve the solubility of the resulting polymers.
Unfortunately, these side chains inevitably impose a negative
effect on the effective conjugation due to severe steric
hindrance-induced twisting between the neighboring aryls.8
By implementing forced planarization via covalently fastening
adjacent aromatic units in the polymer backbone, advantageous
intrinsic properties such as reduced band gap and enhanced
charge mobility can be retained.9 Therefore, it is of interest to
integrate benzene and thieno[3,2-b]thiophene units into a
molecular entity with forced rigidification to simultaneously
INTRODUCTION
■
Polymer solar cells (PSCs) using organic p-type (donor) and n-
type (acceptor) semiconductors have attracted tremendous
scientific and industrial interest.1 The most critical challenge at
molecular level is to develop p-type conjugated polymers that
can simultaneously possess sufficient solubility for process-
ability and miscibility with an n-type material, low band gap
(LBG) for strong and broad absorption spectrum to capture
more solar photons, and high hole mobility for efficient charge
transport.2 The most useful approach to make a LBG polymer
is to connect electron-rich donor and electron-deficient
acceptor segments along the conjugated polymer backbone.
Thiophene and benzene aromatic rings are the most important
ingredients to comprise p-type conjugated polymers. Benzene-
based derivatives such as tricyclic 2,7-fluorene or 2,7-carbazole
units have shown to serve as useful building blocks to construct
donor−acceptor polymers having deep-lying HOMO energy
levels that contribute to achieve high open-circuit voltage (Voc)
(>0.8 V) for PSCs.3 However, the intrinsic drawback is that
these polymers usually possess relatively large optical band gaps
(>2 eV) that limit their ability to harvest sunlight and thus
result in moderate short-circuit current (Jsc). On the other
hand, because of the lower aromaticity to adapt more quinoidal
resonance structure, thiophene-based D−A polymers have
better light absorption ability to permit greater Jsc. However,
Received: September 18, 2012
Revised: November 4, 2012
Published: November 27, 2012
© 2012 American Chemical Society
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extend the conjugation while maintaining the coplanarity.
However, development of ladder-type architectures requires
elegant molecular design and synthesis.10 Recently, we first
reported a multifused hexacyclic diindenothieno[3,2-b]-
thiophene (DITT) unit, where the central TT ring is connected
with two outer phenyl rings through two embedded cyclo-
pentadienyl (CP) rings (Scheme 1).11 This electron-rich unit
thiophene (IDTT), where the central phenylene ring is fused
with two outer TT rings by two carbon bridges. Compared to
hexacyclic DITT unit, this heptacyclic IDTT has extended
conjugation length with greatly increasing the content of the
thiophene moieties (one benzene and two thieno[3,2-b]-
thiophene units). Furthermore, the placement of the
thienothiophene units at the two terminal sides of IDTT is
advantageous for facile α-bromination or stannylation for
subsequent polymerization. Four side chains introduced at the
bridging carbons in IDTT guarantee solubility without twisting
the coplanarity. Meanwhile, exploitation of suitable electron-
deficient acceptors in combination with IDTT donor in the
polymeric backbone is required. Benzothiadiazole (BT)12 and
thieno[3,4-c]pyrrole-4,6-dione (TPD)13 are the widely used
electron-deficient acceptors due to their suitable electron
affinity and easy availability. 5,6-Difluorobenzothiadiazole
(FBT) unit with two fluorine atoms on BT unit also emerges
as a superior derivative for adjusting the molecular properties.14
In this research, we report the detailed synthesis of the
distannyl-IDTT monomer which was copolymerized with BT,
FBT, TPD acceptor moieties to prepare a new class of D−A
alternating IDTT-based copolymers (Scheme 1). Their
thermal, optical and electrochemical properties have been
carefully characterized. Preliminary results showed that the
IDTT-based polymers are promising for photovoltaic solar cell
applications.
Scheme 1. Chemical Structures of Hexacyclic DITT and
Heptacyclic IDTT Arenes and Their Corresponding
Donor−Acceptor Copolymers
Synthesis. The synthetic route for Sn-IDTT monomer is
depicted in Scheme 2. Stille coupling of diethyl 2,5-
dibromoterephthate with 2-(tributylstannyl)thieno[3,2-b]-
thiophene yielded compound 1. Double nucleophilic addition
of the ester groups in 1 by (4-octyloxy)phenyl magnesium
bromide led to the formation of benzylic alcohols in 2 which
was subjected to intramolecular Friedel−Crafts cyclization
under acidic condition to furnish the heptacyclic IDTT arene in
a good yield of 81%. Bromination of IDTT by N-
bromosuccinimide generated Br-IDTT in a high yield of
87%. Finally, Br-IDTT was lithiated by n-butyllithium followed
by quenching with trimethyltin chloride to afford the distannyl
was copolymerized with electron-deficient benzothiadiazole
acceptor to obtain a donor−acceptor copolymer PDITTBT
(Scheme 1). Nevertheless, this polymer is short of absorption
coverage at the visible and near IR region due to the fact that
DITT possesses high content of high-aromaticity benzene rings
(two benzene rings and one thieno[3,2-b]thiophene), leading
to relatively large optical band gap and thus limited
photocurrent. By reversing the arrangement of TT and benzene
units in the DITT framework, we present here a new
multifused heptacyclic structure, indacenodithieno[3,2-b]-
Scheme 2. Synthetic Route of Sn−IDTT Monomer
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Sn-IDTT in a moderate yield of 54%. Sn-IDTT monomer was
copolymerized with 4,7-dibromo-2,1,3-benzothiadiazole (BT),
4,7-dibromo-5,6-difluoro-2,1,3-benzothiadiazole (FBT) and
1,3-dibromo-thieno[3,4-c]pyrrole-4,6-dione (TPD) acceptor
monomers via Stille coupling to afford three donor−acceptor
copolymers, poly(indacenodithieno[3,2-b]thiophene-alt-benzo-
thiadiazole) (PIDTTBT, Mn = 16.6 kDa, PDI = 1.7),
poly(indacenodithieno[3,2-b]thiophene-alt-difluorobenzothia-
diazole) (PIDTTFBT, Mn = 24.0 kDa, PDI = 1.2) and
poly(indacenodithieno[3,2-b]thiophene-alt-thieno[3,4-c]-
pyrrole-4,6-dione) (PIDTTTPD, Mn = 31.3 kDa, PDI = 2.0),
respectively (Scheme 3). These copolymers purified by
bands in the spectra. The longer wavelength absorbance is
attributed to the intramolecular charge transfer (ICT) between
the electron-rich and the electron-deficient segments. However,
the localized transition bands and ICT bands of PIDTTTPD
overlap into a broad band covering the whole visible region
from 400 to 700 nm, indicating that the accepting strength of
TPD is weaker than that of BT unit. The absorption spectra of
the three polymers shift toward longer wavelengths from the
solution state to the solid state, indicating that the coplanar
structure of IDTT is capable of inducing strong interchain
interactions. The optical band gaps (Egopt) deduced from the
onset of absorption in the solid state are determined to be 1.69
eV for PIDTTBT, 1.77 eV for PIDTTFBT and 1.95 eV for
PIDTTTPD. Note that the optical band gap of PDITTBT is
2.15 eV, which is significantly larger than that of PIDTTBT,
suggesting that the heptacyclic IDTT unit with higher content
of thienothiophene moieties indeed facilitates the π-electron
delocalization compared to the hexacyclic DITT unit.
Scheme 3. Synthesis of PIDTTBT, PIDTTFBT, and
PIDTTTPD Copolymers
Electrochemical Properties. Cyclic voltammetry (CV)
was employed to examine the electrochemical properties and
determine the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) energies of
the polymers (Figure 2, Table 1). The three polymers showed
stable and reversible p-doping and n-doping processes, which
are important prerequisites for p-type semiconductor materials.
The LUMO energy levels of PIDTTBT, PIDTTFBT, and
PIDTTTPD are determined to be −3.40, −3.50, and −3.18 eV,
respectively. The LUMO energy levels are higher than that of
the PC71BM acceptor (−3.8 eV) to ensure energetically
favorable electron transfer. It should be noted that the
HOMO energy of PIDTTFBT (−5.30 eV) is lower than that
of PIDTTBT (−5.43 eV) due to the two electron-withdrawing
fluorine atoms on the BT units.14 Furthermore, PIDTTTPD
shows the lowest HOMO energy level of −5.45 eV, indicating
that TPD acceptor unit is also capable of lowering the HOMO
energy level of the resulting polymer. The HOMO energy
levels are within the ideal range to ensure better air-stability and
greater attainable Voc.
successive Soxhlet extraction and precipitation showed
narrower molecular weight. The resulting copolymers flanked
with four side chains on IDTT unit possess excellent
solubilities in common organic solvents, such as chloroform,
toluene, and THF.
Thermal and Optical Properties. The thermal stability of
the polymers was analyzed by thermogravimetric analysis
(TGA) (Figure S1, Supporting Information). PIDTTBT,
PIDTTFBT, and PIDTTTPD exhibited sufficiently high
decomposition temperatures (Td) of 414, 391, and 384 °C,
respectively.
Theoretical Calculations. In order to gain more insight
into the molecular orbital properties of the polyaromatic π-
electron systems, quantum−chemical calculations were per-
formed with the Gaussian09 suite employing the B3LYP and
TD-B3LYP density functionals in combination with the 6-
311G(d,p) basis set. Considering an insignificant effect on
UV−vis absorption spectra of the three polymers in THF
solutions and in thin films are shown in Figure 1 and the
correlated optical parameters are summarized in Table 1.
PIDTTBT and PIDTTFBT showed two obvious absorption
Figure 1. Normalized absorption spectra of PIDTTBT, PIDTTFBT, and PIDTTTPD in (a) THF solutions and (b) in thin films.
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a
Table 1. Summary of the Intrinsic Properties of the Polymers
λmax (nm)
THF
polymer
Mn (kDa)
PDI
1.7
Td (°C)
Egopt (eV) (Film)
film
HOMO (eV)
LUMO(eV)
PIDTTBT
16.6
414
1.69
420
622
415
616
588
430
645
424
635
591
−5.30
−3.40
PIDTTFBT
PIDTTTPD
24.0
31.3
1.2
2.0
391
384
1.77
1.95
−5.43
−5.45
−3.50
−3.18
a
Egopt from the onset of UV spectra in thin film.
The calculated HOMO/LUMO energy, excitation energy,
oscillator strength, and configurations of the excited states are
summarized in Table 2 and the frontier orbitals, HOMO (H),
LUMO (L) and the closeby LUMO+1 (L+1) are illustrated in
Table 3. The HOMO electron density distribution of
IDTTTPD is analogous to that of IDTTFBT and IDTTBT,
where the electron density is not only distributed homoge-
neously along the molecular backbone of the IDTT moiety but
also on parts of the acceptor. Given that the contribution of the
IDTT moiety to the HOMO energy level should be similar
among the three compounds, the difference in the HOMO
energy level therefore mainly depends on the nature of the
acceptors. Consistent with the electrochemical experiments, the
calculated HOMO energy levels of the three model compounds
follow the order: IDTTTPD (−5.25 eV) < IDTTFBT (−5.23
eV) < IDTTBT (−5.18 eV). On the contrary, the LUMO of
IDTTTPD is higher in energy than that of IDTTFBT and
IDTTBT. IDTTFBT and IDTTBT have similar LUMO
electron density patterns of which the electron density is
mainly located on the acceptors (BT and FBT). Instead, the
electron density of LUMO in IDTTTPD is not only localized
on TPD unit but span from TPD to IDTT moieties.
Involvement of the IDTT orbitals in the LUMO of IDTTTPD
may result in a high-lying energy level of LUMO.
Figure 2. Cyclic voltammograms of PIDTTBT, PIDTTFBT, and
PIDTTTPD in the thin films at a scan rate of 50 mV/s.
electronic properties, all the side-chain substituents were
replaced with methyl groups for simplicity. Repeating units,
denoted as IDTTBT, IDTTFBT, and IDTTTPD, in their most
stable conformations were used as simplified model compounds
for PIDTTBT, PIDTTFBT, and PIDTTTPD, respectively.
a
Table 2. Calculated HOMO/LUMO Energy, Excitation Energy, Oscillator Strength, and Configurations (with Large CI
Coefficients) of the Excited States
excitation energy
b
c
compound
HOMO (eV)
LUMO (eV)
λmax,exp (nm)
λcalcd (nm)
oscillator strength
symmetry
configuration
IDTT
−5.22
−1.87
393, 417
420
321
1.1262
0.2295
singlet-A
singlet-A
H→L
H−4→L
H→L+2
H→L
IDTTBT
−5.18
−5.23
−2.78
−2.89
622
420
597
433
430
0.6822
0.7493
0.315
singlet-A
singlet-A
singlet-A
H−1→L
H→L+1
H−1→L
H→L+1
H−2→L
H→L
IDTTFBT
616
415
607
435
0.6499
0.2193
singlet-A
singlet-A
H−3→L
H−2→L
H−1→L
H→L+1
H→L+1
H−3→L
H−2→L
H−1→L
H→L
431
521
0.8399
1.4519
singlet-A
IDTTTPD
−5.25
−2.53
588
singlet-A
a
b
TD−B3LYP/6-311G(d,p), PCM=THF. Experimental values were measured for nonsimplified IDTT, IDTTBT, IDTTFBT, IDTTTPD in THF
c
solution, respectively. Configurations with largest coefficients in the CI expansion of each state are highlighted in boldface.
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Table 3. Plots (Isovalue = 0.02 au) of Frontier Orbitals of IDTT, IDTTBT, IDTTFBT, and IDTTPD Calculated at the Level of
B3LYP/6-311G(d,P) in THF
Figure 3. Electron density difference maps (EDDMs) of selected singlet electronic transitions of IDTTBT (at 597, 433, and 430 nm), IDTTFBT (at
607, 435, and 431 nm), and IDTTTPD (at 521 nm). Red indicates a decrease in charge density, while green indicates an increase. All EDDMS were
plotted with isovalue 0.0012 au.
As listed in Table 2, although there are variations in the
absolute values, the calculated absorptions are still in good
agreement with the experimental values. In order to have
further understanding of these electronic transitions, electron
density difference maps (EDDMs) were conducted (Figure
3).15 The electronic transitions can therefore be visualized
through EDDMs. Red indicates a decrease in charge density,
while green indicates an increase. For IDTTBT, the lowest
UV absorption spectrum, only one absorption peak (λmax,exp=
420 nm) was observed. On the basis of the EDDMs, both
transitions belong to charge separations from the molecular
backbone of IDTT and one 4-methoxyphenyl side chain to the
BT unit. The lowest energy singlet electronic transition (λcalc
=
607 nm; λmax,exp= 616 nm) of IDTTFBT is a ICT band, which
is basically caused by the electron transfer from IDTT to FBT.
The transitions at λcalc = 435 and 431 nm are also inseparable in
energy singlet electronic transition (λcalc = 597 nm; λmax,exp
=
the experimental spectrum (λmax,exp= 415 nm). One at λcalc =
622 nm) indicates a pronounced intramolecular charge transfer
(ICT) from IDTT to BT. The transitions at λcalc = 433 and 430
nm are close in energy and may not be separable. In fact, in the
435 nm is assigned to the ICT from the flanking 4-
methoxyphenyl groups of IDTT to BT, and the other one at
λmax = 431 nm only occurs at localized regions. Lastly, the most
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Table 4. FET Characteristics of the Polymer Thin Films
polymer
SAM layer
annealing temperature (°C)
mobility (cm2 V−1 s−1
)
on/off
Vt (V)
PIDTTBT
PIDTTFBT
PIDTTTPD
ODTS
ODTS
ODTS
200
150
200
7 × 10−3
1 × 10−2
1 × 10−3
1.9 × 103
1.4 × 107
3.2 × 104
−13.8
−21.7
−25.4
Figure 4. . Typical output curves (a, c, e) and transfer plots (b, d, f) of the OFET devices based on PIDTTBT, PIDTTFBT, and PIDTTTPD,
respectively, with ODTS-SAM layer.
probable vertical excitation of IDTTTPD is calculated to be
521 nm, which deviates slightly from the λmax,exp of 589 nm. It
mainly results from the ICT from IDTT to TDP and the π−π*
transition within the IDTT unit has a minor contribution.
Organic Field Effect Transistors. To investigate the
mobilities of the polymers, organic field-effect transistors
(OFETs) were fabricated in the bottom-contact/top-contact
geometry as described in the Experimental Section (Table 4
and Figure 4). The hole mobilities were deduced from the
transfer characteristics of the devices in the saturation regime.
The polymers-based OFETs using SiO2 as gate dielectric was
treated with octadecyltrichlorosilane (ODTS) to form a self-
assembly monolayer (SAM). With the modification of a
ODTS-SAM layer along with annealing temperature at 150
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and 200 °C, the hole mobilities of the three polymers were
determined to be 7 × 10−3 cm2 V−1 s−1, 1 × 10−2 cm2 V−1 s−1,
and 1 × 10−3 cm2 V−1 s−1 with the on−off ratios of 1.9 × 103,
1.4 × 107, and 3.2 × 104 for PIDTTBT, PIDTTFBT, and
PIDTTTPD devices, respectively. It should be mentioned that
PDITTBT using hexacyclic DITT as the donor showed a much
lower mobility of 7 × 10−5 cm2 V−1 s−1.11 This result again
indicates that higher content of thienothiophene unit can
improve the charge transporting properties.
device derived from PIDTTFBT has the highest Voc and Jsc, it
does not show a better PCE than the PIDTTTPD-based
device, which is due to the fact that the device of PIDTTTPD
has a much larger FF than that of PIDTTBT. To confirm the
accuracy of the measurements of the devices, the corresponding
external quantum efficiency (EQE) spectra were measured
under illumination of monochromatic light (Figure 4). The Jsc
values calculated from integration of the EQE spectra agree well
with the Jsc obtained from the J−V measurements.
Photovoltaic Characteristics. Bulk heterojunction photo-
voltaic cells were fabricated by spin-coating the blends from o-
dichlorobenzene solutions at various polymer-to-PC71BM ratios
on the basis of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al
configuration and their performances were measured under a
simulated AM 1.5 G illumination of 100 mW/cm2. The
asymmetric PC71BM was used due to its stronger light
absorption in the visible region than that of PC61BM. The
characterization data are summarized in Table 5 and the J−V
CONCLUSIONS
■
Compared to thiophene unit, C2h-symmetry thieno[3,2-b]-
thiophene (TT) unit with higher aromatic stabilization energy
and coplanar geometry is a promising building block for
donor−acceptor conjugated polymers to obtain higher Voc and
Jsc. However, introducing aliphatic side chains on the β-
positions of TT units leads to severe steric hindrance-induced
twisting between the neighboring aryls in the polymer
backbone. A straightforward approach to circumvent this
deficiency is to embed TT units in a multifused ladder-type
structure. A hexacyclic diindenothieno[3,2-b]thiophene
(DITT) unit has been first developed. Nevertheless, DITT
possesses high content of high-aromaticity benzene rings
resulting in relatively large optical band gap and thus limited
Jsc. By reversing the arrangement of TT and benzene units in
the DITT framework, we have successfully developed a new
multifused heptacyclic structure, indacenodithieno[3,2-b]-
thiophene (IDTT), where the central phenylene ring is fused
with two outer TT rings. The ladder-type IDTT framework can
be easily constructed by Friedel−Crafts annulation. The optical
and electrochemical properties of the resulting polymers have
been characterized experimentally and theoretically. Compared
to the hexacyclic DITT unit, this heptacyclic IDTT has
extended conjugation length with significantly increasing the
content of the thiophene moieties. Because of the more
favorable π-electron delocalization, IDTT-based polymers show
much improved light-harvesting abilities and enhanced charge
mobilities. The device using PIDTTBT:PC71BM (1:4, w/w)
blend exhibited an improved efficiency of 3.8%. Meanwhile, the
device using PIDTTFBT:PC71BM (1:4, w/w) blend exhibited
a greater Voc value of 0.9 V as a result of the fluorine
withdrawing effect on the BT unit, and a larger Jsc of 10.08 mA/
cm2 with a higher PCE of 4.2%. The device based on the
Table 5. PSCs Characteristics
blend ratio
polymer:PC71BM
Voc
Jsc
FF
PCE
(%)
polymer
(V) (mA/cm2) (%)
PIDTTBT
PIDTTFBT
PIDTTTPD
1: 4
1: 4
1: 4
0.84
0.90
0.90
8.32
10.08
7.99
55
46
60
3.8
4.2
4.3
curves of these polymers are shown in Figure 5. The blend ratio
of the active layers shown in the Table 5 is the result of the
optimized conditions for the devices. The device based on the
PIDTTBT:PC71BM (1:4 in wt %) blend exhibited a Voc of 0.84
V, a Jsc of 8.32 mA/cm2, a FF of 55%, delivering a decent PCE
of 3.8%. It is noteworthy that the device based on PDITTBT
polymer using the hexacyclic DITT unit as the donor only
exhibited a lower Voc of 0.88 V, a Jsc of 7.46 mA/cm2, resulting
in a lower PCE of 2.7%.11 Meanwhile, the device using
PIDTTFBT:PC71BM (1:4 in wt %) blend exhibited a greater
Voc value of 0.9 V and a larger Jsc of 10.08 mA/cm2 with an
improved PCE to 4.2%. The enhanced Voc is attributed to the
lower-lying of HOMO energy level of PIDTTFBT as a result of
fluorine withdrawing effect on the FBT unit. More encourag-
ingly, the device based on the PIDTTTPD:PC71BM (1:4 in wt
%) blend exhibited a high Voc of 0.90 V, a Jsc of 7.99 mA/cm2, a
FF of 60%, leading to a highest PCE of 4.3%. Even though the
Figure 5. J−V characteristics of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al under illumination of AM1.5, 100 mW/cm2, and corresponding EQE
spectra.
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vacuo. The residue was purified by column chromatography on silica
PIDTTTPD:PC71BM (1:4 in wt %) blend exhibited a high Voc
of 0.90 V and a highest PCE of 4.3%. We envisage that further
improvement of device performance is highly achievable by
optimizing the processing conditions which are underway in
our laboratories. This research demonstrated that the new
heptacyclic indacenodithieno[3,2-b]thiophene is one of the
most promising building blocks for constructing high-perform-
ance conjugated polymers.
gel (hexane/ethyl acetate, v/v, 20/1) and then recrystallized from
1
hexane to give a light yellow solid 1 (6.9 g, 87%): H NMR (CDCl3,
300 MHz, ppm) δ 1.13 (t, J = 7.1 Hz, 6 H), 4.21−4.28 (q, J = 7.1 Hz,
4 H), 7.26−7.29 (m, 4 H), 7.40 (d, J = 5.1 Hz, 2 H), 7.89 (s, 2 H); 13C
NMR (CDCl3, 75 MHz, ppm) δ 13.8, 61.8, 119.3, 119.4, 127.4, 132.0,
133.8, 134.1, 139.3, 139.9, 142.0, 167.4; MS (EI, M+•, C24H18O4S4)
calcd 498.01, found 498.
Synthesis of Compound 2. A Grignard reagent was prepared by
the following procedure. To a suspension of magnesium turnings (3.2
g, 132.0 mmol) and 3−4 drops of 1,2-dibromoethane in dry THF (120
mL) was slowly added 1-bromo-4-(octyloxy)benzene (34.23 g, 120.0
mmol) dropwise and the mixture was then stirred for 1 h. To a THF
(50 mL) solution of 1 (4.80 g, 9.6 mmol) under nitrogen atmosphere
was added dropwise the freshly prepared 4-(octyloxy)phenyl-1-
magnesium bromide (1 M, 76.8 mL, 76.8 mmol) at room temperature.
The resulting mixture was heated at the refluxing temperature for 16 h,
cooled to room temperature, poured into water (100 mL), and
extracted with ethyl acetate (150 mL × 3). The collected organic layer
was dried over MgSO4. After removal of the solvent under reduced
pressure, the residue was purified by column chromatography on silica
gel (hexane/ethyl acetate, v/v, 30/1) to furnish a yellow solid 2 (5.17
g, 43.7%): 1H NMR (CDCl3, 300 MHz, ppm) δ 0.88, (t, J = 6.8 Hz, 12
H), 1.28−1.45 (m, 40 H), 1.75−1.80 (m, 8 H), 3.42 (s, 2 H), 3.94 (t, J
= 6.6 Hz, 8 H), 6.27 (s, 2 H), 6.80 (d, J = 9 Hz, 8 H), 6.90 (s, 2 H),
7.08 (d, J = 9 Hz, 8 H), 7.13 (d, J = 5.1 Hz, 2 H), 7.30 (d, J = 5.1 Hz, 2
H); 13C NMR (CDCl3, 75 MHz, ppm) δ 14.1, 22.7, 26.0, 29.2, 29.3,
29.4, 31.8, 68.0, 82.4, 113.8, 119.3, 120.3, 127.0, 129.1, 132.3, 136.0,
138.7, 139.5, 139.9, 143.9. 145.4. 158.3; MS (FAB, M+•, C76H94O6S4)
calcd 1230.59, found 1230.
Synthesis of IDTT. To a solution of 2 (1.00 g, 0.81 mmol) in THF
(100 mL) was added concentrated sulfuric acid (0.5 mL). The mixture
was stirred for 2 h at 90 °C, cooled to room temperature, poured into
water (250 mL), and extracted with ethyl acetate (500 mL × 3). The
combined organic layer was dried over MgSO4 and the solvent was
removed under reduced pressure. The residue was then purified by
column chromatography on silica gel (hexane/ethyl acetate, v/v, 80/1)
to afford an yellow solid IDTT (1.58 g, 81%): 1H NMR (CDCl3, 400
MHz, ppm) δ 0.87 (t, J = 6.8 Hz, 12 H), 1.26−1.43 (m, 40 H), 1.70−
1.77 (m, 8 H), 3.89 (t, J = 6.6 Hz, 8 H), 6.79 (d, J = 8.8 Hz, 8 H), 7.19
(d, J = 8.8 Hz, 8 H), 7.25−7.29 (m, 4 H), 7.45 (s, 2 H) ; 13C NMR
(CDCl3, 75 MHz, ppm) δ 14.1, 22.6, 26.1, 29.2, 29.3, 29.3, 31.8, 62.2,
67.9, 114.3, 116.7, 120.4, 126.3, 129.2, 133.6, 134.9, 136.0, 141.7,
143.0. 146.3. 153.5. 158.2; MS (FAB, M+•, C76H90O4S4) calcd 1195.79,
found 1195.
Synthesis of Monomer Br-IDTT. To a solution of IDTT (1.1 g,
0.92 mmol) in THF (30 mL) was added N-bromosuccinimide (0.38 g,
2.12 mmol) at room temperature. The reaction mixture, kept away
from light, was stirred for 12 h at room temperature, quenched by
water (50 mL + 150 mL), and extracted with ethyl acetate (150 mL ×
3). The collected organic layer was dried over MgSO4. After removal
of the solvent under vacuum, the residue was purified by column
chromatography on silica gel (hexane/ethyl acetate, v/v, 100/1) and
then recrystallized from hexane to give a brown solid Br-IDTT(1.08 g,
87%): 1H NMR (CDCl3, 300 MHz, ppm) δ 0.88 (t, J = 6.3 Hz, 12 H),
1.27−1.42 (m, 40 H), 1.72−1.77 (m, 8 H), 3.90 (t, J = 6.5 Hz, 8 H),
6.80 (d, J = 8.6 Hz, 8 H), 7.14 (d, J = 8.6 Hz, 8 H), 7.27 (s, 2 H), 7.44
(s, 2 H); 13C NMR (CDCl3, 75 MHz, ppm) δ 14.1, 22.6, 26.0, 29.2,
29.3, 29.3, 31.8, 62.1, 67.9, 112.5, 114.4, 116.8, 123.1, 129.0, 133.9,
134.5, 135.8, 139.9, 142.1. 146.6. 153.6. 158.3; MS (FAB, M+·, C76H88
Br2O4S4) calcd 1353.58, found 1353.
EXPERIMENTAL SECTION
General Measurement and Characterization. All chemicals are
■
purchased from Aldrich or Acros and used as received unless otherwise
1
specified. H and 13C NMR spectra were measured using Varian 300
and 400 MHz instrument spectrometers. Thermogravimetric analysis
(TGA) was recorded on a Perkin-Elmer Pyris under nitrogen
atmosphere at a heating rate of 10 °C/min. Absorption spectra were
recorded on a HP8453 UV−vis spectrophotometer. The molecular
weight of polymers was measured on a Viscotek VE2001GPC, and
polystyrene was used as the standard (THF as the eluent). The
electrochemical cyclic voltammetry (CV) was conducted on a CH
Instruments Model 611D. A carbon glass coated with a thin polymer
film was used as the working electrode and Ag/Ag+ electrode as the
reference electrode, while 0.1 M tetrabutylammonium hexafluoro-
phosphate (Bu4NPF6) in acetonitrile was the electrolyte. CV curves
were calibrated using ferrocence as the standard, whose oxidation
potential is set at −4.8 eV with respect to zero vacuum level. The
HOMO energy levels were obtained from the equation HOMO =
onset
onset
−(Eox
− E(ferrocene)
+ 4.8) eV. The LUMO levels of polymer
were obtained from the equation LUMO = −(Ered onset − E(ferrocene)
+ 4.8) eV.
onset
OFET Fabrication. An n-type heavily doped Si wafer with a SiO2
layer of 300 nm and a capacitance of 11 nF/cm2 was used as the gate
electrode and dielectric layer. Thin films (40−60 nm in thickness) of
polymers were deposited on octadecyltrichlorosilane (ODTS)-treated
SiO2/Si substrates by spin-coating their o-dichlorobenzene solutions
(5 mg/mL). Then, the thin films were annealed at different
temperatures (150 or 250 °C) for 30 min. Gold source and drain
contacts (40 nm in thickness) were deposited by vacuum evaporation
on the organic layer through a shadow mask, affording a bottom-gate,
top-contact device configuration. Electrical measurements of OTFT
devices were carried out at room temperature in air using a 4156C,
Agilent Technologies. The field-effect mobility was calculated in the
saturation regime by using the equation Ids = (μWCi/2L)(Vg − Vt)2,
where Ids is the drain-source current, μ is the field-effect mobility, W is
the channel width (1 mm), L is the channel length (100 μm), Ci is the
capacitance per unit area of the gate dielectric layer, Vg is the gate
voltage, and Vt is threshold voltage.
PSCs Fabrication. ITO/Glass substrates were ultrasonically
cleaned sequentially in detergent, water, acetone and iso-propanol
(IPA). The cleaned substrates were covered by a 30 nm thick layer of
PEDOT:PSS (Clevios P provided by Stark) by spin coating . After
annealing in a glovebox at 150 °C for 30 min, the samples were cooled
to room temperature. Polymers were dissolved in o-dichlorobenzene
(ODCB), and then PC71BM (purchased from Nano-C) was added.
The solution was then heated at 80 °C and stirred overnight at the
same temperature. Prior to deposition, the solution were filtered (0.45
μm filters). The solution of polymer:PC71BM was then spin coated to
form the active layer. The cathode made of calcium (35 nm thick) and
aluminum (100 nm thick) was sequentially evaporated through a
shadow mask under high vacuum (<10−6 Torr). Each sample consists
of 4 independent pixels defined by an active area of 0.04 cm2. Finally,
the devices were encapsulated and characterized in air.
Synthesis of Sn-IDTT. To a THF (30 mL) solution of Br-IDTT
(0.85 g, 0.63 mmol) was added a hexane solution of n-BuLi (2.5M,
1.58 mmol) dropwise at −78 °C. The mixture was stirred at this
temperature for 1 h and a THF solution of chlorotrimethylstannane
(1.0 M, 1.89 mmol) was then introduced dropwise. It was quenched
with water (50 mL) and extracted with ether (50 mL × 3). The
collected organic layer was dried over MgSO4 and the solvent was
removed in vacuo. The residue was recrystallized from hexane to give a
Synthesis of Compound 1. A mixture of diethyl 2,5-
dibromoterephthalate (6.05 g, 15.9 mmol), 2-(tributylstannyl)thieno-
[3,2-b]thiophene (15.71 g, 36.6 mmol), Pd(PPh3)4 (0.74 g, 0.6 mmol),
and degassed toluene (80 mL) was heated to 130 °C under nitrogen
atmosphere for 16 h. The reaction mixture was poured into water (150
mL) and extracted with ethyl acetate (300 mL × 3). The combined
organic layer was dried over MgSO4 and the solvent was removed in
1
brown solid Sn-IDTT(0.52 g, 54%): H NMR (CDCl3, 300 MHz,
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Article
ppm) δ 0.37 (s, 18 H), 0.87 (t, J = 6.6 Hz, 12 H), 1.27−1.41 (m, 40
H), 1.69−1.78 (m, 8 H), 3.89(t, J = 6.6 Hz, 8 H), 6.79 (d, J = 8.7 Hz, 8
H), 7.19 (d, J = 8.7 Hz, 8 H), 7.30 (s, 2 H), 7.42 (s, 2 H); 13C NMR
(CDCl3, 100 MHz, ppm) δ −8.1, 14.1, 22.6, 26.1, 29.2, 29.3, 29.3,
31.8, 62.1, 67.9, 114.2, 116.6, 127.4, 129.3, 135.2, 136.1, 139.1, 140.3,
143.0, 143.8. 145.8. 153.3. 158.1.
three days. The crude polymer was dissolved in hot THF and the
residual Pd catalyst and Sn metal in the THF solution was removed by
Pd−thiol gel and Pd−TAAcOH (Silicycle Inc.). After filtration and
removal of the solvent, the polymer was redissolved in THF and
reprecipitated by methanol. The resultant polymer was collected by
filtration and dried under vacuum for 1 day to give a dark-purple fiber-
like solid (170 mg, 89%, Mn = 31300, PDI = 2.04): δ 1H NMR
(CDCl3, 300 MHz) δ 0.87 (br, 15 H), 1.27−1.43 (m, 50 H), 1.75 (br,
10 H), 3.92 (br, 10 H), 6.84 (br, 8 H), 7.19 (br, 8 H), 7.50 (br, 2 H),
8.52 (br, 2 H).
Synthesis of PIDTTBT. To a 50 mL round-bottom flask were
introduced Sn-IDTT (200 mg, 0.131 mmol), 4,7-dibromo-2,1,3-
benzothiadiazole (38.5 mg, 0.131 mmol), Pd2(dba)3 (4.8 mg, 0.005
mmol), tri(o-tolyl)phosphine (12.8 mg, 0.042 mmol), and dry
chlorobenzene (7 mL). The mixture was bubbled with nitrogen for
10 min at room temperature. The reaction was then carried out in a
microwave reactor under 270 W for 50 min. In order to end-cap the
resultant polymer, tributyl(thiophen-2-yl)stannane (24.4 mg, 0.059
mmol) was added to the mixture, and the microwave reaction was
continued for 10 min under 270 W. Subsequent to tributyl(thiophen-
2-yl)stannane, another end-capping reagent, 2-bromothiophene (11.6
mg, 0.063 mmol), was added and the reaction was continued for
another 10 min under otherwise identical conditions. The mixture was
then added into methanol dropwise. The precipitate was collected by
filtration and washed by Soxhlet extraction with acetone and hexane
sequentially for 3 days. The crude polymer was dissolved in hot THF
and the residual Pd catalyst and Sn metal in the THF solution was
removed by Pd−thiol gel and Pd-TAAcOH (Silicycle Inc.). After
filtration and removal of the solvent, the polymer was redissolved in
THF and reprecipitated by methanol. The resultant polymer was
collected by filtration and dried under vacuum for 1 day to afford a
dark-purple fiber-like solid (150 mg, 84%, Mn = 16600, PDI = 1.70):
1H NMR (CDCl3, 300 MHz) δ 0.87 (br, 12 H), 1.26−1.42 (br, 40 H),
1.74 (br, 8 H), 3.91 (br, 8 H), 6.84 (br, 8 H), 7.19 (br, 8 H), 7.35−
7.55 (m, 2 H), 7.75 (br, 2 H), 8.57 (br, 2 H).
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Computational details, thermogravimetric analysis, and H and
13C NMR spectra of the new compounds and copolymers. This
material is available free of charge via the Internet at http://
pubs.acs.org/.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yjcheng@mail.nctu.edu.tw.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Council and the “ATU
Program” of the Ministry of Education, Taiwan, for financial
support. We are also grateful to the National Center for High-
performance Computing (NCHC) in Taiwan for computer
time and facilities.
Synthesis of PIDTTFBT. To a 50 mL round-bottom flask were
introduced Sn-IDTT (200 mg, 0.131 mmol), 4,7-dibromo-5,6-
difluoro-2,1,3-benzothiadiazole (43.2 mg, 0.131 mmol), Pd2(dba)3
(4.8 mg, 0.005 mmol), tri(o-tolyl)phosphine (12.8 mg, 0.042
mmol), and dry chlorobenzene (7 mL). The mixture was bubbled
with nitrogen for 10 min at room temperature. The reaction was then
carried out in a microwave reactor under 270 W for 50 min. In order
to end-cap the resultant polymer, tributyl(thiophen-2-yl)stannane
(24.4 mg, 0.059 mmol) was added to the mixture, and the microwave
reaction was continued for 10 min under 270 W. Subsequent to
tributyl(thiophen-2-yl)stannane, another end-capping reagent, 2-
bromothiophene (11.6 mg, 0.063 mmol) was added and the reaction
was continued for another 10 min under otherwise identical
conditions. The mixture was then added into methanol dropwise.
The precipitate was collected by filtration and washed by Soxhlet
extraction with acetone and hexane sequentially for three days. The
crude polymer was dissolved in hot THF and the residual Pd catalyst
and Sn metal in the THF solution was removed by Pd−thiol gel and
Pd−TAAcOH (Silicycle Inc.). After filtration and removal of the
solvent, the polymer was redissolved in THF and reprecipitated by
methanol. The resultant polymer was collected by filtration and dried
under vacuum for 1 day to afford a give a dark-purple fiber-like solid
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