Significant enhancement of polymer solar cell performance via side-chain engineering and simple solvent treatment

Article abstract of DOI:10.1021/cm401618h

Rational design and synthesis of polymeric semiconductors is critical to the development of polymer solar cells (PSCs). In this work, a new series of benzodithiophene-difuranylbenzooxadiazole-based donor-acceptor co-polymers-namely, PBDT-DFBO, PBDTT-DFBO, and PBDTF-DFBO, with various side groups-have been developed for bulk-heterojunction PSCs. These side-group substituents provide the opportunity to tailor the opto-electrical properties of the polymers. In addition, we show that the reduction of the bandgap of polymers and the enhancement of charge mobility in the devices can be accomplished concurrently by substituting the alkylthienyl side group with its furan counterpart. In the preliminary investigation, one could obtain PSCs with a power conversion efficiency (PCE) of 2.1% for PBDT-DFBO with an alkoxyl side chain, 2.2% for PBDTT-DFBO with an alkylthienyl side group, and 3.0% for PBDTF-DFBO with an alkylfuranyl side group. Further optimizing the performance of the devices was conducted via a simple solvent treatment. The PSCs based on PBDTF-DFBO:PC₇₁BM could even achieve 7.0% PCE, which exhibited an enhancement of 130%. To the best of our knowledge, the value of 7.0% is the highest efficiency for furan-containing PSCs to date and is also comparable with the hitherto reported highest efficiency of the single junction PSCs. Through a combination of testing charge transport by the space-charge limited current (SCLC) model and examining the morphology by atomic force microscopy (AFM), we found that the effects of solvent treatment on the improved performance originate from higher and more balanced charge transport and the formation of fiberlike interpenetrating morphologies, which are beneficial to the increase of short-circuit current density (J_sc) and fill factor (FF). This work demonstrates a good example for tuning absorption range, energy level, charge transport, and photovoltaic properties of the polymers by side-chain engineering and the solvent treatment can offer a simple and effective method to improve the efficiency of PSCs.

Full text of DOI:10.1021/cm401618h

Article
pubs.acs.org/cm
Significant Enhancement of Polymer Solar Cell Performance via Side-
Chain Engineering and Simple Solvent Treatment
†
,‡
†
†
†
,†
Yang Wang, Ying Liu, Shaojie Chen, Ruixiang Peng, and Ziyi Ge*
†Ningbo Institute of Material Technology and Engineering (NIMTE), Chinese Academy of Sciences (CAS), Zhuangshi Road 519,
Ningbo, Zhejiang 315201, People’s Republic of China
‡
University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
*
S Supporting Information
ABSTRACT: Rational design and synthesis of polymeric semiconductors is critical to the
development of polymer solar cells (PSCs). In this work, a new series of benzodithiophene−
difuranylbenzooxadiazole-based donor−acceptor co-polymersnamely, PBDT-DFBO,
PBDTT-DFBO, and PBDTF-DFBO, with various side groupshave been developed for
bulk-heterojunction PSCs. These side-group substituents provide the opportunity to tailor the
opto-electrical properties of the polymers. In addition, we show that the reduction of the
bandgap of polymers and the enhancement of charge mobility in the devices can be
accomplished concurrently by substituting the alkylthienyl side group with its furan
counterpart. In the preliminary investigation, one could obtain PSCs with a power conversion
efficiency (PCE) of 2.1% for PBDT-DFBO with an alkoxyl side chain, 2.2% for PBDTT-DFBO
with an alkylthienyl side group, and 3.0% for PBDTF-DFBO with an alkylfuranyl side group.
Further optimizing the performance of the devices was conducted via a simple solvent
treatment. The PSCs based on PBDTF-DFBO:PC BM could even achieve 7.0% PCE, which
71
exhibited an enhancement of 130%. To the best of our knowledge, the value of 7.0% is the highest efficiency for furan-containing
PSCs to date and is also comparable with the hitherto reported highest efficiency of the single junction PSCs. Through a
combination of testing charge transport by the space-charge limited current (SCLC) model and examining the morphology by
atomic force microscopy (AFM), we found that the effects of solvent treatment on the improved performance originate from
higher and more balanced charge transport and the formation of fiberlike interpenetrating morphologies, which are beneficial to
the increase of short-circuit current density (J ) and fill factor (FF). This work demonstrates a good example for tuning
sc
absorption range, energy level, charge transport, and photovoltaic properties of the polymers by side-chain engineering and the
solvent treatment can offer a simple and effective method to improve the efficiency of PSCs.
KEYWORDS: polymer solar cells, side group engineering, solvent treatment, donor−acceptor conjugated copolymer
1
.. INTRODUCTION
substituted BO-based conjugated polymers, which reached
the highest PCE of 5.9%. However, 4,7-difuran-substituted
4
Bulk-heterojunction (BHJ) solar cells, formed from a blend of
electron-donating π-conjugated polymers and electron-accept-
ing fullerene derivatives (such as PCBM), have received
enormous academic and industrial enthusiasm, because of
promising advantages such as low cost, light weight, flexibility,
and large-area manufacturing compatibility. The efficiency of
polymer solar cells (PSCs) has now been enhanced by more
BO-based copolymers remain unexplored in PSCs. It has been
shown that the substitution of furan for thiophene in the main
chain of the polymer will improve polymer solubility, which is
beneficial for obtaining higher molecular weight and the
formation of high-quality films of active layer that provides
1
5
efficient charge generation and collection for PSCs. Besides,
Li’s work demonstrates that the replacement of thiophene in
the backbone of polymers with furan is able to lower the
than 10% through fundamental understanding, new materials
synthesis, and device architecture innovation.
2
HOMO energy level, thus increasing the open circuit voltage
New polymeric semiconductors are crucial in order for PSCs
development. However, nearly all highly efficient PSCs (with a
power conversion efficiency (PCE) of >5%) reported so far
have been dependent on thiophene-based conjugated polymers.
Only a small amount of research has been focused on
nonthiophene heterocycles-based polymers. For instance,
6
(
V ) of solar cells. Nonetheless, the photovoltaic performance
oc
of the furan-bridged copolymer (P(BDT-F-BT)) was lower
than its thiophene counterpart (P(BDT-T-BT)), mainly
because of its larger band gap, narrower absorption spectrum,
7
and thus lower short-circuit current density (J ) of the devices.
sc
On the other hand, we recently have found that poly[2,6-(4,8-
2,1,3-benzooxadiazole (BO) is an electron-deficient hetero-
3
cycle, which is similar to the renowned 2,1,3-benzothiadiazole
BT), yet is much less explored than BT as an acceptor building
(
Received: May 18, 2013
Revised: July 7, 2013
block. To the best of our knowledge, several reports have
described the photovoltaic properties of 4,7-dithiophene-
©
XXXX American Chemical Society
A
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Chemistry of Materials
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8
Figure 1. Chemical structures of furan containing donor−acceptor polymers in the literature (top) and in this work (bottom).
bis(5-(2-ethylhexyl)furan-2-yl)benzo[1,2-b:4,5-b′]-
dithiophene)-alt-3,6-(2,5-bis(2-ethylhexyl)-3,6-di(furan-2-yl)-
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione)] (PBDTF-DPPF)
with furan side group had smaller optical band gap, broader
absorption range, deeper HOMO energy level in comparison
with its corresponding alkoxy-substituted counterpart and a
high PCE of 6.1% was achieved in a typical BHJ device,
exceeding the performance of poly[2,6-(4,8-bis(5-(2-
ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene)-alt-
PBDTF-DFBO, as shown in Figure 1. We showed that the
reduction of the bandgap of copolymers and the enhancement
of charge mobility in the devices could be accomplished
simultaneously by substituting the thiophene side group with its
furan counterpart. The PSCs based on pristine polymers/
PC BM blends showed a moderate PCE value of 2.1% for
71
PBDT-DFBO with an alkoxyl side chain, 2.2% for PBDTT-
DFBO with an alkylthienyl side group, and 3.0% for PBDTF-
DFBO with an alkylfuranyl side group. Further optimizing the
performance of the devices was conducted via a simple solvent
treatment. Encouragingly, the PCE exhibited an enhancement
of 130%−170% and PSCs based on PBDTF-DFBO:PC BM
3
,6-(2,5-bis(2-ethylhexyl)-3,6-di(furan-2-yl)pyrrolo [3,4-c]-
pyrrole-1,4(2H,5H)-dione)] (PBDTT-FDPP) with thiophene
side group; Meanwhile, Li and his co-workers have found that
the furan side chain makes the lowest unoccupied molecular
orbital (LUMO) level shift slightly downward. (All the
7
1
reached the highest efficiency of 7.0% for furan-containing
12
PSCs to date.
8
structures are shown in Figure 1.) In addition, furan-based
materials are known to be biodegradable and can be prepared
from biorenewable sources, which makes them more suitable
for low-cost and large-scale applications. Based on the above
results, more investigation of furan-based PSCs is definitely
needed.
2
.. EXPERIMENTAL SECTION
2
.1. Materials. n-BuLi, Pd(PPh ) , and Sn(C H ) Cl were
3 4 4 9 3
obtained from Tokyo Chemical Industry (TCI) and used as received.
Tetrahydrofuran (THF) was dried over Na/benzophenone ketyl and
freshly distilled prior to use. Other reagents and solvents were
purchased commercially as analytical-grade quality and used without
further purification. The synthesis of (4,8-bis((2-ethylhexyl)oxy)-
benzo-[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(tributylstannane) (BDT),
On the other hand, despite the advantages of the BHJ
structure, the efficiency of PSCs can be limited by the9
unsatisfactory morphology of the polymer−fullerene blends.
Although thermal annealing and solvent vapor treatment are
useful post-treatments to optimize the morphology of the
(
2
4,8-bis(5-(2-ethylhexyl)furan-2-yl)benzo[1,2-b:4,5-b′]dithiophene-
,6-diyl)bis(tributylstannane) (BDTF), (4,8-bis(5-(2-ethylhexyl)-
1
0
thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis-
blends, these methods are time-consuming and require
meticulous control of the experimental parameters. Inspiringly,
several independent groups have found that the performance of
PSCs could be improved simply through solvent treatment
(
[
tributylstannane) (BDTT), 4,7-dibromo-5,6-bis(octyloxy)benzo[c]-
1,2,5]oxadiazole (compound 1); 2-(2-ethylhexyl)furan, 2-(2-
ethylhexyl)thiophene, and 4,8-dehydrobenzo[1,2-b:4,5-b′]-
dithiophene-4,8-dione (compound 3); and 2-(tributylstannyl)furan
11
8
,13
before deposition of the metal electrodes. However, there are
limited instances of employing this method to optimize PSCs,
based on state-of-the-art donor−acceptor copolymers.
was performed similarly to previous literature,
compounds were synthesized following the procedures described
herein.
All of the other
4
,7-Di(furan-2-yl)-5,6-bis(octyloxy)benzo[c][1,2,5]oxadiazole(2).
Herein, we focus our attention on the effects of side-chain
engineering and various polar solvent treatments to optimize
the performance of furan-based PSCs. To carry out our studies,
we first synthesized a series of new p-type semiconducting
polymers with various side groups comprising benzodithio-
phene and difuranylbenzooxadiazole as the electron-donating
and electron-accepting components, respectively. The three co-
polymers were named PBDT-DFBO, PBDTT- DFBO, and
To a solution of 4,7-dibromo-5,6-bis(octyloxy)benzo[c][1,2,5]-
oxadiazole (2.20 g, 4 mmol), Pd(PPh ) (0.51 g, 0.44 mmol) in dry
toluene (100 mL) was added 2-(tributylstannyl)furan (4.50 g, 12
mmol) and the reaction mixture was heated to reflux for 48 h under
argon. The reaction mixture was concentrated directly on Celite under
reduced pressure. The residue was chromatographically purified on a
silica gel column eluting with CH Cl /hexane (1:10, v:v) to afford title
3
4
2
2
1
compound as a yellow oil (1.8 g, yield 90%). H NMR (400 MHz,
B
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Chemistry of Materials
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CDCl ): d (ppm) 7.73 (d, 2H, Ar−H), 7.42 (d, 2H, Ar−H), 6.65 (dd,
energy levels of the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) were then calculated
according to the following equations:
3
2
H, Ar−H), 4.15 (t, 4H, CH ), 1.90 (m, 4H, CH ), 1.47 (m, 4H,
2
2
CH ), 1.29−1.39 (m, 16H, CH ), 0.95 (t, 6H, CH ).
2
2
3
4
,7-Bis(5-bromofuran-2-yl)-5,6-bis(octyloxy)benzo[c][1,2,5]-
EHOMO (eV) = −e(φ + 4.40)
oxadiazole (DFBO). A mixture of compound 2 (0.70 g, 3.8 mmol), N-
bromosuccimide (NBS) (0.46 g, 7.8 mmol), chloroform (50 mL), and
acetic acid (50 mL) was stirred at room temperature in darkness
overnight. Then the reaction mixture was poured into water (200 mL)
and extracted with chloroform (3 × 50 mL). The extracts were
combined and washed with water and saturated sodium bicarbonate
solution then dried over anhydrous sodium sulfate. After filtration, the
solvent was removed under reduced pressure. The residue was purified
by column chromatography on silica, eluting with petroleum ether/
ox
ELUMO (eV) = −e(φ + 4.40)
re
where φox is the onset oxidation potential vs Ag/AgCl and φ is the
re
onset reduction potential vs Ag/AgCl.
2.3. Fabrication and Characterization of BHJ Devices. All
organic photovoltaic devices had a conventional device architecture,
ITO/PEDOT:PSS/polymer:PC BM/Ca/Al. ITO/glass substrates
7
1
were ultrasonically cleaned sequentially in detergent, water, acetone,
and iso-propanol (IPA), followed by treating in an ultraviolet ozone
chamber (Ultraviolet Ozone Cleaner, Jelight Company, USA) for 20
min. The cleaned substrates were covered by a 40-nm-thick layer of
PEDOT:PSS(Baytron PV PAI 4083, Germany) by spin coating. After
annealing in a glovebox at 140 °C for 20 min, the samples were cooled
chloroform (from 20:0 to 20:1), to give a bright yellow solid (0.50 g,
1
6
6
4
0%). H NMR (400 MHz, CDCl ): d (ppm) 7.38 (d, 2H, Ar−H),
3
.54 (d, 2H, Ar−H), 4.11 (t, 4H, CH ), 1.94 (m, 4H, CH ), 1.52 (m,
2
2
13
H, CH ), 1.24−1.36 (m, 16H, CH ), 0.90 (t, 6H, CH ). C NMR
2
2
3
(
400 MHz, CDCl ): d (ppm) 152.42, 148.43, 145.14, 123.85, 116.51,
3
1
13.91, 108.54, 75.24, 31.86, 30.30, 29.59, 29.31, 26.09, 22.70, 14.14.
Polymerization for PBDT-DFBO. DFBO (133 mg, 0.20 mmol) and
to room temperature. Polymers were dissolved in dichlorobenzene
14
(
DCB)/4% (v/v) 1-chloronaphthalene (CN) mixed solvent, and
BDT (205 mg, 0.20 mmol) were dissolved into dry toluene (4 mL).
The solution was flushed with argon for 10 min, and then Pd(PPh3)4
then PC BM (purchased from American Dye Source) was added. The
71
solution was then heated at 70 °C and stirred overnight at the same
(5 mg) was added into the flask. The flask was purged three times with
temperature. The solution of polymer:PC BM was then spin-coated
71
successive vacuum and argon filling cycles. The polymerization
reaction was heated to 120 °C, and the mixture was stirred for 36 h
under argon atmosphere. Then, the reactant was cooled to room
temperature and poured slowly into methanol (200 mL). The
precipitate was filtered and washed with methanol, acetone, and
hexane in a Soxhlet apparatus to remove the oligomers and catalyst
residue. Finally, the polymer was extracted with chloroform. The
solution was condensed by evaporation and precipitated into
to form the active layer (∼100 nm). After drying under vacuum, polar
solvent exposure with various wetting time was carried out using the
spin-coating methanol solvent on the top of active layers at 2500 rpm
for 45 s. The devices were completed after the deposition of 20 nm
Ca/100 nm Al as a cathode through a shadow mask under high
−6
vacuum (<10 Torr). The effective area of the device was measured
2
to be 0.1257 cm . The device characteristics were obtained using a
xenon lamp at AM1.5 solar illumination (Oriel Instruments). The
current−voltage (J−V) characterization of the devices was carried out
on a computer-controlled Keithley 2440 Source Measurement system.
The EQE measurements of the PSCs were performed 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.
methanol. The title polymer was collected as a dark red solid. (205
1
mg, 61%, M = 36.1 kg/mol, PDI = 2.07). H NMR (400 MHz,
n
CDCl , ppm): 7.52−6.91 (br, 6H), 4.25 (br, 8H), 1.25−2.04 (br,
3
4
7
2H), 0.89−1.16 (br, 18H). Anal. Calcd for (C H N O S ) (%): C,
5
8
80
2
7 2 n
0.98; H, 8.22; N, 2.85. Found: C, 70.27; H, 7.81; N, 2.68.
Polymerization for PBDTT-DFBO. PBDTT-DFBO was prepared
using the same procedure as PBDT-DFBO. The resulting co-polymer
PBDTT-DFBO was obtained as a dark purple solid with a yield of
2.4. Hole and Electron Mobility. Hole and electron mobility was
1
6
5%. (M = 29.3 kg/mol, PDI = 2.01). H NMR (400 MHz, CDCl ,
n
3
measured using the space charge limited current model (SCLC), using
ppm): 7.42−7.39 (br, 4H), 7.00 (br, 6H), 4.18 (br, 4H), 2.96 (br,
a diode configuration of ITO/PEDOT:PSS/polymer:PC BM/MoO /
7
1
3
6
7
H), 2.14−0.70 (br, 58H). Anal. Calcd for (C H N O S ) (%): C,
4a
15
6
6
84
2
5 4 n
Al for hole-only device and ITO/Al/polymer:PC BM/Ca/Al for
71
1.18; H, 7.60; N, 2.52. Found (%): C, 70.47; H, 7.30; N, 2.37.
electron-only device, respectively, and taking current−voltage
measurements in the range of 0−6 V and fitting the results to a
space-charge-limited form, where the SCLC is described by the Mott−
Gurney law:
Polymerization for PBDTF-DFBO. PBDTF-DFBO was prepared
using the same procedure as PBDT-DFBO. The resulting copolymer
PBDTF-DFBO was obtained as a dark blue solid, with a yield of 72%.
1
(
M = 38.4 kg/mol, PDI = 2.24). H NMR (400 MHz, CDCl , ppm):
n
3
2
7
2
.52−7.33 (br, 4H), 6.59−6.39 (br, 6H), 4.32−4.17 (br, 4H), 3.01−
⎛ ⎞
⎛
⎞
⎟
8
⎝ ⎠
9
V
J = ⎜ ⎟ε ε μ⎜
.93 (br, 6H), 2.15−0.71 (br, 58H). Anal. Calcd for (C H N O S )
r
0
3
66
84
2
7
2
n
⎝ L ⎠
(%): C, 73.30; H, 7.83; N, 2.59. Found (%): C, 72.57; H, 7.44; N,
2
.46.
2
where ε is the permittivity of free space, ε the dielectric constant of
0
r
.2. General Measurement and Characterization. H and 13C
1
the polymer, μ the charge mobility, V the voltage drop across the
device (V = Vappl − V , where V is the applied voltage to the device
NMR spectra were measured using a Bruker DMX-400 spectrometer.
Chemical shifts of NMR were reported in ppm relative to the singlet of
bi
appl
and V is the built-in voltage, which we estimated from the difference
bi
1
13
CDCl at 7.26 ppm for H NMR spectroscopy and 77.6 ppm for
C
3
between the work function and the HOMO energy level of
1
6
NMR spectroscopy. Absorption spectra were collected on a Perkin−
Elmer Lambda 950. The molecular weights of the polymers were
measured by the GPC method on a Waters 1515, and polystyrene was
used as the standard (room temperature, chloroform as the eluent).
The electrochemical cyclic voltammetry (CV) of the polymer film was
conducted in acetonitrile with 0.1 M of tetrabutylammonium
polymers ), and L is the polymer thickness. The dielectric constant
εr is assumed to be 3, which is a typical value for conjugated polymers.
The thickness of films was measured using a Dektak 6 M surface
profilometer.
2.5. AFM Measurement. AFM samples are prepared as follows.
Films of polymer/PC BM blend are cast from solutions on clean glass
71
−1
hexafluorophosphate (Bu NPF ) using a scan rate of 100 mV s at
slides. Both pristine and treated films are examined using a Veeco
4
6
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 coated from a chloroform solution, ca. 5 mg/
mL. For calibration, the redox potential of ferrocene/ferrocenium (Fc/
Dimension 3100 V atomic force microscopy (AFM) system.
3
.. RESULTS AND DISCUSSION
.1. Material Design and Synthesis. The synthetic routes
3
+
of the new monomer (4,7-bis(5-bromofuran-2-yl)-5,6-bis-
(octyloxy)benzo[c][1,2,5]oxadiazole) (DFBO) and three co-
polymers are shown in Scheme 1. In our synthetic design, the
Fc ) was measured under the same conditions, and it is located at 0.40
V vs the Ag/AgCl electrode. It is assumed that the redox potential of
+
Fc/Fc has an absolute energy level of −4.80 eV to vacuum. The
C
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Scheme 1. Synthetic Routes of the Monomers and Polymers
Figure 2. (a) Absorption spectra of PBDT-DFBO, PBDTT-DFBO, and PBDTF-DFBO in thin films. (b) HOMO and LUMO energy levels of the
polymers; energy levels of PC BM are listed for comparison.
71
π-conjugating bridge (furan) was first attached to the electron-
accepting BO unit. It was synthesized from 2-stannylated furan
by a Stille cross-coupling reaction with compound 1 to give
compound 2, which was then subjected to bromination to
afford DFBO. Consequently, three new furan-π-bridged
donor−acceptor copolymersnamely, PBDT-DFBO,
PBDTT-DFBO, and PBDTF-DFBOwere synthesized via
Stille-coupling polymerization (Scheme 1). All three copoly-
mers were purified by Soxhlet extraction with a washing solvent
sequence of methanol, acetone, hexane, and finally chloroform.
The resulting copolymers showed excellent solubility in
common organic solvents, such as chloroform, toluene,
chlorobenzene, and 1,2-dichlorobenzene. Gel permeation
chromatography (GPC) studies (using polystyrene as the
standard and chloroform as the eluent) showed that these
polymers were purified by successive reprecipitation and
Soxhlet extraction had a number-average molecular weight
(M ): PBDT-DFBO (M = 36.1 kg/mol, PDI = 2.07), PBDTT-
n
n
DFBO (M = 29.3 kg/mol, PDI = 2.01), and PBDTF-DFBO
n
(M = 38.4 kg/mol, PDI = 2.24), respectively.
n
3.2. Properties of Newly Designed Benzodithio-
phene−Difuranylbenzooxadiazole Co-polymers. The
UV−vis absorption spectra of all three co-polymers as thin
films are shown in Figure 2a (UV absorption in dilute
D
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chloroform solution shown in Figure S1 in the Supporting
Information), and the photophysical data of polymers are
summarized in Table 1. One could observe that PBDTT-
HOMO and LUMO of PBDTF-DFBO were 0.06 and 0.18 eV
lower than those of PBDT-DFBO, respectively. On the other
hand, PBDTT-DFBO had deeper HOMO level than PBDTF-
DFBO. This is because the oxygen atom is known to be a richer
electron donor, compared to the sulfur atom, because of the
better overlap of its orbitals with the π-system; therefore, the
alkylfuranyl group has relatively stronger electron-donating
ability than does the alkylthienyl unit. Consequently, by
replacing the alkylthienyl with alkylfuranyl, the HOMO values
of the resulting polymers were expected to be higher, which
Table 1. Optical and Electrochemical Properties of the
Polymers
18
λonset (nm)
Egopt
HOMO
(eV)
LUMO
(eV)
Egec
a
b
polymer
solution
645
film
(eV)
(eV)
PBDT-
DFBO
672
686
700
1.85
−5.34
−5.44
−5.40
−3.43
−3.59
−3.61
1.91
1.85
1.79
would lead to lower open-circuit voltage (V ) in photovoltaic
oc
8
devices. Overall, the results of the UV/vis absorption spectra
PBDTT-
DFBO
661
672
1.81
and the CV measurements agreed quite well.
PBDTF-
DFBO
1.77
3.3. Characteristics and Optimization of Photovoltaic
Devices. To characterize the photovoltaic properties for these
polymers, BHJ PSCs were fabricated with a general device
structure of ITO/PEDOT: PSS/polymer:PC BM/Ca/Al, and
a
b
Measured in chloroform solution. Cast from chloroform solution.
71
2
their performances were measured under 100 mW/cm AM 1.5
G illumination. The ratio of polymer to PC BM was adjusted
DFBO and PBDTF-DFBO had broader absorption range than
PBDT-DFBO, which could be ascribed to the enhanced
intermolecular π−π interaction that was originated from the
extended conjugation by replacing alkoxyl group with alkyl-
aromatic unit. Furthermore, it should be noted that PBDTF-
DFBO with a furan side group showed red-shifted absorption
onset (11 and 14 nm), compared with thiophene counterpart in
both solution and thin film. Thus, an increased light harvesting
by the film of PBDTF-DFBO was expected because of
absorption over a broader spectral range (see Figure 2a), and
the film color was close to blue. From the onset of the thin film
absorptions (λonset), one could estimate the optical band gaps of
the copolymers. The band gap of PBDTF-DFBO was 1.77 eV,
which was reduced by 0.04 eV in comparison with that of
PBDTT-DFBO (1.81 eV). The reduction of the band gap
energy can be mainly attributed to a more planar structure of
71
from 1:1 to 1:3 (by weight) and the optimized condition was
:2 for all of them. The polymer active layers were spin-coated
1
from solutions in dichlorobenzene (DCB)/1-chloronaphtha-
lene (CN) mixed solvent. Moreover, the optimized condition
was 4 vol % CN in DCB solution, and the typical current
density−voltage (J−V) curves of PSCs based on PBDTF-
DFBO with varying CN concentration in DCB are shown in
Figure S3a in the Supporting Information. The J−V curves of
all PSCs are shown in Figure 3a (and Figure S3b in the
Supporting Information), and the device performances are
summarized in Table 2. In the initial investigation, it was found
that the solar cells exhibited moderate performance with PCE
of 2.0% ± 0.1% for PBDT-DFBO with alkoxyl chains, 2.1% ±
0
.1% for PBDTT-DFBO with alkylthienyl groups and a relative
17
high efficiency of 2.8% ± 0.2% for PBDTF-DFBO with
the BDTF unit than that of the BDTT unit.
alkylfuranyl side moiety.
Meanwhile, the HOMO and LUMO energy levels of the
polymers are determined by cyclic voltammetry (CV), and the
results are summarized in Figure 2b and Table 1 (the original
data can be found in the Figure S2 in the Supporting
Information). When the alkoxy side chain is replaced by
aromatic conjugated side chains (such as alkylfuranyl or
alkylthienyl), the HOMO and LUMO levels of the polymers
are shifted to lower energy levels, implying that the weak
electron-donating ability of aromatic side groups is favorable to
reducing the HOMO level of the polymer. For example, the
11
Inspired by recent work on post-solvent treatment, we tried
to employ a variety of polar solvents, including methanol,
ethanol, and propanol, to optimize the performance of PSCs
with a modified procedure: (i) spin-coated the active layer and
dried under vacuum; (ii) methanol (or other polar solvents)
was added atop the active layer and a short wait time (such as 2
min) was allowed; (iii) solvent was removed by spin coating at
high speed (such as 2500 rpm); (iv) the cathode was
evaporated (as shown in Figure S4 in the Supporting
Figure 3. (a) Current density−voltage (J−V) curves of the PSCs based on polymer/PC BM (1:2, w/w) with and without solvent exposure, under
7
1
−2
the illumination of AM1.5G, 100 mW cm . (b) J−V characteristics of devices with (circles) and without polar solvent treatment (squares) in
darkness.
E
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Chemistry of Materials
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Table 2. Performance of the Polymer:PC BM BHJ Solar Cells before and after Various Polar Solvent Treatments with a
7
1
a
Wetting Time of 2 min, at a Blend Ratio of 1:2
Jsc [mA cm−2]b
Voc [V]
b
FF [%]
b
PCE [%]
2.0 ± 0.1
4.0 ± 0.2
5.6 ± 0.3
3.7 ± 0.1
b
solvent treatment
PBDT-DFBO
0.78 ± 0.01
0.82 ± 0.01
0.83 ± 0.01
0.80 ± 0.02
PBDTT-DFBO
0.83 ± 0.01
0.86 ± 0.01
0.83 ± 0.02
0.82 ± 0.01
PBDTF-DFBO
0.79 ± 0.01
0.83 ± 0.01
0.81 ± 0.01
none
5.2 ± 0.1
9.3 ± 0.1
10.6 ± 0.2
9.0 ± 0.4
50.2 ± 0.1
52.9 ± 0.2
64.7 ± 0.4
48.7 ± 0.6
methanol
ethanol
propanol
none
5.5 ± 0.1
9.1 ± 0.3
7.3 ± 0.5
7.4 ± 0.6
46.5 ± 0.1
58.6 ± 0.5
39.4 ± 0.7
46.6 ± 0.5
2.1 ± 0.1
c
methanol
ethanol
propanol
4.5 ± 0.3 (5.0)
2.6 ± 0.1
3.0 ± 0.1
none
6.9 ± 0.1
12.7 ± 0.1
11.0 ± 0.2
10.7 ± 0.4
53.4 ± 0.1
62.0 ± 0.1
53.0 ± 0.4
48.1 ± 0.3
2.8 ± 0.2
c
methanol
ethanol
propanol
6.5 ± 0.1 (7.0)
4.7 ± 0.2
0.84 ± 0.01
4.4 ± 0.2
a
b
Boldface font indicates the best solvent treatment for each polymer solar cell. Photovoltaic properties of co-polymer/PC BM-based devices spin-
71
coated from a DCB/4% (v/v) CN mixed solution. Only the optimized recipes were considered for the estimation of the average PCE. Data have
c
been averaged on eight devices. The efficiency was achieved with a wetting time of 3 min.
Figure 4. Wetting time dependence of PSCs performance of PBDT-DFBO, PBDTT-DFBO, and PBDTF-DFBO: (a) short-circuit current density
(
J ), (b) open-circuit voltage (V ), (c) fill factor (FF), and (d) power conversion efficiency (PCE).
s
c
o
c
Information). From Table 2, Figure 3a, and Figure S3b in the
Supporting Information, one could observe that different
materials had different appropriate solvent options. Cheerfully,
after the solvent exposure, the performance of all PSCs had
been enhanced dramatically. For PBDT-DFBO, the most
appropriate solvent was ethanol and the PCE values achieved
ranged from 2.0% ± 0.1% to 5.6% ± 0.3%, with a largely
ranging from 46.5% ± 0.1% to 58.6% ± 0.5%. For PBDTF-
DFBO, the most appropriate solvent is also the methanol, and
the PCE achieved ranged from 2.8% ± 0.2% to 6.5% ± 0.1%,
with J ranging from 6.9 ± 0.1 mA cm− to 12.7 ± 0.1 mA
2
sc
−2
cm , a V ranging from 0.79 ± 0.01 V to 0.83 ± 0.01 V, and a
oc
FF from 53.4% ± 0.1% to 62.0% ± 0.1%. The J value (12.32
sc
−2
mA cm ) calculated by integrating the EQE data (see Figure
S5 in the Supporting Information) with the AM 1.5G spectrum
increased short-circuit current density (J ), from 5.2 ± 0.1 mA
sc
−2
−2
cm to 10.6 ± 0.2 mA cm , a open-circuit voltage (V )
agreed rather well with the directly measured J value (PCE
oc
sc
ranging from 0.78 ± 0.01 V to 0.83 ± 0.01 V, and a fill factor
estimated from the calculated J value: 6.43%) and the EQE
sc
(
FF) from 50.2% ± 0.1% to 64.7% ± 0.4%. For PBDTT-
curve revealed a broad photoresponse, from 300 nm to 700 nm,
with two maximum peak values of 74% at ∼450 nm and ∼570
nm. Moreover, as shown in the J−V curves (obtained in
darkness) in Figure 3b and Figure S6 in the Supporting
Information), all BHJ solar cells with polar solvent exposure
DFBO, the most appropriate solvent is methanol and the PCE
value reached ranged from 2.1% ± 0.1% to 4.5% ± 0.3%, with J
−2
−2
ranging from 5.5 ± 0.1 mA cm to 9.1 ± 0.3 mA cm , a V
sc
oc
value ranging from 0.83 ± 0.01 V to 0.86 ± 0.01 V, and a FF
F
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0
.5
Figure 5. J −V plots for PBDT-DFBO:PC BM devices without (squares) and with (circles) ethanol treatment: (a) hole-only devices and (b)
71
electron-only devices. The solid lines represent the fit using a model of single carrier SCLC with field-independent mobility. The J−V characteristics
are corrected for the built-in voltage (V ).
bi
Figure 6. Tapping-mode atomic force microscopy (AFM) topography image of the blend film of PBDTF-DFBO/PC BM (1:2, w/w): (a) without
71
solvent treatment, (b) with methanol treatment, (c) with ethanol treatment, (d) with propanol treatment, (e) three-dimensional (3D) surface plot
without solvent treatment, and (f) 3D surface plot with methanol treatment. (All image sizes are 2.0 μm × 2.0 μm.)
exhibited a higher shunt resistance (R ) and a reduced series
pristine PSCs based on PBDTF-DFBO, respectively. These
sh
resistance (R ), compared without solvent treatment. For
data are in agreement with the slightly increased Voc values in
s
5
2
example, the R values were calculated to be 1.4 × 10 Ω cm
the PSCs. Moreover, the series resistance (R ) of the methanol-
sh
s
4
2
2
and 1.0 × 10 Ω cm for the methanol-treated PSCs and the
treated PSCs is 3 Ω cm , which was lower than that of the
G
dx.doi.org/10.1021/cm401618h | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
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2
pristine device (10 Ω cm ). These results demonstrated that
polar solvent exposure via tapping-mode atomic force
microscopy (AFM). The AFM topography images are shown
in Figure 6 and Figure S8 in the Supporting Information). In
better diode characteristics were obtained after solvent
exposure. In addition, note that the J value of the PBDTF-
sc
−
2
DFBO/PC BM solar cell (12.7 ± 0.1 mA cm ) was
the case of PBDTF-DFBO:PC BM blends, it could be
7
1
71
−
2
remarkably improved from 9.1 ± 0.3 mA cm
of the
observed that the blend film without solvent exposure exhibited
relatively course phase separation between the polymer and
PC BM with a root-mean-square (RMS) surface roughness of
PBDTT-DFBO based devices, but accompanied by a slight
reduction in V . This phenomenon is consistent with that
oc
71
which is predicted by theory and the experimental results of the
UV/vis absorption spectra and the CV measurements. The
improved photocurrent of PBDTF-DFBO from PBDTT-
DFBO is attributed to its higher charge carrier mobility (see
Table S2 in the Supporting Information) and broader
absorption range. The slightly decreased Voc value of
PBDTF-DFBO from PBDTT-DFBO is, to a certain degree,
caused by its higher HOMO energy level.
5
.2 nm (see Figure 6a). The relatively large (100−300 nm)
globular clusters that can be attributed to PC BM domains
may originate from the fact that, during the drying of the film,
PC BM crystallizes before the polymer is solidifying. In
contrast, after solvent treatment, the surface of the blend film
demonstrated a more uniform distribution of PBDTF-DFBO
and PC BM (see Figures 6b, 6c, and 6d) and the formation of
71
20
71
71
fiberlike interpenetrating morphologies at the length scale of
20 nm with decreasing roughness of RMS = 2.1 nm for the
To further optimize the performance of all PSCs, we examine
the wetting time effect (the time that polar solvent is present on
top of the active layer before the substrate is spun), since it is
∼
methanol-treated device, RMS = 0.7 nm for the ethanol-treated
device, and RMS = 1.4 nm for the propanol-treated device.
11a
relevant for the observed improvements. Figure 4 presented
11c
According to Park’s work, 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 and weighty, compared to
the small molecules. 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
the wetting time dependence of J , V , FF, and overall PCE of
sc
oc
1
:2 polymer/PC BM devices on their most appropriate
71
solvent treatment (ethanol for PBDT-DFBO, methanol for
PBDTT-DFBO and PBDTF-DFBO) (see also Table S1 in the
Supporting Information. A wetting time of 2 min was found to
work best for PBDT-DFBO, whereas PSCs based on PBDTT-
DFBO and PBDTF-DFBO achieved the highest PCEs of 5.0%
and 7.0%, respectively, with a wetting time of 3 min. Although
there were slight decreases of PCE with wetting time longer
21
solvents, and after some appropriate wetting time, PC BM
7
1
particles aggregated to the tens-of-nanometers length domains,
thus, the formation of nanoscale phase separation of the blend
films (see Figures 6a, 6b, 6e, and 6f). This relatively smooth
surface and more ordered structure are beneficial to the charge
than 3 min, the Voc and J values remained almost the same.
sc
3.4. Effects of Solvent Treatment on Charge-Trans-
port Properties. In order to explore the origin of the largely
enhanced performance, especially the dramatically increased Jsc
and FF values after polar solvent exposure, the charge-transport
8b
transportation, thus leading to an increase in J , as well as the
sc
device efficiency. The cases of PBDT-DFBO/PC BM and
7
1
properties were investigated by examining the charge carrier
mobility of polymer:PC71 BM using single-carrier diodes and
the space-charge limited current (SCLC) model. The photo-
current in the BHJ solar cells combines contributions of both
the hole and the electron; therefore, we must assist the
injection of one type of the charges and suppress the other by
choosing suitable electrodes to manufacture electron-only or
PBDTT-DFBO/PC BM blends are similar to that of PBDTF-
71
DFBO/PC BM blend (see Figure S8 in the Supporting
71
Information).
4
.. CONCLUSION
In summary, a series of furan-based donor−acceptor co-
polymers with various side groups have been developed. These
side-group substituents offer the opportunity to tailor the
optical and electronic properties of the polymers. In the
preliminary investigation, one can obtain solar cells with power
conversion efficiency (PCE) values of 2.1% for PBDT-DFBO,
19
hole-only devices (details can be found in the Experimental
Section). The J−V curves were fit using the Mott−Gurney law
(
see Figure 5 and Figure S7 in the Supporting Information),
and the data were collected in Table S2 in the Supporting
Information. The hole mobilities of all polymer:PC BM
71
devices were enhanced dramatically after polar solvent
exposure. For instance, in pristine PBDT-DFBO:PC BM
2
.2% for PBDTT-DFBO, and 3.0% for PBDTF-DFBO.
7
1
Inspiringly, after simply polar solvent exposure (such as
methanol or ethanol), the PCE could reach up to 5.9%, 5.0%,
and 7.0%, respectively, which showed an improvement of
−
5
2
−1 −1
devices, the hole mobility of 5.4 × 10 cm V
s
was
almost 1 order of magnitude lower than the electron mobility of
3
.1 × 10− cm V s . After methanol treatment, the hole
4
2
−1 −1
1
30%−170%. The effects of polar solvent treatment on the
−4
2
−1 −1
mobility increased to 1.8 × 10 cm V s , while the electron
−
4
2
−1 −1
simultaneous enhancement in the open-circuit voltage, short-
circuit current density, and fill factor of the PSCs are shown to
originate from the increased hole mobilities, more-balanced
charge transport, and the formation of fiberlike interpenetrating
morphologies. Furthermore, this strategy obviates laborious
synthesis of interlayer materials, time-consuming thermal
annealing, and solvent vapor treatment. This work demon-
strates a good example for tuning the absorption range, energy
level, charge transport, and photovoltaic properties of the
polymers by side-chain engineering and the solvent treatment
can offer a simple and effective method to improve the
efficiency of polymer solar cells (PSCs).
mobility almost remained unaltered (3.4 × 10 cm V s ).
As a consequence, on one hand, the larger J value of the polar
sc
solvent-treated devices from prestine devices was, to a certain
degree, attributed to its higher charge carrier mobility; on the
other hand, with the more balanced charge transport in the
devices, one may increase the FF of the PSCs by restricting the
buildup of space charges, and consequently, reducing charge
1
5
recombination.
.5. Modification of Solvent Treatment on the Active
3
Layer surface. Since the morphology of the photoactive layer
played a key role in the photovoltaic performance of PSCs, we
directly examined the surface morphology with and without
H
dx.doi.org/10.1021/cm401618h | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials
Article
Roelofs, W. S. C.; Wienk, M. M.; Janssen, R. A. J. J. Am. Chem. Soc.
ASSOCIATED CONTENT
■
2
(
012, 134, 13787.
6) Wang, X.; Chen, S.; Sun, Y.; Zhang, M.; Li, Y.; Li, X.; Wang, H.
Polym. Chem. 2011, 2, 2872.
7) Wang, X.; Sun, Y.; Chen, S.; Guo, X.; Zhang, M.; Li, X.; Li, Y.;
*
S
Supporting Information
1
H NMR and 13C NMR spectrum of DFBO and co-polymers.
UV−visible absorption spectra, CV, SCLC results, J−V
characteristics of the polymer:PC BM devices, and EQE of
(
7
1
Wang, H. Macromolecules 2012, 45, 1208.
PBDTF-DFBO:PC BM devices. This material is available free
of charge via the Internet at http://pubs.acs.org.
(8) (a) Yuan, J.; Huang, X.; Zhang, F.; Lu, J.; Zhai, Z.; Di, C.; Jiang,
Z.; Ma, W. J. Mater. Chem. 2012, 22, 22734. (b) Wang, Y.; Yang, F.;
Liu, Y.; Peng, R.; Chen, S.; Ge, Z. Macromolecules 2013, 46, 1368.
7
1
(
c) Dou, L.; Gao, J.; Richard, E.; You, J.; Chen, C. C.; Cha, C. C.; He,
AUTHOR INFORMATION
Corresponding Author
E-mail: geziyi@nimte.ac.cn.
■
*
Y.; Li, G.; Yang, Y. J. Am. Chem. Soc. 2012, 134, 10071. (d) Zhang, Y.;
Gao, L.; He, C.; Sun, Q.; Li, Y. Polym. Chem. 2013, 4, 1474.
9) (a) Moule,
(b) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736.
10) (a) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.;
(
́
A. J.; Meerholz, K. Adv. Funct. Mater. 2009, 19, 3028.
Notes
(
The authors declare no competing financial interest.
Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6, 497. (b) Li, G.;
Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat.
Mater. 2005, 4, 864. (c) Miller, S.; Fanchini, G.; Lin, Y.-Y.; Li, C.;
Chen, C.-W.; Su, W.-F.; Chhowalla, M. J. Mater. Chem. 2008, 18, 306.
(11) (a) Liu, X.; Wen, W.; Bazan, G. C. Adv. Mater. 2012, 24, 4505.
(b) Li, H.; Tang, H. W.; Li, L. G.; Xu, W. T.; Zhao, X. L.; Yang, X. N.
J. Mater. Chem. 2011, 21, 6563. (c) Nam, S.; Jang, J.; Cha, H.; Hwang,
J.; An, T. K.; Park, S.; Park, C. E. J. Mater. Chem. 2012, 22, 5543.
ACKNOWLEDGMENTS
We would like to thank the financial support of the National
Natural Science Foundation of China (Nos. 21074144,
1273209, and 21102156), and the External Cooperation
■
5
Program of the Chinese Academy of Scieces (No. GJHZ1219).
(
d) Wang, Q.; Zhou, Y.; Zheng, H.; Shi, J.; Li, C. Z.; Su, C. M. Q.;
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