Angular-Shaped 4,9-Dialkyl α- And β-Naphthodithiophene-Based Donor-Acceptor Copolymers: Investigation of Isomeric Structural Effects on Molecular Properties and Performance of Field-Effect Transistors and Photovoltaics

Article abstract of DOI:10.1002/adfm.201502338

Two angular-shaped 4,9-didodecyl α-aNDT and 4,9-didodecyl β-aNDT isomeric structures have been regiospecifically designed and synthesized. The distannylated α-aNDT and β-aNDT monomers are copolymerized with the Br-DTNT monomer by the Stille coupling to furnish two isomeric copolymers, PαNDTDTNT and PβNDTDTNT, respectively. The geometric shape and coplanarity of the isomeric α-aNDT and β-aNDT segments in the polymers play a decisive role in determining their macroscopic device performance. Theoretical calculations show that PαNDTDTNT possesses more linear polymeric backbone and higher coplanarity than PβNDTDTNT. The less curved conjugated main chain facilitates stronger intermolecular π-π interactions, resulting in more redshifted absorption spectra of PαNDTDTNT in both solution and thin film compared to the PβNDTDTNT counterpart. 2D wide-angle X-ray diffraction analysis reveals that PαNDTDTNT has more ordered π-stacking and lamellar stacking than PβNDTDTNT as a result of the lesser curvature of the PαNDTDTNT backbone. Consistently, PαNDTDTNT exhibits a greater field effect transistor hole mobility of 0.214 cm² V^-1 s^-1 than PβNDTDTNT with a mobility of 0.038 cm² V^-1 s^-1. More significantly, the solar cell device incorporating the PαNDTDTNT:PC₇₁BM blend delivers a superior power conversion efficiency (PCE) of 8.01% that outperforms the PβNDTDTNT:PC₇₁BM-based device with a moderate PCE of 3.6%. Two new 4,9-dialkyl α- and β-naphthodithiophene-based D-A copolymers, PαNDTDTNT and PβNDTDTNT, are presented. With the better ordered structures in the solid state, PαNDTDTNT exhibits a greater field-effect transistor hole mobility of 0.214 cm² V^-1 s^-1 and a superior solar cell efficiency of 8.01% than PβNDTDTNT with a mobility of 0.038 cm² V^-1 s^-1 and a PCE of 3.6%.

Full text of DOI:10.1002/adfm.201502338

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Angular-Shaped 4,9-Dialkyl α- and β-Naphthodithiophene-
Based Donor–Acceptor Copolymers: Investigation of
Isomeric Structural Effects on Molecular Properties and
Performance of Field-Effect Transistors and Photovoltaics
Sheng-Wen Cheng, De-Yang Chiou, Che-En Tsai, Wei-Wei Liang, Yu-Ying Lai, Jhih-Yang Hsu,
Chain-Shu Hsu, Itaru Osaka, Kazuo Takimiya,* and Yen-Ju Cheng*
solution-processability.[1] One of the
Two angular-shaped 4,9-didodecyl α-aNDT and 4,9-didodecyl β-aNDT
isomeric structures have been regiospecifically designed and synthesized.
The distannylated α-aNDT and β-aNDT monomers are copolymerized
with the Br-DTNT monomer by the Stille coupling to furnish two isomeric
copolymers, PαNDTDTNT and PβNDTDTNT, respectively. The geometric
shape and coplanarity of the isomeric α-aNDT and β-aNDT segments in
the polymers play a decisive role in determining their macroscopic device
performance. Theoretical calculations show that PαNDTDTNT possesses
more linear polymeric backbone and higher coplanarity than PβNDTDTNT.
The less curved conjugated main chain facilitates stronger intermolecular π–π
interactions, resulting in more redshifted absorption spectra of PαNDTDTNT
in both solution and thin film compared to the PβNDTDTNT counterpart.
2D wide-angle X-ray diffraction analysis reveals that PαNDTDTNT has more
ordered π-stacking and lamellar stacking than PβNDTDTNT as a result of the
lesser curvature of the PαNDTDTNT backbone. Consistently, PαNDTDTNT
exhibits a greater field effect transistor hole mobility of 0.214 cm2 V−1 s−1
than PβNDTDTNT with a mobility of 0.038 cm2 V−1 s−1. More significantly,
the solar cell device incorporating the PαNDTDTNT:PC71BM blend delivers
a superior power conversion efficiency (PCE) of 8.01% that outperforms the
PβNDTDTNT:PC71BM-based device with a moderate PCE of 3.6%.
important criteria of acquiring high-
performance-conjugated polymers is to
induce strong π–π stacking and ordered
structure that facilitate charge transpor-
tation between intermolecular polymer
chains.[2] Müllen and co-workers dem-
onstrated that the backbone curvature of
the polymers plays an important role in
determining molecular packing and their
charge carrier mobilities.[3] In general,
the curvature of backbone is governed by
the molecular geometry and symmetry
of the conjugated segments in the poly-
mers. The polymer with wavy backbone
results in lesser ordered packing structure
and thus lower mobility. Recently, Hou
and co-workers demonstrated that the
benzodithiophene (BDT)-based polymer
with more linear conformation results
in more ordered interchain packing in
thin film and better photovoltaic perfor-
mance.[4] Similar observations were also
found in the isoindigo-based D-A copoly-
mers reported by Pei and co-workers.[5] In
addition to the main-chain configuration,
side-chain engineering also has an equally
1. Introduction
important impact. The placement of aliphatic side chains on a
polymer backbone also significantly influences the backbone
conformations as well as interchain interactions which in turn
determine the molecular packing, crystallinity, and thereby
charge transportation properties.[6]
Organic photovoltaics (OPVs) and organic field effect tran-
sistors (OFETs) have attracted much interest over the past
decade. Conjugated polymers are an important class of organic
materials in terms of tunable molecular properties and easy
Despite that numerous π-conjugated electron-rich donors
and electron-deficient acceptors have been developed, only a
few donor building blocks such as BDT have successfully led
to the high-efficiency donor–acceptor-conjugated copolymers
for OPVs applications.[7] Along with the widely used tricyclic
BDT that is the shortest member among the acenedithiophene
family, an extended tetracyclic naphthodithiophene (NDT),
where two thiophene units are fused with a central naphtha-
lene unit, emerges as a promising donor building block.[8]
Depending on the structural geometry, the NDT derivatives
are primarily categorized into linear-shaped NDT (lNDT)[9]
and angular-shaped NDT (aNDT) isomers.[10] The aNDT can
S.-W. Cheng, D.-Y. Chiou, C.-E. Tsai, W.-W. Liang,
Dr. Y.-Y. Lai, J.-Y. Hsu, Prof. C.-S. Hsu, Prof. Y.-J. Cheng
Department of Applied Chemistry
National Chiao Tung University
1001 University Road Hsin-Chu 30010, Taiwan
E-mail: yjcheng@mail.nctu.edu.tw
Dr. I. Osaka, Prof. K. Takimiya
Emergent Molecular Function Research Group
RIKEN Center for Emergent Matter Science
Wako, Saitama 351-0198, Japan
E-mail: takimiya@riken.jp
DOI: 10.1002/adfm.201502338
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be further classified into another two isomeric structures
denoted as α-aNDT and β-aNDT where the sulfur atoms of
the thiophene units are attached on the α- and β-position of
the central naphthalene unit, respectively. Takimiya et al. have
demonstrated that the α-aNDT-based copolymer has more
linear polymer backbone (pseudostraight) than the corre-
sponding lNDT-based counterpart,[10d] thereby resulting in the
more ordered crystalline structure and higher mobility. To fully
exploit the aNDT-based polymers, incorporation of aliphatic
side chains onto the aNDT units is essential to improve solu-
bility of the resultant copolymers. Takimiya et al. and Osaka
et al. first reported the synthesis of 5,10-dialkyl α-aNDT[11] unit
and its corresponding donor–acceptor copolymer with superior
solar cell performance.[8c]
On account of the aforementioned considerations, it is of
great interest to design new aNDT derivatives by implementing
molecular engineering in two aspects: (1) shifting the aliphatic
side chains from the outer 5,10-positions to the inner 4,9-posi-
tions, (2) changing the isomeric main chain structure from
the α-form to the β-form. Cheng et al. developed a facile syn-
thetic strategy that can regiospecifically synthesize angular
4,9-dialkylated α-aNDT as well as 4,9-dialkylated β-aNDT.[12]
Two isomeric dithienyl-ene-diyne intermediates undergo base-
induced 6π-cyclizations to construct the central naphthalene
cores of the 4,9-didodecyl-α-aNDT and 4,9-didodecyl-β-aNDT,
respectively (Figure 1).
with an ideal system to investigate the electronic and steric
influences on the microscopic properties such as absorption,
electrochemical, polymeric curvature, and molecular packing
that ultimately govern the macroscopic characteristics of
OFETs and OPVs.
2. Results and Discussion
2.1. Synthesis and Thermal Properties of the Polymers
Synthetic routes of the monomers Sn-α-aNDT and Sn-β-aNDT
are shown in Scheme 1. Olefination of 3-bromothiophene-
2-carbaldehyde (1) and 2-bromothiophene-3-carbaldehyde (4)
by McMurry coupling reactions furnished the (E)-1,2-bis(3-bro-
mothiophen-2-yl)ethene (2) and (E)-1,2-bis(2-bromothiophen-
3-yl)ethene (5), respectively. The dithiophenyl-ene-diyne (3) and
(6) were synthesized by the Sonogashira coupling reactions
of (2) and (5) with tetradec-1-yne in good yields, respectively.
The 1,8-Diazabicycloundec-7-ene (DBU) induced tandem
6π-cyclizations of (3) and (6) resulted in the formation of the
central naphthalene units with the dodecyl groups regiospe-
cifically at the 4,9-positions of the α-aNDT and β-aNDT units.
Subsequently, the Sn-α-aNDT and Sn-β-aNDT monomers were
obtained by the lithiations of α-aNDT and β-aNDT followed by
quenching with trimethyltin chloride. The two monomers were
copolymerized with the Br-DTNT[13] monomer by the Stille cou-
pling to afford PαNDTDTNT and PβNDTDTNT, respectively
(Scheme 1).
The molecular weight was determined to be Mn = 67.3 kDa
with a polydispersity index (PDI) of 1.3 for PαNDTDTNT, and
Mn = 28.3 kDa with a PDI of 2.1 for PβNDTDTNT by gel per-
meation chromatography (GPC) measurements (Table 1).
PβNDTDTNT is soluble well in tetrahydrofuran (THF) at room
temperature, whereas PαNDTDTNT is partially soluble in THF
and can be dissolved in warm chlorobenzene (CB) or ortho-
dichlorobenzene (oDCB), implying that PαNDTDTNT has
stronger intermolecular interactions. The thermal properties
of the polymers were evaluated by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC). Both of the
Compared with the commonly used 2,1,3-benzothiadia-
zole (BT), naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT),[13]
a
doubly BT-fused heterocycle, is a superior acceptor due to its
stronger electron-withdrawing ability and the more π-extended
structure. The centrosymmetric NT with a C2h symmetry can
lead to more straight backbone when combined with cen-
trosymmetric units, compared to the corresponding polymer
with axisymmetric BT with a C2v symmetry, resulting in
better polymer chain ordering.[13c] In this research, the two
isomeric 4,9-didodecyl α-aNDT and 4,9-didodecyl β-aNDT
monomers were copolymerized with an NT-containing
monomer, dithienyl naphthobisthiadiazole (DTNT), to gen-
erate PαNDTDTNT and PβNDTDTNT, respectively. The two
isomeric α-aNDT- and β-aNDT-based copolymers provide us
Figure 1. The chemical structures of α-aNDT, β-aNDT, PαNDTDTNT, and PβNDTDTNT copolymers.
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Scheme 1. Synthetic routes for α-aNDT and β-aNDT and their corresponding copolymers.
polymers showed sufficiently high decomposition temperatures
(Td) of 446 and 450 °C, respectively (Figure 2). According to the
DSC analysis, PαNDTDTNT did not show obvious melting
point (Tm) below 350 °C, while PβNDTDTNT exhibited a Tm at
gradually hypsochromically shifted, suggesting that the strong
polymeric interchain interactions occurring at room tempera-
ture are attenuated at high temperatures as a result of increased
thermal motion around the polymer main chains. Therefore,
the higher-temperature spectra can truly reflect the intrinsic
optical transitions of a single polymer chain. The λmax (580 nm)
of PαNDTDTNT at 140 °C is more redshifted than that of
314 °C during heating and a crystallization point (T ) at 292 °C
c
during cooling (Figure 3), which implies that PαNDTDTNT is
structurally more rigid than PβNDTDTNT.
2.2. UV–Vis Absorption Spectroscopy
The UV–vis absorption spectra of the polymers are shown in
Figure 4. Both of the polymers showed two distinct bands in
the absorption spectra. Compared to PβNDTDTNT showing
the absorption maxima (λmax) at 595 nm in oDCB solution
(Table 2 and Figure 4a), PαNDTDTNT exhibited broader
absorption bands and a bathochromic shift of λmax at 645 nm.
The temperature-dependent absorption spectra were monitored
at T = 25, 40, 60, 80, 100, 120, and 140 °C (Figure 5). As the
temperature increases, the absorbance of two polymers was
Table 1. Molecular weights and thermal properties of the polymers.
Polymer
Mn
Mw
PDI
Td
Tg
Tm
[kDa]
[kDa]
[°C]
[°C]
[°C]
67.3
28.3
86.4
60.5
1.3
2.1
446
450
122
PαNDTDTNT
PβNDTDTNT
Figure 2. Thermogravimetric analysis of PαNDTDTNT and PβNDTDTNT
314
at a ramping rate of 10 °C min−1
.
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Figure 3. Differential scanning calorimetry of a) PαNDTDTNT and b) PβNDTDTNT at a ramping rate of 10 °C min−1
.
PβNDTDTNT (570 nm), suggesting that the αNDT segment
induces more effective conjugation than the βNDT moiety.
From 25 to 140 °C, λmax of PαNDTDTNT and PβNDTDTNT
is blueshifted by 65 and 25 nm, respectively, implying that
PαNDTDTNT has stronger aggregation in solution.
oxidative and reductive scans, which are important prerequisites
for p-type organic semiconductors. The HOMO energy levels
were estimated to be −5.40 eV for PαNDTDTNT and −5.46 eV
for PβNDTDTNT. On the other hand, the LUMO energy
levels are approximately located at −3.64 eV for PαNDTDTNT
and −3.64 eV for PβNDTDTNT, which are higher than the
LUMO level of the PC71BM (−3.8 eV) to ensure energetically
favorable electron transfer. The Gaussian09 suite was applied
to understand the molecular orbital properties of the polymeric
π-electron systems. Two simplified trimer model compounds,
denoted as TαNDTDTNT and TβNDTDTNT, are used for the
calculations (Figure 7). All the aliphatic side chains are replaced
by methyl groups. Quantum chemical calculations show that
the HOMOs are homogeneously delocalized along the polymer
chain, and the LUMOs are mostly localized on the NT acceptor
region, which indicates evident intramolecular charge transfer
between the donor and the acceptor upon excitation. The
calculated HOMO and LUMO energy levels are −5.00 and
−3.05 eV for TαNDTDTNT and −4.93 and −3.04 eV for
TβNDTDTNT, respectively.
In the thin film absorption spectra (Figure 4b), PαNDTDTNT
exhibited a λmax at 673 nm which is 60 nm bathochromic shift
from that of PβNDTDTNT (613 nm). The optical band gaps
(Egopt) calculated from the absorption onset of the solid state
spectra were determined to be 1.58 eV for PαNDTDTNT (the
onset at 786 nm) and 1.65 eV for PβNDTDTNT (the onset at
753 nm). Also note that PαNDTDTNT showed a pronounced
vibronic shoulder in the longer wavelengths compared to
PβNDTDTNT. The difference presumably results from the
more linear polymer backbone of PαNDTDTNT to facilitate
π-stacking in solid state, which will be discussed in the fol-
lowing sections. The broader absorption of PαNDTDTNT than
that of PβNDTDTNT could be a beneficial factor for better
photovoltaic properties.
2.3. Electrochemical Properties
2.4. OFET Characteristics
Cyclic voltammetry (CV) was carried out to determine the
highest ocuppied molecular orbital and lowest unocuppied
molecular orbital (HOMO/LUMO) levels and the electrochem-
ical band gaps of the polymers (Table 2 and Figure 6). The
two polymers showed stable and reversible processes in the
The OFET mobilities of the polymers were measured by
the devices using
a bottom-gate/bottom-contact configu-
ration with evaporated gold source/drain electrodes. The
polymer films were spin-coated from the CB solutions, which
Figure 4. Normalized UV–vis absorption spectra of PαNDTDTNT, and PβNDTDTNT in a) oDCB solutions and b) the solid states.
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Table 2. Optical and electrochemical properties.
opt
elect
Polymer
Eg
EHOMO
ELUMO
Eg
UV–vis λmax
[eV]
[eV]
[eV]
[eV]
[nm]
Film
673
610
oDCB at 25/140 °C
645/580
595/570
1.58
1.65
1.76
1.82
PαNDTDTNT
PβNDTDTNT
−5.40
−5.46
−3.64
−3.64
were subsequently annealed at 120 or 200 °C. The OFET devices
used SiO2 as gate dielectric whose surfaces were modified
with octadecyltrichlorosilane (ODTS) to form a self-assembled
monolayer (SAM). All of the output and transfer plots of the
devices showed typical p-type OFET characteristics (Figure 8).
The hole mobilities listed in Table 3 with on/off ratios were cal-
culated from the transfer characteristics of the devices in the
saturation regime.
After annealed at 120 °C, PαNDTDTNT and PβNDTDTNT
device exhibited similar hole mobilities of 0.019 cm2 V−1 s−1
and 0.013 cm2 V−1 s−1, respectively. However, with thermal
annealing at 200 °C, the mobility of PβNDTDTNT is slightly
improved to 0.038 cm2 V−1 s−1, while PαNDTDTNT showed a
substantial enhancement of the hole mobility reaching 0.214
cm2 V−1 s−1 with an on–off ratio of 7.6 × 105. The high mobility
of PαNDTDTNT is presumably due to the stronger π–π stacking
of the polymer chains induced by the annealing process.
the polymers exhibited (h00) peaks up to the third order at the
small-angle region, indicating the highly ordered structures.
PαNDTDTNT and PβNDTDTNT showed (100) peaks at 2θ of
3.80° and 3.88°, respectively, corresponding to d-spacings of
23.2 and 22.6 Å which are assigned to the backbone-to-backbone
lamellar distances associated with the length of the lateral side
chains. The similar distances of the conjugated polymer back-
bones are due to the fact that PαNDTDTNT and PβNDTDTNT
possess the identical side chains that fill the periphery of the
stacked backbone. On the other hand, at wide-angle region,
PαNDTDTNT showed a pair of distinct diffraction arcs and a
sharp (010) peak at 2θ of 25.0° which corresponds to the peri-
odical π–π stacking distance (dπ) of 3.56 Å between the facing
conjugated backbones, whereas PβNDTDTNT exhibited halo-
like diffractions and a corresponding broad (010) peak at 2θ of
22.6° with a dπ spacing of 3.93 Å. The π-stacking distance (dπ)
of 3.56 Å for PαNDTDTNT is relatively small compared to the
other high-performance crystalline donor–acceptor copolymers.
The more pronounced diffractions and closer dπ spacing sup-
port that PαNDTDTNT has a more ordered crystalline struc-
ture with stronger π–π stacking than PβNDTDTNT. These
results are in good agreement with the stronger interchain
aggregation of PαNDTDTNT observed in the UV–vis absorp-
tion spectra. Furthermore, considering that enhanced ordering
structures generally facilitate the charge mobility, the structure
analysis is also consistent with the trend of the OFET mobility
for PαNDTDTNT and PβNDTDTNT.
2.5. Polymer Ordering Structures in Solid State
In order to further understand the polymer ordering and the
difference of the mobilities in the OFET devices between
PαNDTDTNT and PβNDTDTNT, the ordering structures in
the solid-state were investigated by 2D wide-angle X-ray diffrac-
tion (2D-WAXRD) studies. The polymers were annealed and
sheared at 250 °C to form the polymer fibers. The 2D-WAXRD
patterns were measured at room temperature with the X-ray
incident beam perpendicular to the shearing direction which is
along the meridian (Figure 9a,b). Both of the polymers exhib-
ited diffraction arcs on the equator and meridian, suggesting
the crystalline nature and the ordered phase of PαNDTDTNT
and PβNDTDTNT. The diffractions of 1D-WAXRD at the
equator as a function of 2θ are shown in Figure 9c. Both of
2.6. Geometrical Effect of α-aNDT/β-aNDT Units on the
Polymer Backbones
Both isomeric α-aNDT and β-aNDT are highly rigid and
coplanar structures. Although PαNDTDTNT and PβNDTDTNT
Figure 5. UV–vis absorption spectra of a) PαNDTDTNT and b) PβNDTDTNT in oDCB solutions at different temperatures.
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angle was determined to be 148° for TαNDTDTNT and 126°
for TβNDTDTNT. This analysis concludes that PαNDTDTNT
possesses a more linear-shaped backbone than PβNDTDTNT,
leading to better π–π stacking and more ordered packing struc-
ture in the thin film. Again, the result is in good agreement
with the absorption spectra, OFET mobilities, and 2D-WAXRD
analysis.
2.7. Quinoidal Structures of α-aNDT and β-aNDT in the
Polymers
There are two mesomeric structures of a conjugated polymer
including aromatic form and quinoidal form. The forma-
tion of quinoidal structures upon π-electron delocalization
along the polymer chains would also play an important role
in determining the polymer conformations. The quinoidal
forms of α-aNDT[14] and β-aNDT segments denoted as q-αNDT
and q-βNDT in the two isomeric polymers are illustrated in
Figure 11. The geometry optimization of q-αNDT and q-βNDT
moieties with two ethyl groups at 4,9-positions for simplicity
was thus performed at the B3LYP/6-31G(d) level of theory.
The results are summarized in Figure 12. The dihedral angles
ϕ1(S)–2(C)–3(C)–4(C) and ϕ5(S)–6(C)–7(C)–8(C) between the
two benzothiophene moieties in q-αNDT are very small (≈0.9°),
indicating that the high coplanarity of αNDT moiety can be
preserved from aromatic to quinoidal structures. However, the
q-βNDT has much larger distortion with the dihedral angles
ϕ1(S)–2(C)–3(C)–4(C) of −9.4° and ϕ5(S)–6(C)–7(C)–8(C)
of −17.7°.
Figure 6. Cyclic voltammetry of PαNDTDTNT and PβNDTDTNT.
have similar structural compositions, it is obvious that the mor-
phology of the two polymers in solid state is rather different.
To further understand the nature of the distinct ordering struc-
tures in the isomeric α- and β-aNDT-based copolymers, we
calculated the optimized backbone geometry of TαNDTDTNT
and TβNDTDTNT model compounds as shown in Figure 10.
Both TαNDTDTNT and TβNDTDTNT have sine-wave shapes
but with different amplitudes. The angles between the hori-
zontal axis of α-/β-aNDT donor units and that of NT units are
used to quantify the difference in the backbone curvature. The
Figure 7. The HOMO and LUMO plots of the model compounds, a) TαNDTDTNT and b) TβNDTDTNT, calculated at the level of B3LYP/6-31G(d).
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Figure 8. a,c) Typical output curves and b,d) transfer plots of the OFET devices based on PαNDTDTNT and PβNDTDTNT with the ODTS-SAM layer,
respectively.
2.8. Coplanarity of α-aNDT and β-aNDT Moieties after Exciton
Dissociation
reduce reorganization energy for improving the charge mobility
of PαNDTDTNT.[15]
The coplanarity of a natural polymer might change significantly
after photoinduced electron transfer to generate a radical–cati-
onic species. With the intention of further investigating the
planarity of α-aNDT and β-aNDT units in the polymers after
exciton dissociation, geometry optimization of two simpli-
fied structures, αNDTDT and βNDTDT, was calculated (the
B3LYP/6-31G(d)) in both neutral and cationic states. The results
are summarized in Figure 13. For αNDTDT, the dihedral angles
ϕ1(S)–2(C)–3(C)–4(C) and ϕ5(S)–6(C)–7(C)–8(C) between the
two benzothiophene moieties are close to 0° regardless of the
neutral (Figure 13a) or cationic (Figure 13b) electronic states,
indicating that the structural planarity of α-aNDT does not
vary noticeably subsequent to the loss of one electron. With
regard to β-aNDT system, the neutral βNDTDT already pos-
sesses slight distortion on the β-aNDT core (Figure 13c). The
distortion is changed to a different manner when βNDTDT is
situated in the radical–cationic state (Figure 13d). Overall, the
computation results strongly suggest that α-aNDT moiety has
more rigid and coplanar structure than β-aNDT, which might
2.9. Photovoltaic Characteristics
The bulk heterojunction devices with an inverted architecture
have better long-term stability than the conventional devices.[16]
The inverted devices were thus fabricated based on the configu-
ration of ITO/ZnO/PFN/polymer:PC71BM/MoO3/Ag, where
the poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-
alt-2,7-(9,9-dioctylfluorene)] (PFN)[17] is a buffer layer. The
performance of conventional devices and inverted devices
without using PFN is shown in Table S1 (Supporting Infor-
mation). The current density–voltage characteristics and the
external quantum efficiency (EQE) spectra of the devices under
simulated 100 mW cm−2 AM 1.5 G illumination are shown in
Figure 14 and all the device parameters are shown in Table 4.
The device using PαNDTDTNT:PC71BM (1:2, in wt%) delivered
a PCE of 3.78% with a Voc of 0.78, a moderate Jsc of 9.31, and a
fill factor (FF) of 52%. By introducing 5 vol% diphenyl sulfide
(DPS), the device efficiency further improved to 6.72% due to
the increased Voc of 0.84, FF of 58.4%, and Jsc of 13.70 mA cm−2.
To make a systematic comparison, the PβNDTDTNT:PC71BM-
based device using identical fabrication conditions (1:2 in
wt% with 5 vol% DPS) showed a PCE of 3.6%, with a Jsc of
8.41 mA cm−2, a Voc of 0.80 V, and an FF of 53.5. The Voc values
of the PαNDTDTNT- and PβNDTDTNT-based devices are sim-
ilar due to the small difference in their EHOMO levels. Although
PαNDTDTNT possesses lower extinction coefficient (Figure S1,
Supporting Information), the device with PαNDTDTNT shows
the much higher Jsc than that with PβNDTDTNT. It is likely
Table 3. P-type OFET characteristics of the polymers.
Polymer
Annealing
[°C]
Vth
[V]
Ion/off
Mobility
[cm2 V−1 s−1
]
120
200
120
200
0.019
0.214
0.013
0.038
PαNDTDTNT
−0.54
−0.89
−6.84
−24.3
3.24 × 104
7.60 × 105
1.98 × 104
6.71 × 107
PβNDTDTNT
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Figure 11. The quinoidal structures q-αNDT and q-βNDT in the polymers
upon π–electron delocalization.
that ultimately determines the Jsc in the solar cells. The SCLC
curves are included in Figure S3 (Supporting Information). The
hole mobility of PαNDTDTNT:PC71BM blend (1:2.2 in wt%) is
estimated to be 2.1 × 10−2 cm2 V−1 s−1 which is an order of mag-
nitude higher than that of PβNDTDTNT:PC71BM blend (1:2.2
in wt%) with 2.2 × 10−3 cm2 V−1 s−1.
To further optimize the device performance, the
PαNDTDTNT:PC71BM device with 1:2.2 in wt% blend and
5 v% DPS delivered superior performance with a higher Jsc of
14.45 mA cm−2, a Voc of 0.78 V, an increased FF of 65.4%, and
an improved PCE of 7.37%. By solvent annealing of the active
layer for 30 min, a highest PCE of 8.01% with a Voc of 0.80 V,
an excellent Jsc of 15.55 mA cm−2, and an FF of 64.4% was
achieved. The corresponding EQEs for solar cells were meas-
ured under illumination of monochromatic light to check the
accuracy of the measurements of the devices and the Jsc calcu-
lated from integration of the EQE spectra with an AM 1.5 G
reference spectrum agrees well with the Jsc obtained from the
J–V measurements.
Figure 9. 2D-WAXD patterns of a) PαNDTDTNT and (b) PβNDTDTNT,
c) 1D-WAXD diffractions at the equator.
attributed to the better film morphology of the active layer for
the charge transportation leading to higher hole mobility of
PαNDTDTNT. The grazing-incidence X-ray diffraction was thus
employed to correlate the device performances with the mor-
phology of the polymer/PC71BM blends (Figure S2, Supporting
Information). The patterns of PαNDTDTNT/PC71BM blend
showed a (200) peak and a sharp (100) peak at 2θ = 3.8° with a
d spacing (dl) of 23.3 Å which corresponds to the lamellar struc-
ture along with side-chain interdigitation, whereas only negli-
gible lamellar peak was observed for PβNDTDTNT/PC71BM.
The results again demonstrate that PαNDTDTNT is able to
form a more ordered structure than PβNDTDTNT even when
they are blended with PC71BM, thereby producing higher Jsc
in the PβNDTDTNT/PC71BM-based device. We also conducted
the space-charge-limited current (SCLC) mobility measure-
ments for the two blend systems to evaluate vertical transport
2.10. Thin Film Morphology and AFM Images
Atomic force microscopy (AFM) was employed to examine the
surface morphology of the blend films for better understanding
the performance enhancement (Figure 15). Compared to the
film without the additive (Figure 15a), the additive-processed
PαNDTDTNT:PC71BM film (1:2, in wt%) with 5 vol% DPS
Figure 10. Optimized polymer backbone configurations of a) TαNDTDTNT and b) TβNDTDTNT.
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Figure 12. Optimized structures and dihedral angles between two benzothiophenes moieties of a) q-αNDT and b) q-βNDT. aϕ = dihedral angle.
(Figure 15b) showed larger separation domains and greater
surface roughness (5.52 vs 14.9 nm), suggesting that the DPS
additive could influence the domains of the active layer to form
the more favorable morphology leading to an increase of Jsc
from 9.31 to 13.70 mA cm−2. On the other hand, although the
PαNDTDTNT:PC71BM blend (1:2.2 in wt%) with 5 vol% DPS
film showed very similar surface roughness to that of the film
treated with 30 min solvent annealing (Figure 15c,d), more pro-
nounced fiber-like nanostructures were observed after solvent
annealing, implying its more favorable morphological property
for efficient charge separation and transporting properties to
reach higher Jsc and PCE.
3. Conclusions
We have successfully developed two isomeric angular
NDT building blocks, 4,9-dialkylated α-aNDT and β-aNDT,
Figure 13. Optimized structures and dihedral angles between two benzothiophene moieties for a) αNDTDT (neutral species), b) αNDTDT (radical–
cationic species), c) βNDTDT (neutral species), and d) βNDTDT (radical–cationic species). aϕ = dihedral angle.
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Figure 14. Current density–voltage characteristics of a) devices 1–6. b) EQE spectra of the devices under illumination of AM 1.5 G, 100 mW cm−2
.
from Sigma-Aldrich, TCI, Alfa Aesar, or Acros and used as received
unless otherwise specified and all solvents were distilled before using.
3-bromothiophene-2-carbaldehyde (1), (E)-1,2-bis(3-bromothiophen-
2-yl)ethene (2), 2-bromothiophene-3-carbaldehyde (4), and (E)-1,2-
bis(2-bromothiophen-3-yl)ethane (5) were synthesized as reported.[12]
Polymerization was operated with a microwave reactor, CEM Discover
System. Polymer Average Molecular weights were determined by GPC
with a Shodex KF-804 using THF as a solvent and calibrated with
polystyrene standards.
Synthesis of (E)-1,2-Bis(3-(tetradec-1-yn-1-yl)thiophen-2-yl)ethene (3) :
To a deoxygenated solution of compound 2 (1.50 g, 4.31 mmol) in THF
(45 mL) and diisopropylamine (45 mL) was added Pd(PPh3)2Cl2 (800
mg, 1.08 mmol, 25 mol%), CuI (196 mg, 1.08 mmol, 25 mol%), PPh3
(282 mg, 1.08 mmol, 25 mol%) and tetradec-1-yne (2.10 g, 10.78 mmol).
The mixture was stirred for 12 h at 80 °C, and water (60 mL) and
hydrochloric acid (1 M, 60 mL) were then added. The resulting mixture
was extracted with ethyl acetate (EA) (60 mL × 3), and the combined
organic layer was dried over anhydrous MgSO4, filtered, and concentrated
in vacuo. The residue was purified by column chromatography on silica
gel (hexane) to give a yellow solid 3 (yield: 95%). 1H NMR (CDCl3,
400 MHz): δ 7.31 (s, 2 H), 7.05 (d, J = 5.2 Hz, 2 H), 6.94 (d, J = 5.2 Hz,
2 H), 2.48 (t, J = 7.0 Hz, 4 H), 1.70–1.60 (m, 4 H), 1.56–1.55 (m, 4 H),
1.40–1.20 (m, 32 H), 0.88 (t, J = 6.8 Hz, 6 H); 13C NMR (CDCl3,
100 MHz): δ 143.85, 130.54, 123.07, 121.81, 121.17, 95.47, 75.07, 31.92,
29.69, 29.68, 29.65, 29.58, 29.56, 29.25, 28.98, 28.82, 22.68, 19.70, 14.11;
MS (EI, C38H56S2): calcd, 576.3823; found, 576.3823.
synthesized by a regiospecific 6π-cyclization of the dithio-
phenyl-ene-diyne precursors. The distannylated α-aNDT and
β-aNDT monomers are copolymerized to afford two isomeric
donor–acceptor copolymers PαNDTDTNT and PβNDTDTNT.
The molecular shape and geometry of the isomeric α-aNDT
and β-aNDT subunits play a pivotal role in governing the con-
formation and packing of the resulting polymers. Theoretical
calculations revealed that PαNDTDTNT has higher coplanarity
and more linear polymer backbone than PβNDTDTNT. Con-
sequently, PαNDTDTNT can assemble into more ordered
π-stacking as well as lamellar stacking in the solid-state evi-
denced by the 2D-WAXD analysis. UV–vis spectroscopy also
showed that PαNDTDTNT exhibited much more redshifted
absorption than PβNDTDTNT both in solution and thin film
because of the stronger intermolecular π−π interactions and
better effective conjugation. Consistently, PαNDTDTNT exhib-
ited a greater FET hole mobility of 0.214 cm2 V−1 s−1 with an
on–off ratio of 7.6 × 105, compared to PβNDTDTNT with a
mobility of 0.038 cm2 V−1 s−1 due to the higher ordering struc-
ture. More importantly, the solar cell devices based on the
PαNDTDTNT:PC71BM blend delivered a superior efficiency of
8.01%.
Synthesis of (E)-1,2-Bis(2-(tetradec-1-yn-1-yl)thiophen-3-yl)ethene (6) : In
a similar manner to preparation of 3, 6 was synthesized from compound
1
5 (1.00 g, 2.86 mmol) (yield: 97%). H NMR (CDCl3, 400 MHz): δ 7.24
4. Experimental Section
(d, J = 5.4 Hz, 2 H), 7.18 (s, 2 H), 7.10 (d, J = 5.4 Hz, 2 H), 2.51 (t, J = 7.0
Hz, 4 H), 1.70–1.60 (m, 4 H), 1.50–1.40 (m, 4 H), 1.40–1.20 (m, 32 H),
0.88 (t, J = 7.0 Hz, 6 H); 13C NMR (CDCl3, 100 MHz): δ 142.12, 125.21,
Materials: Unless otherwise stated, all reactions were carried out on
a vacuum line under N2 atmosphere. All chemicals were purchased
Table 4. Photovoltaic properties of ITO/ZnO/PFN/polymer:PC71BM/MoO3/Ag-based solar cells.
Devices
Polymer
Polymer: PCB71
M
DPS
[v/v,%]
Voc
[V]
Jsc
FF
[%]
PCE
[%]
[w/w]
[mA cm−2
]
1
2
3
4
5
6
1:2
1:2
No
5
0.78
0.84
0.78
0.80
0.84
0.80
9.31
13.70
14.45
15.55
4.64
52.0
58.4
65.4
64.4
43.4
53.5
3.78
6.72
7.37
8.01
1.69
3.60
PαNDTDTNT
PαNDTDTNT
PαNDTDTNT
PαNDTDTNT
PβNDTDTNT
PβNDTDTNT
1:2.2
1:2.2
1:2
5
5a)
No
5
1:2
8.41
a)Solvent annealing for 30 min.
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Figure 15. AFM height images (5.0 µm × 5.0 µm) of the surface of PαNDTDTNT:PC71BM blends a) 1:2 without DPS additives, b) 1:2 with 5 vol% DPS,
c) 1:2.2 with 5 vol% DPS before solvent annealing, and d) 1:2.2 with 5 vol% DPS after 30 min solvent annealing.
124.00, 122.63, 121.07, 99.24, 73.11, 31.90, 29.66, 29.65, 29.62, 29.57,
29.34, 29.20, 29.00, 28.68, 22.67, 19.97, 14.10; MS (EI, C38H56S2): calcd,
576.3823; found, 576.3822.
(0.76 mL, 2.5 M) slowly at −78 °C. The stirring was continued for 30 min
at −78 °C and subsequently for 30 min at RT. After cooling to −78 °C,
trimethyltin chloride (2.17 mL, 1 M) was added in one portion. The
mixture was slowly warmed to RT and stirred for 6 h. It was quenched
with saturated NH4Cl solution (10 mL). The organic layer was separated
and the aqueous layer was extracted with EA (10 mL). The combined
organic extract was washed with brine solution (20 mL × 3) and dried
over anhydrous MgSO4. After filtration, the solvent was removed under
Synthesis of 4,9-Didodecylnaphtho[1,2-b:5,6-b′]dithiophene (α-aNDT):
a deoxygenated N-methyl-2-pyrrolidone (10 mL) solution of
To
compound 3 (1.94 g, 3.36 mmol) was added DBU (0.95 mL, 6.72 mmol).
The resulting mixture was refluxed for 2 d, cooled to room temperature
(RT), diluted with water (100 mL), and extracted with EA (15 mL × 3).
The combined organic layer was washed with brine solution (15 mL) and
dried over anhydrous MgSO4. After filtration, the solvent was removed
under vacuum and the residue was purified by column chromatography
on silica gel (hexane) to give a white solid α-aNDT (yield: 70%). 1H NMR
(CDCl3, 400 MHz): δ 7.91 (s, 2 H), 7.59 (d, J = 5.6 Hz, 2 H), 7.49 (d, J =
5.6 Hz, 2 H), 3.52 (t, J = 8.0 Hz, 4 H), 1.95–1.80 (m, 4 H), 1.65–1.58 (m,
4 H), 1.50–1.20 (m, 32 H), 0.89 (t, J = 6.8 Hz, 6 H); 13C NMR (CDCl3,
100 MHz): δ 137.59, 135.91, 134.25, 128.17, 125.85, 123.71, 123.60,
37.02, 31.93, 31.23, 29.82, 29.70, 29.68, 29.66, 29.36, 22.69, 14.12; MS
(EI, C38H56S2): calcd, 576.3823; found, 576.3827.
Synthesis of 4,9-Didodecylnaphtho[2,1-b:6,5-b′]dithiophene (β-aNDT):
In a similar manner to preparation of α-aNDT, β-aNDT was synthesized
from 6 (2.0 g, 3.47 mmol) (yield: 82%). 1H NMR (CDCl3, 400 MHz):
8.15 (d, J = 5.6 Hz, 2 H), δ 7.94 (s, 2 H), 7.54 (d, J = 5.6 Hz, 2 H), 3.43
(t, J = 8.0 Hz, 4 H), 1.90–1.80 (m, 4 H), 1.60–1.50 (m, 4 H), 1.40–1.20
(m, 32 H), 0.88 (t, J = 6.8 Hz, 6 H); 13C NMR (CDCl3, 100 MHz): δ
138.64, 136.20, 133.66, 128.37, 126.63, 123.48, 122.71, 38.27, 31.91,
30.80, 29.81, 29.67, 29.65, 29.64, 29.63, 29.59, 29.34, 22.68, 14.11; MS
(EI, C38H56S2) calcd, 576.3823; found, 576.3822.
1
vacuum to give a white solid Sn-α-aNDT (yield: 88%). H NMR (CDCl3,
400 MHz): δ 7.90 (s, 2 H), 7.57 (s, 2 H), 3.55 (t, J = 8.0 Hz, 4 H),
1.90–1.80 (m, 4 H), 1.70–1.58 (m, 4 H), 1.50–1.20 (m, 32 H), 0.89 (t,
J = 6.8 Hz, 6 H), 0.49 (s, 18 H); 13C NMR (CDCl3, 100 MHz): δ 139.43,
138.84, 138.82, 136.06, 131.78, 127.62, 122.87, 37.06, 31.94, 31.28,
29.81, 29.72, 29.71, 29.68, 29.67, 29.37, 22.69, 14.12, −8.24; MS (APCI,
C44H72S2Sn2): calcd, 904.3114; found, 904.3145.
Synthesis of 2,7-Bis(trimethylstannyl)-4,9-didodecylnaphtho[2,1-b:6,5-b′]
dithiophene (Sn-β-aNDT): In a similar manner to preparation of Sn-α-
aNDT, Sn-β-aNDT was synthesized from β-aNDT (1.0 g, 1.73 mmol)
1
(yield: 85%). H NMR (CDCl3, 400 MHz): δ 8.18 (s, 2 H), 7.95 (s, 2 H),
3.44 (t, J = 8.0 Hz, 4 H), 1.90–1.80 (m, 4 H), 1.60–1.50 (m, 4 H), 1.40–
1.10 (m, 32 H), 0.89 (t, J = 6.8 Hz, 6 H), 0.48 (s, 18 H); 13C NMR (CDCl3,
100 MHz): δ 143.23, 136.49, 135.69, 135.20, 134.30, 127.87, 122.38,
38.69, 31.92, 31.30, 30.14, 29.76, 29.71, 29.68, 29.64, 29.35, 22.69, 14.12,
−8.25; MS (APCI, C44H72S2Sn2) calcd, 904.3114; found, 904.3131.
SynthesisofPαNDTDTNT:Toa50mLround-bottomflaskwasintroduced
Sn-α-aNDT (84 mg, 0.093 mmol), Br-DTNT-HD (94 mg, 0.093 mmol),
tris(dibenzylideneacetone)dipalladium (3.40 mg, 0.0037 mmol),
tri(2-methylphenyl)phosphine (9.1 mg, 0.03 mmol), and deoxygenated
CB (4 mL). The mixture was then degassed by bubbling nitrogen for
10 min at room temperature. The round-bottom flask was placed into
Synthesis of 2,7-Bis(trimethylstannyl)-4,9-didodecylnaphtho[1,2-b:5,6-
b′]dithiophene (Sn-α-aNDT): To an anhydrous THF (20 mL) solution
of α-aNDT (500 mg, 0.867 mmol) was added n-butyllithium
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the microwave reactor and refluxed for 50 min under 270 W. Then,
tributyl(thiophen-2-yl)stannane (17.3 mg, 0.046 mmol) was added
to the mixture solution and reacted for 10 min under 270 W. Finally,
2-bromothiophene (8.2 mg, 0.050 mmol) was added to the mixture
solution and reacted for 10 min under 270 W. The solution was added
into methanol dropwise. The precipitate was collected by filtration
and washed by Soxhlet extraction with acetone (24 h) and hexane
(24 h) sequentially. The product was redissolved in hot CB (200 mL).
The Pd-thiol gel (Silicycle Inc.) was added to the above CB solution
to remove the residual Pd catalyst at 80 °C for 1 h. After filtration of
solution and removal of the solvent under reduced pressure, the
polymer solution was added into methanol to reprecipitate. The purified
polymer was collected by filtration and dried under vacuum for 1 d to
give a purple black solid (110 mg, yield 82%).
electrodes. An n-type heavily doped Si wafer with an SiO2 layer of 300 nm
and a capacitance of 11 nF cm−2 was used as the gate electrode and
dielectric layer. Gold source and drain contacts (40 nm in thickness)
were deposited by vacuum evaporation after titanium on ODTS-treated
SiO2/Si substrates through the same shadow mask. The gold source
and drain contacts were treated with the pentafluorothiophenol SAM.
The channel width and length of the transistors are 1.00 and 0.100 mm,
respectively. The polymers were dissolved in CB (10 mg mL−1) and spin-
coated at 1000 rpm from a hot solution before being annealed at 200 °C
for 10 min. Electrical measurements of OTFT devices were carried out at
room temperature in vacuum 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, L is the channel
length, Ci is the capacitance per unit area of the gate dielectric layer, Vg is
the gate voltage, and Vt is the threshold voltage.
Synthesis of PβNDTDTNT: To a 50 mL round-bottom flask was
introduced Sn-β-aNDT (88.9 mg, 0.0985 mmol), Br-DTNT-HD
(100.0 mg, 0.0985 mmol), tris(dibenzylideneacetone)dipalladium
(3.61 mg, 0.00394 mmol), tri(2-methylphenyl)phosphine (9.6 mg,
0.0315 mmol), and deoxygenated CB (4.0 mL). The mixture was then
degassed by bubbling nitrogen for 10 min at room temperature. The
round-bottom flask was placed into the microwave reactor and refluxed for
50 min under 270 W. Then, tributyl(thiophen-2-yl)stannane (18.38 mg,
0.0493 mmol) was added to the mixture solution and reacted for 10 min
under 270 W. Finally, 2-bromothiophene (8.67 mg, 0.0532 mmol) was
added to the mixture solution and reacted for 10 min under 270 W. The
solution was added into methanol dropwise. The precipitate was collected
by filtration and washed by Soxhlet extraction with acetone (24 h) and
hexane (24 h) sequentially. The product was redissolved in hot CB (200 mL).
The Pd-thiol gel (Silicycle Inc.) was added to the above CB solution to
remove the residual Pd catalyst at 80 °C for 1 h. After filtration of solution
and removal of the solvent under reduced pressure, the polymer solution
was added into methanol to reprecipitate. The purified polymer was
collected by filtration and dried under vacuum for 1 d to give a purple
black solid (100 mg, yield 71%). 1H NMR (CDCl3, 400 MHz): δ 8.80–8.00
(br, 4 H), 7.90–7.30 (br, 4 H), 3.60–2.60 (br, 8 H), 2.10–0.40 (br, 108 H).
General Measurement and Characterization: 1H and 13C NMR spectra
were measured using a Varian 400 MHz instrument spectrometer and
obtained in deuterated chloroform (CDCl3) with tetramethylsilane (TMS)
as internal reference unless otherwise stated, and chemical shifts (δ) are
reported in parts per million. TGA was recorded on a Perkin Elmer Pyris
under nitrogen atmosphere at a heating rate of 10 °C min−1. DSC was
measured on a TA Q200 Instrument under nitrogen atmosphere at a
heating rate of 10 °C min−1. Absorption spectra were taken on an HP8453
UV–vis spectrophotometer. Electrochemical CV was conducted on a CH
instruments electrochemical analyzer. A carbon glass was used as the
working electrode, Pt wire was used as the counter electrode, and Ag/
Ag+ electrode (0.01 M AgNO3, 0.1 M TBAP in acetonitrile) was used as the
reference electrode in a solution of dichloromethane with 0.1 M TBAPF6
(tetrabutylammonium hexafluorophosphate) at 80 mV s−1. CV curves
were calibrated using ferrocene as the standard, and the E1/2 of ferrocene
is 0.09 V calculated as the average of the two peak maxima Epa and Epc.
OPV Device Fabrication and Characterization: Indium tin oxide (ITO)/
glass substrates were ultrasonically cleaned sequentially in detergent,
water, acetone, and isopropyl alcohol. Then, the ZnO precursor solution
was spin-coated onto ITO-coated glass and followed by thermal
annealing at 170 °C in air for 15 min to crystallize the film (thickness
≈50 nm). Subsequently, the PFN interlayer material was dissolved in
methanol in the presence of a small amount of acetic acid (2 µL mL−1
)
and its solution (concentration, 2 mg mL−1) was spin-coated on top of
the ZnO layer. Polymers were dissolved in o-dichlorobenzene (ODCB)
and PC71BM (purchased from UR) was added to reach the desired ratio.
The solution was stirred at 80 °C overnight and then heated at 110 °C
for 1 h before spin-coating. The photoactive layers were then spin-coated
in a glovebox. The polymer/ODCB concentration is 10 mg mL−1 and the
spin-coating speed is 600 rpm. An MoO3 layer (thickness = ≈7 nm) and
silver top anode (thickness = ≈150 nm) were then thermally evaporated
through a shadow mask under high vacuum (<1 × 10−6 Torr) to complete
the inverted devices. Each device is constituted of four pixels defined by
an active area of 0.04 cm2. Finally, the J–V curves were measured in air
under AM 1.5 G spectrum from a solar simulator.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors thank the Ministry of Science and Technology, the Ministry
of Education, and the Center for Interdisciplinary Science (CIS) of the
National Chiao Tung University, Taiwan, for financial support and the
National Center of High-Performance Computing (NCHC) in Taiwan
for computer time and facilities. They also thank Prof. Chien-Lung
Wang at NCTU for help with the 2D-WAXRD measurements. Y.J.C.
acknowledges support from the Golden–Jade fellowship of the Kenda
Foundation, Taiwan. This work was partially supported by Grants-in-Aid
for Scientific Research (24685030 and 15H02196) from Japan Society for
the Promotion of Science.
The HOMO energy levels were obtained from the equation HOMO
onset
= −(Eox
− E1/2(ferrocene) + 4.8) eV and the LUMO energy levels were
onset
obtained from the equation LUMO = −(Ered
− E1/2(ferrocene) + 4.8) eV.
Received: June 9, 2015
Revised: July 10, 2015
Published online: September 14, 2015
For 2D-WAXD patterns, Bruker APEX DUO single crystal diffraction was
applied and an INCOATEC 18 kW rotating I microfocus X-ray generator
(Cu-Kα radiation (λ = 0.1542 nm)) attached to an APEX II CCD camera
was used. The exposure time to obtain high-quality patterns was 40 s.
Quantum chemical calculations were performed with the Gaussian09
suite employing the B3LYP density functional in combination with the
6-31G(d) basis set. Geometry optimizations were performed by applying
tight self-consistent field (SCF) and convergence criteria and an ultrafine
integration grid by applying the a geometry optimization method
using an energy-represented direct inversion in the iterative subspace
algorithm (GEDIIS) optimization algorithm. The minimum nature of
each stationary point was confirmed by a frequency analysis.
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OFET Device Fabrication and Characterization: Bottom-gate, bottom-
contact OFETs were fabricated on Si/SiO2 substrates with Ti/Au
©
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Adv. Funct. Mater. 2015, 25, 6131–6143
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6143