Comprehensive study of pyrido[3,4-b]pyrazine-based D-π-a copolymer for efficient polymer solar cells

Article abstract of DOI:10.1002/pola.28044

Two D-π-A copolymers, based on the benzo[1,2-b:4,5-b′]-dithiophene (BDT) as a donor unit and benzo-quinoxaline (BQ) or pyrido-quinoxaline (PQ) analog as an acceptor (PBDT-TBQ and PBDT-TPQ), were designed and synthesized as a p-type material for bulk heterojunction (BHJ) photovoltaic cells. When compared with the PBDT-TBQ polymer, PBDT-TPQ exhibits stronger intramolecular charge transfer, showing a broad absorption coverage at the red region and narrower optical bandgap of 1.69 eV with a relatively low-lying HOMO energy level at -5.24 eV. The experimental data show that the exciton dissociation efficiency of PBDT-TPQ:PC₇₁BM blend is better than that in the PBDT-TBQ:PC₇₁BM blend, which can explain that the IPCE spectra of the PBDT-TPQ-based solar cell were higher than that of the PBDT-TBQ-based solar cell. The maximum efficiency of PBDT-TPQ-based device reaches 4.40% which is much higher than 2.45% of PBDT-TBQ, indicating that PQ unit is a promising electron-acceptor moiety for BHJ solar cells.

Full text of DOI:10.1002/pola.28044

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Comprehensive Study of Pyrido[3,4-b]pyrazine-Based
D–p–a Copolymer for Efficient Polymer Solar Cells
Yu-Qi Wu,1 Hsieh-Chih Chen,2,3 Yun-Siou Yang,1 Sheng Hsiung Chang,2
Pin-Jiun Wu,4 Yen-Yi Chu,4 Chun-Guey Wu1,2
1Department of Chemistry, National Central University, Taoyuan City 32001, Taiwan
2Research Center for New Generation Photovoltaics, National Central University, Taoyuan City 32001, Taiwan
3Graduate Institute of Energy Engineering, National Central University, Taoyuan City 32001, Taiwan
4National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
Correspondence to: H.-C. Chen (E-mail: austinchen@ncu.edu.tw) or C.-G. Wu (E-mail: t610002@cc.ncu.edu.tw)
Received 24 September 2015; accepted 5 January 2016; published online 29 January 2016
DOI: 10.1002/pola.28044
ABSTRACT: Two D–p–A copolymers, based on the benzo[1,2-
b:4,5-b0]-dithiophene (BDT) as
donor unit and benzo-
blend is better than that in the PBDT-TBQ:PC71BM blend, which
can explain that the IPCE spectra of the PBDT-TPQ-based solar
cell were higher than that of the PBDT-TBQ-based solar cell.
The maximum efficiency of PBDT-TPQ-based device reaches
4.40% which is much higher than 2.45% of PBDT-TBQ, indicat-
a
quinoxaline (BQ) or pyrido-quinoxaline (PQ) analog as an
acceptor (PBDT-TBQ and PBDT-TPQ), were designed and syn-
thesized as a p-type material for bulk heterojunction (BHJ) pho-
tovoltaic cells. When compared with the PBDT-TBQ polymer,
PBDT-TPQ exhibits stronger intramolecular charge transfer,
showing a broad absorption coverage at the red region and
narrower optical bandgap of 1.69 eV with a relatively low-lying
HOMO energy level at 25.24 eV. The experimental data show
that the exciton dissociation efficiency of PBDT-TPQ:PC71BM
ing that PQ unit is a promising electron-acceptor moiety for
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BHJ solar cells.
2016 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2016, 54, 1822–1833
KEYWORDS: bulk heterojunction; quinoxaline; solar cells; struc-
ture–function relationship; time-resolved photoluminescence
p-type conjugated polymers is of great significance in BHJ
solar cells. Through prolonged effort, the record of power
conversion efficiencies (PCEs) has been achieved in excess of
8–10% for single-junction solar cells.15–23 Nowadays, a num-
ber of strategies have been developed to achieve the PCEs of
10–15% for commercialization on various principles, such as
the design of novel material systems and the device architec-
ture amelioration, with a view to enhance the light absorp-
tion and tuning of the energy band structure of the blended
active layers.24–32
INTRODUCTION Organic photovoltaics (OPVs) have recently
become popular because of their advantages, including low
cost, light weight, flexibility, and solution processability over
conventional solar cells.1 One of the most commonly used
OPV device architectures is a bulk heterojunction (BHJ),
which is composed of an active layer of a p-type polymer
and an n-type fullerene blended together to form a bicontin-
uous interpenetrating network for efficient exciton diffusion,
charge separation, and charge transport flanked by two elec-
trodes.2,3 It has been recognized that an ideal polymer donor
in OPVs should represent broader absorption with high
absorption coefficient in the visible and near-infrared region,
high hole mobility, and suitable electronic energy levels
matching the fullerene acceptor.2,3 In this regard, the combi-
nation of alternating electron-rich donor (D) and electron-
deficient acceptor (A) moieties along the backbone is a
powerful strategy in designing narrow bandgap conjugated
polymer with broad absorbance.4–14 Thus, seeking novel effi-
cient donor and acceptor units to develop well-performed
Benzo[1,2-b:4,5-b0]-dithiophene (BDT) is a representative donor
block for high-mobility polymer semiconductors because of its
rigidity, coplanarity by fusing a benzene with two flanking
thiophene units, high hole mobility, extremely extended p-
conjugation, favorable interchain p–p stacking, and proper side
chain patterns for enhanced solubility.33–35 In addition, quinoxa-
line derivatives with two electron-withdrawing imine nitrogen
atoms are of particular interest as electron-accepting moieties
are not only easily modified but also side-substituted to finely
Y.-Q. Wu and H.-C. Chen contributed equally to this work.
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Schematic illustration of the procedure for the preparation of PBDT-TBQ and PBDT-TPQ copolymers.
tune the optoelectronic properties of the resulting polymers.12–15
Therefore, how to rationally design suitable substituents within
quinoxaline derivatives in combination with BDT unit to achieve
high photovoltaic performance p-type polymer is highly desirable
and of particular importance.
Synthesis of 2,5-Dibromopyridine-3,4-diamine (M2)
Pyridine-3,4-diamine (5.0 g, 45.8 mmol) and 48% HBr(aq)
(62 mL) were added into a 250-mL flask, followed by slowly
adding Br2 (7.5 mL) dropwise. The reaction mixture was
refluxed for 12 h. After cooling, the raw product was collected
by filtration and washed using saturated Na2S2O3 aqueous
solution, saturated NaHCO3 aqueous solution, and deionized
water successively. The crude product was purified by column
chromatography on silica gel using ethyl acetate/hexane (5 1/2)
as an eluent to afford compound M2 (6.5 g, 53%).
In this context, we introduced a new strategy to modify quinoxaline
moiety by incorporating pyridine substitution into the quinoxaline
unit. Therefore, the replacement of substitution of the 6-position
carbon with nitrogen could offer an additional opportunity to alter-
nate the electronegativity of quinoxaline derivatives. Here, we
report a straightforward synthetic methodology to prepare two
medium bandgap conjugated D–p–A copolymers composed of a
BDT donor unit coupled with benzo-quinoxaline (BQ) or pyrido-
quinoxaline (PQ) acceptor segments bridged by a thiophene (see
Scheme 1). The preliminary results showed that PBDT-TPQ poly-
mer could be a promising candidate for solar cell applications.
1H NMR (CDCl3, 300 MHz, d): 7.86 (s, 1H), 4.47 (br, 2H),
3.65 (br, 2H). 13C NMR (CDCl3, 300 MHz, d): 106.88, 128.41,
130.14, 141.22, 142.10. HR-FAB MS m/z calcd for C5H5Br2N3
[M1]: 266.92; found: 266.8827.
Synthesis of 4,40-Dihexyloxybenzil (M3)
A mixture of 4,40-dihydroxybenzil (1.0 g, 4.1 mmol), 1-
bromohexane (1.3 mL, 9.0 mmol), tetra-n-butylammonium
bromide (1.2 g, 4.1 mol), and potassium carbonate (1.2 g,
8.2 mol) in DMF (10 mL) was stirred at 100–120 8C for 2 h.
The reaction mixture was diluted with water, and the redun-
dant precipitate was filtered and washed with water (3 3
50 mL). The crude product was purified by column chroma-
tography on silica gel using ethyl acetate/hexane (5 1/4) as
an eluent to afford compound M3 (0.86 g, 51%).
EXPERIMENTAL
Synthesis of Precursors and Polymers
Synthesis of 3,6-Dibromo-1,2-benzenediamine (M1)
4,7-Dibromo-2,1,3-benzothiadiazole (0.7 g, 2.4 mmol) in
ethyl alcohol (80 mL) was cooled to 0 8C, and then under
argon, sodium borohydride (2.0 g, 52.0 mmol) was added.
After being stirred at 0 8C for 10 min, the reaction mixture
was stirred at room temperature for another 20 h. After
being cooled to 0 8C, the reaction mixture was treated with
distilled water (20 mL) to quench the reaction. After remov-
ing the solvent under reduced pressure, the residue was
diluted with ether and subsequently purified by column
chromatography on silica gel using ethyl acetate/hexane (5
1/3) as an eluent to afford compound M1 (0.56 g, 86%).
1H NMR (CDCl3, 300 MHz, d): 7.92 (d, 4H), 6.94 (d, 4H), 4.02
(t, 4H), 1.80 (m, 4H), 4.00 (t, 4H), 1.35 (m, 12H), 0.96 (m, 6H).
13C NMR (CDCl3, 300 MHz, d): 14.00, 22.56, 25.61, 20.97,
31.50, 68.47, 114.71, 126.09, 132.35, 164.50, 196.40. HR-FAB
MS m/z calcd for C26H30O4 [M1]: 410.55; found: 410.5138.
Synthesis of 5,8-Dibromo-2,3-bis[4-(hexyloxy)phenyl]
quinoxaline (A1)
A solution of 3,6-dibromo-1,2-benzenediamine (M1; 0.9 g,
3.6 mmol), 4,40-dihexyloxybenzil (M3; 1.35 g, 3.6 mmol), and
1H NMR (CDCl3, 300 MHz, d): 6.84 (s, 2H), 3.89 (br, 4H). 13C
NMR (CDCl3, 300 MHz, d): 109.41, 123.41, 133.86. HR-FAB MS
m/z calcd for C6H6Br2N2 [M1]: 265.9364; found: 265.8875.
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p-TSA (0.12 g, 0.69 mmol) in ethyl alcohol (30 mL) was
heated to reflux for 14 h. The solution was cooled to room
temperature, and the reaction mixture was filtered to get the
crude solid. After washing with cold ethanol, the product
was purified by column chromatography on silica gel using
dichloromethane/hexane (5 1/1) as an eluent to afford com-
pound A1 (0.3 mg, 51%).
Synthesis of Poly{2,6-di-2-thienyl-4,8-bis(20-
ethylhexyloxy)-benzo[1,2-b:4,5-b0]dithiophene-alt-
2,3-bis[4-(hexyloxy)phenyl]pyrido[3,4-b]pyrazine}
(PBDT-TPQ)
A mixture of 5,8-dibromo-2,3-bis[4-(hexyloxy)phenyl]pyrido[3,4-
b]pyrazine (A2; 0.3 g, 0.46 mmol), bis(trimethyltin)-2,6-di-2-
thienyl-4,8-bis(20-ethylhexyloxy)-benzo[1,2-b:4,5-b0]dithiophene
(0.4 g, 0.46 mmol), and Pd(PPh3)4 (0.02 g) with toluene
(15 mL) was heated under reflux for 72 h. After cooling to room
temperature, the polymer was precipitated by slow dropwise
addition of the reaction mixture into MeOH (250 mL), filtered
off, and dried in air. The crude polymer was Soxhlet extracted
subsequently with methanol, hexane, and chloroform. The chlo-
roform fraction was concentrated by evaporation under reduced
pressure and precipitated in methanol. The precipitate was
filtered off and dried to afford PBDT-TPQ copolymer (0.28 g,
55%).
1H NMR (CDCl3, 300 MHz, d): 7.84 (s, 2H), 7.64 (d, 4H), 6.87
(d, 4H), 3.99 (t, 4H), 1.79 (m, 4H), 1.36 (m, 12H), 0.91 (m,
6H). 13C NMR (CDCl3, 300 MHz, d): 160.43, 153.54, 138.97,
132.44, 131.63, 130.29, 123.40, 114.32, 68.05, 31.55, 29.13,
25.67, 22.57, 14.02. HR-FAB MS m/z calcd for C32H36Br2N2O2
[M1]: 638.1144; found: 638.1854.
Synthesis of 5,8-Dibromo-2,3-bis[4-
(hexyloxy)phenyl]pyrido[3,4-b]pyrazine (A2)
A solution of 2,5-dibromopyridine-3,4-diamine (M2; 1.0 g,
3.7 mmol), 4,40-dihexyloxybenzil (M3; 1.5 g, 3.7 mmol),
and p-TSA (0.15 g, 0.7 mmol) in ethyl alcohol (30 mL) was
heated to reflux for 16 h. The solution was cooled to room
temperature, and the reaction mixture was filtered
followed by washing with cold ethanol and purified by col-
umn chromatography on silica gel using dichloromethane/
hexane (5 1/1) as an eluent to afford compound A2
(1.2 g, 48%).
1H NMR (CDCl3, 300 MHz, d): 7.79–7.74 (br, 6 H), 6.99–6.90 (br,
7 H), 4.20–4.06 (br, 8 H), 1.92–1.84 (br, 11H), 1.44–1.25 (br,
18H), 1.05–0.88 (br, 24H). Anal. calcd (%) for (C70H81N3O4S5):
C, 70.73; H, 6.87; N, 3.35; S, 13.49; found: C, 70.58; H, 6.60; N,
2.50; S, 10.33.
Measurement and Characterization
All NMR spectra were recorded in deuterated chloroform
(CDCl3) containing 0.003% TMS as an internal reference using a
Bruker 300 spectrometer. Chemical shifts (d), quoted in parts
per million (ppm), and coupling constants (J) were recorded in
hertz (Hz). The molecular weight and molecular weight distri-
bution of the copolymers were measured with a Waters gel
permeation chromatography (GPC; Breeze system) using tetra-
hydrofuran (THF) as an eluent at 35 8C. The apparatus was
equipped with two Waters Styragel columns (HR3 and HR4E), a
refractive index detector (Waters 2414), and a dual-wavelength
absorbance detector (Waters 2487). Polystyrene standards
(Waters) were used for calibration. The absorption spectra
were recorded on a Hitachi U-4100 spectrophotometer. Cyclic
voltammetry (CV) was conducted on an Autolab PGSTAT 30
Electrochemical Workstation with glass carbon, Ag/Ag1, and Pt
wire as working electrode, reference electrode, and counter
electrode, respectively, in a solution of 0.1 M tetrabutylammo-
nium hexafluorophosphate (Bu4NPF6) in deoxygenated acetoni-
trile (CH3CN) at a scan rate of 50 mV s21. The redox potential of
the polymer was corrected using the internal standard of
Fc/Fc1 in CH3CN (0.45 V vs. Ag/Ag1 electrode). Thermogravi-
metric analysis (TGA) and differential scanning calorimetry
(DSC) measurements were performed under a nitrogen atmos-
phere at heating rates of 10 and 5 8C min21 using a TA Instru-
ments (TGA-Q500 and DSC-910S, respectively). Permanent
dipole moment of dyes was calculated using the density func-
tional theory (DFT) with Gaussian 09. Ground-state geometry
optimization was performed using the B3LYP exchange correla-
tion functional. The thickness of the active films was measured
with a depth-profile meter (Veeco Dektak 150, USA). Five lines
on a film of 1 cm 3 1 cm were made by carefully scratching
with a plastic tip, and the average height between the hills and
valleys is used to represent the film thickness. GIXRD patterns
1H NMR (CDCl3, 300 MHz, d): 8.68 (s, 1H), 7.66 (t, 4H), 6.90
(d, 4H), 4.00 (t, 4H), 1.80 (m, 4H), 1.36 (m, 12H), 0.91 (m,
6H). 13C NMR (CDCl3, 300 MHz, d): 193.37, 164.33, 161.41,
132.16, 131.81, 131.53, 125.92, 114.55, 68.31, 31.36, 28.83,
25.46, 22.43, 13.88. HR-FAB MS m/z calcd for C31H35Br2N3O2
[M1]: 639.1096; found: 639.1710.
Synthesis of Poly{2,6-di-2-thienyl-4,8-bis
(20-ethylhexyloxy)-benzo[1,2-b:4,5-b0]dithiophene-alt-2,3-
bis[4-(hexyloxy)phenyl]quinoxaline} (PBDT-TBQ)
A mixture of 5,8-dibromo-2,3-bis[4-(hexyloxy)phenyl]quinoxa-
line (A1; 0.3 g, 0.46 mmol), bis(trimethyltin)-2,6-di-2-thienyl-
4,8-bis(20-ethylhexyloxy)-benzo[1,2-b:4,5-b0]dithiophene (0.4 g,
0.46 mmol), and Pd(PPh3)4 (0.02 g) with toluene (15 mL)
was heated under reflux for 72 h. After cooling to room tem-
perature, the polymer was precipitated by slow dropwise
addition of the reaction mixture into MeOH (250 mL), filtered
off, and dried in air. The crude polymer was Soxhlet extracted
subsequently with methanol, hexane, and chloroform. The
chloroform fraction was concentrated by evaporation under
reduced pressure and then precipitated in methanol. The
precipitate was filtered off and dried to afford PBDT-TBQ
copolymer (0.29 g, 58%).
1H NMR (CDCl3, 300 MHz, d): 7.81–7.67 (br, 8.5 H), 6.97–
6.90 (br, 7 H), 4.25–4.00 (br, 8 H), 1.82–1.74 (br, 10H), 1.41–
1.25 (br, 18H), 1.06–0.94 (br, 24H). Anal. calcd (%) for
(C71H82N2O4S5): C, 71.80; H, 6.96; N, 2.36; S, 13.5; found: C,
68.32; H, 6.78; N, 1.86; S, 14.18.
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were collected using a Rigaku-TTRAX III diffractometer, operat-
ing at an accelerating voltage of 50 kV and current of 300 mA. A
Ni-filtered Cu Ka radiation source (0.154 nm) was used. A 0.48
incident angle, a 0.018 sampling step, a 58 min21 scanning rate,
and 2h of ꢀ38–408 of scanning range were carefully chosen to
allow for complete X-ray penetration into the polymer film.
AFM measurements were performed using PeakForce tapping
mode with PeakForce Quantitative Nanomechanical Mapping
software (Bruker). The set point of the PeakForce applied dur-
ing the measurements was in the range of a few nanonewtons;
the PeakForce frequency was 2 kHz. The PeakForce error imag-
ing technique represents the feedback signal that controls the Z
piezo in the PeakForce tapping mode and is equivalent to the
deflection error in contact mode or amplitude error in tapping
mode. These feedback signals are often used to create 3D
images which help to visualize the topography. Hole mobility
was measured according to a method based on the space
charge–limited current (SCLC) model described in the litera-
ture,36,37 using a diode configuration of ITO/PEDOT:PSS/
polymer: PC71BM/Pd. The SCLC current was measured under
dark conditions using a Keithley Model 2400 unit. Femtosecond
time-resolved photoluminescence (FTR-PL) was then conducted
by the sum-frequency technique (FluoMax, IB Photonics Ltd.)
along with the instrument response function (IRF) of 150 fs,
and the excitation wavelength was 420 nm. To prevent laser-
induced thermal effects, the diameter of the spot size on the
sample was increased to 300 lm, and the excitation power was
reduced to 4 mW. Furthermore, to resolve the exciton dynamics,
the time-dependent photoluminescence was fitted in an appro-
priate exponentially decaying function.38
8C and stirred 8 h for complete dissolution. Then, the blend
solution with 3 vol % 1,8-diiodooctane (DIO) additive was
spin-casted. The wet film was slowly dried in a covered Petri
dish for 3 h in the glove box and then annealed at 110 8C for
10 min. Afterward, methanol was dropped onto the active layer
with a spin-casting rate at 2000 rpm for 60 s, and subse-
quently, the coated substrates were transferred to a thermal
evaporator and evacuated to ꢂ5 3 1026 Torr. Molybdenum
oxide was thermally deposited on top of the active layer with
an evaporation rate of 0.1 Å s21. Ultimately, 100-nm silver film
was deposited on top of the molybdenum oxide layer through
a shade mask. The J–V curves were measured using a Keithley
4200 source-measuring unit. A calibrated solar simulator
(Oriel) with a 100 mW cm22 power density was used as the
light source. An IPCE (QE-R3011) characterization platform
supplied by Enlitech was used for data acquirement. All IPCE
spectra were recorded using a lock-in technique at a chopping
frequency of 100 Hz.
RESULTS AND DISCUSSION
Synthesis and Characterization
Scheme 1 outlines the chemical structure and the synthetic
route for the two copolymers by Stille cross-coupling reac-
tions under thermal heating conditions using Pd(PPh3)4 as a
catalyst. Crude polymers were purified by Soxhlet extraction
with methanol, hexane, and chloroform.
The chloroform solution was concentrated, and the product
was reprecipitated in methanol to obtain the target copoly-
mers .The 1H NMR spectra of the synthesized copolymers
showed a general broadening when compared with those of
the monomers, as depicted in the spectrum of PBDT-TBQ
(Fig. 1) and PBDT-TPQ (Fig. 2). The number–average molecu-
lar weights (Mns) of the synthesized PBDT-TBQ and PBDT-
TPQ polymers were determined by GPC against polystyrene
standards in a THF eluent and were found to be as 10.13
and 10.95 kDa, with polydispersity indices of 1.6 and 1.9,
respectively.
Solar Cell Device Fabrication and Characterization
The inverted PSCs were fabricated with the device structure of
ITO/ZnO (40 nm)/poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis
(30-(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) (5 nm)/
polymer: PC71BM (65-80 nm)/MoO3 (8 nm)/Ag (100 nm).
Prior to use, patterned ITO-coated glass substrates (10 X/
square) were successively cleaned by ultrasonication in 1%
neutral detergent in water, then deionized water, followed by
acetone, and finally isopropanol for 10 min each and subse-
quently dried under a stream of dry nitrogen. The substrates
then underwent UV–ozone treatment for 10 min prior to the
deposition of a ꢀ40-nm-thick layer of ZnO. The ZnO precursor
was prepared using zinc acetate dehydrate [Zn(CH3COO)2ꢁ2H2O,
99.999%; Aldrich) as starting material, and 2-ethoxyethanol
and monoethanolamine (MEA, 99.7%; Aldrich) as the nontoxic
solvent and stabilizer, respectively, under vigorous stirring for
12 h for the hydrolysis reaction in air. The respective solutions
were spin-coated on ITO-coated glass substrates at 5000 rpm
for 30 s and preheated at 180 8C for 30 min in air to remove
residual organic materials and then transferred into an inert
N2-filled glove box (<0.1 ppm O2 and H2O). Subsequently, PFN
interfacial layer was spin-casted onto the active layer according
to a similar method described in the literature.20 Blend
solutions were prepared by dissolving the target polymer
(PBDT-TBQ or PBDT-TPQ) and PC71BM (purchased from Nano-
C) in o-DCB solution in the blend ratios of 1:1 and 1:2 with a
polymer concentration of 12 mg mL21 and were heated to 100
The thermal properties of the polymers were determined by
TGA under a nitrogen atmosphere at a heating rate of 10
8C min21 as shown in Supporting Information Figure S1. Both
polymers are thermally stable with onset decomposition tem-
peratures at 5% weight loss (Td) above 300 8C, which indicates
that the thermal stability of the study copolymers is adequate
for application in optoelectronic devices. Additionally, these
target polymers show neither glass transition temperature nor
melting point in the DSC curves of the second heating and
cooling runs, suggesting that the rigid backbones of these
copolymers limit the chain motion.
Optical and Electrochemical Properties
UV–visible absorption spectra together with energy-level dia-
gram of the study polymers of PBDT-TBQ and PBDT-TPQ are
displayed in Figure 3, and the relevant data are summarized
in Table 1. The absorption spectra of the polymers [Fig.
3(a)] show two distinct absorption bands. The short-
wavelength absorption bands below 430 nm are assigned to
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FIGURE 1 1H NMR spectra (300 MHz) of PBDT-TBQ in CDCl3. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
a delocalized excitonic p–p* transition in the conjugated
main chains, whereas the long-wavelength absorption
bands at around 500–620 nm are reasonably ascribed to
the strong internal charge transfer interaction between the
electron-donating and electron-accepting units in the poly-
mer backbone. The PBDT-TBQ and BDT-TPQ solutions show
their maximum absorptions at 405 and 530 nm and 415
and 575 nm, respectively, whereas those of the correspond-
ing polymer films shift to the longer wavelength region and
show maximum absorptions at 414 and 558 nm and 426
and 590 nm, respectively, indicating intramolecular and/or
intermolecular interactions existing in the solid state. The
optical bandgap (Eogpt) deduced from the absorption edges
of the thin film spectra are in the following order: PBDT-
TPQ (1.69 eV) < PBDT-TBQ (1.81 eV). Furthermore, kmax of
PBDT-TPQ film is red-shifted by 32 nm when compared
with that of PBDT-TBQ, indicating the pyrido-substituted
quinoxaline PQ unit offering stronger electron-withdrawing
capacity than the benzo-substituted quinoxaline BQ analog.
Such significantly enhanced absorption in the red region
may uplift the exciton formation as the solar flux peaks are
here. In addition, PBDT-TPQ film displays
a vibronic
shoulder at ꢀ650 nm, indicating that there exists some
ordered structural organization in the solid state,39 which
should benefit the charge mobility and photovoltaic per-
formance of the polymer.
FIGURE 2 1H NMR spectra (300 MHz) of PBDT-TPQ in CDCl3. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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FIGURE 3 (a) UV–vis absorption spectra of PBDT-TBQ and PBDT-TPQ in chloroform solution and as thin films. (b) Energy-level dia-
gram of the active layer components. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Cyclic voltammetry was used to evaluate the electrochemical
properties and to estimate the energy levels of the polymers.
The HOMO levels were estimated from the onsets of their
corresponding oxidative peaks (Supporting Information Fig.
S2). The LUMO levels were then calculated from the differen-
ces between the HOMO energy level and the optical bandg-
aps. Figure 3(b) depicts a sketch of the energy levels for
PBDT-TBQ, PBDT-TPQ, and [6,6]-phenyl-C71-butyric acid
methyl ester (PC71BM). PBDT-TPQ exhibits a relatively lower-
lying HOMO energy level at 25.24 eV, which is in an ideal
range to ensure better air stability and can be anticipated to
produce a higher open-circuit voltage (Voc), in accordance
with the known linear correlation of Voc with the difference
between the HOMO energy level of a donor and the LUMO
energy level of an acceptor. The LUMO energy levels of these
two polymers are all located within a suitable range from
23.36 to 23.55 eV and are clearly higher-lying than that of
PC71BM, providing sufficient driving force for charge separa-
tion and electron transfer without too much energy loss.
whereas the electron density associated with the LUMO is
fairly well localized at the electron acceptor site. Additionally,
in LUMO wave function, PBDT-TPQ appears to be more effi-
cient in intracharge transfer than that of PBDT-TBQ. The aver-
age torsion angles of PBDT-TBQ between D–p and p–A repeat
units are of 8.18 and 23.58, respectively, whereas the average
torsion angles of PBDT-TPQ between D–p and p–A repeat units
decrease to 7.98 and 20.38, respectively. DFT calculations of the
ground state of the polymers show the permanent dipole
moment of 3.95 and 5.83 debyes for PBDT-TBQ and PBDT-TPQ,
respectively. The slightly higher coplanarity of PBDT-TPQ may
result from the higher dipole moment, which can induce
strong intermolecular interaction (see Supporting Information
Fig. S3).
Evaluation of Charge Carrier Mobility and
Photovoltaic Properties
The hole mobility of PBDT-TBQ and PBDT-TPQ was measured
by SCLC method. PBDT-TPQ demonstrates a relatively higher
hole mobility of 1.4 3 1025 cm2 V21 s21 and is nearly four
times higher than that of the PBDT-TBQ (3.3 3 1026
cm2 V21 s21; Supporting Information Fig. S4). This variation
in the charge transport behavior can be attributed to the rel-
atively ordered structure and stronger intermolecular inter-
actions of PBDT-TPQ polymer as proven by synchrotron
grazing incidence wide-angle X-ray scattering (GIWAXS) anal-
ysis [Fig. 5(a,b)], thereby enabling crystalline ordering and
good uniformity in the molecular conformation. As expected,
the higher hole mobility of PBDT-TPQ yielded a balanced
charge carrier transport in the active layer, which is expected
To evaluate the impact of the different acceptors on the molec-
ular structures and electronic properties, theoretical calcula-
tions by DFT model at the B3LYP/6-31G* level were
performed on trimers. As the side-chain substituents have lit-
tle influence on the oxidative and reductive properties of the
molecule, the alkyl side chains were all truncated to methyl
groups for simplicity. The wave functions of the frontier molec-
ular orbital of model compounds are depicted in Figure 4. As
can be observed, the electron density in the HOMO wave func-
tion is mostly delocalized along the whole polymer backbone,
TABLE 1 Optical and Electrical Characteristics of the Target Polymers
kmax (nm)
kedge (nm)
Polymer
Solution
Film
Film
Eogpt (eV)a
EHOMO (eV)b
ELUMO (eV)c
Td (8C)d
PBDT-TBQ
PBDT-TPQ
405, 530
415, 575
414, 558
426, 590
685
734
1.81
1.69
25.17
25.24
23.36
23.55
329
325
a
c
Estimated from the optical absorption edge in thin films.
Estimated from the onset of the oxidation peaks of cyclic
Values deducted from EHOMO and optical bandgap.
Decomposition temperature (5% weight loss) determined by thermog-
b
d
voltammogram.
ravimetric analysis under nitrogen.
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FIGURE 4 Minimum energy conformations of optimized geometries of (a) PBDT-TBQ and (b) PBDT-TPQ trimers. Gaussian opti-
mized at B3LYP/6-31G* level with visualization of the corresponding HOMO/LUMO energy distributions. Color code: gray (C), white
(H), red (O), blue (N), and yellow (S). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 5 Two-dimensional grazing incidence wide-angle X-ray scattering (GIWAXS) patterns of (a) PBDT-TBQ, (b) PBDT-TPQ neat
films, (c) PBDT-TBQ:PC71BM (1:1), and (d) PBDT-TPQ:PC71BM (1:2) blend films. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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ing along the qz axis at q 5 1.64 Å21 represents the (010) face-
on p–p stacking packing reflection peak of the polymer back-
bones (dp–p 5 3.84 Å). These results manifest that the PBDT-
TPQ film possesses a relatively ordered structure and stronger
intermolecular interactions, which can be supported by the
observation in the UV–vis absorption spectra [Fig. 3(a)], where
the PBDT-TPQ film shows a distinct vibronic shoulder in the
long wavelength.39 On adding PC71BM in PBDT-TBQ and PBDT-
TPQ blend systems, both GIWAXS patterns show a new iso-
tropic broad reflection at q 5 1.34 Å21 due to scattering from
the PC71BM domains. Furthermore, an additional (200) reflec-
tion peak can be clearly observed, indicating that lamellar
stacking of the blend film has further been improved [Fig.
5(c,d)]. The result may arise from the strong polymer–fuller-
ene interactions that delay the fullerene self-assembly in the
final drying process. Therefore, fullerene aggregation during
film formation may promote the self-organization of the
polymer.40,41
To characterize the photovoltaic properties of PBDT-TBQ and
PBDT-TPQ, inverted BHJ PSCs with a device configuration of
ITO/ZnO (40 nm)/PFN (5 nm)/polymer:PC71BM/MoO3 (8 nm)/
Ag (100 nm) were fabricated and examined under simulated
100 mW cm22 AM 1.5G illumination. The incorporation of a thin
PFN cathode interlayer can construct interfacial dipole, which
could result in the reduced electron injection barrier and
enhanced built-in potential across the device.20,27,32 The opti-
mized polymer to PC71BM ratios of PBDT-TBQ and PBDT-TPQ
used to form the active layers of the PSCs were 1:1 and 1:2,
respectively. The polymer active layers of PBDT-TBQ and PBDT-
TPQ were spin-coated from o-DCB solutions with 3 vol % DIO
additives. DIO is commonly used as a solvent additive in PSCs to
improve the device performances owing to its high boiling point
(363 8C) and good solubility for fullerene aggregates to form
favorable nanostructures. We optimized the thickness of the
active layers and found that 65–80 nm was the best thickness
for all devices. Figure 6(a) shows the J–V curves of the champion
PSC for PBDT-TBQ:PC71BM- and PBDT-TPQ:PC71BM-based
devices. The optimal photovoltaic parameters of the devices are
summarized in Table 2. In the optimized 1:2 PBDT-TPQ:PC71BM
device with thermal annealing at 110 8C for 10 min, the Voc
reaches to 0.72 V with a Jsc of 9.69 mA cm22 and an FF of
63.1%, offering a PCE of 4.40%. Conversely, the device based on
optimized 1:1 PBDT-TBQ:PC71BM exhibits an inferior PCE of
2.45%, with a Voc of 0.70 V, a Jsc of 6.50 mA cm22, and an FF of
53.8%, presumably due to the inefficient exciton dissociation
FIGURE 6 (a) J–V curves of optimized devices based on 1:1
PBDT-TBQ:PC71BM and 1:2 PBDT-TPQ:PC71BM solar cells under
AM 1.5G solar illumination, and (b) the corresponding IPCE
spectra of optimized devices illuminated by monochromatic
light. Legend: orange diamonds (᭜) represent the PBDT-TBQ
device (Voc 5 0.70 V, Jsc 5 26.50 mA cm22, FF 5 53.8%, and
PCE 5 2.45%), and royal blue circles (•) represent the PBDT-TPQ
device (Voc 5 0.72 V, Jsc 5 29.69 mA cm22, FF 5 63.1%, and
PCE 5 4.40%). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
to contribute in part, not just to the higher short-circuit cur-
rent density (Jsc) but also to higher fill factor (FF) and over-
all better device performance elaborated below.
To gain deeper insight into the molecular packing and nano-
structural order within the active layer, GIWAXS analysis was
performed to examine the neat PBDT-TBQ and PBDT-TPQ films
as well as the optimized 1:1 PBDT-TBQ:PC71BM and 1:2 PBDT-
TPQ:PC71BM blend films. In Figure 5(a), the 2D GIWAXS pat-
tern of the neat PBDT-TBQ film shows an anisotropic ring-
scattering pattern at q 5 0.34 Å21 with an interlayer d100 spac-
ing of 18.52 Å, corresponding to the periodic lamellae of poly-
mer backbones. In contrast, the neat PBDT-TPQ film [Fig. 5(b)]
shows arc and anisotropic ring-scattering patterns. The Bragg
scattering peak of (100) appears at q 5 0.31 Å21 with an inter-
layer d100 spacing of 20.06 Å, whereas a distinctive arc scatter-
TABLE 2 Photovoltaic Parameters of Optimized Solar Cells
Polymer:
PC71BM
Jsc (mA
Voc (V) cm22
Polymer
)
FF (%) PCEa (%)
PBDT-TBQ 1:1
PBDT-TBQ 1:2
PBDT-TPQ 1:1
PBDT-TPQ 1:2
0.70
0.68
0.72
0.72
6.50
5.12
8.26
9.69
53.8
51.7
54.7
63.1
2.45 (2.28)
1.80 (1.57)
3.25 (3.09)
4.40 (4.28)
a
Average PCE of more than 20 optimized devices in parentheses.
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8.92 mA cm22 for PBDT-TBQ and PBDT-TPQ devices, respec-
tively. The Jsc values calculated from the integration of the
IPCE spectra are within a 5% error, which agrees well with
those obtained from the J–V measurements, supporting the
reliability of the photovoltaic measurement.
To better understand the morphological effect of the optimized
active layers on the OPV performance, we investigated the
morphological discrepancies of the polymer:PC71BM blends by
PeakForce tapping-mode atomic force microscopy. Figure 7
shows the topographic and PeakForce error images of the best
performing active films on 1:1 PBDT-TBQ:PC71BM and 1:2
PBDT-TPQ:PC71BM blends. The topography of these two active
films is prominently different. PBDT-TBQ:PC71BM blend film
clearly exhibits a quite large spherical domain (ꢀ100 nm)
with a root-mean-square (rms) surface roughness of 3.5 nm.
The relatively larger globular clusters and poorly formed
bicontinuous percolation pathways result in the geminate
recombination, limited charge transport, and/or collection
processes, as confirmed by the low charge mobility, photocur-
rent, and FF of the corresponding devices.2,3 In marked con-
trast, the surface of PBDT-TPQ:PC71BM blend film is extremely
uniform in height with an rms value of 2.2 nm. The long well-
defined nanoscale phase separations can be seen more clearly,
which is beneficial to efficient exciton dissociation at the D/A
interfaces and efficient charge carrier transport to the electro-
des.2,3 This may be attributable to the fact that the polar nitro-
gen atom in the PQ moiety may possess better interaction with
PCBM when compared with the nonpolar BQ moiety. These
results could demonstrate the merit of using PQ as an acceptor
subunit in the D–A polymer to some extent.
FIGURE 7 Morphology characterization of PeakForce tapping-
mode atomic force microscopy topographic images (upper
row) and PeakForce error images (lower row) of 1:1 PBDT-
TBQ:PC71BM blend films with 3 vol % DIO [Panels (a) and (c)]
and 1:2 PBDT-TPQ:PC71BM blend films with 3 vol % DIO [Pan-
els (b) and (d)]. The imaging size is 2 lm 3 2 lm for each
panel. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
and low carrier mobility. Further increase of the fullerene ratio
in PBDT-TBQ:PC71BM blends yielded negative effect in both Jsc
and FF, which resulted in a decreased PCE. As expected from the
low-lying HOMO energy levels of the polymers, the trend in Voc
agrees well with the electrochemical potentials. Note that PBDT-
TPQ-based devices show dramatically higher Jsc and FF than
those of PBDT-TBQ-based devices. The reasons can be attribut-
able to the better backbone coplanarity, enhancing absorption in
the long wavelength region, optimal morphology with proper
domain size of the active layer, and higher charge carrier mobil-
ity that can promote the exciton separation and charge trans-
port, as well as enhanced charge collection efficiency. The
moderate PCE may be attributed to a lower molecular weight as
observed in other studies.15,42,43 Nevertheless, the better device
performance of PBDT-TPQ over that of PBDT-TBQ clearly sug-
gests that the newly developed PQ acceptor unit is better than
the widely used BQ acceptor unit to construct highly efficient
PSCs.
To address the exciton lifetime and charge carrier dynamics
of the neat and polymer blend films, FTR-PL was then con-
ducted along with the IRF of 150 fs, and the excitation wave-
length was 420 nm. The broadband exciton dynamics of the
PBDT-TBQ and PBDT-TPQ dissolved in THF are shown in Fig-
ure 8. The spectral width of the PBDT-TPQ solution is nar-
rower when compared with that of PBDT-TBQ solution,
indicating that the molecular structure of PBDT-TPQ is more
coplanar than that of PBDT-TBQ.
Figure 9(a) depicts the dynamics of the FTR-PL for the pris-
tine PBDT-TBQ and PBDT-TPQ films with various emission
peak wavelengths. To acquire the relaxation time, the experi-
mental results were fitted in a two-constant exponentially
decaying function as follows:
To evaluate the photoresponse of PBDT-TBQ and PBDT-TPQ
and to calibrate the Jsc data, the corresponding IPCE spectra
of the devices elaborated above were measured under illumi-
nation of monochromatic light [Fig. 6(b)]. When compared
with the absorption spectra of the pristine polymers, the
conspicuously broadened IPCE responses in the visible
region can be attributed to both the intrinsic absorptions of
the polymers and the response of PC71BM. In the IPCE spec-
tra, PBDT-TPQ blend exhibits a very broad panchromatic
spectrum over the entire excitation spectral range with rela-
tively high IPCE values above 50% in the 458–572 nm range.
The integrated Jsc values from the IPCE spectra are 6.10 and
2t=se
I
PL5Aeee2t=s 1Aee
;
(1)
ee
where Aee is the amplitude for the exciton–exciton annihila-
tion, Ae is the amplitude for exciton, see is the time constant
of the exciton–exciton annihilation, and se is the exciton life-
time. The exciton lifetimes for the pristine PBDT-TBQ and
PBDT-TPQ films are 712 ps and 491 ps, respectively (see
Table 3), which are inversely proportional to the exciton
binding energy (the energy for the exciton to dissociate in
the polymer blends). To gain a deeper insight into the origin
of IPCE of PBDT-TBQ- and PBDT-TPQ-based solar cells, PL
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FIGURE 8 Broadband time-resolved photoluminescence. The
pulse duration, repetition rate, and average power of the excitation
are 100 fs, 80 MHz, and 4 mW, respectively: (a) PBDT-TBQ polymer
and (b) PBDT-TPQ polymer dissolved in THF. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.
com.]
FIGURE 9 (a) Normalized time-resolved photoluminescence
intensities of PBDT-TBQ and PBDT-TPQ polymers dissolved in
THF at the maximum peak wavelength. (b) Transient photolumi-
nescence spectra of PBDT-TBQ:PC71BM and PBDT-TPQ:PC71BM
blend films along with the excitation wavelength of 420 nm and
the initial time of 0.5 ps. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
quenching experiments were performed for the blend films,
and the exciton dissociation at the region of charge transfer
radius44 was also calculated. The PL spectra of PBDT-
TBQ:PC71BM and PBDT-TPQ:PC71BM blends [Fig. 9(b)] at the
initial time (t 5 0.5 ps) infer that the photo-excited electrons
in PBDT-TBQ and PBDT-TPQ originally relax to a lower meta-
stable state via a rapid electron–phonon interaction (thermal
relaxation). In this meta-stable state, the main exciton disso-
ciation can be conspicuously observed at the emission wave-
lengths of 650 and 700 nm for the PBDT-TBQ:PC71BM and
PBDT-TPQ:PC71BM blends, respectively.
ton diffusion; s is the time constant of the exciton–exciton
annihilation or the exciton dissociation; and se is the exciton
diffusion lifetime. The corresponding data are also summar-
ized in Table 4. For the neat PBDT-TBQ and PBDT-TPQ films,
the trends in the exciton lifetimes are similar to those of the
solution obtained from the FTR-PL results. In addition, simi-
lar time constants (0.62 and 0.67 ps, within the experimental
error) of the excition dissociation are also obtained for these
two blends because of the balanced competition between the
built-in driving force in the charge transfer radius and the
exciton mobility. It is noteworthy that the addition of
PC71BM reduces the exciton diffusion lifetime in the PBDT-
PL dynamics of the pristine and polymer blend films are
shown in Figure 10. To obtain the relaxation time, the exper-
imental results were fitted into a two-constant exponentially
decaying function as follows:45
TABLE 3 Decay Parameters of Target Polymers in THF Solutions
Polymer
A1 (%)
see (ps)
A2 (%)
se (ps)
PL5Ae2t=s1Aee2t=s
;
(2)
e
I
PBDT-TBQ
PBDT-TPQ
35
41
9.74
9.56
65
59
712
491
where A is the amplitude for the exciton–exciton annihilation
or the exciton dissociation; Ae is the amplitude for the exci-
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TABLE 4 PL Decay Parameters of the Neat Polymers and Blend
Films
Sample
A (%)
s (ps)
Ae (%)
se (ps)
PBDT-TBQ
83
83
78
82
0.68
0.62
0.81
0.67
17
17
22
18
9.23
7.64
8.67
11.74
PBDT-TBQ:PC71BM
PBDT-TPQ
PBDT-TPQ:PC71BM
CONCLUSIONS
In conclusion, we have designed and synthesized two new D–
p–A copolymers, PBDT-TBQ and PBDT-TPQ, based on a BDT
donor moiety together with a BQ or a PQ acceptor unit bridged
by the thiophene between the donor and acceptor units, for
studies on the relationship of the molecular structure and pho-
tovoltaic performance. The results showed that by the incorpo-
ration of strong electron-withdrawing PQ unit, PBDT-TPQ
copolymer not only offered excellent solubility and thermal
stability but also exhibited many desirable advantages such as
deeper HOMO and LUMO energy levels, semicrystallinity, and
higher hole mobility. In comparison with PBDT-TBQ, PBDT-TPQ
promoted the formation of well-organized domains in the
polymer blend, in agreement with the red-shifted absorption
spectra, enhanced charge mobility, photocurrent, and FF of the
corresponding devices. The experimental data showed that the
exciton dissociation efficiency of PBDT-TPQ:PC71BM blend was
better than that in the PBDT-TBQ:PC71BM blend, which can be
used to explain that the IPCE spectra of the PBDT-TPQ-based
solar cell were higher than that of the PBDT-TBQ-based solar
cell. Solar efficiencies of the best performing PBDT-TBQ- and
PBDT-TPQ-based devices were 2.45% and 4.40% in an
inverted device architecture, suggesting that PQ moiety could
be a novel promising building block for organic semiconduct-
ing materials.
FIGURE 10 Photoluminescence quenching characteristics of (a)
the pristine PBDT-TBQ and PBDT-TBQ:PC71BM blend film and
(b) the pristine PBDT-TPQ and PBDT-TPQ:PC71BM blend film.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
TBQ owing to the built-in driving force in the region of
charge transfer radius. Conversely, the exciton diffusion life-
time in PBDT-TPQ is extended from 8.67 to 11.74 ps with
the addition of PC71BM, which is beneficial for the exciton
dissociation. It seems that the addition of PC71BM appears to
improve the crystallinity of the PBDT-TPQ blend to some
extent and quite likely is responsible for the differences in
exciton lifetime of these two polymer blends. Moreover, the
efficiency of the exciton dissociation in the region of charge
transfer radius can be evaluated from the following
equation:
ACKNOWLEDGMENTS
This work was financially supported by the Ministry of Science
and Technology of Taiwan. The device fabrication was carried
out in the Advanced Laboratory of Accommodation and
Research for Organic Photovoltaics, Ministry of Science and
Technology of Taiwan. The authors thank Ting-Yang Kuo and
Chun-Hsien Chen, National Taiwan University, for their support
with the atomic force microscopy.
g5ð1=sÞ=ð1=s11=seÞ;
REFERENCES AND NOTES
where s is the exciton dissociation time and se is the exciton dif-
fusion lifetime. The experimental data show that the exciton dis-
sociation efficiency is better in the PBDT-TPQ:PC71BM blend
(g 5 94.6%) than that in the PBDT-TBQ:PC71BM blend
(g 5 92.5%), which can be used to explain that the IPCE spectra
of the PBDT-TPQ-based OPV (ꢀ41%) is higher than that of the
PBDT-TBQ-based OPV (ꢀ35%) at the wavelength of 420 nm.
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