Dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DTBDAT)-based copolymers for high-performance organic solar cells

Article abstract of DOI:10.1002/pola.28205

P(BDT-TCNT) and P(DTBDAT-TCNT), which has an extended conjugation length, were designed and synthesized for applications in organic solar cell (OSCs). The solution absorption maxima of P(DTBDAT-TCNT) with the extended conjugation were red-shifted by 5–15 nm compared with those of P(BDT-TCNT). The optical band gaps and highest occupied molecular orbital (HOMO) energy levels of both P(BDT-TCNT) and P(DTBDAT-TCNT) were similar. The structure properties of thin films of these materials were characterized using grazing-incidence wide-angle X-ray scattering and tapping-mode atomic force microscopy, and charge carrier mobilities were characterized using the space-charge limited current method. OSCs were formed using [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as the electron acceptor and 3% diphenylether as additive suppress aggregation. OSCs with P(BDT-TCNT) as the electron donor exhibited a power conversion efficiency (PCE) of 4.10% with a short-circuit current density of J_SC = 9.06 mA/cm², an open-circuit voltage of V_OC = 0.77 V, and a fill factor of FF = 0.58. OSCs formed using P(DTBDAT-TCNT) as the electron donor layer exhibited a PCE of 5.83% with J_SC = 12.2 mA/cm², V_OC = 0.77 V, and FF = 0.62.

Full text of DOI:10.1002/pola.28205

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Dithieno[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene (DTBDAT)-Based
Copolymers for High-Performance Organic Solar Cells
Ye Seul Lee,1 Seyeong Song,2 Yung Jin Yoon,2 Yun-Ji Lee,3 Soon-Ki Kwon,3 Jin Young Kim,2
Yun-Hi Kim1
1Department of Chemistry & RINS, Gyeongsang National University, Jin-ju 660-701, Republic of Korea
2Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST) Ulsan 689-798, South Korea
3Department of Materials Engineering and Convergence Technology, Gyeongsang National University, Jin-ju 660-701, Republic
of Korea
Correspondence to: Y.-H. Kim (E-mail: ykim@gnu.ac.kr) or J. Y. Kim (E-mail: jykim@unist.ac.kr) or S.-K. Kwon (E-mail: skwon@gnu.ac.kr)
Received 17 May 2016; accepted 8 June 2016; published online 2 July 2016
DOI: 10.1002/pola.28205
ABSTRACT: P(BDT-TCNT) and P(DTBDAT-TCNT), which has an
extended conjugation length, were designed and synthesized
for applications in organic solar cell (OSCs). The solution
absorption maxima of P(DTBDAT-TCNT) with the extended
conjugation were red-shifted by 5–15 nm compared with those
of P(BDT-TCNT). The optical band gaps and highest occupied
molecular orbital (HOMO) energy levels of both P(BDT-TCNT)
and P(DTBDAT-TCNT) were similar. The structure properties of
thin films of these materials were characterized using grazing-
incidence wide-angle X-ray scattering and tapping-mode atom-
ic force microscopy, and charge carrier mobilities were charac-
terized using the space-charge limited current method. OSCs
were formed using [6,6]-phenyl-C71-butyric acid methyl ester
(PC71BM) as the electron acceptor and 3% diphenylether as
additive suppress aggregation. OSCs with P(BDT-TCNT) as the
electron donor exhibited a power conversion efficiency (PCE)
of 4.10% with a short-circuit current density of JSC 5 9.06 mA/
cm2, an open-circuit voltage of VOC 5 0.77 V, and a fill factor of
FF 5 0.58. OSCs formed using P(DTBDAT-TCNT) as the electron
donor layer exhibited a PCE of 5.83% with JSC 5 12.2 mA/cm2,
C
V
V
OC 5 0.77 V, and FF 5 0.62.
2016 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2016, 54, 3182–3192
KEYWORDS: conjugated polymers; synthesis; structure-property
relations
lead to a decrease in the band gap. Moreover, the nitrile group
can attract electrons through the vinyl linkage. Therefore, 2,3-
bis-(thiophene-2-yl)-acrylronitrile can be used as an acceptor.
The copolymer unit (4,8-bis(4,5-didecylthiophen-2-yl)benzo[1,2-
b:4,5-b’]dithiophene-2,6-iyl)bis(trimethylstannane) (BDT) is
known to be electron-rich unit with a high charge carrier mobili-
ty and high Voc and BDT has been widely used as the donor unit
for high-performance organic photovoltaics.12–16 Fused deriva-
tives of BDT such as dithieno[2,3-d:2’,3’-d’]benzo[1,2-b:4,5-
b’]dithiophene (DTBDAT) for use as donor units in polymers
provide a higher PCE than BDT when combined with a 2,3-bis-
(thiophene-2-yl)-acrylronitrile acceptor unit.17,18 Such fused aro-
matic rings typically exhibit excellent intermolecular charge-
transport properties because of the planarity of the backbone.
There have been several reports describing the high charge-
carrier mobilities of these moieties when used in organic field-
effect transistors.2,19 Derivatives of DTBDAT exhibit relatively
low HOMO levels and large band gaps compared with those of
other fused aromatic rings, which suggests that the DTBDAT
INTRODUCTION Polymer solar cells (PSCs) have received
growing attention in recent years owing to the flexible, col-
orful and low-mass properties of the devices, and the broad
range of potential small- and large-area applications.1,2 PSCs
typically employ bulk-heterojunction (BHJ) architecture, and
the power conversion efficiencies (PCEs) of PSCs that have
thus far been achieved range from 1% to more than 9%.3–6
One strategy that has been employed to achieve a PCE is the
use of donor–acceptor (D-A) conjugated polymers with alter-
nating electron-rich (donor) and electron-deficient (acceptor)
moieties. The electronic properties of these layers can be
easily tuned by controlling the intermolecular charge trans-
fer from the donor to the acceptor.7–10 The most important
parameter of PCE is the open-circuit voltage Voc, which is
related to the optical band gap Eg.11
The copolymer unit 2,3-bis-(thiophene-2-yl)-acrylronitrile
has a vinyl linkage with a nitrile group between thiophene
units. This group can increase the conjugation length, which
Y. S. Lee and S. Song contributed equally to this work.
C
V
2016 Wiley Periodicals, Inc.
3182
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
motif is a particularly stable organic semiconductor. Although
the development of materials based on DTBDAT is expected to
lead to promising printable organic semiconductors, materials
formed from DTBDAT moieties have not been reported.20
DTBDAT can be prepared using a TCNT moiety as an electron
acceptor unit, which helps to strengthen the intermolecular
interactions between polymer chains, resulting in relatively
large charge carrier mobility. Here we investigate the structural
properties of P(BDT-TCNT) and P(DTBDAT-TCNT) with
extended conjugations for applications in organic thin film
transistor (TFTs) and organic solar cells (OSCs).23
Materials
The Pd2(dba)3 and P(o-tol)3 were purchased from Umicore.
All reactions were carried out in a nitrogen atmosphere
using the usual Schlenk techniques. All chemical reagents
were purchased from Aldrich and used as received. Other
chemicals were used unless otherwise specified.
Synthesis of (E)22,3-Bis(5-bromothiophene-
2-yl)acrylonitrile (1)
Compound 1 was synthesized referring to the literature.21
1H NMR (300 MHz, CDCl3, d): 7.27 (d, 1 H), 7.22 (s, 1 H),
7.10–7.08 (m, 2 H), 7.03 (d, 1 H). 13C NMR (75 MHz, CDCl3,
d): 140.3, 139.2, 139.0, 132.7, 131.9, 127.9, 127.5, 120.1,
118.8, 117.1, 109.3. FTIR (cm21): 3025, 2214, 1584, 1429,
1409, 1313, 905, 787, 500.
EXPERIMENTAL
Characterization
1H-NMR and 13C NMR spectra were recorded with a Bruker
Advance-300 and 500 spectrometer. The thermal analysis
were performed on a TA TGA 2100 thermogravimetric ana-
lyzer in a nitrogen atmosphere at a rate of 10 8C/min. Differ-
ential scanning calorimeter (DSC) was conducted under
nitrogen on a TA instrument 2100 DSC. The sample was
heated with 10 8C/min from 30 to 300 8C. UV–vis absorption
spectra were measured by UV-1650PC spectrophotometer.
Molecular weights and polydispersities of the copolymers
were determined by gel permeation chromatography (GPC)
analysis with polystyrene standard calibration (waters high-
pressure GPC assembly Model M515 pump, l-Styragel col-
umns of HR4, HR4E, HR5E, with 500 and 100 Å, refractive
index detectors, solvent: tetrahydrofuran). Cyclic voltamme-
try (CV) was performed on an EG and G Parc model 273 Å
potentiostat/galvanostat system with a three-electrode cell
in a solution of 0.1 M tetrabutylammonium perchlorate
(Bu4NClO4) in acetonitrile at a scan rate of 50 mV/s. The
polymer films were coated on a square carbon electrode by
dipping the electrode into the corresponding solvents and
then dried under nitrogen. A Pt wire was used as the coun-
ter electrode, and an Ag/AgNO3 (0.1 M) electrode was used
as the reference electrode. Atomic force microscopy (AFM)
images of the polymer:PC71BM blend films were recorded on
a XE-100 (Park systems) with an AC160-TS cantilever in the
tapping mode. AFM samples were fabricated in the same
method for OPV devices without TiO2/Al deposition. Photo-
luminescence (PL) spectra of the polymer:PC71BM blends
Synthesis of (E)22,3-Bis(5-(trimethylstannyl)
thiophene-2-yl)acrylonitrile (2)
Compound 2 was synthesized referring to the literature.22
1H NMR (300 MHz, CDCl3, d): 7.66–7.65 (d, 1 H), 7.56 (s,
1 H), 7.43–7.42 (d, 1 H), 7.22–7.21 (d, 1 H), 7.15–7.13 (d,
1 H), 0.53–0.32 (m, 18 H). 13C NMR (75 MHz, CDCl3, d)
144.38, 143.30, 139.77, 136.12, 135.67, 132.82, 131.57,
127.79, 117.50, 102.37, 28.10, 28.16. FTIR (cm21): 3058
(aromatic, C-H), 2996–2800 (aliphatic, C-H), 2218 (-CN),
1576–1408 (C 5 C).
Synthesis of 4,8-Bis(4,5-didecylthiophen-2-yl)benzo
[1,2-b:4,5-b’]dithiophene (3)
Compound 3 was synthesized referring to the literature.23
1H NMR (300 MHz, CDCl3, d): 7.69 (d, 2 H), 7.46 (d, 2 H),
7.23 (s, 2 H), 2.85 (br, 4 H), 2.62 (t, 4 H), 1.72 (m, 8 H),
1.72 (m, 8 H), 1.30 (m, 56 H), 0.90 (t, 12 H).
Synthesis of (4,8-Bis(4,5-didecylthiophen-2-yl)benzo
[1,2-b:4,5-b’]dithiophene-2,6-diyl)bis(trimethylstannane) (4)
Compound
literature.23
4
was also synthesized referring to the
1H NMR (300 MHz, CDCl3, d): 7.74 (s, 2 H), 7.24 (s, 2 H),
2.85 (t, 4 H), 2.63 (t, 4 H), 1.70 (m, 8 H), 1.29 (m, 56 H),
0.90 (t, 12 H), 0.41 (t, 18 H). FTIR (cm21): 3108 (aromatic,
C-H), 2923–2855 (aliphatic, C-H).
were obtained on
a Horiba Jobin Yvon spectrometer
(iHR320, FL 3-1 iHR, S1 detector). Transmission electron
microscopy (TEM) images were taken on Philips CM30
microscope at an operating voltage of 300 keV. For TEM
experiments, the polymer:PC71BM blend films spin-coated on
the ITO/PEDOT:PSS substrates were floated in water, and
then lifted on a carbon-coated square mesh copper grid
(#200 mesh). The electronic structures and conformations of
model compounds of polymer backbone in study were esti-
mated by density functional theory (DFT) calculations. The
model compound consisted of three repeating units of poly-
mers were geometrically optimized to an energy minimum
using Gaussian 09 at the DFT B3LYP level with a 6-
31 1 G(d,p) basis set. To save the calculation time, all the
alkyl chains were shortened to methyl groups.
Synthesis of 5-10-Bis(4,5-didecylthiophen-2-yl)benzo
[1,2-b:4,5-b’]diithieno[3,2-b]thiophene (5)
Compound 5 was synthesized referring to the literature.19
1H NMR (300 MHz, CDCl3, d): 7.47–7.45 (d, J 5 5.12 Hz, 2 H),
7.31–7.29 (d, J 5 6.02, 2 H), 7.12 (s, 2 H), 2.93–2.70 (m,
8 H), 2.17–2.15 (m, 8 H), 1.60–1.41 (m, 56 H), 0.92–0.89 (m,
12 H); 13C NMR (75 MHz, CDCl3, d): 143.8, 143.4, 142.3,
139.7, 139.6, 134.4, 132.8, 131.4, 130.7, 130.2, 124.7, 120.5,
32.77, 32.71, 31.61, 30.49, 30.43, 30.37, 30.23, 30.15, 30.06,
29.07, 28.78, 23.47, 14.63; EIMS (m/z (%)): 1027 [M
1
]
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192
3183

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
SCHEME 1 Synthetic scheme of polymers (P(BDT-TCNT) and P(DTBDAT-TCNT)).
Synthesis of 2,7-Dibromo-5,10-bis(4,5-didecylthiophen-2-
yl)benzo[1,2-b:4,5-b’]diithieno[3,2-b]thiophene (6)
Under nitrogen atmosphere, compound 5 (4.0 g, 3.43 mmol)
was dissolved into chloroform. N-bromosuccinimide (NBS)
(1.5 g, 8.59 mmol) was added to the solution slowly. The
reaction mixture was stirred at room temperature for 12 h.
The reaction mixture was poured into water and extracted
with dichloromethane. The organic solution was dried with
plenty of anhydrous magnesium sulfate and evaporation
with low pressure. The compound was further purified by
recrystallization from dichloromethane and isopropyl alcohol
(Yield: 4.1g, 72%).
1H NMR (300 MHz, CDCl3, d): 7.31 (s, 2 H), 7.09 (s, 2 H),
2.95–2.90 (t, 4 H), 2.72–2.67 (t, 4 H), 1.82–1.70 (m, 8 H),
1.51–1.31 (m, 56 H), 0.92–0.87 (m, 12 H). FTIR (cm21):
3092 (aromatic, C-H), 2916–2848 (aliphatic, C-H).
Polymerization of P(BDT-TCNT)
In a 25 mL dry flask, compound 4 (0.4000 g, 0.322 mmol)
and compound 1 (0.1208 g, 0.322 mmol) was dissolved in
6 mL chlorobenzene, then bubbling with nitrogen for 30
min. Pd2 (dba)3 (5.9 mg, 0.006 mmol) and P(o-tol)3 (7.8 mg,
0.025 mmol) were added to the mixture, which was stirred
for 48
h at 110 8C afterwards. Tributyl(thiophen-2-
3184
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
FIGURE 1 TGA thermogram and DSC curve of (a) P(BDT-TCNT) and (b) P(DTBDAT-TCNT) at a heating rate of 10 8C/min. [Color fig-
ure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
yl)stannane and 2-bromothiophene were injected sequential-
ly into the reaction mixture for end-capping, and the solution
was stirred for 6 h after each addition. After cooling to room
temperature, the reaction mixture was then dropped into
methanol (100 mL). The crude polymer was collected by fil-
tration and purified by soxhlet extraction with methanol,
acetone, hexane and chloroform in sequence. The P(BDT-
TCNT) was obtained by precipitation of the chloroform solu-
tion into methanol (Yield: 0.26 g)
stirred for 6 h after each addition. After cooling to room
temperature, the reaction mixture was then dropped into
methanol (100 mL). The crude polymer was collected by fil-
tration and purified by soxhlet extraction with methanol,
acetone, hexane, and chloroform in sequence. The
P(DTBDAT-TCNT) was obtained by precipitation of the chlo-
roform solution into methanol (Yield: 0.13 g).
FTIR (cm21): 3071 (aromatic, C-H), 2930–2857 (aliphatic, C-
H), 2215 (-CN), 1650–1453 (C 5 C).
1H NMR (500 MHz, CDCl3, d): 7.80–7.46 (br, 3 H), 7.25–6.52
(br, 5 H), 2.94–2.74 (br, 8 H), 1.79–1.35 (br, 64 H), 0.91 (br,
12 H). FTIR (cm21): 3073 (aromatic, C-H), 2917–2854 (ali-
phatic, C-H), 2215 (-CN), 1658–1459 (C 5 C).
RESULTS AND DISCUSSION
Scheme
1 shows the synthesis of P(BDT-TCNT) and
P(DTBDAT-TCNT). The monomers were synthesized accord-
ing to the methods in references.2,19,20,22 Polymers were
obtained via a Stille coupling reaction using Pd2 (dba)3/P(o-
tol)3 (1/4) in chlorobenzene. P(BDT-TCNT) and P(DTBDAT-
TCNT) were purified via Soxhlet extraction in methanol, hex-
ane, and chloroform. The chloroform fraction was collected
and reprecipitated in methanol. The chemical structures of
P(BDT-TCNT) and P(DTBDAT-TCNT) were analyzed using
proton nuclear magnetic resonance (H NMR) spectroscopy.
Polymerization of P(DTBDAT-TCNT)
In a 25-mL dry flask, compound 2 (0.4000 g, 0.337 mmol)
and compound 6 (0.1831 g, 0.337 mmol) was dissolved in 6-
mL chlorobenzene, then bubbling with nitrogen for 30 min.
Pd2 (dba)3 (5.9 mg, 0.006 mmol) and P(o-tol)3 (7.8 mg,
0.025 mmol) were added to the mixture, which was stirred
for 48 h at 110 8C afterwards. Tributyl(thiophen-2-yl)stan-
nane and 2-bromothiophene were injected sequentially into
the reaction mixture for end-capping, and the solution was
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192
3185

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 2 UV–vis absorption spectra and Cyclovoltammograms of (a) P(BDT-TCNT) and (b) P(DTBDAT-TCNT). [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
Both polymers were soluble in chloroform, chlorobenzene,
and dichlorobenzene.
This suggests planarity and rigidity of the polymer backbone,
even in solution. Table 1 lists a summary of the optical proper-
ties of the two copolymers. The optical band gap of P(BDT-
TCNT) was 1.73 eV and that of P(DTBDAT-TCNT) was 1.71 eV.
These results are based on the onset of the absorption spectra.
The electrochemical properties of two polymers were investi-
gated using cyclic voltammetry. The oxidation onset of P(BDT-
TCNT) and P(DTBDAT-TCNT) was 1.09 and 1.07 V, respective-
ly. We determined the HOMOs based on the onset of oxidation
from the equation EHOMO 5 -(EOX 1 ferroceneOX), where EOX is
the oxidation onset potential of both polymers and ferroceneOX
The thermal stability of the polymers was investigated using
thermogravimetry analysis (TGA) with a heating rate of 10
8C/min in a nitrogen atmosphere. As shown in Figure 1, both
P(BDT-TCNT) and P(DTBDAT-TCNT) exhibited good thermal
stability, with 5% weight loss at 423 and 437 8C, respective-
ly. Differencial scanning calorimetry (DSC) was used to inves-
tigate the phase behavior, and neither polymer exhibited any
transitions between 50 and 300 8C.
UV–vis absorption spectra were measured in solution phase
and as thin films. Figure 2 shows UV–vis absorption spectra of
the two copolymers. In chloroform, P(BDT-TCNT) exhibited
absorption maxima at 571 and 615 nm, and P(DTBDAT-
TCNT) exhibited absorption maxima at 587 and 620 nm. The
solution absorption maxima of P(DTBDAT-TCNT), with the
extended conjugation, red-shifted by 5–15 nm compared with
those of P(BDT-TCNT). In the thin film state, P(BDT-TCNT)
exhibited absorption maxima at 592 and 634 nm, that is, the
absorbance bands were red shifted by ꢀ20 nm, compared
with solution-phase absorption spectra. P(DTBDAT-TCNT)
exhibited absorption maxima at 584 and 626 nm, that is, little
spectral shift was observed relative to solution phase spectra.
TABLE 1 The Optical Properties of P(BDT-TCNT) and
P(DTBDAT-TCNT)
P(BDT-TCNT)
P(DTBDAT-TCNT)
UV-S (max) (nm)
UV-F (max) (nm)
UV-F-ann (max) (nm)
UV-edge (nm)
615, 571, 348
634, 592, 353
630
620, 587, 336
626, 584, 344
633
714
724
Band gap (optical) (ev)
HOMO (ev)
1.73
1.71
25.49
25.49
LUMO (ev)
23.76
23.78
3186
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
TABLE 2 Photovoltaic Properties of the Polymers Blended with
PC71BM at 1:1 Weight Chlorobenzene (CB) with or Without
Diphenylether (DPE)
P(DTBDAT-
P(BDT-TCNT)
TCNT)
CB:DPE (v/v)
Jsc (mA/cm2)
Voc (V)
100:0
97:3
9.06
0.78
0.58
4.10
8.71
100:0
5.69
0.75
0.38
1.62
5.55
97:3
12.20
0.77
0.62
5.83
11.93
2.65
0.77
0.40
0.82
2.64
FF
PCE (%)
Cal. Jsc (mA/cm2)
The calculated energy levels are in good agreement with
experimental data and both P(DTBDAT- TCNT) and P(BDT-
TCNT) had similar HOMO and LUMO energy levels.
Fabrication and Characterization of Organic Solar Cells
Organic solar cells (OSCs) were fabricated with structure of
indium tin-oxide-coated glass (ITO)/PEDOT:PSS/P(BDT-
TCNT):PC71BM/Al. The patterned ITO was cleaned with
detergent and rinsed by ultrasonic cleaner of distilled water,
acetone, and isopropanol. Substrates were treated in an
ultraviolet-ozone chamber for 20 min after dried in an oven
at 100 8C. Poly(3,4-ethylenedioxythiophene):poly(styrene sul-
fonate) (PEDOT:PSS, Baytron P cleviosTM AI 4083, Germany)
was spin-coated onto the ITO glass as 45 nm thin layer and
dried at 140 8C for 10 min. Solutions containing mixture of
P(BDT-TCNT):PC71BM in chlorobenzene (CB) solvent with
concentration of 1 wt % were spin-cast on the PEDOT:PSS
layer under a nitrogen filled glove box. The mixed solutions
of P(BDT-TCNT):PC71BM include diphenylether (DPE) as an
additive by 3% (v/v). Then, Al cathode (100 nm) was depos-
ited on the active layer by thermal evaporation in vacuum
(<1026 torr) condition. The thickness of the photoactive lay-
er was measured using a surface profiler (KLA_tencor). The
active area of the device was 13 mm2. J–V characteristics
were measured by a computer-controlled Keithley 2635A
Source Measurement under an AM 1.5G illumination at 100
FIGURE 3 Calculated stereostructure and frontier orbital of
P(BDT-TCNT) (upper) and P(DTBDAT-TCNT) (bottom). [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
is the oxidation onset potential of a ferrocene reference. The
HOMO values are 25.49 eV for P(BDT-TCNT) and 25.47 eV
for P(DTBDAT-TCNT). For both compounds, the LUMO levels
were 23.76 eV, which were calculated from optical band gaps
and HOMO levels of them. The difference between the LUMO
levels of these polymers and PC71BM suggests that the energy
offset will be sufficient to give rise to efficient charge transfer/
separation at polymer/PC71BM interfaces. Moreover, the rela-
tively low HOMO levels are expected to give rise to a large Voc.
The electronic density distributions of the polymers were
analyzed using density functional theory (DFT). Figure 3
shows the resulting molecular orbitals and energy levels.
TABLE 3 The Parameters of OPV at Various Conditions
Polymer
Polymer:PC71BM (w/w)
1:1
Solvent (v/v)
JSC (mA/cm2)
VOC (V)
FF
PCE (%)
P(BDT-TCNT)
CB
2.65
9.06
8.01
7.85
5.02
6.37
5.69
12.2
11.3
11.3
11.1
0.77
0.78
0.78
0.75
0.87
0.84
0.75
0.77
0.75
0.77
0.77
0.40
0.58
0.58
0.59
0.39
0.57
0.38
0.62
0.53
0.59
0.55
0.82
4.10
3.63
3.49
1.69
3.06
1.62
5.83
4.55
4.76
4.69
97%CB: 3%DPE
95%CB: 5%DPE
90%CB: 10%DPE
DCB
97%DCB: 3%DPE
CB
P(DTBDAT-TCNT)
1:1
97%CB: 3%DPE
95%CB: 5%DPE
DCB
97%DCB: 3%DPE
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192
3187

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 4 J–V characteristics and EQE spectra. [Color figure can be viewed in the online issue, which is available at wileyonlineli-
brary.com.]
mW/cm2. EQE (PV Measurements) spectra were obtained by
applied monochromatic light from a xenon lamp.
photoactive material. To obtain optimal photoactive material,
a small amount of additive a diphenylether (DPE) was
used.24 25
Table 2 lists a summary of the device performance
,
Film Morphology and Solar Cell Performance
parameters and Table 3 lists parameters for each polymer
device. The corresponding current density–voltage (J–V)
characteristics and external quantum efficiency (EQE) spec-
tra are shown in Figure 4. The best performance of device
based on P(DTBDAT-TCNT):PC71BM (1:1 w/w) with 3%
DPE exhibited a power conversion efficiency (PCE) of 5.83%
with a current density JSC 5 12.2 mA/cm2, open circuit volt-
age VOC 5 0.77 V, and a fill factor FF 5 0.62.
We investigated the effect of the benzodithiophene-based
conjugate polymer including the cyano group as an electron
unit and acceptor PC71BM matierials on the performance
and film property of bulkheterojunction (BHJ) organic solar
cells. Organic solar cells with configuration of conventional
structure were fabricated with the ratio of 1:1 (w/w) blend
of the conjugate polymer:PC71BM in chlorobenzene (CB) as a
FIGURE 5 2D GIWAXS patterns of thin films as neat, Polymer:PC71BM blend and blend with DPE as a processing additive for
P(BDT-TCNT) (upper) and P(DTBDAT-TCNT) (bottom). [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
3188
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
FIGURE 6 In-plane (qxy) and out-of-plane (qz) linecuts of polymer films: (a) The result of P(BDT-TCNT) (b) P(DTBDAT-TCNT). [Color
figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
The device formed using P(BDT-TCNT) exhibited VOC 5 0.78
V, which is similar to that of P(DTBDAT-TCNT); the PCE was
4.10%, with JSC 5 9.06 mA/cm2 and FF 5 0.58. Employing
DPE as morphology processing additive in the CB solution
significantly enhanced the short-circuit current density,
which increased from JSC 5 2.65 and JSC 5 5.69 without DPE
to JSC 5 9.06 and JSC 512.2 mA/cm2 with DPE for OSCs based
on P(BDT-TCNT):PC71BM and P(DTBDAT-TCNT):PC71BM, res-
pectively, and the corresponding PCEs increased to 4.10%
and 5.83%. In the data shown in Figure 4, JSC was calculated
by integrating the EQE spectra. The data are consistent with
the JSC calculated from J–V curves (agreement to within
63%), which indicates that these OSC measurements were
reliable.
To investigate the relationship between the microstructural
arrangement of the polymers in thin films and the perfor-
mance of OSCs, we measured grazing-incidence wide-angle
X-ray scattering (GIWAXS) and analyzed the film patterns of
pristine polymers and optimal polymer:PC71BM blends.
Both pristine and blended films exhibited (h00) diffraction
patterns along qz (out-of-plane) with qxy (in-plane) corre-
sponding to lamella ordering. Furthermore, (010) peaks
were observed along all axes, which indicates p-p stacking of
TABLE 4 The Parameters of GIWAXS Data
Scattering
Scattering
Vector (q)
of p–p
Lamella
Interaction
(h00)
Vector of
Lamella
Lamella
d-Spacing
Interaction
21
21
˚
˚
˚
˚
Direction
Stacking (A
)
(A)
Peak (A
)
Distance (A)
Neat-P(BDT-TCNT)
In-plane (qxy
)
1.72
1.72
1.73
1.75
1.72
1.75
1.71
1.70
1.72
1.77
1.72
1.77
3.65
3.65
3.63
3.59
3.65
3.59
3.67
3.70
3.65
3.55
3.65
3.55
(100)
(100)
(100)
(100)
(100)
(100)
(100)
(100)
(100)
(100)
(100)
(100)
0.23
0.25
0.24
0.28
0.23
0.27
0.25
0.29
0.26
0.32
0.25
0.30
27.32
25.13
26.18
22.44
27.32
23.27
25.13
21.67
24.17
19.63
25.13
20.94
Out-of-plane (qz)
In-plane (qxy
Out-of-plane (qz)
In-plane (qxy
Out-of-plane (qz)
In-plane (qxy
Out-of-plane (qz)
In-plane (qxy
Out-of-plane (qz)
In-plane (qxy
Out-of-plane (qz)
P(BDT-TCNT):PC71BM
)
P(BDT-TCNT):PC71BM w/DPE
Neat-P(DTBDAT-TCNT)
)
)
P(DTBDAT-TCNT):PC71BM
P(DTBDAT-TCNT):PC71BM W/DPE
)
)
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192
3189

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 7 AFM morphology images (2 lm 3 2 lm) of the pristine, Polymer:PC71BM blend and blend with 3% DPE (v/v) in Chloro-
bezene for P(BDT-TCNT) (a–c) and P(DTBDAT-TCNT (d–f). [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
the molecule backbones, as shown in Figures 5 and 6.
Table 4 lists the crystallographic parameters.
P(DTBDAT-TCNT). Large agglomerated clusters were
observed for the blended polymer:PC71BM film, which is con-
sistent with inferior OSC performance; we found the RMS of
41.25 nm for P(BDT-TCNT):PC71BM, and 28.10 nm for
P(DTBDAT-TCNT):PC71BM. Solution-phase DPE was added to
control the morphology of the thin films, resulting in sufficient
phase separation to enable efficient electron transport from
the polymer to the PC71BM layer. The performances of PSCs
formed devices with DPE processing were dramatically
enhanced by over twice of JSC value arising from evidently
reduced RMS of 8.37 and 7.29 nm of P(BDT-TCNT):PC71BM
Both of the pristine polymer films exhibited first- and
second-order diffraction peaks up in the out-of-plane direc-
tions, indicating lamella stacking. The pristine P(BDT-TCNT)
thin film exhibited strong inter-lamellar scattering peaks at
0.23–0.25 Å21 on both axis. In contrast, the pristine-
P(DTBDAT-TCNT) film exhibited reflection at 0.25–0.29 Å21
,
indicating that the P(DTBDAT- TCNT)-based film had a
shorter inter-lamella interaction distance (25.13–21.67 Å)
than the films based on PBDT-TCNT (27.32–25.13 Å). These
trends were also observed for the blended system, with the
halo patterns at 1.31–1.35 Å21 which are attributed to the
PC71BM acceptor. The scattering vector due to lamella stack-
ing increased slightly from 0.23–0.25 to 0.23–0.28 Å21 of
films based P(BDT-TCNT), and increased from 0.25–0.29 Å
to 0.26–0.32 Å21 for films based on P(DTBDAT-TCNT). The
p-p stacking peaks of the blended thin films along the qz
axis also changed following the addition of fullerene deriva-
tives. For P(BDT-TCNT), qz increased from 1.72 Å21 !1.75
Å21, and for PDTBDAT-TCNT, qz increased from 1.70 Å21
!1.77 Å21) with narrower d-spacing by adding fullerene
derivative materials compared to pristine films.
To investigate the surface morphology of thin films, we carried
out tapping-mode atomic force microscopy (AFM) measure-
ments, as shown in Figure 7. The topographic images of the
neat-polymer indicate a root-mean-square (RMS) roughness of
Rq 5 3.35 nm for P(BDT-TCNT) and Rq 5 1.87 nm of
FIGURE 8 (a) Logarithmic plot of dark J–V characteristics from
the hole-only devices and (b) electron-only device. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
3190
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
TABLE 5 Results of the SCLC Device Performance
Polymer
Thicknesselectron (nm)
lelectron (cm2V/s)
Thicknesshole (nm)
lhole (cm2V/s)
le/lh
P(BDT-TCNT)
95
90
1.2 x 1025
6.9 x 1025
140
140
4.2 x 1024
6.6 x 1025
0.03
1.04
P(DTBDAT-TCNT)
REFERENCES AND NOTES
and P(DTBDAT-TCNT):PC71BM films. The external photocur-
rent in bulkheterojunction (BHJ) solar cell systems corre-
sponds to the photogenerated charge carriers that are
extracted from the photoactive layer to the electrodes. There-
fore, the charge carrier mobilities for holes and electrons are
significant for device performance. We used the space-charge
limited current (SCLC) method to investigate vertical trans-
port of charge carrier. We fabricated hole only (ITO/
PEDOT:PSS/polymer:PC71BM/Au) and electron only (FTO/
polymer:PC71BM) devices using the optimized conditions
described above. The SCLC carrier transport properties can be
found using J 5 9e0erlV2/8L3, where e0 is permittivity of free-
space, er is the relative dielectric constant of the semiconduc-
tor, l is the charge carrier mobility, V is the applied voltage
and L is the thickness of the layer. Figure 8 shows the dark
current versus the applied bias voltage, which was revised the
built-in potential. Table 5 lists the electron and hole mobilities.
We found lhole 5 4.231024 cm2/Vs and lelectron 5 1.231025
cm2/Vs for P(BDT-TCNT):PC71BM, and lhole 5 6.631025 cm2/Vs
and lelectron 5 6.931025 cm2/Vs for P(DTBDAT-TCNT):PC71BM
devices, respectively. The lhole/lelectron ratios greater than 1.04
were observed for devices formed of P(DTBDAT-TCNT):PC71BM
and the ratio lhole/lelectron was 0.03 for P(BDT-TCNT):PC71BM,
resulting in superior performance with a larger FF for the
P(DTBDAT-TCNT)-based device.
1 M. Wang, X. Hu, P. Liu, W. Li, X. Gong, F. Huang, Y. Cao, J.
Am. Chem. Soc. 2011, 133, 9638–9641.
2 Kwon, J. Ho, An, J.–Y. Jang, H. Choi, S. Chung, D. S.
Lee, M. Jung, Cha, H.–J. Park, J.–H. Park, C.–E. Kim,
Y. –H. J. Polym. Sci. Part A: Polym. Chem. 2011, 49, 1119–
1128.
3 H. Choi, J. P. Lee, S. J. Ko, J. W. Jung, H. Park, S. Yoo, O.
Park, J. R. Jeong, S. Park, J. Y. Kim, Nano Lett. 2013, 13, 2204–
2208.
4 T. L. Nguyen, H. Choi, S. J. Ko, M. A. Uddin, B. Walker,
S. Yum, J. E. Jeong, M. H. Yun, T. J. Shin, S. Hwang, J.
Y. Kim, H. Y. Woo, Energ. Environ. Sci. 2014, 7, 3040–
3051.
5 H. Bin, Zhi-Guo Zhang, L. Gao, S. Chen, Lian Zhong, L.
Xue, C. Yang, Y. Li, J. Am. Chem. Soc. 2016, 138, 4657–
4664.
6 Jing-De Chen, C. Cui, Yan-Qing Li, L. Zhou, Qing-Dong
Ou, C. Li, Y. Li, Jian-Xin Tang, A. Material 2015, 27, 1035–
1041.
7 N. Wang, Z. Chen, W. Wei, Z. Jiang, J. Am. Chem. Soc. 2013,
135, 17060–17068.
8 E. Wang, Z. Ma, Z. Zhang, K. Vandewal, P. Henriksson, O.
€
Inganas, F. Zhang, M. R. Andersson, J. Am. Chem. Soc. 2011,
133, 14244–14247.
9 I. Meager, R. S. Ashraf, S. Mollinger, B. C. Schroeder, H.
Bronstein, D. Beatrup, M. S. Vezie, T. Kirchartz, A. Salleo, J.
Nelson, I. McCulloch,, J. Am. Chem. Soc. 2013, 135, 11537–
11540.
CONCLUSIONS
10 M. A. Uddin, T. H. Lee, S. Xu, S. Y. Park, T. Kim, S. Song, T.
L. Nguyen, S. Ko, S. Hwang, J. Y. Kim, H. Y. Woo, Chem.
Mater. 2015, 27, 5997–6007.
We investigate the structure–property relations of newly
designed and synthesized P(BDT-TCNT) and P(DTBDAT-
TCNT) materials, the latter of which exhibits an extended
conjugation length, for application in OSCs. The optical band
gaps and HOMO energy levels were similar, OSCs formed
with P(BDT-TCNT) exhibited a PCE of 4.10% with JSC 5 9.06
mA/cm2, with VOC 5 0.77 V, and FF 5 0.58. OSCs formed
using P(DTBDAT-TCNT) exhibited a PCE of 5.83% with
11 W. Li, K. H. Hendriks, A. Furlan, M. M. Wienk, R. A. Janssen,
J. Am. Chem. Soc. 2015, 137, 2231–2234.
12 K. C. Lee, S. Song, J. Lee, D. S. Kim, J. Y. Kim, C. Yang,
ChemPhysChem 2015, 16, 1305–1314.
13 Y. Li, Acc. Chem. Res. 2012, 45, 723–733.
14 C. Cui, Wai-Yeung Wong, Y. Li, Energy Environ. Sci. 2014, 7,
2276–2284.
J
SC 5 12.2 mA/cm2,
VOC 5 0.77 V, and FF 5 0.62. The
improved performance of the P(DTBDAT-TCNT)-based devi-
ces may be explained by considering the larger current due
to shorter interlamella interaction, as a more balanced ratio
15 C. Cui, Z. He, Y. Wu, X. Cheng, H. Wu, Y. Li, Yong Cao, Wai-
Yeung Wong, Energy Environ. Sci. 2016, 9, 885–891.
16 M. Gao, J. Subbiah, Paul, B. Geraghty, M. Chen, B.
Purushothaman, X. Chen, T. Qin, D. Vak, Fiona, H. Scholes,
Scott, E. Watkins, M. Skidmore, Gerard, J. Wilson, Andrew, B.
Holmes, David, J. Jones, Wallace, W. H. Wong, Chem. Mater.
2016, 28, 3481–3487.
l
hole/lelectron
,
resulting from extended conjugation of
P(DTBDAT-TCNT).
17 L. Huo, T. Liu, X. Sun, Y. Cai, Alan, J. Heeger, Y. Sun, Adv.
Mater. 2015, 27, 2938–2944.
ACKNOWLEDGMENTS
This work was supported by the National Research Foundation
(NRF) of Korea (2015R1A2A1A10055620 and 2015
M1A2A2057506) and MOTIE/KEIT(10048317).
18 T. I. Ryu, Y. Yoon, J.-H. Kim, D.-H. Hwang, M. J. Ko, D.-K.
Lee, J. Y. Kim, H. Kim, N.-G. Park, B. Kim, H. J. Son, Macromo-
lecules 2014, 47, 6270–6280.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192
3191

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
19 H.–J. Yun, Y.–J. Lee, S.–J. Yoo, D. S. Chung, Y.–H. Kim, S.
K. Kwon, Chem. Eur. J. 2013, 19, 13242–13248.
23 H.–J. Yun, M. C. Hwang, S. M. Park, R. Kim, D. S. Chung,
Y.–H. Kim, S.–K. Kwon, ACS Appl. Mater. Interfaces 2013, 5,
6045–6053.
20 J. Kim, A.–R. Han, J. H. Seo, J. H. Oh, C. Yang, Chem.
Mater. 2012, 24, 3464–3472.
24 G. Kim, S. Song, J. Lee, T. Kim, T. H. Lee, B. Walker, J. Y.
Kim, C. Yang, Adv. Energ. Mater. 2015, 5, n/a–n/a.
21 S. Park, B. T. Lim, B. S. Kim, H. J. Son, D. S. Chung, Nature
2014, 4, 5482.
25 J. Kim, M. H. Yun, G. H. Kim, J. Lee, S. M. Lee, S. J. Ko, Y.
Kim, G. K. Dutta, M. Moon, S. Y. Park, D. S. Kim, J. Y. Kim, C.
Yang, ACS Appl. Mater. Interfaces 2014, 6, 7523–7534.
22 Yun, H.–J. Kang, S.–J. Yong, X. Kim, S. O. Kim, Y.–H. Noh,
Y.–Y. Kwon, S.–K. Adv. Mater. 2014, 26, 73.00–7307.
3192
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 3182–3192