Effective side chain selection for enhanced open circuit voltage of polymer solar cells based on 2D-conjugated anthracene derivatives

Article abstract of DOI:10.1016/j.dyepig.2014.12.010

Two novel conjugated donor-acceptor polymers were synthesized by combining anthracene donor and benzothiadiazole acceptor with two different side chains (conjugated and non-conjugated). Their absorption spectroscopy, thermogravimetric analysis, electrochemical cyclic voltammetry, theoretical prediction, surface morphology, and photovoltaic performance were investigated. The resultant two-dimensional configuration showed good optical and electrochemical properties. By selectively introducing branched conjugated side chains and non-conjugated linear side chains on the polymer backbone, the highest occupied molecular orbital levels are low lying which results in an increased open circuit voltage for polymer solar cells. The open circuit voltage of 1.02 V in this work was among the highest value for anthracene-based polymer solar cells ever. Our results suggest a good way to regulate the molecular energy levels by selecting appropriate side chains.

Full text of DOI:10.1016/j.dyepig.2014.12.010

Accepted Manuscript
Effective side chain selection for enhanced open circuit voltage of polymer solar cells
based on 2D-conjugated anthracene derivatives
L. Ai, X.H. Ouyang, Q.D. Liu, S.Y. Wang, R.X. Peng, A. Islam, Z.Y. Ge
PII:
S0143-7208(14)00464-1
10.1016/j.dyepig.2014.12.010
DYPI 4610
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 21 October 2014
Revised Date: 28 November 2014
Accepted Date: 10 December 2014
Please cite this article as: Ai L, Ouyang XH, Liu QD, Wang SY, Peng RX, Islam A, Ge ZY, Effective
side chain selection for enhanced open circuit voltage of polymer solar cells based on 2D-conjugated
anthracene derivatives, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2014.12.010.
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ACCEPTED MANUSCRIPT
Effective side chain selection for enhanced open circuit voltage of
polymer solar cells based on 2D-conjugated anthracene derivatives
L. Ai, a, b X. H. Ouyang, a Q. D. Liu,a S. Y. Wang,a R. X. Peng, a A. Islam a and Z. Y. Ge *a
aNingbo Institute of Material Technology and Engineering (NIMTE), Chinese Academy of
Sciences (CAS), Zhuangshi Road 519, Ningbo, Zhejiang 315201, P. R. China.
bUniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China.
Fax: + 86-574-86685043; Tel: + 86-574-86690273
E-mail: geziyi@nimte.ac.cn
Abstract
Two novel conjugated donor-acceptor polymers were synthesized by combining anthracene donor
and benzothiadiazole acceptor with two different side chains (conjugated and non-conjugated). Their
absorption spectroscopy, thermogravimetric analysis, electrochemical cyclic voltammetry, theoretical
prediction, surface morphology, and photovoltaic performance were investigated. The resultant
two-dimensional configuration showed good optical and electrochemical properties. By selectively
introducing branched conjugated side chains and nonconjugated linear side chains on the polymer
back-bone, the highest occupied molecular orbital levels are low lying which results in an increased
open circuit voltage for polymer solar cells. The open circuit voltage of 1.02 V in this work was among
the highest value for anthracene-based polymer solar cells ever. Our results suggest a good way to
regulate the molecular energy levels by selecting appropriate side chains.
Key words: side chains, open circuit voltage, 2D-anthracene-based polymers, polymer solar cells
1. Introduction
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Polymer solar cells (PSCs) have attracted much attention due to their potential as a competitive
technology for green energy with the advantages of low cost, flexibility, light weight, transparency and
large-area manufacturing compatibility [1–6]. State-of-the-art solution-processed PSCs have achieved
power conversion efficiency (PCEs) of 8%-10% [7-11]. Great efforts have been devoted to modulate
the properties of the bulk heterojuntion (BHJ) layers in PSC devices in the past decades. Several
molecular design strategies have been successfully used to modulate absorption spectra, band gaps, and
molecular energy levels of the active-layer materials, resulting in significant improvements in the
photovoltaic performance of PSC devices [12-16].
Much of the early progress in the field of organic photovoltaics focused on improving fill factor
(FF) and short circuit current (JSC), either by optimizing film morphology with structures such as the
bulk and planar-mixed heterojunctions, or by developing suitable functional materials with increased
absorption. As a result, PSCs with high JSC over 17 mA/cm2 and FF over 70% were achieved [17-20].
In addition, the molecular energy levels of donor and acceptor materials can be modified by molecular
design to increase the open circuit voltage (VOC) and PCEs. One feasible approach is to introduce
appropriately sized and shaped side chains appended to polymer back-bone, a feature which was
recognized as a critical factor impacting the photovoltaic parameters of the corresponding PSCs as
reported [12,21,22]. By introducing different conjugated side chains onto the back-bone of 2D-based
polymers, their molecular energy levels including the HOMO levels and the lowest unoccupied
molecular orbital (LUMO) levels can be effectively tuned without changing their optical properties.
Conjugated branched side chains have a positive influence in adjusting molecular energy levels but
have adverse effects on the structural order and solubility, so the nonconjugated side chains should be
carefully selected. Several studies have reported that the dodecylthienyl-based polymers can deliver a
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significantly improved VOC but with a slightly reduced JSC relative to the dodecylphenyl-based
polymers. For instance, Lidzeyet al. [23] presented a photovoltaic polymer with relatively low VOC
(0.59V) by incorporating the 4-dodecyloxyphenyl group into the anthracene unit of the conjugated
polymer, PPATBT (see Scheme 1a). After that, 2-dodecylthienyl was also introduced in
anthracene-based polymer back-bone and the similar linear dodecyloxy chains were attached on
4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) unit, an improved VOC of 0.96 V was obtained
with slightly reduced JSC in the PSCs based on PTA12OTBT (see Scheme 1a) [24]. In view of this, the
design and synthesis of novel anthracene-based polymers with suitable side chains are thought to be a
crucial component for highly-efficient PSCs.
In an effort to tune the molecular structure to manipulate the desired high VOC for bulk
hetero-junction polymer solar cells, we design and synthesize two novel alternating conjugated
polymers with 2-(2-ethylhexyl)-thienyl group attached on anthracene and linear alkyl chains on DTBT
unit, named as P1 and P2. The synthetic routes of the monomers and polymers are outlined in Scheme
1.
2. Experiment
2.1. Materials
All reagents and chemicals were purchased from commercial sources (Aldrich, Acros, TCI) and
used without further purification unless stated otherwise. Tetrahydrofuran (THF) was dried over
Na/benzophenone and freshly distilled prior to use. 2,6-dibromoanthracene-9,10-dione 2,
4,7-bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole
5
and
4,7-bis(5-bromo-4-hexylthiophen-2-yl)benzo[c][1,2,5]thiadiazole
6 were prepared according to
reported literature [25,26]. Other solvents of analytical grade were purchased and used without further
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purification.
2.2. Characterization
Fourier transform-infrared (FTIR) spectra were performed using Thermo Nicolet 6700
spectrophotometer. Nuclear magnetic resonance (NMR) spectra were measured on a Bruker DRX-400
spectrometer (in CDCl3, TMS as internal standard). UV-vis absorption spectra were recorded using a
1
Perkin -Elmer Lambda 950. H NMR spectra were recorded on a Bruker DMX-400 spectrometer.
Chemical shifts were calibrated against TMS as an internal standard. Molecular weight was measured
by a Waters 1515 gel permeation chromatograph with a refractive index detector at room temperature
(THF as the eluent), and polystyrene was used as a standard. Differential scanning calorimetry (DSC)
o
curves were obtained with a Mettler Toledo DSC 822 instrument at 20 C min-1 under a flow of
nitrogen. Thermogravimetric analyses (TGA) were carried out using a PerkineElmer Pyris
thermogravimeter under a dry nitrogen gas flow at a heating rate of 10 oC min-1. Cyclic voltammetry of
the
polymer
films
were
measured
in
acetonitrile
with
0.1
M
of
tetrabutylammoniumhexafluorophosphate (Bu4NPF6) using a scan rate of 100 mv s−1 at room
temperature. Platinum disk, Ag/AgCl, and platinum plate were used as the working electrode, reference
electrode, and counter electrode, respectively. The polymer films for electrochemical measurements
were cast from a chloroform solution, ca. 5 mg mL-1. Ferrocene was used as the internal standard in
each measurement and it is located at 0.40 V vs. the Ag/AgCl electrode. HOMO and LUMO energy
levels were calculated according to the following equations:
E HOMO (eV) = -e (φox+4.40)
E LUMO (eV) = -e (φre+4.40)
Egec (eV) = e (φox-φre)
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Where φox is the onset oxidation potential vs Ag/AgCl and φre is the onset reduction potential vs
Ag/AgCl.
2.3. Charge Mobility
Charge mobility was measured using the space charge limited current model (SCLC) [14,27].
Electron-only and hole-only devices were fabricated with a cell structure of ITO/Al/polymer:
PC71BM/Ca/Al and ITO/PEDOT: PSS/polymer: PC71BM/Au, respectively. The mobility was
determined by fitting the dark current to the model of a single carrier SCLC, which is described by the
following equation: J = (9/8) ε0εrꢀ ((V2)/ (d3)), where J is the current, ꢀ is the zero-field mobility, ε0 is
the permittivity of free space, εr is the relative permittivity of the material, d is the thickness of the
active layer, and V is the effective voltage. The effective voltage can be obtained by subtracting the
built-in voltage (Vbi) and the voltage drop (Vs) from the substrate’s series resistance from the applied
voltage (Vappl), V=Vappl – Vbi – Vs. The charge mobility can be calculated from the slope of the J1/2 ~ V
curves.
2.4. Device Fabrication
All devices were fabricated with the structure of ITO/PEDOT: PSS/polymer: PC71BM/Ca/Al.
ITO/glass substrates were cleaned ultrasonically in detergent, water, acetone, and iso-propanol (IPA)
sequentially, followed by treating in an ultraviolet ozone chamber (Ultraviolet Ozone Cleaner, Jelight
Company, USA) for 20 min. The cleaned substrates were covered with a 40-nm-thick layer of PEDOT:
PSS (Baytron PV PAI 4083, Germany) by spin coating. After annealing in a glove box with argon at
150 °C for 20 min, the samples were cooled to room temperature. Polymers and PC71BM (purchased
from American Dye Source) were dissolved in dichlorobenzene (DCB), and 3% diiodooctane (DIO)
was added. The solution was heated at 70 °C and stirred overnight at the same temperature before use.
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The solution of polymer: PC71BM was then spin-coated to form the active layer (~100 nm). Finally,
two layers of 20 nm Ca and 100 nm Al were evaporated as a cathode through a shadow mask under
high vacuum (<10−6Torr). The effective area of the device was measured to be 0.04 cm2. The device
characteristics were obtained using a xenon lamp at AM 1.5 solar illumination (Oriel Instruments). The
current-voltage (J-V) characteristics of the devices were measured out on a computer-controlled
Keithley 2440 Source Measurement system. The EQE of the PSCs were measured using a Stanford
Research Systems model SR830 DSP lock-in amplifier coupled with WDG3 monochromator and 500
W xenon lamp. The light intensity at each wavelength was calibrated with a standard single-crystal Si
photovoltaic cell.
2.5. Synthesis
5,5'-(2,6-Dibromoanthracene-9,10-diyl)bis(2-(2-ethylhexyl)thiophene) (3).
2-(2-Ethylhexyl)thiophene (8.6 g, 44.0 mmol) was dissolved in anhydrous THF (100 mL) in a
three-necked round-bottom flask under the nitrogen protection. The solution was cooled to 0 °C, and a
solution of n-BuLi (2.4 M in hexane, 18.3 mL, 44.0 mmol) was added dropwise over a period of 30
min with stirring. After this addition, the reaction mixture was warmed to room temperature, stirred for
0.5 h and heated up to 50 °C for another 2 h. 2,6-dibromo-9,10-anthraquinone (4.0 g, 11.0 mmol) was
added to the mixture and the temperature was kept at 50 °C for another 1.5 h. Then the reaction mixture
was cooled to 0°C. A solution of SnCl2·2H2O (22.4 g, 99.0 mmol) in HCl (10%, 50 mL) was added.
After stirring for an additional 1.5 h, the solvent was removed under reduced pressure and the product
was purified by column chromatography using hexane as the eluent and recrystallized from methanol
to afford the pure product as light yellow needles (4.5 g, 6.6 mmol, yield 60%). FTIR (KBr, cm-1) 3064
1
(aromatic C-H), 2958, 2925, 2856 (alkyl C-H), 1504, 1493, 1459, 1436 (aromatic C=C). H NMR
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(CDCl3, 400 MHz, δ/ppm): 8.06 (s, 2H), 7.78 (d, 2H, J=9.35 Hz), 7.46 - 7.43 (m, 2H), 6.98 (dd, 4H,
J=3.51 Hz), 2.90 (t, 4H, J=6.61 Hz), 1.54 - 1.36 (m, 18H), 0.99 - 0.92 (m, 12H). Anal. Calcd. for
C38H44Br2S2: C, 62.98; H, 6.12; Br, 22.05. Found: C, 62.69; H, 6.30; Br, 22.04%.
2,2’-(9,10-Bis(5-ethylhexylthiophen-2-yl)anthracene-2,6-diyl)-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborol
ane) (4).
A mixture of 5,5'-(2,6-Dibromoanthracene-9,10-diyl)bis(2-(2-ethylhexyl)thiophene) (3) (0.6 g, 2.77
mmol), bis(pinacolato)diboron (2.81 g, 11.08 mmol), CH3COOK (0.54 g, 5.54 mmol), and
[1,1-bis(diphenylphosphino)ferrocene]dichloro palladium (II) ( 154 mg, 0.15 mmol) were dissolved in
dioxane (90 mL) under nitrogen. The mixture was stirred for 48 h at 85 °C and allowed to cool down to
room temperature. Dioxane was distilled off using a rotary evaporator. The crude product was
dissolved in dichloromethane and washed with brine for 3 times; the organic layer was dried over
anhydrous sodium sulfate. The solvent was removed at a reduced pressure to give the crude product.
The crude solid was purified by column chromatography on silica gel using ethyl
acetate/dichloromethane as an eluent (DCM/EA (2:1) (v/v)). Recrystallization product from ethyl
acetate gave light yellow powder. (1.35 g, 1.66 mmol, yield 60%). FTIR (KBr, cm-1) 3063 (aromatic
1
C-H), 2957, 2922, 2854 (alkyl C-H), 1505, 1490, 1462, 1440 (aromatic C=C). H NMR (400 MHz,
CDCl3, δ/ppm): 8.59 (s, 2H), 8.03 (d, 2H, J=9.01 Hz), 7.83 (d, 2H, J=8.84 Hz), 7.11 (dd, 4H, J=3.51
Hz), 3.05 - 2.99 (m, 4H), 1.85 - 1.38 (m, 42H), 1.09 - 0.92 (m, 12H). Anal. Calcd. for C50H68B2O4S2: C,
73.34; H, 8.37. Found: C, 73.11; H, 8.42%.
Preparation of polymer (P1)
In
a
25
mL
dry
two-necked
flask,
2,2’-(9,10-bis(5-ethylhexylthiophen-2-yl)anthracene-2,6-diyl)-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborol
7

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ane) (4) (199.5 mg, 0.25 mmol), monomer 5 (206.5 mg, 0.25 mmol), Pd(PPh3)4 (5 mg) and K2CO3 (2
M, 1 mL) were dissolved in degassed toluene (5 mL). The mixture was placed under an argon
atmosphere, refluxed with vigorous stirring at 95 °C for 3 h and then bromobenzene (1 mL) was added.
After 12 h, phenylboronic acid (1 mL, 2 M in THF) was added and the reaction mixture was stirred for
another 24 h. After cooling to room temperature, the mixture was dissolved in chloroform, washed with
brine for 3 times and dried over anhydrous sodium sulfate. The solvent was concentrated to about
10mL and the polymer precipitated in methanol and collected by filtration. Then the solid was washed
in a Soxhlet extractor sequentially with MeOH, acetone and hexane. After that, the solid was
Soxhlet-extracted with chloroform. The chloroform fraction was collected and concentrated under
reduced pressure, and precipitated in methanol. The final product was collected by filtration and dried
under vacuum at 40 °C overnight (yield 70%). FTIR (KBr, cm-1) 3065 (aromatic C-H), 2954, 2920,
2854 (alkyl C-H), 1619 (aromatic C=N), 1504, 1492, 1455, 1435 (aromatic C=C). 1H NMR (400 MHz,
CDCl3, δ/ppm): 8.19 - 7.51 (br, 10H), 7.13 - 6.94 (br, 4H), 2.97 - 2.89 (br, 3H), 2.84 - 2.73 (br, 3H),
1.81 - 0.78 (br, 28H). Anal.Calcd.for (C64H76N2S5)n: C, 74.37; H, 7.41; N, 2.71. Found: C, 74.55; H,
7.22; N, 2.86%.
P2
In
a
25
mL
dry
two-necked
flask,
a
mixture
of
2,2’-(9,10-bis(5-ethylhexylthiophen-2-yl)anthracene-2,6-diyl)-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborol
ane) (4) (199.5 mg, 0.25 mmol), monomer 6 (206.5 mg, 0.25 mmol) and Pd(PPh3)4 (5 mg) were
dissolved in a mixture of degassed toluene (5 mL) and an aqueous solution of K2CO3 (2 M, 1 mL). The
mixture was placed under an argon atmosphere, refluxed with vigorous stirring at 95 °C for 4 h and
then bromobenzene (1 mL) was added. After 12 h, phenylboronic acid (1 mL, 2 M in THF) was added
8

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and the mixture was stirred for another 12 h. After cooling to room temperature, the mixture was
dissolved in chloroform, washed with brine for 3 times and dried over anhydrous sodium sulfate. The
solvent was concentrated to about 10mL and the polymer precipitated in methanol and collected by
filtration. Then the resulting solid was washed in a Soxhlet extractor sequentially with MeOH, acetone
and hexane. After that, the polymer was Soxhlet-extracted with chloroform. The chloroform fraction
was collected, concentrated under reduced pressure, and precipitated in methanol. The final product
was collected by filtration and dried under vacuum at 40 °C overnight (yield 80%). FTIR (KBr, cm-1)
3064 (aromatic C-H), 2958, 2925, 2856 (alkyl C-H), 1606 (aromatic C=N), 1510, 1490, 1465, 1439
1
(aromatic C=C). H NMR (400 MHz, CDCl3, δ/ppm): 8.19 - 7.51 (br, 8H), 7.13 - 6.94 (br, 4H), 2.97 -
2.89 (br, 3H), 2.84 - 2.73 (br, 3H), 1.81 - 0.78 (br, 54H). Anal. Calcd. for (C52H52N2S5)n: C, 72.18; H,
6.06; N, 3.24. Found: C, 72.22; H, 5.98; N, 3.43%.
3. Results and Discussion
3.1.
Synthesis and thermal stability
A key intermediate for the synthesis of the polymers was the bis-boronate ester monomer 4,
which was obtained in high yield from 2,6-dibromo-9,10-bis(4-(ethylhexyl)thiophene) anthracene upon
reaction with bis(pinacolato)diboron in the presence of a palladium catalyst. Suzuki coupling reactions
between monomer 4 and dibromides 5 or 6 afforded polymers P1 and P2. The chloroform fraction of
P1 with Mn of 5800 Da and an Mw of 7900 Da is able to be processed into films in BHJ devices. The
analogous polymer P2 with linear alkyl substituents on the thiophene-bridged units had Mn=15000 Da
and Mw=23900 Da in its chloroform fraction.(Table 1) The thermo-gravimetric analysis (TGA) curves
are shown in Fig. 1c. The results found that the polymers exhibit good thermal stability with 5%
o
o
weight-loss temperature (Td) value of 340 C and 406 C for P1 and P2, respectively. Thermal
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properties of the copolymers were determined by differential scanning calorimetry (DSC). As shown in
Figure S1 (Supporting Information), DSC analysis of all the polymers demonstrated no apparent
thermal transition up to 275 °C. This indicated that all polymers possessed the amorphous nature.
3.2.
Absorption spectra
Absorption spectra of the polymers in chloroform and solid thin films are shown in Fig. 1 and the
values of the absorption maxima are illustrated in Table 2. In solution, the polymers P1 and P2 exhibit
the short-wavelength absorption at 351 nm which originates from the π– π∗ transition of the conjugated
backbones, whereas the long-wavelength absorption peak at 526 nm for P1 and 511 nm for P2 are
attributed to the internal charge transfer interaction from the donor to the acceptor units. As shown in
the film (Fig. 1), the absorption bands of P1 are broad and red-shifted to 531 nm, which can be
attributed to stronger interchain packing in the ordered structure of P1 in the solid film. Notably, P2
also showed red-shift absorption maxima in the thin film as compared to those in chloroform solution,
which can be ascribed to the weak interaction of polymer chains in the solid state, caused by the large
dihedral angle of 80.9° between the anthracene moiety and the thiophene rings at the 9, 10-positions
[28]. The onsets absorption spectra of the films are 633 nm for P1 and 644 nm for P2, corresponding to
optical band gaps (Egopt) of 1.95 and 1.92 eV, individually.
3.3.
Electronic energy levels and theoretical calculations
Cyclic voltammetry (CV) was performed to determine the HOMO and LUMO levels of the
polymers. The oxidation onsets of P1 and P2 films measured by cyclic voltammetry (CV) were 0.72 V
and 1.17 V, respectively. The CV curves are shown in Fig. 1d. According to the equation EHOMO = −e
(Eox+4.40) (eV), the EHOMO levels were calculated to be −5.12 eV for P1 and −5.57 eV for P2. Since
VOC is determined by the difference of LUMO energy level of acceptor and HOMO energy level of the
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donor, the lower HOMO energy levels of P1 and P2 are beneficial to the high VOC of the photovoltaic
devices. To provide further insight into the fundamentals of molecular architecture, molecular
simulation was carried out for P1 and P2, with a chain length of n=1 at the DFT B3LYP/6-31G (d)
level with the Gaussian 09 program package. As depicted in Scheme 2a, the LUMOs of the
donor-acceptor copolymers are well distributed along the conjugated chains, while their HOMOs are
localized on the DTBT acceptor unit, suggesting effective delocalization of the HOMO band and
p-type behavior. Although the HOMO and LUMO isosurface configurations of P1 were quite similar to
those of P2, the backbone of P2 was markedly distorted compared to P1. Such steric hindrance by the
alkyl side chains in P2 twisted the structure of the conjugated backbone, which is attributed to the
interrupted internal charge transfer (ICT). However, for P2, both the HOMO and LUMO energy level
are decreased. The electronic energy level diagrams of the donor and acceptor materials are shown in
Scheme 2b.
3.4.
Photovoltaic performance and electron mobility
PSCs were fabricated and characterized to investigate the photovoltaic properties of the polymers.
The device structure used in this work is ITO/PEDOT: PSS/polymer: PC71BM/Ca/Al. After the device
optimization, solar cells were fabricated with blends of polymers and PC71BM. Solar cells fabricated in
a weight ratio of 1:3 in dichlorobenzene (DCB) solution gave the best performance. A PCE of 1.12%
with VOC of 0.82 V, a JSC of 4.24 mA/cm2, and a FF of 30.0% was obtained for P1: PC71BM under 1
sun of simulated AM 1.5 G solar radiation (100 mW cm −2), while P2 exhibited a relatively higher PCE
of 1.94% with a VOC of 0.86 V, a JSC of 6.29 mA/cm2 and a FF of 35.9%. The J−V curves of PSCs are
shown in Fig.2a, and the device performances are summarized in Table 3.
To further improve the photovoltaic performance of the device, we tried to employ a variety of
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polar solvents, including methanol, ethanol, and propanol, to optimize the performance of PSCs.
Inspired by recent work on post-solvent treatment, [29-33]. We modified the literature procedure as
follows: (i) spin-coated the active layer and dried under vacuum; (ii) ethanol (or other polar solvents)
was added dropwise on top of the active layer and a short wet time (such as 45 s) was required; (iii)
polar solvent was removed by spin coating at high speed (such as 2500 rpm); (iv) dried under vacuum
o
and annealed at 100 C for 10 min. From Table 3 and Fig. 2b, P2 was treated with different solvents.
Remarkably, after the solvent exposure, the performance of P2-based PSCs was enhanced. The most
optimized solvent was ethanol and the PCE values changed from 1.61% ±0.1% to 3.43% ±0.2%, with a
largely increased VOC ranging from 0.86±0.02 V to 1.02±0.01 V, and FF from 35.9% to 46.6%. The
reduced HOMO level of the donor polymer resulted in a higher VOC. However, polar solvent treatment
on the P1: PC71BM blend film did not show large enhancement, probably because of its non-uniform
film formation and poor solubility of the DTBT unit. The effect of wetting time was investigated, and
the results are plotted in Fig. 2c. In P2-based cells, the carrier mobility allowed improvement relative to
P1-based cells. The electron and hole mobility were studied across the film by applying single-carrier
space charge limited conduction (SCLC) method, giving electron mobility values of 3. 62 × 10-7 and
4.78 × 10-7cm2 V-1 S-1, and hole mobility values of 6.99 × 10-8 and 2.27 × 10-7 cm2 V-1 S-1 for P1 and P2,
individually. (J1/2~V curves were shown in Fig. 2d) The slightly different carrier mobility of the two
copolymers demonstrates that introduction of long or branched chains such as hexyl and 2-ethyhexyl
on the molecular back-bone has an impact on their carrier mobility. This is probably due to their
different packing and ordering structure. Additionally, the electron mobility of P2 is very close to the
hole mobility, which indicates a good balance of carriers in the blend film and it could help to explain
the improved device performance.
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3.5.
Morphologies of the active layer
Besides the above-mentioned parameters such as thermal stability, absorption, and energy levels,
the morphology of the photoactive layer plays a key role in the photovoltaic performance of PSCs. We
directly examined the surface morphology with or without polar solvent exposure via tapping-mode
atomic force microscopy (AFM). The AFM topography images are shown in Fig.3. In the P2: PC71BM
blends, it could be observed that the blend film without solvent exposure exhibited relatively course
phase separation between the polymer and PC71BM with a root-mean-square (RMS) surface roughness
of 0.47 nm (see Fig.3a). Globular clusters were formed on the surface by methanol treatment. It could
be attributed to the PC71BM domains, in which PC71BM crystallized before the polymer solidified. The
propanol-treated device (with roughness 0.84 nm) and ethanol-treated device (with roughness 0.38 nm)
formed fiber-like interpenetrating morphologies. Tens of seconds of polar solvent treatment did not
provide an adequate driving force for reconstructing the morphologies of the polymer films, possibly
because polymers are relatively bulky as compared to the small molecules [34]. However, as some of
the polar solvent molecules penetrated the active layer, PC71BM particles could be redistributed to
reduce the contact areas with the polar solvents, and after appropriate wetting time, PC71BM particles
aggregated to the tens-of-nanometers length domains, thus, forming nanoscale phase separation of the
blend films (see Fig.3a, 3d, 3e, and 3f). This relatively smooth surface and more ordered structure are
beneficial to the charge transport, thus leading to an increase in JSC and VOC, as well as the device
efficiency.
4. Conclusions
In summary, two 2, 6-substituted anthracene-based polymers consist of anthracene and DTBT
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units as building blocks in the main chain were synthesized by Suzuki coupling polymerization. The
resulting two-dimensional polymers revealed good absorption and thermal stability. By directly
attaching branched alkylthienyl side chains instead of linear alkyl-conjugated side chains on the
anthracene unit, the molecular energy levels were effectively tuned with deeper HOMO levels of 5.12
eV and attainable VOC of 0.82 eV for P1.The nonconjugated hexyl side chains were introduced in the
DTBT back-bones to improve the solubility of the polymer and film formation of the active layer. As a
result, an enhanced VOC of 1.02 eV from the anthracene and DTBT copolymers (anthracene-based
polymer) was obtained by introducing branched alkylthienyl chains in anthracene unit and linear alkyl
chains in DTBT without sacrificing the JSC of the original backbone. To the best of our knowledge, a
VOC of 1.02 V is one of the highest values for two dimensional anthracene-based polymers solar cells.
Overall, this work demonstrates a good way to regulate the photovoltaic properties by effective
selection of the side chains.
Acknowledgements
The present research was financially supported by the National Natural Science Foundation (No.
21074144, No. 51273209, No. 21102156), CAS External Cooperation Program (GJHZ1219) and
Projects from Ningbo Science and Technology Bureau (2012B10022, 2012D10009, 2013D10013,
2013A610032 and 2013A610015).
Notes and references
[1] F. C. Krebs, Fabrication and processing of polymer solar cells: a review of printing and coating
14

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techniques. Sol Energy Mater Sol Cells 2009; 93: 394-412.
[2] G. Li, R. Zhu and Y. Yang, Polymer solar cells. Nat Photonics 2012; 6: 153-161.
[3] S. Guenes, H. Neugebauer, N. S. Sariciftci, Conjugated polymer-based organic solar cells. Chem
Rev 2007; 107: 1324-1338.
[4] M. Jφrgensen, K. Norrman, S. A. Gevorgyan, T. Tromholt, B. Andreasen and F. C. Krebs, Stability
of polymer solar cells. Adv Mater 2012; 24: 580-612.
[5] Y. Z. Lin, Y. F. Li, X. W. Zhan, Small molecule semiconductors for high-efficiency organic
photovoltaics. Chem Soc Rev 2012; 41: 4245-4272.
[6] J. W. Chen, Y. Cao, Development of novel conjugated donor polymers for high efficiency bulk
heterojunction photovoltaic devices. Acc Chem Res 2009; 42: 1709-1718.
[7] Z. C. He, C. M. Zhong, S. J. Su, M. Xu, H. B. Wu and Y. Cao, Enhanced power-conversion
efficiency in polymer solar cells using an inverted device structure. Nat Photon 2012; 6: 591-595.
[8] H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang, Y. Yang, L. P. Yu, Y. Wu and G. Li,
Polymer solar cells with enhanced open-circuit voltage and efficiency. Nat Photonics 2009; 3:
649-653.
[9] X. Guo, M. J. Zhang, W. Ma, L. Ye, S. Q. Zhang, S. J. Liu, H. Ade, F. Huang, J. H. Hou, Enhanced
photovoltaic performance by modulating surface composition in bulk heterojunction polymer
solar cells based on PBDTTT-C-T/PC71BM. Adv Mater 2014; 26: 4043-4049.
[10] 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, Semi-crystalline photovoltaic polymers with efficiency
exceeding 9% in a ~300 nm thick conventional single-cell device. Energy Environ Sci 2014; 7:
3040-3051.
15

ACCEPTED MANUSCRIPT
[11] L. Ye, S. Q. Zhang, W. C. Zhao, H. F. Yao, J. H. Hou, Highly efficient 2D-conjugated
benzodithiophene-based photovoltaic polymer with linear alkylthio side chain. Chem Mater 2014;
26: 3603-3605.
[12] Y. F. Li, Molecular design of photovoltaic materials for polymer solar cells: toward suitable
electronic energy levels and broad absorption. Acc Chem Res 2012; 45: 723-733.
[13] L. T. Dou, C. C. Chen, K. Yoshimura, K. Ohya, W. –H. Chang, J. Cao, Y. S. Liu, E. Richard and Y.
Yang, Synthesis of 5H-dithieno [3, 2-b: 2’, 3’-d] pyran as an electron-rich building block for
donor-acceptor type low-band gap polymers. Macromolecules 2013; 46: 3384-3390.
[14] S. J. Liu, K. Zhang, J. M. Lu, J. Zhang, H. –L. Yip, F. Huang and Y. Cao, High-efficiency polymer
solar cells via the incorporation of an amino-functionalized conjugated metallopolymer as a
cathode interlayer. J Am Chem Soc 2013; 135: 15326-15329.
[15] M. J. Zhang, Y. Gu, X. Guo, F. Liu, S. Q. Zhang, L. J. Huo, T. P. Russell and J. H. Hou, Efficient
polymer solar cells based on benzothiadiazole and alkylphenyl substituted benzodithiophene with a
power conversion efficiency over 8%. Adv Mater 2013; 25: 4944-4949.
[16] S. –H. Liao, H. –J. Jhuo, Y. –S. Cheng and S. –A. Chen, Fellerene derivative-doped zinc oxide
nanofilm as the cathode of inverted polymer solar cells with low-band gap polymer (PTB7-Th) for
high performance. Adv Mater 2013; 25: 4766-4771.
[17] W. W. Li, K. H. Hendriks, W. S. C. Roelolf, Y. H. Kim, M. M. Wienk and R. A. J. Janssen,
Efficient small band gap polymer solar cells with high fill factors for 300 nm thick films. Adv
Mater 2013; 25: 3182-3186.
[18] Q. D. Zheng, B. J. Jung, J. Sun and H. E. Katz, Ladder- type oligo-p-phenylene-containing
copolymers with high open circuit voltages and ambient photovoltaic activity. J Am Chem Soc
16

ACCEPTED MANUSCRIPT
2010; 132: 5394-5404.
[19] K. Vandewal, K. Tvingstedt, A. Gadisa, O. Inganas and J. V. Manca, On the origin of the open-
circuit voltage of polymer – fullerene solar cells. Nature 2009; 8: 904-909.
[20] A. Gadisa, W. Mammo, L. M. Andersson, S. Admassie, F. Zhang, M. R. Andersson and O. Inganas,
A new donor–acceptor–donor polyfluorene copolymer with balanced electron and hole mobility.
Adv Funct Mater 2007; 17: 3836-3842.
[21] L. Ye, S. Q. Zhang, L. J. Huo, M. J. Zhang, J. H. Hou, Molecular desigh toward highly efficient
photovoltaic polymers based on two-dimensional conjugated benzodithiophene. Acc Chem Res
2014; 47: 1595-1603.
[22] C. –Y. Kuo, W. Y. Nie, H. Tsai, H. –J. Yen, A. D. Mohite, G. Gupta, A. M. Dattelbaum, D. J.
William, K. C. Cha, Y. Yang, L. Wang, H. –L. Wang, Structural design of benzo [1, 2-b: 4, 5-b’]
dithiophene-based 2D conjugated polymers with bithienyl and terthienyl substituents toward
photovoltaic applications. Macromolecules 2014; 47: 1008-1020.
[23] M. S. Almeataq, H. N. Yi, S. A. –Faifi, A. B. B. Alghamdi, A. Iraqi, N. W. Scarratt, T. Wang, D. G.
Lidzey, Anthracene-based donor-acceptor low band gap polymers for application in solar cells.
Chem Commun 2013; 49: 2252-2254.
[24] C. C. Liu, W. Z. Cai, X. Guan, C. H. Duan, Q. F. Xue, L. Ying, F. Huang, Y. Cao, Synthesis of
donor-acceptor copolymers based on anthracene derivatives for polymer solar cells. Polym Chem
2013; 4: 3949-3958.
[25] H. C. Zhang, E. Q. Guo, Y. L. Zhang, P. H. Ren, W. J. Yang, Donor-acceptor-substituted
anthracene-centered cruciforms: synthesis, enhanced two-Photon absorptions, and spatially
separated frontier molecular orbitals. Chem Mater 2009; 21: 5125-5135.
17

ACCEPTED MANUSCRIPT
[26] E. G. Wang, M. Wang, L. Wang, C. H. Duan, J. Zhang, W. Z. Cai, C. He, H. B. Wu, and Y.
Cao, Donor polymers containing benzothiadiazole and four thiophene rings in their repeating units
with improved photovoltaic performance. Macromolecules 2009; 42: 4410-4415.
[27] Y. Wang, F. Yang, Y. Liu, R. X. Peng, S. J. Chen, Z. Y. Ge, New alkylfuranyl-substituted benzo [1,
2-b: 4, 5-b’] dithiophene-based donor-acceptor polymers for highly efficient solar cells.
Macromolecules 2013; 46: 1368-1375.
[28] C. Teng, X. C. Yang, C. Yang, S. F. Li, M. Cheng, A. Hagfeldt and L. C. Sun, Molecular design of
anthracene-bridged metal-free organic dyes for efficient dye-sensitized solar cells. J Phy Chem C
2010; 114: 9101-9110.
[29] X. F. Liu, W. Wen, G. C. Bazan, Post-deposition treatment of an arylated-carbazole conjugated
polymer for solar cell fabrication. Adv Mater 2012; 24: 4505-4510.
[30] H. Zhou, Y. Zhang, J. Seifter, S. D. Collins, C. Luo, G. C. Bazan, T. –Q. Nguyen, A. J. Heeger,
High-efficiency polymer solar cells enhanced by solvent treatment. Adv Mater 2013; 25:
1646-1652.
[31] Y. Wang, Y. Liu, S. J. Chen, R. X. Peng, Z. Y. Ge, Significant enhancement of polymer solar cell
performance via side-chain engineering and simple solvent treatment. Chem Mater 2013; 25:
3196-3204.
[32] E. –P. Yao, C. –C. Chen, J. Gao, Y. S. Liu, Q. Chen, M. Cai, W. –C. Hsu, Z. R. Hong, G. Li, Y.
Yang, The study of solvent additive effects in efficient polymer photovoltaics via impedance
spectroscopy. Sol Energy Mater Sol Cells 2014; 130: 20-26.
[33] P. –N. Yeh, S. –H. Liao, Y. –L. Li, H. –R. Syue, S. –A. Chen, Large active area inverted tandem
polymer solar cell with high performance via alcohol treatment on the surface of bottom active
18

ACCEPTED MANUSCRIPT
layer P3HT: ICBA. Sol Energy Mater Sol Cells 2014; 128: 240-247.
[34] S. Nam, D. S. Chung, J. Jang, S. H. Kim, C. Yang, S. –K. Kwon, C. E. Park, Effects of poor
solvent for solution-processing passivation of organic field effect transistors. J Electrochem Soc
2010; 157: H90-H93.
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Figure Lengends
Table 1 Molecular weights and thermal properties of P1 and P2.
Scheme1 (a) Molecular structure of two reference polymers and target polymer; (b) Synthetic
procedure of P1 and P2: (i) CuBr2, t-butyl-nitrite, acetonitrile, 65 oC, 4 h;(ii) n-BuLi, THF, 0 oC then 50
oC, 2 h; 2, 6-dibromo-9, 10-anthraquinone, 1 h; and then SnCl2•2H2O in 20% HCl, 1.5 h;(iii)
bis(pinacolato)diboron, Pd(dppf)2Cl2, KOAc, 1, 4-dioxane, 20 h;(iv) 2 M K2CO3 (aq.), Pd(PPh3)4,
toluene, Aliquat 336, 95 oC, 48 h; (v) 2 M K2CO3(aq.), Pd(PPh3)4, toluene, Aliquat 336, 95 oC, 24 h.
Figure 1 The UV/Vis absorption spectra of P1 and P2 in (a) chloroform solution and (b) as a thin film;
o
(c) TGA curves of P1 and P2 with a heating rate of 10 C min−1 under a N2 atmosphere; (d) Cyclic
voltammetry curve of P1 and P2 films.
Table 2 Optical and electrochemical properties of the polymers.
Scheme 2 (a) Calculated HOMOs and LUMOs of P1 and P2 using the DFT method at the B3LYP-6-31
g (d) level. (b) Energy levels of different components in a photovoltaic device.
Table 3 Performance of the polymer: PC71BM BHJ solar cells before and after various polar solvent
treatments with a wetting time of 45 s, at a blend ratio of 1:3 under the illumination with AM 1.5G, 100
mW cm-2.
Figure 2 (a) J–V characteristics of the devices with the configuration of ITO/PEDOT: PSS/Polymer:
PC71BM/Ca/Al under the illumination of AM 1.5G, 100 mW cm−2. (b) Current density-voltage (J-V)
curves of the PSCs based on P2/PC71BM (1:3, w/w) with solvent exposure. (c) Wetting time
dependence of PSCs performance of P2. (d) J1/2~V characteristics of electron-only (ITO/Al/Polymer:
PC71BM/Ca/Al). The inset depicts the hole-only (ITO/PEDOT: PSS/Polymer: PC71BM/Au) devices.
Figure 3 Tapping-mode atomic force microscopy (AFM) topography image of the blend film of
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P2/PC71BM (1:4, w/w); (a) without solvent treatment, (b) with methanol treatment, (c) with propanol
treatment, (d) with ethanol treatment, (e) three-dimensional (3D) surface plot without solvent treatment,
and (f) 3D surface plot with ethanol treatment. (All image sizes are 2.0 ꢀm×2.0 ꢀm.)
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Figures and Schemes
Table 1
Polymer
Mn (Da)a
5800
Mn (Da)a
7900
PDIa
1.36
1.59
Td (ꢀ)b
340
P1
P2
a
15000
23900
406
Mn, Mw and PDI of polymers were determined by GPC using polystyrenes as standards with
chloroform as eluent.
b The temperature of 5% weight loss under nitrogen.
Scheme 1
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Figure 1
Table 2
UV-Vis absorption
solution
film
Cyclic Voltammetry
Polymer
λmax(nm) λoneset(nm)
λmax(nm) λoneset(nm) Band gap
HOMO(eV) LUMO(eV)
P1
P2
351,526
351,511
594
632
354,531
359,535
633
644
1.95
1.93
-5.12
-5.57
-3.50
-3.60
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Scheme 2
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Figure 2
Table 3
Solvent treatment
JSC [mA cm-2]
VOC [V]
FF [%]
PCE [%]
P1
none
4.24±0.2
3.82±0.4
5.00±0.4
5.64±0.3
0.82±0.02
0.84±0.03
0.82±0.02
0.86±0.02
30.0±0.3
28.5±0.5
30.5±0.5
32.2±0.3
1.12±0.2
0.90±0.3
1.23±0.2
1.51±0.4
propanol
methanol
ethanol
P2
none
6.29±0.3
5.64±0.2
6.08±0.2
7.23±0.1
0.86±0.02
0.87±0.02
0.97±0.02
1.02±0.01
35.9±0.2
32.7±0.5
34.7±0.2
46.6±0.3
1.94±0.2
1.61±0.1
2.05±0.2
3.43±0.2
propanol
methanol
ethanol
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Figure 3
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Highlights
1. Two novel conjugated donor-acceptor polymers were synthesized and
characterized.
2. By fine tuning side chains, polymers with low lying HOMO and high
VOC of 1.02 V were obtained.
3. The VOC of the polymer containing 3-hexylthiophene was enhanced
without sacrificing the JSC.
4. The VOC of 1.02 V is among the highest value for anthracene-based
polymer solar cells reported.
5. This work exhibits a good way to regulate the molecular energy levels
by fine tuning side chains.

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Effective side chain selection for enhanced open circuit voltage of
polymer solar cells based on 2D-conjugated anthracene derivatives
L. Ai, a, b X. H. Ouyang, a Q. D. Liu,a S. Y. Wang,a R. X. Peng, a A. Islam a and Z. Y. Ge *a
aNingbo Institute of Material Technology and Engineering (NIMTE), Chinese Academy of
Sciences (CAS), Zhuangshi Road 519, Ningbo, Zhejiang 315201, P. R. China.
bUniversity of Chinese Academy of Sciences, Beijing 100049, P. R. China.
Fax: + 86-574-86685043; Tel: + 86-574-86690273
E-mail: geziyi@nimte.ac.cn
Fig. S1 Differential scanning calorimetry (DSC) curves of P1 and P2.