A new benzo[1,2-b:4,5-b′]difuran-based copolymer for efficient polymer solar cells

Article abstract of DOI:10.1039/c2jm32843g

A new donor-acceptor type copolymer, namely poly{4,8-bis(2-ethylhexyloxy) benzo[1,2-b:3,4-b′]difuran-alt-6-octylnaphtho[2,3-c]thiophene-4,9-dione} (PBDFNTDO) was synthesized by a Stille coupling reaction and characterized by ¹H NMR, GPC, TGA, UV-Vis absorption spectroscopy and cyclic voltammetry. PBDFNTDO is readily soluble in common organic solvents with a number-average molecular weight (M_n) of 10.7 kDa mol^-1 and a polydispersity index of 1.71. TGA analysis shows the copolymer exhibits good thermal stability with 5% weight loss at a temperature of 341 °C. PBDFNTDO possesses a broad absorption band at 300-750 nm with an optical bandgap of 1.65 eV. Cyclic voltammetry gives HOMO and LUMO energy levels of -5.33 eV and -3.40 eV, respectively. The hole mobility of PBDFNTDO:PC₇₁BM (1:1.5, w/w) reaches up to 5.0 × 10^-3 cm² V^-1 s ^-1 by the space-charge-limited current (SCLC) method. A polymer solar cell with the configuration of ITO/PEDOT:PSS/PBDFNTDO:PC₇₁BM (1:1.5, w/w)/Ca/Al demonstrates a promising power conversion efficiency of 4.71% under the illumination of AM 1.5 G, 100 mW cm^-2.

Full text of DOI:10.1039/c2jm32843g

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PAPER
A new benzo[1,2-b:4,5-b0]difuran-based copolymer for efficient polymer solar
cells†
ab
Xuewen Chen,‡a Bo Liu,‡ab Yingping Zou,
Lu Xiao,a Xiuping Guo,a Yuehui Heb and Yongfang Lic
*
Received 6th May 2012, Accepted 9th June 2012
DOI: 10.1039/c2jm32843g
A new donor–acceptor type copolymer, namely poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4-b0]
difuran-alt-6-octylnaphtho[2,3-c]thiophene-4,9-dione} (PBDFNTDO) was synthesized by a Stille
1
coupling reaction and characterized by H NMR, GPC, TGA, UV-Vis absorption spectroscopy and
cyclic voltammetry. PBDFNTDO is readily soluble in common organic solvents with a number-
average molecular weight (Mn) of 10.7 kDa molꢀ1 and a polydispersity index of 1.71. TGA analysis
ꢁ
shows the copolymer exhibits good thermal stability with 5% weight loss at a temperature of 341 C.
PBDFNTDO possesses a broad absorption band at 300–750 nm with an optical bandgap of 1.65 eV.
Cyclic voltammetry gives HOMO and LUMO energy levels of ꢀ5.33 eV and ꢀ3.40 eV, respectively.
The hole mobility of PBDFNTDO:PC71BM (1 : 1.5, w/w) reaches up to 5.0 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1 by the
space-charge-limited current (SCLC) method. A polymer solar cell with the configuration of ITO/
PEDOT:PSS/PBDFNTDO:PC71BM (1 : 1.5, w/w)/Ca/Al demonstrates a promising power conversion
efficiency of 4.71% under the illumination of AM 1.5 G, 100 mW cmꢀ2
.
as 2,1,3-benzothiadiazole (BT),15 thieno[3,4-c]pyrrole-4,6-dione
(TPD),16 thieno[3,4-b]thiophene (TT),17 pyrrolo[3,4-c]-pyrrole-
1,4-dione (DPP),18 etc. exhibited promising photovoltaic prop-
erties. For example, Hou et al. reported an alternating copolymer
of BDT and TT, namely PBDTTT-CF, and obtained a high
power conversion efficiency (PCE) of up to 7.7%.19
Benzo[1,2-b:4,5-b0]difuran (BDF) has a similar structure
compared with the BDT unit, although the oxygen atom has a
smaller diameter and higher electronegativity than a sulfur atom.
BDF-based copolymers may form a good planar and stacking
structure when combined with a suitable acceptor moiety, and
then better absorbance and higher mobility are expected, which
can lead to high photovoltaic properties.
1. Introduction
In the past two decades, conjugated polymers have attracted
much interest in polymer solar cells (PSCs),1 field-effect transis-
tors (FETs)2 and light-emitting diodes (LEDs),3 because of their
advantages of being light-weight, flexible and low-cost.4–8 PSCs
are composed of a photoactive blend layer of a conjugated
polymer donor and a soluble fullerene derivative acceptor
sandwiched between an indium-tin oxide (ITO) positive electrode
and a low work function metal negative electrode.9–11 The pho-
toactive layer of PSCs consists of a network formed by an elec-
tron-donor (D) blended with an electron-acceptor (A).12
Recently, a D–A type conjugated polymer showed excellent
properties and this kind of polymer plays an important role in the
The
naphtho[2,3-c]thiophene-4,9-dione (NTDO)
unit
PSCs field.13
possesses a relatively simple, planar structure and has strong
electron withdrawing ability resulting in lower HOMO and
LUMO energy levels of the D–A copolymers.20 Usually a lower
HOMO level is beneficial to get higher open circuit voltages (Voc)
of the PSCs.
Benzo[1,2-b:4,5-b0]dithiophene (BDT) as electron-donor unit
has attracted broad attention because BDT-based copolymers
possessed high efficiencies of up to 8%.14 Many BDT based
conjugated polymers with different electron acceptor units, such
Taking all of these results into account, the development of
new low band-gap BDF based copolymers with NTDO as the
electron accepting unit should therefore lead to some interesting
features for photovoltaic applications. In this work, we synthe-
sized a new D–A copolymer, namely poly{4,8-bis(2-ethyl-
hexyloxy)benzo[1,2-b:3,4-b0]difuran-alt-6-octylnaphtho[2,3-c]
thiophene-4,9-dione} PBDFNTDO and our recently synthesized
similar polymer PBDTNTDO-C3 is introduced for comparison
(see Scheme 1). However, when preparing our manuscript, Huo
et al. reported an interesting BDF based copolymer
(PBDFDTBT) with a high PCE of up to 5% after thermal
aCollege of Chemistry and Chemical Engineering, Central South
University, Changsha 410083, China. E-mail: yingpingzou@csu.edu.cn
bState key Laboratory for Powder Metallurgy, Central South University,
Changsha 410083, China
cBeijing National Laboratory for Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
† Electronic supplementary information (ESI) available: Fabrications of
OFET devices and Fig. S1 giving the output characteristics and transfer
characteristics of PBDFNTDO thin films, Fig. S2 giving the J–V curves
of the devices with some modifications and Table S1 giving the
corresponding photovoltaic data. See DOI: 10.1039/c2jm32843g
‡ X. Chen and B. Liu contributed equally to this work.
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annealing,21 which is much higher than that of its most similar
BDT based polymer-Z3 with a PCE of 1.95%:22 this first publi-
cation about BDF based polymers further confirmed our
prediction. Herein, our new polymer is well characterized and the
properties are investigated in detail. PBDFNTDO is soluble in
common organic solvents, such as chloroform, toluene and
o-dichlorobenzene, etc. with excellent film forming properties.
PBDFNTDO film shows a broad absorption edge at 750 nm with
an optical band-gap (Eg) of 1.65 eV. The PSC devices based on
PBDFNTDO as the donor and PC71BM as the acceptor indicate
a high PCE of 4.71%, with a short circuit current (Jsc) of 9.14 mA
cmꢀ2, an open circuit voltage (Voc) of 0.87 V and a fill factor (FF)
acetonitrile) reference electrode in an anhydrous and argon-
saturated solution, as working electrode, counter electrode and
reference electrode, respectively, in a 0.1 mol Lꢀ1 tetrabuty-
lammonium hexafluorophosphate (Bu4NPF6) acetonitrile solu-
tion at a scan rate of 50 mV sꢀ1. The HOMO and LUMO energy
levels were determined from the oxidation and reduction onsets
from CV spectra. Electrochemical onsets were determined at the
position where the current started to differ from the baseline. X-
ray diffraction (XRD) measurements of the polymer thin films
were carried out with a 2 kW Rigaku X-ray diffraction system.
XRD patterns were obtained using Bragg-Brentano Geometry
(q–2q) with Cu Ka radiation as an X-ray source in the reflection
mode at 45 kV and 300 mA. AFM images were obtained using a
Veeco’s Dimension V atomic force microscopic (AFM) in the
tapping mode.
of 0.59 under the illumination of AM 1.5 G, 100 mW cmꢀ2
.
These results indicate that BDF probably would be a promising
electron donor building block for efficient PSCs.
2. Experimental
2.3 Fabrication and characterization of polymer solar cells
2.1 Materials
The PSCs were fabricated in the configuration of the traditional
sandwich structure with an indium-tin oxide (ITO) glass positive
electrode and a metal negative electrode. Patterned ITO glass
with a sheet resistance of 10 U ,ꢀ1 was purchased from CSG
HOLDING Co. LTD. (China). The ITO glass was cleaned by
sequential ultrasonic treatments in detergent, deionized water,
acetone and isopropanol, and then treated in an ultraviolet-
ozone chamber (Ultraviolet Ozone Cleaner, Jelight Company,
USA) for 20 min. The PEDOT:PSS (poly(3,4-ethylene dioxy-
thiophene):poly(styrene sulfonate)) (Baytron P 4083, Germany)
was filtered through a 0.45 mm filter and spin coated at 2000 rpm
for 60 s on the ITO electrode. Subsequently, the PEDOT:PSS
film was baked at 150 ꢁC for 15 min in air to give a thin film with
a thickness of 40 nm. A blend of the polymer and PC71BM (1 : 1
w/w, 10 mg mLꢀ1 of polymer) was dissolved in o-dichloroben-
zene (ODCB), and spin-cast at 3000 rpm for 45 s onto the
PEDOT:PSS layer. The substrates were then dried at 70 ꢁC for 15
min. The thickness of the photoactive layer is in the range of 70–
80 nm measured by an Ambios Technology XP-2 profilometer. A
bilayer cathode consisting of Ca (ꢃ20 nm) capped with Al (ꢃ60
nm) was thermally evaporated under a shadow mask with a base
pressure of ca. 10ꢀ5 Pa. The active area of the PSCs is 4 mm2.
Device characterization was carried out under AM 1.5 G irra-
diation with the intensity of 100 mW cmꢀ2 (Oriel 67005, 500 W)
calibrated by a standard silicon cell. J–V curves were recorded
with a Keithley 236 digital source meter.
Furan-3-carboxylic acid, n-BuLi, Pd(PPh3)4 and Sn(CH3)3Cl
were obtained from Acros Organics, and they were used as
received. Tetrahydrofuran (THF) was dried over Na/benzophe-
none ketyl and freshly distilled prior to use. Other reagents and
solvents were purchased commercially as analytical-grade
quality and used without further purification. 1,3-Dibromo-
6-octylnaphtho[2,3-c]thiophene-4,9-dione has been synthesized
according to our group’s recently submitted manuscript.23 All the
other compounds were synthesized following the procedures
described here.
2.2 Characterization
1H NMR and 13C NMR spectra were recorded using a Bruker
DMX-400 spectrometer, chemical shifts were reported as d
values (ppm) relative to an internal tetramethylsilane (TMS)
standard. Number-average (Mn) and weight-average (Mw)
molecular weights were measured by GPC, using polystyrene as a
standard. TGA was performed on a PE TGA-7 at a heating rate
of 20 K minꢀ1 under a nitrogen atmosphere. UV-vis absorption
spectra were taken using a Hitachi U-3010 UV-Vis spectropho-
tometer. For solid state measurements, a polymer solution in
chloroform was cast onto quartz plates. Optical bandgaps were
calculated from the onset of the absorption band. Cyclic vol-
tammograms (CV) were recorded on a Zahner IM6e Electro-
chemical Workstation using a platinum disk coated with the
polymer film, Pt wire and a Ag/Ag+ (0.1 M AgNO3 in
2.4 Synthesis of the monomer and polymer
Furan-3-carbonyl chloride (1). A mixture of furan-3-carboxylic
acid (25.1 g, 0.224 mol) and thionyl chloride (65.3 mL, 0.90 mol)
was refluxed for 4 h. To the reaction mixture was added dry
benzene (60 mL), and the solution was distilled to remove any
excess of thionyl chloride and benzene, the brown liquid was used
in the next step without further purification.
N,N-Diethylfuran-3-carboxamide (2). Compound 1 was added
dropwise to a solution of diethylamine (92.6 mL) in dry CH2Cl2
(100 mL) at 0 ꢁC, the solution was then stirred at room
temperature and poured into iced water. The reaction solution
was extracted three times with CH2Cl2. The combined organic
Scheme 1 Chemical structures of PBDTNTDO-C3 and PBDFNTDO.
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Scheme 2 Synthesis of the comonomer and PBDFNTDO: (i) SOCl2, reflux, 4 h; (ii) N(CH3CH2)2, CH2Cl2, rt, 1 h; (iii) n-BuLi, THF, 0 ꢁC; then rt,
overnight; (iv) 2-ethylhexyl bromide, NaOH, Zn, H2O, N(C4H9)4Br, reflux, overnight; (v) n-BuLi, THF, ꢀ78 ꢁC; then Sn(CH3)3Cl, rt, overnight; (vi)
Pd(PPh3)4, toluene, 110 ꢁC, 48 h.
extractions were washed with dilute hydrochloric acid and water
successively, dried over MgSO4 and then evaporated. The crude
product was purified by chromatography on silica gel (elution
with ethyl acetate/hexane ¼ 2/1). Compound 2 was isolated (30.3
g, 81%) as a colorless oil: GC-MS: m/z ¼ 167. 1H NMR
(400 MHz, CDCl3, ppm): 7.72 (d, 1H), 7.44 (d, 1H), 6.65 (d, 1H),
3.48 (q, 4H), 1.20 (t, 6H).
30 mmol) and a catalytic amount of tetrabutylammonium
bromide were added to the flask. Then, the reaction mixture
was refluxed overnight. The reaction mixture was poured into
cold water and extracted by CH2Cl2. The organic layer was
dried over anhydrous MgSO4. After removing the solvent, the
crude product was purified by column chromatography on
silica gel using petroleum ether as eluent. Compound 4
was obtained as a pale yellow oil (2.2 g, yield: 53%). GC-MS:
1
Benzo[1,2-b:4,5-b0]difuran-4,8-dione (3). Compound 2 (8.35 g,
0.05 mol) was put into a well-dried flask with 50 mL THF under
an inert atmosphere. The solution was cooled down in an ice-
m/z ¼ 414. H NMR (400 MHz, CDCl3, ppm): 7.87 (m, 2H),
7.37 (m, 2H), 4.72 (t, 2H), 2.12 (m, 2H), 1.34–1.22 (m, 10H),
0.86 (t, 3H). 13C NMR (CDCl3, 100 MHz), d (ppm): d 144.09,
142.47, 131.35, 119.42, 104.65, 75.52, 40.24, 30.40, 29.15, 23.68,
23.18, 14.15, 11.19.
water bath, and 21 mL n-butyllithium (0.05 mol, 2.4 mol Lꢀ1
)
were added to the flask dropwise over 30 min. Then the reaction
mixture was stirred at ambient temperature for 14 h. The reac-
tion mixture was poured into 500 g ice-water and stirred over-
night. The mixture was filtered, the yellow precipitate was
washed by water, methanol, and hexane successively. Compound
3 was obtained as a yellow powder (1.6 g, 34%). GC-MS: m/z ¼
188. 1H NMR (400 MHz, CDCl3, ppm): 7.72 (s, 2H), 6.94
(s, 2H).
2,6-Bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4-
b0]difuran (5). Compound 4 (0.83 g, 2 mmol) and 30 mL dry THF
were added to a flask under an inert atmosphere. The solution
was cooled down to ꢀ78 ꢁC and 2.9 mL n-butyllithium
(7.0 mmol, 2.4 mol Lꢀ1) were added dropwise. After being stirred
ꢁ
at ꢀ78 C for 1 h, 8 mL trimethyltin chloride (8.0 mmol, 1 mol
L
ꢀ1) were added in one portion, the cooling bath was removed,
4,8-Bis(2-ethylhexyloxy)benzo[1,2-b:3,4-b0]difuran (4).
Compound 3 (4.4 g, 20 mmol), zinc powder (1.88 g, 10 mmol)
and 33 mL water were put into a 100 mL flask; then 6 g of
NaOH were added to the mixture. The mixture was stirred well
and heated to reflux for 1 h. Then, 1-bromoethylhexane (5.8 g,
and the reaction mixture was stirred at ambient temperature
overnight. Finally it was poured into 100 mL ice-water and
extracted by CH2Cl2. The organic layer was washed by water
twice, then dried over anhydrous MgSO4, and evaporated to give
a yellow oil which was slowly crystallized. The residue was
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recrystallized from isopropanol to yield the target monomer as
1
colorless needles (0.74 g, 50%). GC-MS: m/z ¼ 740. H NMR
(400 MHz, CDCl3, ppm): 7.04 (s, 2H), 4.32 (d, 4H), 1.75 (m, 2H),
1.55–1.34 (m, 16H), 0.97 (m, 12H), 0.41 (s, 18H). 13C NMR (100
MHz, CDCl3, ppm): d 164.15, 146.21, 129.95, 120.01, 115.40,
75.34, 40.25, 30.63, 29.20, 23.93, 23.27, 14.28, 11.26, ꢀ9.12.
Poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4-b0]difuran-alt-
6-hexylnaphtho[2,3-c]thiophene-4,9-dione} (PBDFNTDO).
Compound 5 (0.148 g, 0.2 mmol), 1,3-dibromo-6-octylnaphtho
[2,3-c]thiophene-4,9-dione (0.097 g, 0.2 mmol) and 10 mL dry
toluene were put into a two-necked flask. The solution was
flushed with N2 for 10 min, then Pd(PPh3)4 (12 mg) was added to
the flask. The solution was flushed with N2 again for 25 min. The
oil bath was heated to 110 ꢁC carefully, and the reaction mixture
was stirred for 48 h at this temperature under N2 atmosphere.
Then the reaction mixture was cooled to room temperature and
the polymer was precipitated by the addition of 100 mL meth-
anol and filtered through a Soxhlet thimble, which was then
subjected to Soxhlet extraction with methanol, hexane and
chloroform. The polymer was recovered as a solid from the
chloroform fraction by rotary evaporation. The blue solid was
Fig. 1 TGA thermogram of PBDFNTDO with a heating rate of 10 K
minꢀ1
.
3.3 Optical properties
Fig. 2 shows the ultraviolet-visible (UV-vis) absorption spectra
of PBDFNTDO in chloroform solution and film states on quartz
substrates, the absorption curve of PBDTNTDO-C3 film is also
included for clear comparison. The corresponding results are
listed in Table 1. The PBDFNTDO solution displays an
absorption band in the wavelength range of 300–680 nm and the
absorption peak is at 622 nm. The PBDFNTDO film shows
an absorption band in the wavelength range of 300–750 nm and
the absorption peak is at 627 nm. In the film state, the polymer
shows red-shifted and broader absorption than that of the
solution, the broader and better absorption originates from
better planarity of the polymer and stronger electronic interac-
tion between the individual polymer chains in the film states.21
Compared to the PBDTNTDO-C3 film, the absorption peak of
PBDFNTDO film has red-shifted ca. 70 nm, which indicates that
this BDF based copolymer has stronger intermolecular interac-
tions in the polymer chains or better chain stacking than that of
the BDT based copolymer. The optical band gap (Eg) of the
PBDFNTDO film calculated from the absorption band edge is
ca. 1.65 eV. From the above analysis, PBDFNTDO is a lower
bandgap polymer than PBDTNTDO-C3 (Eogpt: 1.79 eV), which
1
ꢁ
dried under vacuum at 40 C overnight (64 mg, yield: 26%). H
NMR (400 MHz, CDCl3): 8.31 (d, 2H), 8.22 (s, 1H), 7.66 (d, 1H),
7.08 (s, 2H), 4.34 (d, 4H), 1.78–1.04 (m, 42H), 0.81–0.96 (m,
15H). Anal. calcd for (C58H76N2O4S2)n (%): C, 75.16; H, 8.15; O,
12.51; S, 4.18. Found (%): C, 75.48; H, 8.23; O, 12.19; S, 4.22%.
3. Results and discussion
3.1 Synthesis and characterization of PBDFNTDO
The synthetic routes to the monomer and PBDFNTDO are
illustrated in Scheme 2. The polymer was synthesized with
2,6-bis(trimethyltin)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4-b0]
difuran (5) and 1,3-dibromo-6-octylnaphtho[2,3-c]thiophene-
4,9-dione using a typical Stille coupling polymerization reaction.
PBDFNTDO was purified by continuous extraction with meth-
anol, hexane and chloroform, the chloroform fractions were
recovered. Gel permeation chromatography (GPC) results (using
polystyrene as the standard and THF as eluent) have shown that
the polymer has a number-average molecular weight (Mn) of 10.7
kDa molꢀ1 with a polydispersity index of 1.71. The copolymer is
readily soluble in common organic solvents such as chloroben-
zene, dichlorobenzene and tetrahydrofuran, etc.
3.2 Thermal stability
Thermal stability of the polymer is important for device fabri-
cation. Fig. 1 displays the TGA thermogram of PBDFNTDO.
TGA analysis reveals that, under the protection of an inert
atmosphere, the onset point of the weight loss (5%) of
PBDFNTDO is ca. 341 ꢁC. High thermal stability of the
resulting copolymer prevents the deformation of the copolymer
morphology and the degradation of the polymeric active layer
under applied electric fields.
Fig. 2 UV-vis absorption spectra of PBDFNTDO in dilute CHCl3
solution and PBDFNTDO, PBDTNTDO-C3 in the film states.
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Table 1 Optical and electrochemical properties of PBDFNTDO and PBDTNTDO-C3
Absorption spectra
Cyclic voltammetry
Film
Solution
p-doping
Eooxn/HOMO
(V)/(eV)
n-doping
lmax
(nm)
lonset
(nm)
Eogpt
(eV)
Polymers
lmax (nm)
Eroend/LUMO (V)/(eV)
EEg C (eV)
PBDFNTDO
PBDTNTDO-C3
622
557
627
557
750
693
1.65
1.79
0.63/ꢀ5.33
0.67/ꢀ5.27
ꢀ1.3/ꢀ3.40
ꢀ1.0/ꢀ3.53
1.93
1.67
may originate from the higher electronegativity of benzodifuran
than benzodithiophene and stronger interactions in the polymer
chains.
3.4 Electrochemical properties
The HOMO and LUMO levels of PBDFNTDO were measured
by electrochemical cyclic voltammetry (CV). Fig. 3 shows the CV
curve of PBDFNTDO film on a Pt electrode with 0.1 mol Lꢀ1
tetrabutylammonium hexafluorophosphate (Bu4NPF6)/acetoni-
trile (CH3CN) as the electrolyte at a scan rate of 50 mV sꢀ1. The
electrochemical data of PBDFNTDO are summarized in Table 1
and the corresponding data of PBDTNTDO-C3 are included for
comparison. As shown in Fig. 3, it can be seen that PBDFNTDO
exhibits quasi-reversible or reversible p-doping/dedoping
(oxidation/re-reduction) processes over a positive potential range
and quasi-reversible n-doping/dedoping (reduction/re-oxidation)
processes over a negative potential range. We calculated the
Fig. 4 Schematic energy level diagram of the PBDFNTDO based
device.
HOMO and LUMO energy levels of the polymer according to
3.5 Hole mobility
red
on
the equations:13 HOMO ¼ ꢀe(Eooxn + 4.7) (eV); LUMO ¼ ꢀe(E
+ 4.7) (eV). The onset reduction potential of PBDFNTDO
is ꢀ1.3 V. The LUMO energy level of PBDFNTDO is ꢀ3.40 eV.
The onset oxidation potential of PBDFNTDO is 0.63 V. The
HOMO energy level of the polymer is ꢀ5.33 eV (see Fig. 4). The
deep HOMO energy level of the PBDFNTDO is desirable for a
high open circuit voltage of the PSCs based on the polymer
donors. The electrochemical band-gap of 1.93 eV deduced from
the electrochemical measurement is relatively higher than the
optical bandgap due probably to the interface barrier for the
charge injection from electrochemical measurement.
Besides the absorption and energy levels, hole mobility is another
important factor for photovoltaic applications. We investigated
the hole mobility of PBDFNTDO by SCLC method which is
based on Poole–Frenkel Law24 with a device structure of ITO/
PEDOT:PSS/PBDFNTDO:PC71BM (1 : 1.5, w/w)/Au. SCLC is
described by
"
#
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
9
8
ðV ꢀ VbiÞ
V ꢀ Vbi
JSCLC
¼
303rm0
exp 0:89g
(1)
d3
d
The results are plotted as ln(Jd3/V2) vs. (V/d)0.5, as shown in
Fig. 5. Here, J stands for current density, d is the thickness of the
device, and V ¼ Vappl ꢀ Vbi, where Vappl is the applied potential
and Vbi is the built-in potential. According to eqn (1),13 the
average hole mobility of the PBDFNTDO/PC71BM blend is
5.0 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1 over five devices. The hole mobility of
the PBDFNTDO blend was much higher than that of the
PBDTNTDO-C3 blend (3.0 ꢂ 10ꢀ5 cm2 Vꢀ1
s
ꢀ1) and the
PBDFDTBT blend (8.9 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1
)
in the same
21
conditions, presumably because of the larger planar structure
and close stacking of PBDFNTDO, which is beneficial for
obtaining a higher mobility. At the same time, we measured the
hole mobility of the single polymer PBDFNTDO using SCLC
method, interestingly PBDFNTDO exhibits very similar hole
mobility compared with its blend with PCBM. To further verify
this result, we measured the field effect transistors (FETs) of
PBDFNTDO, however, the PBDFNTDO just showed
a
Fig. 3 Cyclic voltammogram of the PBDFNTDO film cast on a plat-
inum disk in 0.1 mol Lꢀ1 Bu4NPF6/CH3CN solution at 50 mV sꢀ1
.
moderate FET hole mobility of 2.1 ꢂ 10ꢀ4 cm2 Vꢀ1 sꢀ1
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Fig. 5 ln(Jd3/V2) vs. (V/d)0.5 plot of PBDFNTDO blend with PC71BM
for measurement of the hole mobility by SCLC method.
(Fig. S1†). The FET mobility is not high, which might arise from
the unoptimized device fabrications and the different mecha-
nisms for PSCs and FETs.
3.6 X-ray analysis
To evaluate the crystallinity of PBDFNTDO and PBDTNTDO-
C3, X-ray diffraction (XRD) measurements were taken from the
spin-coated thick films on SiO2 substrate. As shown in Fig. 6, the
shapes of two copolymers are similar, no Bragg refraction peaks
were observed for the two polymers, which show the copolymers
are amorphous.
Fig.
7
J–V curves of the polymer solar cells based on (a)
PBDFNTDO:PC60BM
with
different
weight
ratios;
(b)
PBDFNTDO:PC71BM with different weight ratios and PBDTNTDO-
C3:PC71BM (1 : 1.5) in ODCB.
power conversion efficiency (PCE) are also summarized in
Table 2. First, the different weight ratios of PBDFNTDO to
PCBM were investigated to optimize the device performance.
With blends of PBDFNTDO and PC60BM in weight ratios of
1 : 1, 1 : 2, 1 : 3, respectively, their J–V curves are shown in
Fig. 7a. With PBDFNTDO and PC60BM in : 1 weight ratio,
the best PCE of 3.84% was obtained with a Voc of 0.96 V, Jsc of
7.6 mA cmꢀ2 and FF of 0.56, respectively. With the content of
PC60BM increasing, the PCE of the devices reduced signifi-
cantly, which is due to PC60BM having a narrow absorption
band, resulting in decreased Jsc of the devices. In order to
obtain a better PCE of the PSCs, we replaced PC60BM with
PC71BM, because PC71BM has a better absorption than
PC60BM while keeping a similar energy level, meaning that
higher Jsc of the devices is expected. As shown in Fig. 7b,
compared with PC60BM as acceptor, for PBDFNTDO and
PC71BM in weight ratios of 1 : 1, 1 : 1.5, 1 : 2, in all cases PCEs
of above 4.0% were obtained. The higher PCEs obtained are
mainly from the increased Jsc, as expected. When the weight
ratio of PBDFNTDO to PC71BM is 1 : 1.5, the best PCE of
4.71% with a Voc of 0.87 V, a Jsc of 9.14 mA cmꢀ2, and a FF of
0.59 was achieved. Such a high Voc of almost 0.9 V is consistent
with the deep-lying HOMO energy level of PBDFNTDO.
Compared with PBDTNTDO-C3 (the highest PCE ¼ 2.27%
for a weight ratio of 1 : 1.5 of PBDTNTDO-C3 to PC71BM
blend in the same conditions), PBDFNTDO achieved a PCE
Fig. 6 XRD patterns of the PBDFNTDO and PBDTNTDO-C3 blends
with PCBM thin films.
3.7 Photovoltaic properties
With its broad absorption and appropriate energy levels
combined with high hole mobility, our newly synthesized
polymer should lead to some high photovoltaic properties. The
bulk heterojunction PSCs were fabricated with a device struc-
ture of ITO/PEDOT:PSS/PBDFNTDO:PCBM/Ca/Al to inves-
tigate the photovoltaic properties of PBDFNTDO. Fig. 7
shows the J–V curves of the devices under the illumination of
AM 1.5 G, 100 mW cmꢀ2 and the corresponding open-circuit
voltage (Voc), short circuit current (Jsc), fill factor (FF), and
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Table 2 Photovoltaic properties of PSCs based on PBDFNTDO:PCBM and PBDTNTDO-C3:PC71BM (1 : 1.5)
Active layer
Voc (V)
Jsc (mA cmꢀ2
)
FF (%)
PCE (%)
PBDFNTDO:PC60BM ¼ 1 : 1
PBDFNTDO:PC60BM ¼ 1 : 2
PBDFNTDO:PC60BM ¼ 1 : 3
PBDFNTDO:PC71BM ¼ 1 : 1
PBDFNTDO:PC71BM ¼ 1 : 1.5
PBDFNTDO:PC71BM ¼ 1 : 2
PBDTNTDO-C3:PC71BM ¼ 1 : 1.5
0.89
0.83
0.80
0.86
0.87
0.83
0.84
7.58
7.17
5.79
8.32
9.14
8.90
5.34
57.0
54.8
58.9
62.6
59.2
54.1
50.6
3.84
3.26
2.73
4.48
4.71
4.00
2.27
more than two-fold higher, because PBDFNTDO possessed
better, broad absorption and especially higher hole mobility
than BDTNTDO-C3 which led to a higher Jsc and FF. PCE of
the devices with a D/A ratio of 1 : 1.5 is higher than that of the
other ratios. The devices with 1 : 1.5 weight ratio of
PBDFNTDO/PC71BM were further optimized by using 1,8-
diiodooctane (DIO) (0.5%, v/v) as the processing additive and
thermal annealing (90 ꢁC, 10 min) according to reported
work.25 Unregretfully, the results show DIO and thermal
annealing do not make big changes to the device properties,
even PCEs are lowered because of slightly lower Jsc and FF
with a slightly higher Voc of up to 0.93 V as shown in Fig. S2
and Table S1†. Further device modifications are in progress, we
firmly believe PCEs can be improved after careful device
optimization. To clarify the high photovoltaic properties of
PBDFNTDO based devices, we did external quantum efficiency
(EQE) measurements. Fig. 8 shows EQE plots of the PSC
devices under the illumination of monochromatic light. In both
graphs, the shapes of the EQE plots are similar to the
absorption spectra of the films, indicating that all the absorp-
tion wavelengths of PBDFNTDO contributed to photocurrent
generation. The EQE of the blend from PBDFNTDO and
PC71BM is higher for the absorption wavelengths especially
from 300–400 nm, which originates from the better absorption
from PC71BM than PC60BM. The higher value of EQE is
consistent with the higher Jsc measured in the PSCs.
3.8 Morphology
Besides absorption, energy level and mobility, the morphology of
the photoactive layer is very important for PSCs, and in some
cases the performance of the device strongly depends on its
morphological characteristics. Therefore, we investigated the
morphologies of PBDFNTDO and PC71BM blends spin-coated
from their o-dichlorobenzene (ODCB) solutions using tapping-
mode atomic-force microscopy (AFM). Fig. 9 shows the height
and phase images of PBDFNTDO:PC71BM (1 : 1.5, w/w) films.
From the height image, rough surfaces with relatively large
aggregates were observed, which may be compact chain packing
and aggregate crystallization, beneficial for obtaining high hole
mobility. As shown in the corresponding phase image,
pronounced nanophase separation was formed, which to some
extent explains the high photovoltaic properties of the
PBDFNTDO/PCBM blend.26
Fig. 8 The EQE values of the PSCs based on (a) PBDFNTDO:PC60BM
with different weight ratios; (b) PBDFNTDO:PC71BM with different
weight ratios and PBDTNTDO-C3:PC71BM (1 : 1.5) in ODCB for
comparison.
Fig. 9 AFM images (5 mm ꢂ 5 mm) of PBDFNTDO:PC71BM (1 : 1.5,
w/w) in ODCB: (a) height image; (b) phase image.
17730 | J. Mater. Chem., 2012, 22, 17724–17731
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applied in PSCs. The polymer possesses good thermal stability,
broad absorption and low-lying HOMO energy levels. The hole
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mobility of the blend reached up to 5.0 ꢂ 10ꢀ3 cm2 Vꢀ1
s
ꢀ1. A
PCE of 4.71% from a PBDFNTDO based polymer solar cell with
Voc ¼ 0.87 V, Jsc ¼ 9.14 mA cmꢀ2, and FF ¼ 59% was obtained,
and higher photovoltaic performance will be highly anticipated
after combining elaborate device fabrications with rational
material designs. Compared to its similar polymer PBDTNTDO-
C3, PBDFNTDO exhibited much better properties, which indi-
cates that PBDFNTDO is a promising candidate for high-effi-
ciency polymer solar cells and the benzodifuran unit probably is
an excellent electron donating building block for photovoltaic
polymeric materials.
15 J. Hou, M. Park, S. Zhang, Y. Yao, L. Chen, J. Li and Y. Yang,
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ꢀ
16 Y. Zou, A. Najari, P. Berrouard, S. Beaupre, R. Aich, Y. Tao and
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In this work, Dr Bo Liu did the organic synthesis and Xuewen
Chen performed the characterizations. We also thank Dr Xinjun
Xu and Prof. Lidong Li from University of Science and Tech-
nology Beijing for FET measurements. This work was supported
by NSFC (nos. 51173206, 21161160443), National High Tech-
nology
Research
and
Development
Program
(no.
2011AA050523), the Natural Science Foundation of Hunan
Province, China (no. 11JJ4010), China Postdoctoral Science
Foundation (no. 20110490150) and the Fundamental Research
Funds for the Central Universities (no. 2010QZZD0112).
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