Structure-property relationship of D-A type copolymers based on phenanthrene and naphthalene units for organic electronics

Article abstract of DOI:10.1039/c7tc02925j

Four donor-acceptor (D-A) type conjugated polymers (PA1, PA2, PA3 and PA4) based on phenanthrene and naphthalene as the donating units with or without dimethoxy substitution were synthesized for organic field effect transistors (OFETs) and bulk-heterojunction organic photovoltaics (OPVs). Dimethoxy substituents have significant effects on the optical, electrochemical, charge transport and photovoltaic properties depending on the donor-polyaromatic (PA) compounds. The optical band gaps of these PA-based copolymers from the smallest to the largest are as follows: 1.52 eV (1,5-dimethoxy substituted naphthalene (PA4)), 1.59 eV (unsubstituted naphthalene (PA3)), and 1.63 eV (unsubstituted phenanthrene (PA1), and substituted 9,10-dimethoxy phenanthrene (PA2)). While the values vary depending on the compounds, both PA2 and PA4 are found to have higher highest occupied molecular orbital (HOMO) energy levels than those of PA1 and PA3 due to the electron donating nature of dimethoxy substituents. The PA based copolymers without dimethoxy substituents showed highly balanced ambipolar behavior with ～1 cm² V^-1 s^-1, whereas the electron mobility of dimethoxy modified PA (MeOPA) based copolymers was suppressed. The inverted bulk heterojunction OPVs based on PA1 and PA3 exhibited power conversion efficiency (PCE) as high as 5.3% and 5.8%, respectively. The PCEs of PA copolymer-based OPV devices were mainly affected by an increase in the open circuit voltage rather than by the photocurrent or fill factor.

Full text of DOI:10.1039/c7tc02925j

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Materials Chemistry C
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Structure–property relationship of D–A type
copolymers based on phenanthrene and
Cite this:DOI: 10.1039/c7tc02925j
naphthalene units for organic electronics†
Yeong-A Kim,a Minji Kang,a Ye-Jin Jeon,a Kyeongil Hwang,a Yeon-Ju Kim,a
ab
Soo-Young Jang,‡a In-Bok Kim,b Gucheol Kwona and Dong-Yu Kim
*
Four donor–acceptor (D–A) type conjugated polymers (PA1, PA2, PA3 and PA4) based on phenanthrene
and naphthalene as the donating units with or without dimethoxy substitution were synthesized for
organic field effect transistors (OFETs) and bulk-heterojunction organic photovoltaics (OPVs). Dimethoxy
substituents have significant effects on the optical, electrochemical, charge transport and photovoltaic
properties depending on the donor-polyaromatic (PA) compounds. The optical band gaps of these
PA-based copolymers from the smallest to the largest are as follows: 1.52 eV (1,5-dimethoxy substituted
naphthalene (PA4)), 1.59 eV (unsubstituted naphthalene (PA3)), and 1.63 eV (unsubstituted phenanthrene
(PA1), and substituted 9,10-dimethoxy phenanthrene (PA2)). While the values vary depending on the
compounds, both PA2 and PA4 are found to have higher highest occupied molecular orbital (HOMO)
energy levels than those of PA1 and PA3 due to the electron donating nature of dimethoxy substituents.
The PA based copolymers without dimethoxy substituents showed highly balanced ambipolar behavior
with B1 cm2 Vꢀ1
s
ꢀ1, whereas the electron mobility of dimethoxy modified PA (MeOPA) based
Received 29th June 2017,
Accepted 19th September 2017
copolymers was suppressed. The inverted bulk heterojunction OPVs based on PA1 and PA3 exhibited
power conversion efficiency (PCE) as high as 5.3% and 5.8%, respectively. The PCEs of PA copolymer-
based OPV devices were mainly affected by an increase in the open circuit voltage rather than by the
photocurrent or fill factor.
DOI: 10.1039/c7tc02925j
rsc.li/materials-c
affected by many factors such as device architectures, processing
conditions, and choice of electrodes, the importance of molecular
Introduction
Organic electronics including organic field effect transistors design for a polymer structure is evident as a determining factor
(OFETs) and organic photovoltaics (OPVs) using conjugated for high performance.8–11
polymers as organic semiconducting materials have attracted
As semiconducting materials for OFETs, conjugated polymers
wide interest in recent years because of their advantages such are required to have several important properties.12 First, p-orbitals
as low production cost, low weight, mechanical flexibility, and should be well-overlapped through the conjugated backbone of
solution processibility.1–7 Furthermore, conjugated polymers polymers for efficient charge transport. To obtain high carrier
can also be processed easily over a large-area device and their mobilities, both intramolecular charge transport in the conjugated
electronic properties can be easily tuned through diversification backbone and intermolecular charge transport between the adjacent
of the chemical structures. Although the device performance is molecules need to be maximized. In this regard, polymers need
to adopt a coplanar conjugated backbone structure for efficient
intramolecular charge transport and favorable orientation of
polymer chains for strong intermolecular interaction between
a School of Materials Science & Engineering, Gwangju Institute of Science and
the neighboring polymer molecules.13–15 Second, energy levels
Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61002,
of polymers should be matched with the work functions of
electrodes in order to minimize the energy barrier for charge
injection. While many studies of p-type semiconducting materials
have been published reporting high mobilities, research on n-type
and ambipolar materials is relatively lagging.1,16 For n-type or
ambipolar characteristics, a low-lying lowest unoccupied mole-
cular orbital (LUMO) level is necessary for adequate electron
Republic of Korea
b Research Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of
Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61002,
Republic of Korea. E-mail: kimdy@gist.ac.kr
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7tc02925j
‡ Current address: Department of Chemistry and Centre for Plastic Electronics,
Imperial College London, Exhibition Rd, London, SW7 2AZ, UK. E-mail:
s.jang@imperial.ac.uk
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injection. In addition, environmental, electrical, and thermal units and a DPP unit with 2-decyltetradecyl chains as an A unit.
stability is required for the overall operational stability of the The PA compounds which are introduced in this study are as
device.
follows: unsubstituted phenanthrene (for PA1), 9,10-dimethoxy,
Furthermore, for achieving high performance in bulk hetero- shortest alkoxy chain, substituted phenanthrene (for PA2),
junction (BHJ) OPVs, conjugated polymers as donor materials unsubstituted naphthalene (for PA3), and 1,5-dimethoxy sub-
have to be designed to satisfy several requirements. The power stituted naphthalene (for PA4). These PA-based copolymers
conversion efficiency (PCE) of photovoltaics is determined by the were synthesized via Suzuki polymerization, as represented in
open circuit voltage (Voc), the short-circuit current ( Jsc), and the Scheme 2.
fill factor (FF). These factors can be controlled by varying
the chemical structure of polymers.17,18 The value of Voc is mainly
affected by the energy levels of donor materials. The most widely
Results and discussion
used acceptor material for OPVs is [6,6]-phenyl-C71-butyric acid
methyl ester (PC71BM). When PC71BM is employed, Voc increases
are represented in Scheme 1. MO1, MO2, MO3, MO4 and
proportionally as a function of the difference between the highest
The chemical structures of the four (MeO)PA–DPP copolymers
DTDPP were synthesized using the same procedure followed
in the literature, and specific synthetic steps are depicted in
occupied molecular orbital (HOMO) energy level of a polymer and
the LUMO energy level of PC71BM.19–21 The value of Jsc is
influenced by the amount of photon absorption and exciton
dissociation.22,23 Jsc can be improved by lowering the optical
band gap of a polymer which results in a broader absorption in
the solar spectrum.24 In addition, donor materials are required to
have high hole mobility in order to obtain a reasonable FF.25
To achieve high device performance, conjugated polymers
have to be carefully designed in accordance with the above
requirements. Therefore, the HOMO energy level of a polymer
material should be lowered and the optical energy band gap of
the polymer should be lowered in order to obtain high Voc
and Jsc in OPVs.21 To meet these requirements, we reported
donor–acceptor (D–A) type conjugated copolymers with different
solubilizing groups based on 9,10-alkoxy phenanthrene and
diketopyrrolopyrrole (DPP) units, one kind of strong acceptor,
depending on a ‘‘weak donor–strong acceptor design’’ strategy.26–33
Incorporation of phenanthrene units as weak donor units into
conjugated backbones led to a deep HOMO energy level for high
Voc and introducing a kind of strong acceptor, the DPP unit,
induced low band gap properties for high photocurrent.34
Fig. S1 (ESI†).44–47 Poly{phenanthrene-2,7-diyl-alt-3,6-dithiophene-
2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione} (PA1),
poly{9,10-dimethoxyphenanthrene-2,7-diyl-alt-3,6-dithiophene-2-yl-
2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione} (PA2),
poly{naphthalene-2,6-diyl-alt-3,6-dithiophene-2-yl-2,5-di(2-decyl-
tetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione} (PA3) and poly{1,5-
dimethoxynaphthalene-3,7-diyl-alt-3,6-dithiophene-2-yl-2,5-di(2-
decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione} (PA4) were
synthesized via Suzuki polymerization. The resulting polymers
showed good solubility in chloroform and poor solubility in
chlorobenzene, o-dichlorobenzene and toluene. However, PA4
has somewhat lower molecular weight than the other polymers
due to its low solubility. All polymers exhibited excellent thermal
stability with a decomposition temperature (Td) of up to 412, 385,
417 and 409 1C for PA1, PA2, PA3, and PA4, respectively. On DSC
measurement, they did not show any thermal transition between
40 1C and 300 1C (Fig. S2 and S3, ESI†).
Although phenanthrene molecules comprising three-fused
benzene rings have the potential to be a good candidate as
electron donating units, the previously reported device perfor-
mance of (phenanthrene-alt-diketopyrrolopyrrole) PDPP copoly-
mers was poor.35,36 The copolymers with a longer alkoxy side
chain on phenanthrene showed a worse device performance
compared to their counterpart polymers. In addition, the con-
jugated polymers containing DPP units are known to have
limited solubility which could result in low molecular weight
of the polymers in the absence of a long and bulky side chain
due to their strong intermolecular interactions.37–42 In our
previous study, we confirmed that an alkoxy side chain on
phenanthrene should be shorter and an alkyl side chain on
DPP units should be longer in order to improve polymer
solubility without sacrificing the optoelectronic properties.34
Similarly, research about PDPPT–NAP based on dodecyloxy
naphthalene and DPP was reported by Sonar P. et al. PDPPT–
NAP showed p-type characteristics with the highest hole mobility
Scheme 1 Chemical structure of P3 and P4.34
of around 0.0046 cm2 Vꢀ1 sꢀ1 43
.
In this work, we synthesized a series of D–A copolymers
containing four different polyaromatic (PA) compounds as D
Scheme 2 Synthetic scheme and chemical structures of (MeO)PA–DPP
copolymers.
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Theoretical calculations
substituents due to the positive mesomeric effect of methoxy
groups. However, methoxy groups can also weakly withdraw the
electrons through the s bond due to higher electronegativity of
the oxygen atom and induce a fractional positive charge (d+) in
the aromatic carbon, called a negative inductive effect. As can
be seen in Fig. S5 (ESI†), the conjugation system of PA2
containing 9,10-dimethoxyphenanthrene showed a similar
distribution of the HOMO level to those of the unsubstituted
polymer, PA1. When the lone pair electrons are donated owing
to the mesomeric effect, it could be stabilized through the
electron-withdrawing inductive effect by the adjacent oxygen
atom. However, in the case of naphthalene derivatives, PA4,
dimethoxy groups are substituted in opposite directions. Such
placement of two methoxy substituents makes non-pair electrons
from the oxygen atom well delocalized over the p-conjugated
system.48
Dimethoxy substituents on the PA units had a stronger effect
on the HOMO than on the LUMO energy level of copolymers.
A similar trend was observed for (MeO)PA–DPP copolymers
for the LUMO distribution localized on the DPP units. The
calculated HOMO/LUMO energies were ꢀ5.04/ꢀ2.89, ꢀ4.99/
ꢀ2.85, ꢀ5.01/ꢀ2.94, and ꢀ4.82/ꢀ2.79 eV for PA1, PA2, PA3,
and PA4, respectively.
Density functional theory (DFT) was employed for computational
studies of the effect of dimethoxy substitution on molecular
geometries and electronic wave functions of the frontier orbitals
in the four conjugated systems. Computations were performed
using the respective trimers of each copolymer with Gaussian 09
at the B3LYP/6-311G(d,p) level of theory. The optimized geo-
metries of all compounds are shown in Fig. S4 (ESI†).
All (MeO)PA-based copolymers showed a fairly twisted structure
due to a large dihedral angle (j) between PA compounds and
thiophene units. The respective dihedral angles between the
(MeO)PA unit and DPP unit of each polymer were 25.11, 25.11,
24.91, and 25.661 for PA1, PA2, PA3, and PA4, respectively
(Scheme 3).
In Fig. S5 (ESI†), these polymers showed a slight difference in
both HOMO and LUMO distribution. The orbital distributions of
phenanthrene-based polymers, PA1 and PA2, are rather strongly
localized within the DPP units and remain similar regardless of
dimethoxy substituents. Therefore, dimethoxy substituents make
minor contributions to either the HOMO or LUMO of phenan-
threne units. On the other hand, the electron density of PA3 is
relatively localized similar to that of phenanthrene-based polymers,
but the HOMO of PA4 is distributed over the entire conjugated
backbone. The introduction of dimethoxy substituents at the
1,5-position of naphthalene highly affected the HOMO energy levels
of the polymers relative to those of the 9,10-dimethoxy phenan-
threne units, which could be caused by their different mesomeric
(resonance) structures.
Optical properties
Normalized absorption spectra of the (MeO)PA copolymers in
both o-dichlorobenzene solution and thin films are shown
in Fig. 1. Both in solution and in thin films, (MeO)PA–DPP
copolymers have two-band absorption spectra with a weak
intensity band at shorter wavelengths (300–500 nm) attributed
to localized p–p* electron transitions and with relatively strong
absorption at a longer-wavelength (500–800 nm) arising due to
internal charge transfer (ICT) between electron-rich poly-
aromatic units and electron-deficient DPP units. All (MeO)PA
based copolymers showed a significant vibronic peak which
comes from the strong interchain p–p stacking and inter-
molecular packing of polymer molecules even in the solution
state at room temperature. Temperature-dependent absorption
spectra of (MeO)PA copolymers in o-dichlorobenzene were
examined by increasing the temperature from 40 1C to 110 1C
(Fig. S7, ESI†). We observed hypsochromic shifts in absorption
spectra and weaker vibronic peaks at higher temperatures.
However, PA1, PA2, and PA3 showed similar trends in absorp-
tion spectra with intense aggregation even at 110 1C which is
likely due to a strong intermolecular interaction among their
The lone pair electrons of the oxygen atom in the methoxy
substituent can be released toward the PA rings and can be
delocalized through the conjugated system. Thus the electron
density of MeOPA is higher than PA without dimethoxy
polymer chains. By contrast, in PA4, the shoulder peak (lmax
=
740 nm at 40 1C) was suppressed and largely hypsochromically
shifted from 740 nm to 721 nm due to disaggregation of
polymer chains and steric repulsion upon heating.
As shown in Fig. 1b, the absorption spectra of polymer thin
films are broader than those of polymer solutions due to the
strong intermolecular interaction. In addition, the absorption
spectra of PA1, PA2, PA3 and PA4 films have absorption maxima
(lmax) at 705, 702, 724, and 754 nm, respectively while their
optical band gaps that were measured from the absorption onset
were 1.63 eV, 1.63 eV, 1.59 eV, and 1.52 eV, respectively.
Scheme 3 Mesomeric structure of (a) 9,10-dimethoxyphenanthrene and
(b) 1,5-dimethoxynaphthalene.
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For E1/2(ferrocene) = 0.42 V, the HOMO energy levels of PA1,
PA2, PA3, and PA4 are equal to ꢀ5.22, ꢀ5.12, ꢀ5.25, and ꢀ4.92 eV,
respectively. With the exception of PA4, the polymers have low-
lying HOMO energy levels indicating that they could have high Voc
in OPVs because Voc is mainly determined by the difference
between the HOMO energy level of the polymer and the LUMO
energy level of the acceptor. The LUMO energy levels are calculated
from the optical band gap and the HOMO energy level as
ꢀ2.89, ꢀ2.85, ꢀ2.94, and ꢀ2.82 eV for PA1, PA2, PA3, and PA4,
respectively. The difference between the LUMO levels of the
polymers and the LUMO level of PC71BM is sufficient for efficient
exciton dissociation.50,51 PA1 and PA3 with unsubstituted PA
compounds have higher oxidation potential than dimethoxy sub-
stituted PA based copolymers. Two methoxy substituents induce
different electronic effects in phenanthrene and naphthalene.
The same as the meta- and para-substituted alkoxy groups
on benzene have different influences on electronic effects, PA2
and PA4 do not have the same energy levels because of their
different mesomeric structures.52 In the case of 9,10-dimethoxy
phenanthrene, methoxy substituents are in immediate proximity
to each other and they cannot donate their non-bonding
electrons to the conjugated backbone simultaneously. While
the oxygen atom can donate its unpaired electron through the
mesomeric effect to the conjugated backbone, electrons can
also be pulled again by an electron withdrawing inductive
effect due to the high electronegativity of oxygen. Thus, PA2
showed a slightly higher HOMO energy level than PA1 with
a difference of about 0.1 eV. In contrast, the HOMO energy
level of 1,5-dimethoxy substituted naphthalene-based polymer,
PA4, is much higher compared to that of PA3 due to a strong
resonance effect through naphthalene units.
Fig. 1 UV-vis absorption spectra of (MeO)PA–DPP copolymers (a) in
o-dichlorobenzene solution and (b) in films. (c) Schematic energy diagrams
of (MeO)PA–DPP copolymers.
Thin-film microstructure analysis
Interestingly, the absorption spectrum of PA2 was similar
to that of PA1 regardless of the dimethoxy groups at the 9,10-
position of phenanthrene, while PA4 exhibited an optical
behaviour, which was quite different from that of PA3. The
absorption spectrum of PA4 was fairly red-shifted, and PA4 had
the lowest band gap caused by efficient ICT. With the acceptor
strength being equal, 1,5-dimethoxy substituted naphthalene
is possibly the donor with the strongest electron donating
ability among the four samples studied based on band-gap
measurements.49,50
Tapping-mode atomic force microscopy (AFM) analysis was
carried out for the investigation of the influence of (MeO)PA
units on thin film morphologies, and AFM images of each
polymer film on a glass substrate are shown in Fig. S9 (ESI†).
All polymer films were spin-coated and then thermally treated
with a temperature selected to optimize the OFET device
performance. The pristine films of (MeO)PA-based copolymers
formed fine fibrillar structures with root-mean-square (rms)
roughness values of 1.33, 0.52, 0.91, and 2.81 nm for PA1, PA2,
PA3, and PA4, respectively. Thermally annealed polymer films
formed fibrillar aggregates with higher rms roughness, making
the enhancement of intermolecular interactions feasible for
efficient charge transport (Table 1).
Grazing-incidence wide-angle X-ray scattering (GIWAXS)
analysis of neat polymer films was performed to obtain infor-
mation on the order and molecular stacking mode of the
polymer films. The X-ray diffraction (XRD) profiles of the
pristine and thermally annealed films at the optimum tempera-
ture for the OFET devices are shown in Fig. 2. PA1 and PA3
showed (h00) diffraction that is related to lamella d-spacing up to
Electrochemical properties
Cyclic voltammetry (CV) was employed to investigate the electro-
chemical properties of PADPP copolymers. CV was performed
in 0.1 M tetrabutylammonium perchlorate (Bu4NClO4) and
calibrated with ferrocene/ferrocenium as a reference. The HOMO
energy levels of polymers were derived from the following
equation.
EHOMO = [(Eox ꢀ E1/2(ferrocene)) + 4.8] eV
Here, Eox and E1/2(ferrocene) are the onset of oxidation the fourth order peak even in pristine films in the out-of-plane
potential of a polymer and half-wave potential of ferrocene, direction, indicating a highly ordered and crystalline structure
respectively.
with an edge-on orientation. After thermal treatment, (MeO)PA
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Table 1 Photophysical and electrochemical properties
solution
max
film
max
film
a
Polymer
l
(nm)
l
(nm)
l (nm)
onset
Eg (eV)
Eox (V)
HOMOb (eV)
LUMOc (eV)
PA1
PA2
PA3
PA4
705
702
724
754
706
702
728
754
760
760
780
815
1.63
1.63
1.59
1.52
0.85
0.75
0.87
0.55
ꢀ5.22
ꢀ5.12
ꢀ5.25
ꢀ4.92
ꢀ3.59
ꢀ3.49
ꢀ3.66
ꢀ3.40
a
b
c
Calculated from absorption onset of thin film UV-vis spectra. Determined by cyclic voltammetry from the oxidation potential. Determined
from the optical energy band gap and HOMO energy level.
optimized by annealing the polymer films at 150, 200, 250, and
300 1C for 30 min at each temperature. The filed-effect mobilities
of the copolymers were calculated from the saturation regime.
The key parameters for OFET performance, the hole mobility
(mh), the electron mobility (me), and the threshold voltage (Vth) are
summarized in Table 2. The corresponding transfer and output
characteristics of the optimized devices are depicted in Fig. 3
and Fig. S10 (ESI†). All copolymers exhibited ambipolar behavior
with V-shaped transfer curves with reasonably balanced charge
transport properties. The average hole mobilities (mh,ave) of
pristine PA1, PA2, PA3, and PA4 in the saturation regime are
as high as 0.11, 0.09, 0.35 and 0.17 cm2 Vꢀ1 ꢀ1, respectively.
s
After thermal annealing, the hole mobilities (mh,ave) increased to
0.74, 0.53, 1.10, and 0.62 cm2 Vꢀ1 sꢀ1 for PA1, PA2, PA3 and PA4,
respectively.
Fig. 2 The XRD patterns of the pristine (black line) and annealed (red line)
PA1 film (a) and PA2 film (b), PA3 film (c) and PA4 film (d), respectively. The
out of plane and in plane (insets) GIXRD patterns of the four polymers.
This mobility optimization implies that molecular packing
became more ordered resulting in a closer p–p stacking of
the polymer chain, thereby facilitating the charge transport.
Furthermore, the average values of optimal electron mobilities
(me,ave) have also been increased upon annealing to 0.40, 0.09,
0.90, and 0.17 cm2 Vꢀ1 sꢀ1 for PA1, PA2, PA3, and PA4,
respectively. While the unsubstituted PA-based polymers, PA1
and PA3, exhibited reasonably balanced ambipolar charac-
teristics, dimethoxy substituted PA-based polymers showed
hole-dominant ambipolar transport properties accompanied
by a larger threshold voltage. This may be caused by a large
energy barrier for electron injection due to their higher LUMO
energy levels. In addition, both the hole and electron mobilities
of PA1 and PA3 that adopt an edge-on orientation of polymer
chains are higher than those of PA2 and PA4 with the mixed
orientation.
Also, PA2 demonstrated an increase by about 2 orders of
magnitude in both hole and electron mobilities compared to
those of the polymers with a longer alkoxy side chain on
phenanthrene, which were studied previously. It is expected
that this improvement in charge transport properties is due to
the optimized molecular orientation. PA1 demonstrated further
enhanced OFET device performance in the films coated from
their trichloroethylene solution with off-center spin coating
methods.53 Annealed PA1 films at 200 1C showed mh,ave = 1.27
and maximum hole mobilities (mh,max) up to 1.78 cm2 Vꢀ1 sꢀ1
(Fig. S12, ESI†).53,54
polymers exhibited improved molecular ordering with strong
(h00) diffraction peaks at 0.31, 0.30, 0.29, and 0.28 Åꢀ1 corres-
ponding to a lamella d-spacing of 20.6, 21.3, 21.7, and 22.1 nm,
respectively. Both phenanthrene-based polymers have slightly
shorter d-spacing than naphthalene-based polymers, which
seems to be caused by the wider molecular dimension of
phenanthrene. The thermally treated thin films of dimethoxy
substituted PA based copolymers, PA2 and PA4, showed arced
diffraction in 2D-GIXD patterns (Fig. S10, ESI†) and enhanced
(010) diffraction corresponding to the p–p stacking distance in
the Qxy direction accompanied by (h00) diffraction in the Qz
direction. This indicates that the dimethoxy-substituted polymer
chains adopt a mixed edge-on and face-on orientation after
thermal treatment. In contrast, the film of unsubstituted
PA-based copolymers showed centralized points along the Qz
axis with an edge-on orientation. From the (010) peaks at 1.66
and 1.64 Åꢀ1 in the Qxy direction, the respective p–p stacking
distance in PA1 and PA3 is about 3.78 nm with a different degree
of p–p interactions. In addition, the p–p stacking distances in
a polymer chain with a face-on orientation are 3.76 nm and
3.64 nm for PA2 and PA4, respectively. The shortest p–p stacking
distance of PA4 is expected to originate from strong inter-
molecular interactions between the neighboring molecules.
OFETs and charge transport properties
Photovoltaic characteristics
The charge transport properties of the four (MeO)PA-based
copolymers were investigated using top-gate bottom-contact (TGBC) We fabricated BHJ OPV devices to examine the photovoltaic
OFETs. For achieving the highest mobility, the film morphology was properties of the (MeO)PA-based copolymers as donor materials.
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Table 2 OFET characteristics of (MeO)PA based copolymers
Mobility (cm2 Vꢀ1 Sꢀ1
)
Vth (V)
Polymer
Ta (1C)
mh,ave(max)
me,ave(max)
p
n
PA1
None
110
150
200
250
1.12 ꢁ 10ꢀ1 (1.23 ꢁ 10ꢀ1
2.08 ꢁ 10ꢀ1 (2.14 ꢁ 10ꢀ1
3.51 ꢁ 10ꢀ1 (4.20 ꢁ 10ꢀ1
7.43 ꢁ 10ꢀ1 (9.30 ꢁ 10ꢀ1
4.55 ꢁ 10ꢀ1 (5.67 ꢁ 10ꢀ1
)
)
)
)
)
4.27 ꢁ 10ꢀ2 (5.32 ꢁ 10ꢀ2
1.67 ꢁ 10ꢀ1 (1.91 ꢁ 10ꢀ1
2.60 ꢁ 10ꢀ1 (3.00 ꢁ 10ꢀ1
3.97 ꢁ 10ꢀ1 (5.47 ꢁ 10ꢀ1
2.76 ꢁ 10ꢀ1 (3.22 ꢁ 10ꢀ1
)
)
)
)
)
ꢀ35.2 (ꢂ0.84)
ꢀ37.7 (ꢂ1.47)
ꢀ39.0 (ꢂ1.85)
ꢀ42.2 (ꢂ1.92)
ꢀ43.3 (ꢂ2.03)
64.7 (ꢂ1.44)
51.9 (ꢂ2.31)
57.7 (ꢂ2.31)
56.8 (ꢂ0.93)
57.3 (ꢂ0.86)
PA2
PA3
None
110
150
200
250
9.31 ꢁ 10ꢀ2 (1.37 ꢁ 10ꢀ1
2.12 ꢁ 10ꢀ1 (2.98 ꢁ 10ꢀ1
3.31 ꢁ 10ꢀ1 (4.39 ꢁ 10ꢀ1
5.31 ꢁ 10ꢀ1 (6.61 ꢁ 10ꢀ1
4.61 ꢁ 10ꢀ1 (5.35 ꢁ 10ꢀ1
)
)
)
)
)
1.58 ꢁ 10ꢀ2 (2.03 ꢁ 10ꢀ2
3.22 ꢁ 10ꢀ2 (4.33 ꢁ 10ꢀ2
4.06 ꢁ 10ꢀ2 (4.80 ꢁ 10ꢀ2
9.42 ꢁ 10ꢀ2 (1.28 ꢁ 10ꢀ1
7.32 ꢁ 10ꢀ2 (1.01 ꢁ 10ꢀ1
)
)
)
)
)
ꢀ36.9 (ꢂ3.12)
ꢀ45.8 (ꢂ2.69)
ꢀ55.3 (ꢂ2.12)
ꢀ52.3 (ꢂ2.52)
ꢀ54.2 (ꢂ1.94)
63.9 (ꢂ1.92)
61.9 (ꢂ0.76)
62.6 (ꢂ2.26)
66.4 (ꢂ0.83)
66.9 (ꢂ1.95)
None
110
150
200
250
300
3.50 ꢁ 10ꢀ1 (4.17 ꢁ 10ꢀ1
4.13 ꢁ 10ꢀ1 (4.92 ꢁ 10ꢀ1
4.84 ꢁ 10ꢀ1 (5.65 ꢁ 10ꢀ1
5.06 ꢁ 10ꢀ1 (6.22 ꢁ 10ꢀ1
1.10 (1.30)
)
)
)
)
2.81 ꢁ 10ꢀ1 (3.27 ꢁ 10ꢀ1
2.14 ꢁ 10ꢀ1 (2.27 ꢁ 10ꢀ1
5.61 ꢁ 10ꢀ1 (5.95 ꢁ 10ꢀ1
4.79 ꢁ 10ꢀ1 (5.87 ꢁ 10ꢀ1
9.00 ꢁ 10ꢀ1 (1.38)
)
)
)
)
ꢀ33.5 (ꢂ2.54)
ꢀ44.9 (ꢂ1.43)
ꢀ38.3 (ꢂ1.32)
ꢀ43.7 (ꢂ1.24)
ꢀ47.0 (ꢂ2.10)
ꢀ44.2 (ꢂ1.51)
49.87 (ꢂ0.64)
50.1 (ꢂ1.41)
52.2 (ꢂ2.05)
52.2 (ꢂ1.26)
50.7 (ꢂ1.27)
47.8 (ꢂ2.54)
4.97 ꢁ 10ꢀ1 (6.02 ꢁ 10ꢀ1
)
4.55 ꢁ 10ꢀ1 (5.12 ꢁ 10ꢀ1
)
PA4
None
100
150
200
250
1.68 ꢁ 10ꢀ1 (2.13 ꢁ 10ꢀ1
1.92 ꢁ 10ꢀ1 (2.08 ꢁ 10ꢀ1
6.19 ꢁ 10ꢀ1 (7.11 ꢁ 10ꢀ1
5.36 ꢁ 10ꢀ1 (6.99 ꢁ 10ꢀ1
3.02 ꢁ 10ꢀ1 (3.80 ꢁ 10ꢀ1
)
)
)
)
)
1.55 ꢁ 10ꢀ2 (2.13 ꢁ 10ꢀ2
2.80 ꢁ 10ꢀ2 (3.17 ꢁ 10ꢀ2
1.33 ꢁ 10ꢀ1 (1.45 ꢁ 10ꢀ1
1.66 ꢁ 10ꢀ1 (1.87 ꢁ 10ꢀ1
1.35 ꢁ 10ꢀ1 (1.48 ꢁ 10ꢀ1
)
)
)
)
)
ꢀ34.6 (ꢂ1.56)
ꢀ35.6 (ꢂ0.96)
ꢀ44.3 (ꢂ2.03)
ꢀ42.7 (ꢂ1.74)
ꢀ49.0 (ꢂ0.52)
62.16 (ꢂ1.79)
61.8 (ꢂ2.54)
60.7 (ꢂ0.76)
57.3 (ꢂ1.25)
54.5 (ꢂ1.03)
a
Annealing temperature.
best solubility in chloroform. However, chloroform is difficult to
handle due to its low boiling point. To achieve good processability
and nanoscale phase separation of polymer:PC71BM, we use 3% v/v
of diphenyl ether (DPE) with a high boiling point as a processing
additive. The optimized devices were made with a weight ratio of
1 : 3 for each polymer:PC71BM and the best device exhibited a PCE
of 5.3, 4.8, 5.8, and 4.1% for PA1, PA2, PA3 and PA4, respectively.
The PA1-based devices had a PCE of 5.3%, a Jsc of 11.5 mA cmꢀ2, a
FF of 63%, and a Voc of 0.72 V. The PA2 based device exhibited a
similar Jsc (12.2 mA cmꢀ2) and FF (0.68) but slightly lower PCE
(4.8%) than PA1 because of the lower Voc (0.68 V) due to its higher
HOMO energy level. Upon replacing the longer alkoxy side chain on
phenanthrene with the shortest methoxy group, PA2:PC71BM based
OPVs exhibited enhanced device performance especially in terms of
Jsc, compared to PA3 and PA4 in our previous work. There could be
several reasons for this improvement. PA2 showed better charge
transport properties than previous materials, and the PA2:PC71BM
blend film exhibited much finer phase separation between the
donor and acceptor for efficient exciton dissociation. In addition,
the use of PC71BM instead of PC61BM led to stronger photon
absorption in the shorter wavelength region.
Fig. 3 Transfer characteristics of OFET devices of (MeO)PA-based
polymer films after annealing: (a) PA1 at 200 1C, (b) PA2 at 200 1C,
(c) PA3 at 250 1C and (d) PA4 at 150 1C.
The inverted device structure of indium tin oxide (ITO) ITO/ZnO/
polymer:PC71BM/MoO3/Ag was employed under the illumination
of AM 1.5G at 100 mW cmꢀ2. The PCEs and all key parameters of
the optimized devices for each polymer are summarized in Table 3
and representative current density–voltage ( J–V) characteristics are
shown in Fig. 4a. As already mentioned, all copolymers showed the
Table 3 Photovoltaic characteristics of optimized devices
Polymera
Jsc (mA cmꢀ2
b
)
Voc (V)
FF (%)
PCE (%)
dc (nm)
Jsc (mA cmꢀ2
d
)
PA1
PA2
PA3
PA4
11.5
12.2
12.6
12.5
0.72
0.68
0.75
0.57
63
57
61
58
5.3
4.8
5.8
4.1
100
120
120
100
11.9
11.6
12.0
12.2
a
b
c
d
Polymer : PC71BM (1 : 3, w/w). Short circuit current from the J–V curve. Thickness of the active layer. Determined by integrating the EQE
spectrum with the AM 1.5G spectrum.
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Fig. 5 TEM images of (a) PA1:PC71BM (1 : 3, w/w), (b) PA2:PC71BM (1 : 3, w/w),
(c) PA3:PC71BM (1 : 3, w/w) and (d) PA4:PC71BM (1:3, w/w), respectively.
Fig. 4 (a) Current density–voltage (J–V) curves of OPV devices with the
structure of ITO/ZnO/polymer:PC71BM/MoO3/Ag under AM 1.5G illumina-
tion (100 mW cmꢀ2). (b) EQE curves of the corresponding OPV devices.
and the polymer phase. PA2 based devices exhibited a maximum
EQE of 65% at 475 nm but lowest EQE in the polymer region. The
overall Jsc values integrated from EQE curves are quite similar to
other polymers.
The photoactive layer morphology of the optimized devices
was observed using transmission electron microscopy (TEM),
as shown in Fig. 5. The film morphologies of phenanthrene-
based copolymers (PA1 and PA2):PC71BM show finer dispersed
fibrils with an interpenetrating bicontinuous network that
facilitates efficient exciton dissociation, which is directly related
to photocurrent generation.22,57,58 Also, PA3:PC71BM films exhi-
bited a similar morphology to phenanthrene-based polymers but
slightly coarser. In contrast, PA4:PC71BM films displayed fibrillar
polymer aggregation and a large domain size of over 20 nm, which
While PA3 showed the highest PCE of 5.8% with a Jsc of
12.6 mA cmꢀ2, a FF of 61%, and a Voc of 0.75 V, the PCE of PA4
was only 4% with the lowest Voc (0.57 V) in accordance with the
position of the HOMO energy level. In BHJ OPVs, Voc is
determined as
ꢀ
ꢁ
À
Á
1
kT
e
nenh
Nc2
acceptor
LUMO
donor
HOMO
Voc
¼
E
ꢀ E
ꢀ D ꢀ
ln
e
donor
HOMO
Here, ELUMO
acceptor is the LUMO energy level of the acceptor; E
is the HOMO energy level of the donor polymer; ne and nh are is considered to be close to the limit of exciton diffusion length,
the electron and hole densities in the acceptor and donor due to limited solubility of PA4. Thus, despite the lowest optical
polymer domains at open circuit, respectively; and Nc is the band gap of PA4, the photocurrent is reasonably similar to that of
effective density of states.55,56 The Voc value is directly related to other copolymers.
the energy difference between the HOMO level of the donor
The light intensity dependencies of Jsc and Voc were observed
polymers and the LUMO energy level of PC71BM.19 Thus, the by examining the charge recombination characteristics using a
OPV devices based on PA4 had the lowest Voc values because filter with different densities. Fig. S13a (ESI†) shows a log–log
PA4 has the highest HOMO energy level. The optimized device plot of the corresponding Jsc as a function of light intensity (I).
of each polymer showed a large difference in the Voc value Because the generation rate of electron and hole pairs is related
which is consistent with the HOMO energy level of polymers to photon absorption, the photocurrent is expected to be
rather than the photocurrent or FF.
proportional to the light intensity. From a power law depen-
External quantum efficiencies (EQEs) were measured for dence of Jsc upon I (Jsc p Ia, where a is typically from 0.85 to 1),
each OPV under optimized device conditions in order to when a is close to 1, carrier loss induced by bimolecular
evaluate the photoresponse of (MeO)PA-copolymers and calibrate recombination between mobile electrons and holes at the inter-
the Jsc value (Fig. 4b). The Jsc values derived from integrating the face of the donor/acceptor heterojunction is reduced.10,55,56,59–61
EQE curves are within 5% error compared to the respective Jsc Although all (MeO)PA-based copolymers showed a close to 1,
obtained from the J–V curve. All devices show a broad spectral phenanthrene based copolymers had slightly lower a values (0.978
response from the 300 nm to near 800 nm region. The EQE plots for PA1 and 0.977 for PA2) than naphthalene based copolymers
of the (MeO)PA polymer based devices yielded similar results to (0.992 for both PA3 and PA4), which indicates that the bimolecular
the absorption spectra of the polymers in the long wavelength recombination strength is slightly higher in the device composed
region and the PCBM absorption spectra in the short wavelength of phenanthrene based copolymers. In addition, Fig. S13b (ESI†)
region. This indicates that excitons were generated in both PCBM shows the dependence of the Voc on light intensity for the
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optimized devices with each of the (MeO)PA-based copolymers. (GPC) was performed on an Acme 9000 using 1,2,4-trichlorobezene
When Shockley–Read–Hall (SRH) recombination, also called as an eluent at 150 1C and calibrated with polystyrene standards.
trap-assisted recombination, is dominant, the slope of dVoc The optimized geometry of isolated polymer repeating units were
versus ln(I) increases from kT/e (bimolecular recombination calculated using the DFT calculation based on the B3LYP/
only) to 2kT/e, where k is the Boltzmann constant, T is the 6-311G(d,p) basis set using the GAUSSIAN 09 program. UV-vis
absolute temperature, and e is the electron charge.62–64 The absorption spectra and time-dependent UV-vis absorption spectra
OPVs processed with (MeO)PA-based copolymers exhibited a were recorded using a Lambda 750 (Perkin-Elmer) spectrometer
slope in the range from 1.12kT/e to 1.18kT/e. Therefore, both and VARIAN-5000 spectrophotometer, respectively. Cyclic voltam-
geminate recombination and non-geminate recombination metry was performed on a potentiostat (Eco Chemie AUTOLAB)
coexist, though bimolecular recombination dominates.
with an electrolyte solution of 0.1 M tetrabutylammonium perchlo-
rate in acetonitrile at a scan rate of 100 mV sꢀ1. Ag/Ag+, Pt wire, and
polymer coated ITO substrates were used as a reference, a counter,
and a working electrode, respectively. The atomic force microscopy
(AFM) images were obtained using a Veeco AFM microscope in
tapping mode under ambient conditions and transmission elec-
tron microscopy (TEM) was performed using a Tecnai G2 S-Twin
3600 keV electron microscope. GIWAXD of polymer thin films on a
Si/SiO2 wafer was performed using the beamline 9A with an
incident energy of 11.57 keV at the Pohang Accelerator Laboratory.
Conclusion
Four kinds of D–A type copolymers based on polyaromatic
compounds were synthesized via Suzuki polymerization for
OFETs and OPVs. All (MeO)PA-based copolymers exhibited
small band gap properties (Eg = 1.63–1.52 eV) due to their weak
donor–strong acceptor structural strategies. Dimethoxy substi-
tuents affected the absorption spectra and energy levels slightly
differently depending on the polyaromatic units. As expected,
dimethoxy substituents elevated the HOMO and LUMO energy
levels, and especially, the energy level of the naphthalene-
based copolymer was highly affected compared to those of the
phenanthrene-based copolymers. PA4 containing 1,5-dimethoxy-
nanphthalene had the smallest energy band gap (1.52 eV) and
the highest HOMO level (ꢀ4.92 eV) among the copolymers. The
electron density of PA4 is highly increased through dimethoxy
substituents at opposite sides of the naphthalene unit. The HOMO
distribution of 1,5-dimehoxynaphthalene is a well-delocalized
conjugated system while 9,10-dimethoxyphenanthrene showed
similar distribution with unsubstituted phenanthrene. In the
case of phenanthrene, the electron-donating mesomeric effect
of one methoxy group is relatively decreased due to the
electron-withdrawing inductive effect of the methoxy-oxygen
which is substituted close to the mesomeric methoxy group.
With their relatively higher HOMO levels, the MeOPA based
polymers, PA2 and PA4, showed a hole-dominant ambipolar
charge transport and PA1 and PA3, without dimethoxy substi-
tuents, exhibited reasonably balanced ambipolar charge-
transport characteristics with hole and electron mobilities of
B1 cm2 Vꢀ1 sꢀ1. The PA1-based OFET casted from TCE solution
using an off-center spin-coating method exhibited the best
Synthesis of monomers
2,7-Dibromophenanthrene and 2,7-dibromo-9,10-dimethoxy-
phenanthrene were synthesized following the procedures
described in the literature.44–47 2,6-Dibromonaphthalene was pur-
chased from TCI chemical and used without further purification.
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenanthrene
(MO1). To a solution of 2,7-dibromophenanthrene (1.00 g, 3 mmol),
bis(pinacolato)diboron (1.82 g, 7.2 mmol) and potassium acetate
(1.47 g, 15 mmol) in anhydrous dioxane (50 ml) was added
Pd(dppp)Cl2 (0.05 g, 0.06 mmol), and the reaction mixture was
heated to reflux for 72 h under a nitrogen atmosphere. After
cooling down to room temperature, the reaction mixture was
extracted with chloroform and stirred with Celite and activated
carbon for 1 hour. After this, the solvent was removed and the
residue was purified by column chromatography on a silica gel
with chloroform as an eluent. The solution was condensed and a
solid compound was recrystallized in methanol. The product was
afforded as white crystals (1.08 g, yield 84%). 1H NMR (400 MHz,
CDCl3, ppm): 8.69 (d, 2H), 8.38 (s, 2H), 8.04 (d, 2H), 7.76 (s, 2H),
1.40 (s, 24H).
2,20-(9,10-Dimethoxyphenanthrene-2,7-diyl)bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane) (MO2). MO2 was prepared following
the same procedure as for MO1. The product was afforded as
white crystals (1.3 g, yield 89%). 1H NMR (400 MHz, CDCl3,
ppm): 8.71 (s, 2H), 8.64 (d, 2H), 7.99 (d, 2H), 4.11 (s, 6H), 1.40
(s, 24H).
2,6-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)naphthalene
(MO3). MO3 was prepared following the same procedure as for
MO1. The product was afforded as white crystals (1.19 g, yield
91%). 1H NMR (400 MHz, CDCl3, ppm): 8.35 (s, 2H), 7.84 (q, 4H),
1.39 (s, 24H).
field-effect performance with a maximum hole mobility (mh,max
)
of 1.77 cm2 Vꢀ1 sꢀ1. The inverted OPV devices fabricated using
(MeO)PA based copolymer:PC71BM blends exhibited PCEs of
5.3%, 4.8%, 5.8%, and 4.1% for PA1, PA2, PA3, and PA4,
respectively. However, dimethoxy substituents on polyaromatic
compounds had a negative effect on Voc in particular and a
slight influence on Jsc or FF.
2,20-(4,8-Dimethoxynaphthalene-2,6-diyl)bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolane) (MO4). To a solution of 1,5-dimethoxy-
naphthalene (1.88 g, 10 mmol), pinacolborane (3.20 g, 25 mmol),
Experimental
Characterization
1H NMR spectra were recorded using a JEOL JNM-ECX400 spectro- and 4,40-di-tert-butyl-2,20-bipyridine (0.02 g, 0.1 mmol) in anhy-
meter using CDCl3 as a solvent. Gel permeation chromatography drous THF (50 ml) was added [Ir(COD)OMe]2 (0.02 g, 2.5 mmol),
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and the reaction mixture was heated to reflux for 48 h under a The substrates were cleaned using acetone, DI water, and iso-
nitrogen atmosphere. After cooling down to room temperature, propanol for 10 min each in an ultrasonicator, and dried in an
the reaction mixture was extracted with chloroform and stirred oven. Before polymer deposition, the substrates were UV–ozone
with Celite, and then the residue was purified by column treated for 20 min. Each polymer was dissolved in a cosolvent
chromatography on a silica gel with chloroform: hexane (1: 1) composed of chloroform : o-dichlorobenzene (4 : 1, v/v%) with
as an eluent. The product was afforded as white crystals (1.32 g, a concentration of 3.5 mg mlꢀ1. The polymer solution was spin-
1
yield 31%). H NMR (400 MHz, CDCl3, ppm): 8.37 (s, 2H), 7.18 coated onto a substrate under a nitrogen atmosphere. The polymer
(s, 2H), 4.03 (s, 2H), 1.38 (s, 24H).
films were thermally annealed at 150, 200, 250, and 300 1C for
3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(2-decyltetradecyl)-2,5- 30 min. Then, PMMA (80 mg mlꢀ1 in n-butyl acetate) was deposited
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DTDPP). 1H NMR as a gate dielectric layer and annealed at 80 1C for 2 hours in the
(400 MHz, CDCl3, ppm): 8.63 (d, 2H), 7.22 (d, 2H), 3.92 (d, 4H), nitrogen-filled glove box. The gate electrode was thermally evapo-
1.92-1.82 (m, 2H), 1.38-1.12 (m, 48H), 0.87 (t, 12H).
rated with aluminium through a shadow mask. The electrical
characteristics of individual OFET devices were measured using a
Keithley 4200-SCS under a nitrogen atmosphere.
General procedures for polymerization
(MeO)PA–DPP alternating copolymers were synthesized via Suzuki
polymerization with the same procedure as that followed in the
literature.65 0.4 mmol of each comonomer and reactant was
refluxed for 3 days, then 0.1 mg of phenyl boronic acid and 1 ml
of bromobenzene were added as end-cappers for an additional
12 hours each. After cooling down to room temperature, the
reaction mixture was poured into methanol containing 20 v/v%
hydrochloric acid and stirred for 3 hours. The solid compound
was filtered and then purified via Soxhlet extraction with
methanol, acetone, hexane and chloroform. The chloroform
solution fraction was concentrated and precipitated into methanol.
The solid was filtered and dried in a vacuum oven for 12 hours
at 70 1C.
Fabrication of OPV devices
OPVs were fabricated with an inverted configuration of
ITO/ZnO/active layer/MoO3/Ag in order to evaluate the photo-
voltaic properties of the polymers. ITO-patterned glass substrates
were cleaned using acetone, DI water, and isopropanol for 10 min
each in an ultrasonicator, and dried in an oven. Before polymer
deposition, the substrates were UV–ozone treated for 20 min. The
zinc oxide (ZnO) layer was deposited using ZnO nanoparticle
dispersion in isopropanol (2.5 wt%), then baked at 120 1C for
10 min. Then the substrates were transferred into a nitrogen-filled
glove box. The photoactive layer was spin-coated using each
polymer:PC71BM solution which was dissolved in chloroform
containing 3 v/v% of diphenyl ether (DPE) and stirred for 2 hours.
Then, molybdenum oxide and silver electrodes were thermally
evaporated with a thickness of 10 nm and 100 nm, respectively.
The photovoltaic characteristics were measured using a Keithley
Poly{phenanthrene-2,7-diyl-alt-3,6-dithiophene-2-yl-2,5-di(2-
decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione} (PA1) (401 mg,
yield: 87.2%). Anal. calcd for (C76H110N2O2S2)n: C, 79.51; H,
9.68; N, 2.44; O, 2.79; S, 5.58. Found: C, 78.28; H, 9.83; N, 2.54;
O, 2.91; S, 5.32. Mn = 54.2 kg molꢀ1, Mw = 175 kg molꢀ1
,
4200 under the illumination of AM 1.5G at 100 mW cmꢀ2
.
PDI = 3.23.
Poly{9,10-dimethoxyphenanthrene-2,7-diyl-alt-3,6-dithiophene-
2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione} (PA2)
(361 mg, yield: 74.6%). Anal. calcd for (C78H114N2O4S2)n: C,
77.54; H, 9.53; N, 2.32; O, 5.30; S, 5.31. Found: C, 76.21; H, 9.69;
Conflicts of interest
There are no conflicts to declare.
N, 2.41; O, 5.67; S, 5.19. Mn = 66.5 kg molꢀ1, Mw = 135 kg molꢀ1
PDI = 2.03.
Poly{naphthalene-2,6-diyl-alt-3,6-dithiophene-2-yl-2,5-di(2-decyl-
,
Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (NRF-
2015R1A2A1A10054466), the Korea Institute of Energy Technology
Evaluation and Planning (KETEP) and the Ministry of Trade,
Industry & Energy (MOTIE) of the Republic of Korea (No.
20153010012110), and the GIST Research Institute (GRI) grant
funded by the GIST in 2017. The GIXD was measured using the
synchrotron radiation source 9A beamline at the Pohang Accel-
erator Laboratory (PAL).
tetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione} (PA3) (386 mg, yield:
87.6%). Anal. calcd for (C72H108N2O2S2)n: C, 78.76; H, 9.94;
N, 2.55; O, 2.91; S, 5.84. Found: C, 79.00; H, 9.73; N, 2.72; O,
3.09; S, 5.49. Mn = 56.8 kg molꢀ1, Mw = 105 kg molꢀ1, PDI = 1.84.
Poly{1,5-dimethoxynaphthalene-3,7-diyl-alt-3,6-dithiophene-
2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione} (PA4)
(392 mg, yield: 84.3%). Anal. calcd for (C74H112N2O4S2)n: C,
76.75; H, 9.77; N, 2.42; O, 5.53; S, 5.54. Found: C, 77.02; H, 9.61;
N, 2.24; O, 5.81; S, 5.41. Mn = 23.4 kg molꢀ1, Mw = 48.9 kg molꢀ1
,
PDI = 2.09.
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