Synthesis and photovoltaic application of low-bandgap conjugated polymers by incorporating highly electron-deficient pyrrolo[3,4-d]pyridazine-5,7-dione units

Article abstract of DOI:10.1016/j.polymer.2016.04.011

Two donor-acceptor polymer semiconductors based on highly electron-deficient pyrrolo[3,4-d]pyridazine-5,7-dione unit were synthesized by Stille cross-coupling polymerization, and their thermal property, photophysical property, electrochemical property, microstructure, application as organic thin-film transistors, and photovoltaic property were investigated. Because of the strong electron-accepting characteristic of pyrrolo[3,4-d]pyridazine-5,7-dione, the new polymers (P1 and P2) exhibited much wider absorption, smaller bandgaps (1.70 vs 1.98 eV), and deeper LUMO levels (-3.60 vs -3.37 eV) than those of a phthalimide-based polymer PBDT-PhBT (Figure 1). The fabricated organic thin-film transistor devices exhibited hole-transport behavior, and the highest mobility of 1.14 × 10^-3 cm² V^-1 s^-1 was obtained. The bulk-heterojunction solar cells based on the two polymers as the electron donors and PC₇₁BM as the electron acceptor showed a high open-circuit voltage and achieved a power conversion efficiency of 2.71% and 3.66% for polymers P1 and P2, respectively.

Full text of DOI:10.1016/j.polymer.2016.04.011

Accepted Manuscript
Synthesis and photovoltaic application of low-bandgap conjugated polymers by
incorporating highly electron-deficient pyrrolo[3,4-d]pyridazine-5,7-dione units
Guobing Zhang, Jie Zhang, Guanqun Ding, Jinghua Guo, Hongbo Lu, Longzhen Qiu,
Wanli Ma
PII:
S0032-3861(16)30278-6
10.1016/j.polymer.2016.04.011
JPOL 18598
DOI:
Reference:
To appear in: Polymer
Received Date: 7 January 2016
Revised Date: 14 March 2016
Accepted Date: 8 April 2016
Please cite this article as: Zhang G, Zhang J, Ding G, Guo J, Lu H, Qiu L, Ma W, Synthesis and
photovoltaic application of low-bandgap conjugated polymers by incorporating highly electron-deficient
pyrrolo[3,4-d]pyridazine-5,7-dione units, Polymer (2016), doi: 10.1016/j.polymer.2016.04.011.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis and photovoltaic application of low-bandgap conjugated polymers by
incorporating highly electron-deficient pyrrolo[3,4-d]pyridazine-5,7-dione units
Guobing Zhang a, c, d, *, Jie Zhang a, c, Guanqun Ding b, Jinghua Guoa, c, Hongbo Lua, c,
d, Longzhen Qiu a, c, d*, Wanli Ma b,*
a
Key Lab of Special Display Technology, Ministry of Education, National
Engineering Lab of Special Display Technology, State Key Lab of Advanced Display
Technology, Academy of Opto-Electronic Technology, Hefei University of
Technology, Hefei, 230009, China. E-mail: gbzhang@hfut.edu.cn
b Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University,
199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, P. R. China.
E-mail: wlma@suda.edu.cn
c
Department of Polymer Science and Engineering, School of Chemistry and
Chemical Engineering, Hefei University of Technology, Hefei, 230009, China
d
Anhui Key Laboratory of Advanced Functional Materials and Devices, Anhui
Province, China
Abstract
Two donor–acceptor polymer semiconductors based on highly electron-deficient
pyrrolo[3,4-d]pyridazine-5,7-dione unit were synthesized by Stille cross-coupling
polymerization, and their thermal property, photophysical property, electrochemical
property, microstructure, application as organic thin-film transistors, and photovoltaic
property were investigated. Because of the strong electron-accepting characteristic of
pyrrolo[3,4-d]pyridazine-5,7-dione, the new polymers (P1 and P2) exhibited much
wider absorption, smaller bandgaps (1.70 vs 1.98 eV), and deeper LUMO levels
(–3.60 vs –3.37 eV) than those of a phthalimide-based polymer PBDT-PhBT (Figure
1). The fabricated organic thin-film transistor devices exhibited hole-transport
behavior, and the highest mobility of 1.14 × 10–3 cm2V–1s–1 was obtained. The
bulk-heterojunction solar cells based on the two polymers as the electron donors and
PC71BM as the electron acceptor showed a high open-circuit voltage and achieved a
power conversion efficiency of 2.71% and 3.66% for polymers P1 and P2,
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respectively. This is the first report on photovoltaic performances of
pyrrolo[3,4-d]pyridazine-5,7-dione-based polymers.
Key words: Pyrrolo[3,4-d]pyridazine-5,7-dione Low bandgap Polymer solar cells
1. Introduction
Over the past decade, polymer solar cells (PSCs) have attracted tremendous attention
owing to their significant advantages of low cost, light weight, large area, flexibility,
and application as environmentally friendly energy-converting devices [1-9]. So far, the
bulk hetero-junction (BHJ)-type PSCs based on the blending of electron-donating
conjugated polymers and high-electron-affinity fullerene derivatives have become the
most successful device structure for PSCs [10-12]. Power conversion efficiency (PCE) is
the product of the short-circuit current density (Jsc), open-circuit voltage (Voc), and
fill factor (FF) divided by the incoming light power density. The established design
rules for electron donor conjugated polymers in fullerene-based BHJ devices are as
follows: a low level of the highest occupied molecular orbital (HOMO) for a high Voc,
a low bandgap for a large solar photon harvest, a suitable energy offset between the
lowest unoccupied molecular orbital (LUMO) of the electron donor polymer and that
of the fullerene for efficient charge separation [13-16]. As one of the building units in
conjugated copolymers, phthalimide (Ph) unit has attracted much interest because of
its inherent advantages. Phthalimide can be easily synthesized and has a symmetric,
rigidly fused, and coplanar structure, which can facilitate the electron delocalization
when incorporated into conjugated polymers [17, 18]. Moreover, its relatively high
electron-withdrawing ability would lead to lower HOMO energy levels
(approximately from –5.2 to –5.4 eV) of the conjugated polymers. Usually, a lower
[17, 19]
HOMO energy level is beneficial to obtain a higher Voc of PSCs
.
Phthalimide-based conjugated polymers have been widely explored for organic
electronic applications and shown high mobility in organic thin-film transistors
(OTFTs) [20]. However, phthalimide-based donor–acceptor (D–A) polymers have
relatively high LUMO levels and thus exhibit a wide bandgap (Eg > 2 eV), which
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adversely effects the solar photon harvest. Moreover, an energy difference between
the LUMOs (donor/PCBM) is larger than the minimum value (~0.3 eV) may not be
advantageous, and indeed wastes energy that does not contribute to the device
[11, 21-22].
performance
Further improvement of phthalimide-based polymers by
reducing the bandgap while maintaining a low HOMO level is needed for fully
exploring their potential as low-bandgap polymers for PSCs applications. Previous
studies indicate that the HOMO level of D–A polymers was determined by the
electron-donor unit, whereas the LUMO level was mainly controlled by the
electron-acceptor unit [23, 24]. Therefore, the incorporation of a more electron-deficient
acceptor to lower the LUMO level, which would result in a smaller bandgap and
maintain a low HOMO level, may be an efficient method.
Compared to benzene, pyridazine is highly electron deficient. If the
pyrrolo[3,4-d]pyridazine-5,7-dione unit is introduced into D–A polymers by replacing
the benzene ring of Ph with pyridazine, the polymers would have lower LUMO
energy and smaller bandgap than the phthalimide-based polymers [25, 26]. In this study,
this strategy was applied. Herein, we reported the synthesis and characterization of
two D–A polymers (Figure 1, polymers P1 and P2) using highly electron-deficient
pyrrolo[3,4-d]pyridazine-5,7-dione as the electron acceptor and benzodithiophene
(BDT) as the electron donor. Consequently, compared to polymer PBDT-PhBT,
polymers P1 and P2 exhibited much wider absorptions, much smaller bandgaps,
lower LUMO levels, and similar deep HOMO levels. The hole-transport and
photovoltaic performances with polymers P1 and P2 as electron donors and PCBM as
the electron acceptor were also investigated.
2. Experimental part
2.1 Material and instruments
All chemicals were purchased from commercial suppliers and used without further
purification. Toluene was distilled over sodium under nitrogen prior to use.
2,6-Bis(trimethylstannyl)-4,8-bis(2-ethylhexyloxy)benzol[1,2-b:4,5-b’]dithiophene
and 2,6-bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzol[1,2-b:4,5-b’]-
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dithiophene was purchased from Suna Tech Inc. Monomer 1-3 and M were prepared
[25,
1
according to the literature
26]. The H NMR spectra were recorded by a
VNMRS600 MHz spectrometer. The molecular weights of the polymers were
measured by GPC using a Waters Series 1525 gel at 100 oC, and
1,2,4-trichlorobenzene was used as eluent and polystyrene as the standard. The
thermal analyses were performed on a TA instrument QS000IR under a nitrogen
atmosphere and a heating rate of 10 °C/min. DSC measurements were conducted
under nitrogen on a TA Instruments Q2000, and the samples were heated at a rate of
10 °C/min. The UV–visible absorption spectra were obtained from a Perkin Elmer
model UV–visible spectrophotometer. The cyclic voltammetry (CV) was performed
on a CHI 660D electrochemical workstation equipped with a three-electrode system
in an acetonitrile solution of 0.1 M Bu4NPF6 and at a scan rate of 100 mV/s.
Ferrocene was used as the standard, and the polymer thin films were coated on a
platinum disc electrode, which was used as the working electrode. A platinum wire
was used as the auxiliary electrode, and an Ag/Ag+ electrode was used as the
reference electrode. The morphology of the polymer films was investigated using a
Veeco Multimode V AFM in tapping mode equipped with a 1 ꢀm scanner.
2.2 Fabrication and characterization of polymer solar cells
PSCs were fabricated with a general structure of ITO/PEDOT-PSS (40 nm)/
polymer: PCBM/LiF/Al. Patterned ITO glass substrates were cleaned by sequential
ultrasonic treatment in detergent, isopropyl alcohol and acetone, and the organic
residue was further removed by treating with UV-ozone for 15 min. ITO substrates
were spin-coated with a thin film about 40 nm of PEDOT:PSS and dried at 150 oC for
10 min. A blend of polymer and PCBM was solubilized in chloroform solvent with or
without DIO, filtered through a 0.45 mm poly(tetra-uoroethylene) (PTFE) filter,
spin-coated at different rpm (1500 rpm) for 40 s, 0.6 nm of LiF (0.1 A s-1) and 100 nm
Al (2 A s-1) layers were then thermally evaporated on the active layer at a pressure of
1.0 × 10-6 mbar through a shadow mask (active area 72.5 mm2). The current–voltage
characteristics of the photovoltaic cells were measured on a Keithley 2400 (I–V)
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Digital source meter under one sun of simulated air mass 1.5G (AM 1.5G) solar
irradiation of 100 mW/cm2 (Newport Stratford, Inc., 94023A).
2.3 Fabrication and characterization of field-effect transistors
The hole transport properties of the two polymers were investigated using a
bottom-gated/top-contact OTFTs configuration, with a 300-nm-thick SiO2 dielectric
on a heavily N-type-doped silicon substrate as the gate electrode. A fluoropolymer
Cytop was spin-coated to reduce the number of hydroxyl groups present on the SiO2
surface. The organic semiconductor thin films were prepared by spin-coating of a 5
mg/mL chloroform solution at 8000 rpm. The polymer films were subsequently
annealed at 180 °C for 30 min in a nitrogen-filled glove box. Then, the Au
source/drain electrodes were evaporated on top of these semiconductor layers (40 nm).
The OTFTs devices had a channel length (L) of 100 ꢀm and a channel width (W) of
800 ꢀm. The OTFTs measurements were carried out under ambient environment
using a Keithley 4200-SCS semiconductor parametric analyzer on the probe stage.
The mobilities of holes (µh) were obtained from the following equation at the
saturation regime: Id = (W/2L)Ciµ(Vg− Vth)2, where W/L is the channel width/length, Id
is the drain current in the saturated regime, Ci is the capacitance of the Cytop gate
dielectric, and Vth is the threshold voltage.
2.4 Synthesis
2.4.1 Synthesis of P1
Pd2(dba)3 (0.0072 g, 0.0079 mmol), P(o-tolyl)3, (0.0096 g, 0.032 mmol),
2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b']dithiophene (0.152 g,
0.20
mmol)
and1,4-bis(5-bromothiophene-2-yl)-6-decyltetradecyl-5H-pyrrolo[3,4-d]pyridazine-5,
7-dione (0.159 g, 0.20 mmol) were mixed in toluene (8 mL). The solution was
subjected to three cycles of admission and evacuation of nitrogen before heated to
105 ꢀ for 48h.After cooled to room temperature, methanol and HCl were added and
stirred for 2 h. The precipitate was collected by filtration. The product was purified by
washing with methanol and dichloromethane in a Soxhlet extractor for 24 h each. It
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was extracted with hot chloroform in an extractor for 24 h. After removing solvent,
the purple solid was collected (0.14 g, 62.9%). 1H NMR (600 MHz, CDCl3, ppm): δ =
8.45–8.95 (br, 2H), 6.28–7.21 (br, 4H), 3.54–4.59 (br, 6H), 0.99–2.10 (br, 56H),
0.60–0.95 (br, 18H). Anal. calcd for C64H91N3O4S4 (%): C, 70.22, H, 8.38, N, 3.84,
found (%) C, 70.06, H, 8.56, N, 3.68.
2.4.2 Synthesis of P2
The same procedures were used as for P1. Compounds used were Pd2(dba)3
(0.068
2,6-bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)-thiophen-2-yl)benzo[1,2-b:4,5-b']dithio
phene (0.17 g, 0.19 mmol),
g,
0.0074
mmol),
P(o-tolyl)3
(0.0090
g,
0.030
mmol),
1,4-bis(5-bromothiophene-2-yl)-6-decyltetradecyl-5H-pyrrolo[3,4-d]pyridazine-5,7-di
one (0.15 g, 0.19 mmol). After workup, the purple solid were obtained (0.13 g,
1
55.1%). H NMR (600 MHz, CDCl3, ppm): δ = 8.30–8.88 (br, 2H), 6.31–7.59 (br,
8H), 3.45–4.08 (br, 2H), 2.65–3.36 (br, 4H), 0.92–2.13 (br, 56H), 0.68–0.92 (br, 18H).
Anal. calcd for C72H95N3O2S6 (%): C, 70.48, H, 7.80, N, 3.42, found (%) C, 70.36, H,
7.76, N, 3.35.
3. Results and discussion
3.1 Synthesis and characterization
Scheme 1 shows the syntheses of the monomers and polymers. The monomers
were synthesized according to a previous literature report [25, 26]. Polymers P1 and P2
were synthesized by Stille cross-coupling reactions in a 1:1 monomer ratio with the
presence of Pd2(dba)3 as the catalyst and P(o-tolyl)3 as the ligand. The two polymers
showed good solubility in organic solvents such as tetrahydrofuran (THF), chloroform,
and chlorobenzene at room temperature. The number-average molecular weights and
polydispersity indexes (PDI) of the polymers were determined by gel permeation
chromatography (GPC) with trichlorobenzene as the eluent and polystyrene as the
o
standard at a column temperature of 100 C. The GPC plots are shown in Figure S1
and S2 (Supporting Information). The number-average molecular weights of polymers
P1 and P2 were 12.4 kDa and 11.9 kDa with PDIs of 1.58 and 1.73, respectively.
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Figure 2 shows the thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) curves of the two polymers. A weight loss of 5% was selected as
the onset decomposition point. Polymers P1 and P2 were thermally stable up to 377
and 380 °C, respectively. The two polymers showed a good thermal stability for
applications in optoelectronic devices; none of them showed significant transition in
the DSC analyses.
3.2 Optical properties
The UV–visible absorption spectra of polymers PBDT-PhBT, P1, and P2 in
chloroform solutions and as thin films are shown in Figure 3. The corresponding
optical data are summarized in Table 1. As shown in Figure 3a, the three polymers
PBDT-PhBT, P1, and P2 showed absorption maxima at 462, 532, and 536 nm in
solutions, respectively. After the spin-coating into a thin film, the absorption bands of
polymers P1 and P2 were broadened, and the absorption maxima of polymers P1 and
P2 were located at 543 and 546 nm (Figure 3b), red-shifted by approximately 11 and
10 nm, respectively. The absence of strong red-shifts from the solutions to the films
can be attributed to the probable preaggregation in solutions [27, 28]. The absorption
spectra of polymers P1 and P2 covered the UV–visible region ranging from 300−730
nm, much broadened than that of polymer PBDT-PhBT. The edges of the film
absorption bands for polymers PBDT-PhBT, P1, and P2 were located at 627, 730,
and 730 nm, respectively. The corresponding optical bandgaps of polymers
PBDT-PhBT, P1, and P2 were 1.98, 1.70, and 1.70 eV, respectively. The
replacement of the phthalimide unit with the stronger electron-deficient
pyrrolo[3,4-d]pyridazine-5,7-dione unit may result in stronger D–A interactions,
which may effectively extend the absorption spectra. As a result, the corresponding
polymers exhibited much smaller bandgaps.
3.3 Electrochemical properties
The electrochemical properties of the polymers were investigated by cyclic
voltammetry (CV). Figure 4 shows the CVs of polymers P1 and P2 films on a
platinum disc electrode in an acetonitrile solution containing 0.1 M Bu4NPF6. The
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corresponding data were summarized in Table 1. The onset oxidation potentials of
polymers P1 and P2 were 0.47 and 0.59 V, respectively, whereas the onset reduction
potential of polymers P1 and P2 had the same value of –1.15 V. The HOMO levels of
the two polymers P1 and P2 were calculated to be –5.22 and –5.34 eV, respectively,
close to the HOMO level (–5.32 eV) of polymer PBDT-PhBT. A deep HOMO level
should help the two polymers achieve a good stability against oxidation. The LUMO
levels of the two polymers were both –3.60 eV, significantly deeper than that of
polymer PBDT-PhBT (–3.37 eV). The low LUMO levels of the two polymers can be
attributed to the strong electron-deficient pyrrolo[3,4-d]pyridazine-5,7-dione units in
the polymer backbone. It can be concluded that the replacement of alkoxy side chains
with alkylthienyl groups lowers the HOMO level, thus increasing the Voc of the PSCs.
The electrochemical band gaps of polymers P1 and P2 (1.62/1.74 eV) are matched
well with their optical band gaps (1.70/1.70 eV) within the experimental error.
3.4 Theoretical calculations
For better understanding of the molecular architecture of the polymers, molecular
simulation was carried out for polymers PBDT-PhBT, P1 and P2 using density
functional theory (DFT) at B3LYP/6-31g level with the Gaussian 09. To simplify the
calculation, all the alkyl and alkoxy groups were replaced with methyl and methoxy
groups, respectively. Figure 5 shows the calculated molecular orbital geometry and
energy levels on the model compound of the polymers. The HOMO levels of
polymers are mainly delocalized over the benzothiophene and thiophene units,
therefore, the three polymers have similar deep HOMO levels. In contrast to the
HOMO levels, the LUMO levels are distributed on the acceptor units. Since the
pyrrolo[3,4-d]pyridazine-5,7-dione shows stronger electron-deficient than that of
phthalimide unit, polymers P1 and P2 have much lower LUMO levels compared to
that of PBDT-PhBT. This result indicates that the LUMO levels can be effectively
lowered by changing the acceptor unit.
3.5 Hole mobility
The hole mobilities of pristine polymers P1 and P2 in films were investigated by
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fabricating OTFTs with bottom-gate, top-contact configurations. The polymer films
were subsequently annealed at 180 °C in a glove box under nitrogen atmosphere. The
output and transfer characteristics are shown in Figure 6 and the corresponding device
performances are listed in Table S1 (Supporting Information). Both polymers P1 and
P2 showed typical hole-transport characteristics. The nonannealing (N/A) devices
showed mobilities of 3.29 × 10–5 cm2V–1s–1 for polymer P1 and 6.13 × 10–4 cm2V–1s–1
for polymer P2. Thermal annealing was beneficial to optimize the device performance,
the thermally annealed thin film of polymer P1 exhibited a hole mobility of 1.08 ×
10–4 cm2V–1s–1. Replacement of alkoxyl BDT with the alkylthienyl BDT yield a
significant increase of hole transport, P2 exhibited a hole mobility of 1.14 × 10–3
cm2V–1s–1.
3.6 Photovoltaic performance
The photovoltaic performances of polymers P1 and P2 were also investigated in
BHJ solar cell devices. The polymers were used as the electron donor and PCBM was
used as the electron acceptor. The conventional device structure was
ITO/PEDOT-PSS/polymer:PCBM/LiF/Al. The devices were characterized under AM
1.5 G illumination at 100 mW/cm2 using a solar simulator. The device optimization
was carried out using the ratio shown in Table S2. The devices with PC61BM as the
electron acceptor were initially fabricated with different D:A blend ratios from 1:1 to
1:4. As shown in Table S2, the optimized ratio of polymer to PC61BM was 1:2. At the
1:2 weight ratio of polymer/PC61BM, the device based on P1/PC61BM as the active
layer afforded a Voc of 0.91 V, a Jsc of 1.92 mA/cm2, a FF of 47%, and a PCE of
0.83%. The device using P2/PC61BM as the active layer exhibited a Voc of 0.91 V, a
Jsc of 2.49 mA/cm2, a FF of 46%, and a PCE of 1.04%. Additives have been widely
used to optimize the morphology of BHJ PSCs, and 1,8-diiodooctane (DIO) has been
reported as one of the most efficient additives [29]. Therefore, DIO was also added
from 1 to 4 vol% to optimize the device performance. Table 2 shows the
performances of the devices with different amounts of additive. The best PCE was
achieved with 3 vol% DIO as the additive for both polymers P1 and P2. The current
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density-voltage (J-V) curves of PSCs with blends of polymers, PC61BM, and 3 vol%
DIO are shown in Figure 7. The optimized devices exhibited PCE of 1.73% and 1.84%
with Jsc value of 3.66 and 3.74 mA/cm2 for polymers P1 and P2, respectively. The
PC71BM was selected as the electron acceptor to further optimize the device
performance. The current J-V curves of devices are presented in Figure 8 and the
corresponding results are summarized in Table 3. The P1/PC71BM based devices
displayed a PCE of 2.71% with a Voc of 0.86 V and a Jsc of 5.96 mA/cm2, the
P2/PC71BM based devices exhibited a PCE of 3.66% with a Voc of 0.91 V and a Jsc of
6.04 mA/cm2. The significant increase of Jsc and PCE was attributed to the
improvement of complementary absorption of PC71BM and charge separation in BHJ.
The morphology of the active layer was characterized by Atomic force microscopy
(AFM). The AFM topography and phase images of the film casted from the
polymer/PC71BM blend without and with DIO additive were reordered. As shown in
Figure 9, the blend films showed large aggregations when spin-coated without DIO.
This could cause inefficient exciton dissociation. After addition of DIO, the
aggregations in the blend films were suppressed and the domain sizes were relatively
reduced. The films processed with DIO exhibited fine domains and no large phase
separation was observed compared to the film processed without DIO.
The pyrrolo[3,4-d]pyridazine-5,7-dione-based polymers (P1 and P2) exhibited
moderate performances. This might be attributed to the relatively low molecular
weights and unoptimized composite morphology. Also, when straight alkyl chains
(dodecyl) and short branched chains (ethylhexyl) were used in
pyrrolo[3,4-d]pyridazine-5,7-dione monomers, the resulting polymers were
completely insoluble in solvent. Large branched side chains were used in the
synthesis of these polymers, which efficiently improved the solubility of the resulting
polymers. However, large and bulky side chains may also significantly reduce the
crystallinity and hole mobility of polymers, thus causing a low device performance.
4. Conclusion
Two D–A polymers based on the pyrrolo[3,4-d]pyridazine-5,7-dione unit, which
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replaced the benzene ring of phthalimide with pyridazine, were successfully
synthesized by Stille cross-coupling polymerization. The results indicated that the
replacement of the benzene ring with pyridazine significantly affected the absorption
spectrum, bandgap, and LUMO level of the resulting polymers. The BHJ solar cells
based on the two polymers as the electron donors and PC71BM as the electron
acceptor achieved moderate PCE of 2.71% and 3.66% for polymers P1 and P2,
respectively. However, after the structure and composite morphology optimizations,
polymers P1 and P2 exhibited wide absorptions, low bandgaps, suitable LUMO levels,
and deep HOMO levels. This indicated that pyrrolo[3,4-d]pyridazine-5,7-dione-based
D–A polymers have great potential as semiconductors in photovoltaic applications.
Acknowledgements
This work was supported by National Nature Science Foundation of China (NSFC
Grant Nos. 21204017, 21174036)..
References
1. J. Chen, Y. Cao, Acc Chem Res, 42 (11) (2009) 1709–1718.
2. S. Günes, H. Neugebauer, N. S. Sariciftci, Chem Rev, 107 (4) (2007) 1324–1338.
3. A. Pershin, S. Donets, S. A. Baeurle, Polymer, 55 (16) (2014) 3736–3745.
4. G. Zhang, Y. Fu, L. Qiu, Z. Xie, Polymer, 53 (20) (2012) 4407–4412.
5. J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger, G. C. Bazan,
Nat Mater, 6 (7) (2007) 497–500.
6. M. H. Hoang, D. N. Nguyen, T. T. Ngo, H. A. Um, M. J. Cho, D. H. Choi, Polymer,
83 (2016) 77-84.
7. X. Gong, Polymer, 53 (24) (2012) 5437-5448.
8. P. Shen, H. Bin, L. Chen, Z-G. Zhang, Y. Li, Polymer, 79 (19) (2015) 119-127.
9. J. Yuan, Z. Zhai, H. Dong, J. Li, Z. Jiang, Y. Li, W. Ma, Adv. Funct. Mater., 23 (7)
(2013) 885-892.
10. G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Science, 270 (5243) (1995)
1789–1791.
11

ACCEPTED MANUSCRIPT
11. B. C. Thompson, J. M. J. Fréchet, Angew Chem Int Ed, 47 (1) (2008) 58–77.
12. K. Kranthiraja, K. Gunasekar, W. Cho, M. Song, Y. G. Park, J. Y. Lee, Y. Shin, I.
N. Kang, A. Kim, H. Kim, B. S. Kim, B. S. Kim, S. H. Jin, Macromolecules, 47 (20)
(2014) 7060-7069.
13. G. Zhang, J. Yuan, P. Li, J. Ma, H. Lu, L. Qiu, W. Ma, Polym Chem, 4 (11) (2013)
3390–3397.
14. T. Yang, M. Wang, C. Duan, X. Hu, L. Huang, J. Peng, F. Huang, X. Gong,
Energy Environ Sci, 5 (8) (2012) 8208–8214.
15. G. Zhang, Y. Fu, Z. Xie, Q. Zhang, Polymer, 52 (2) (2011) 415–421.
16. L. Ye, S. Zhang, D. Qian, Q. Wang, J. Hou, J. Phys. Chem. C, 117 (48) (2013)
25360-25366.
17. H. Xin, X. Guo, F. S. Kim, G. Ren, M. D. Watson, S. A. Jenekhe, J Mater Chem,
19 (30) (2009) 5303–5310.
18. J. Huang, X. Wang, C. Zhan, Y. Zhao, Y. Sun, Q. Pei, Y. Liu, J. Yao, Polym
Chem, 4 (6) (2013) 2174–2182.
19. G. Zhang, Y. Fu, Q. Zhang, Z. Xie, Macromol Chem Phys, 211 (24) (2010)
2596–2601.
20. X. Guo, F. S. Kim, S. A. Jenekhe, M. D. Watson, J Am Chem Soc, 131 (21) (2009)
7206–7207.
21. L. J. A. Koster, V. D. Mihailetchi, P. W. M. Blom, Appl Phys Lett, 88 (9) (2006)
093511.
22. J. Huang, Y. Zhao, W. He, H. Jia, Z. Lu, B. Jiang, C. Zhan, Q. Pei, Y. Liu, J. Yao,
Polym. Chem., 3 (10) (2012) 2832-2841.
23. J. Chen, Y. Cao, Acc. Chem. Res., 42 (11) (2009) 1709-1718.
24. H. Yao, H. Zhang, L. Ye, W. Zhao, S. Zhang, J. Hou, ACS Appl. Mater. Interfaces,
8 (6) (2016) 3575-3583.
25. Q. Ye, W. T. Neo, C. M. Cho, S. W. Yang, T. Lin, H. Zhou, H. Yan, X. Lu, C.
Chi, J. Xu, Org Lett, 16 (24) (2014) 6386–6389.
26. C. M. Cho, Q. Ye, W. T. Neo, T. Lin, X. Lu, J. Xu, Polym. Chem., 6 (43) (2015)
12

ACCEPTED MANUSCRIPT
7570-7579.
27. J. Kim, A. R. Han, J. Hong, G. Kim, J. Lee, T. J. Shin, J. H. Oh, C. Yang, Chem
Mater, 26 (17) (2014) 4933–4942.
28. N. Zhou, X. Guo, R. P. Ortiz, S. Li, S. Zhang, R. P. H. Chang, A. Facchetti, T. J.
Marks, Adv Mater, 24 (17) (2012) 2242–2248.
29. J. K. Lee, W. L. Ma, C. J. Brabec, J. Yuen, J. S. Moon, J. Y. Kim, K. Lee, G. C.
Bazan, A. J. Heeger, J Am Chem Soc, 130 (11) (2008) 3619–3623.
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Figure Captions
Scheme 1 Synthetic routes of the monomers and polymers.
Figure 1 Molecular structures of polymers PBDT-PhBT, P1, and P2.
Figure 2 (a) TGA plots of copolymers at a heating rate of 10 °C/min, (b) DSC curves
of the copolymers in inert atmosphere.
Figure 3 Normalized absorption spectra of copolymers (a) in solutions and (b) as thin
films.
Figure 4 Cyclic voltammograms of the polymers at a scan rate of 100 mV/s.
Figure 5 Molecular orbital distribution of HOMO and LUMO for energy-minimized
structure (B3LYP/6-31g*) of the model compounds.
Figure 6 Output (a, b) and transfer characteristics (c and d) of polymers P1 and P2
annealed at 180 °C.
Figure 7 Illuminated I–Vcurves of PSC devices based on polymers/PC61BM
fabricated from chloroform with DIO.
Figure 8 Illuminated I–Vcurves of PSC devices based on polymers/PC71BM fabricated
from chloroform with DIO.
Figure 9 AFM images of P1/PC71BM and P2/PC71BM with and without DIO
processed from chloroform.
14

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Scheme 1
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Figure 1
Figure 2
Figure 3
16

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Figure 4
Figure 5
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Figure 6
Figure 7
18

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Figure 8
Figure 9
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Table 1 Optical and redox properties of polymers.
Egopt
Egec
c
λ abs
max (nm)
abs
onset
a
λ
HOMOb
(ev)
LUMOb
(ev)
(ev)
(ev)
Solution
Film
Film
Polymer
PBDT-PhBT d
462
532
498
543
546
627
730
1.98
1.70
1.70
−5.32
−5.22
−5.34
−3.37
−3.60
−3.60
−
1.62
1.74
P1
P2
536
730
b
a Egopt =1240 λoanbsset (in film). HOMO = −(4.75 +
E
), LUMO = −(4.75 +
E
). c Egec = (HOMO −
ox
onset
red
onset
LUMO). d The data were referred from our previous work [13]
.
Table 2 Device performances of polymer/PC61BM solar cells.
P1：PC61BM = 1:2
P2：PC61BM = 1:2
1%DO 2%DIO 3% DIO 4%DIO
1% DIO 2% DIO 3% DIO 4% DIO
Jsc/(mA.cm-2) 5.23
3.59
0.81
0.56
1.63
3.66
0.81
0.58
1.73
3.55
0.81
0.55
1.61
3.43
0.80
0.57
1.58
3.73
0.74
0.57
1.57
3.74
0.77
0.63
1.84
3.64
0.79
0.63
1.82
Voc/V
FF
0.60
0.36
1.13
PCE/%
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Table 3 Device performances of polymer/PC71BM solar cells.
P1：PC71BM = 1:2
P2：PC71BM = 1:2
1%
DIO
4.76
2%
DIO
5.69
3%
DIO
5.96
4%
DIO
5.95
1%
2%
DIO
5.08
3%
4%
5%
DIO
6.16
DIO
4.41
DIO DIO
5.97 6.04
Jsc/(mA.cm-2)
0.86
0.55
2.29
0.85
0.52
2.53
0.86
0.53
2.71
0.86
0.48
2.47
0.91
0.61
2.46
0.91
0.66
3.05
0.91 0.91
0.67 0.66
3.63 3.66
0.90
0.62
3.48
Voc/V
FF
PCE/%
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Research highlights
1. Two polymers based on pyrrolo[3,4-d]pyridazine-5,7-dione unit were synthesized
and characterized.
2. The LUMO and bandgap were lowered by introducing a more electron deficient
unit.
3. The polymers maintained the deep HOMO level and the devices exhibited a PCE
of 3.66% with a high Voc of 0.91 V.