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Photovoltaics of Donor-Acceptor Polymers based on Benzodithiophene with
Lateral Thiophenyl and Fluorinated Benzothiadiazole
Feiyan Wu1, Daijun Zha1, Lie Chen1,2, Yiwang Chen*1,2
1
Institute of Polymers/Department of Chemistry, Nanchang University, 999 Xuefu
Avenue, Nanchang 330031, China; 2Jiangxi Provincial Key Laboratory of New
Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031,
China
Instruments and measurements
1
H NMR and
13
C NMR spectra are recorded in deuterated solvents on a Bruker
ADVANCE 400 NMR Spectrometer. 1H NMR chemical shifts are reported in ppm
downfield from tetramethylsilane (TMS) reference using the residual protonated
solvent as an internal standard. Molecular weights of the polymers are determined
using a Waters 2410 gel permeation chromatograph (GPC) with a refractive index
detector in tetrahydrofuran (THF) using a calibration curve of polystyrene standards.
Thermo gravimetric analysis (TGA) is performed on a TA-600 for thermal analysis at
a heating rate of 10 °C/min under nitrogen with a sample weight of 8-10 mg.
Differential scanning calorimetry (DSC) is performed on Shimadzu DSC-60. UV-vis
absorption
spectra
are
measured
with
Hitachi
(model
U-3010)
UV-Vis
spectrophotometer. Cyclic voltammetry (CV) is performed with a Zahner IM6e
electrochemical workstation. The X-ray diffraction (XRD) study of the samples is
carried out on a Bruker D8 Focus X-ray diffractometer operating at 30 kV and 20 mA
with a copper target (λ = 1.54 Å) and at a scanning rate of 1°/min. Atomic force
microscopic (AFM) images are measured on a Nanoscope III A (Digital Instruments)
scanning probe microscope using the tapping mode. Current-voltage (J-V)
characteristics are recorded using a Keithley 2400 Source Meter under 100 mW/cm2
simulated AM 1.5 G irradiation (Sciencetech SS-0.5K Solar Simulator). EQE are
detected under monochromatic illumination (Oriel Cornerstone 260 1/4 m
*
Corresponding author. Tel.: +86 791 83969562; fax: +86 791 83969561. E-mail address: ywchen@ncu.edu.cn (Y.
Chen).
monochromator equipped with Oriel 70613NS QTH lamp), and the calibration of the
incident light is performed with a monocrystalline silicon diode.
Materials: Al (99.998%), LiF (99.99%), tetrakis(triphenylphosphine)palladium,
PCBM (99.9%) and other materials are purchased from Alfa, or Aldrich and without
further purification. Tetrahydrofuran (THF) is dried over sodium. Other chemicals are
obtained from Shanghai Reagent Co., Ltd. and used as received. Indium-tin oxide
(ITO) glass is purchased from Delta Technologies Limited, whereas PEDOT:PSS
(Baytron PAl4083) is obtained from Bayer Inc. The monomer TBDT and DTBT are
purchased from solarmar materials Inc. while ffDTBT is prepared according to
literatures and the polymer is synthesized via Stille cross-coupling polymerization
using Pd(PPh3)4 catalyst
[1] [2]
(scheme S1), characterized by 1H NMR (shown in
Figure S1-3).
Scheme S1. synthetic route of polymers
Synthesis of monomer and polymers
Fluorinated 4,7-dithien-2-yl-2,1,3-benzothiadiazole (ffDTBT): A solution of
4,5-difluoro-2-nitroaniline (10 g, 57.4 mmol) dissolved in anhydrous ethanol, then Sn
power (15 g, 126 mmol ) and HCl (25 mL) are added and stirred at 80 °C for 4 hours.
Modulate pH to 7 and filtered to remove insoluble residue, then remove ethanol on
rotary evaporator. The outcome is extracted with ethyl acetate, washed with water and
saturated NaCl solution twice. The brown product 4,5-difluorobenzene-1,2-diamine
(5.7 g) is obtained. Yiel: 69%. 1H NMR(400MHz, CDCl3): δ 6.52 (t, 2H), 3.31 (br,
4H).
4,5-difluorobenzene-1,2-diamine (0.2 g, 1.38 mmol), CHCl3 (20 mL) and
triethylamine (0.57 mL, 5.6 mmol). are added to a round bottom flask. The solution is
stirred until compound is completely dissolved. Thionyl chloride (0.36 g, 2.8 mmol) is
added dropwise and the mixture is heated to reflux for 5 h. The mixture then cooled to
room temperature before it is extracted with CH2Cl2 (100 mL×3). The organic layer is
combined and dried over MgSO4. Solvent is evaporated and the product
5,6-difluorobenzo[c][1,2,5]thiadiazole as white needle-like crystal (0.18 g) is obtained
by column chromatography using hexane/ethyl acetate (1:4) as the eluent. Yield: 76%.
1
HNMR(400MHz, CDCl3): δ 7.76 (t, 2H).
A mixture of 5,6-difluorobenzo[1,2,5]thiadiazole (0.89 g, 5 mmol), I2 (5 g, 20 mmol)
and fuming sulfuric acid (25 mL) in a round-bottom flask is stirred at 60°C for 24 h.
After cooling to room temperature, the mixture is poured into a 500 mL beaker with
crushed ice. Chloroform is added and the mixture is transferred into a separatory
funnel and washed with distilled water (150 mL×3), followed by 1M NaOH solution
several times to remove excess iodine and finally washed with saturated NaHCO3.
The organic layer is then dried over MgSO4. After the solvent removal, the crystalline
product 5,6-difluoro-4,7-diiodobenzo[c][1,2,5]thiadiazole is used without further
purification.
In a 250 mL flame-dried 2-neck round-bottom flask with a condenser, the crystal from
last step (5 mmol), excess of trimethyl(thiophen-2-yl)stannane (2.7 g, 11mmol) and
dry toluene 20 mL are added. The mixture is then purged with argon for 15min. Then,
Pd(PPh3)4 (80 mg) is added and the reaction mixture is heated to reflux for 2d. The
reaction mixture is then cooled to room temperature and the solvent is evaporated.
The crude orange product is purified by column chromatography with hexane/ethyl
acetate (100:1) as eluent. The solvent is evaporated and the product is recrystallized
from isopropanol as orange solid. Yield: 0.6 g (40% from compound
5,6-difluorobenzo[c][1,2,5]thiadiazole). 1HNMR (400MHz, CDCl3): 7.21 (s, 2H),
7.01(s, 4H).
ffDTBT (0.15 g, 0.46 mmol) and N-bromosuccinimide (NBS) (178 mg, 1 mmol) are
added into THF under stirring. The reaction mixture is stirred at a room temperature
for 8 h, then washed with brine and dried over anhydrous sodium sulfate. The solvent
is removed under a reduced pressure to give the product as an orange solid.
Needle-like
crystal
4,7-bis(5-bromothiophen-2-yl)-5,6-difluorobenzo[c][1,2,5]thiadiazole
(dibromoffDTBT) is obtained by recrystallization from iso-propanol. Yield: 159 mg
(70%). 1H NMR (400MHz, CDCl3): δ8.01 (s, 2H), δ7.23 (s, 2H)
Synthesis of polymer PTBDTffDTBT via Stille Coupling Polymerization:
Distannylated alkylthienyl-BDT (452 mg, 0.5 mmol) and dibromoffDTBT (247 mg,
0.5 mmol) are added in 10 mL of dry toluene in a flask under argon. The reaction
mixture is purged with argon for 15 minutes. The catalyst Pd(PPh3)4 (16 mg, 3%) are
added quickly under a stream of argon, and then the reaction mixture is purged with
argon again for 15 minutes. Subsequently, the reaction mixture is heated to reflux for
48 hours with stirring. Next, the reaction mixture is cooled to ambient temperature
and dropped into methanol. The polymer is precipitated and then collected by
filtration. The crude polymer is then extracted subsequently with methanol, ethyl
acetate, hexane and CHCl3 in a Soxhlet’s extractor. The residue after extracting with
CHCl3 is collected and dried under reduced pressure and to give the polymer
PTBDTffDTBT (586 mg, 80%) as a dark green solid. The analogue polymer
PTBDTDTBT is obtained with the same method.
Figure S1. 1H NMR spectra of the monomer ffDTBT.
Figure S2. 1H NMR spectra of the copolymer PTBDTDTBT.
Figure S3. 1H NMR spectra of the copolymer PTBDTffDTBT
4
PTBDTffDTBT
PTBDTDTBT
Heat Flow (W/g)
3
2
1
0
-1
-2
100
150
200
Temperature (C)
Figure S4. DSC plots of PTBDTDTBT and PTBDTffDTBT with a heating rate of 5 °C·min-1
under an inert atmosphere.
Electrochemical Studies Cyclic voltammetry (CV) is performed with a Zahner IM6e
electrochemical workstation using glassy carbon discs as the working electrode, Pt
wire as the counter electrode, SCE electrode as the reference electrode, and
ferrocene/ferrocenium as an internal potential marker. 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6) dissolved in THF is employed as the supporting
electrolyte. The potential of the saturated calomel reference electrode is internally
calibrated using the ferrocene/ferrocenium redox couple (Fc/Fc+), which has a known
reduction potential of -4.4 eV. The HOMO and LUMO energy levels are calculated by
the following equations, the Eg is obtained from the UV-Vis absorption spectra. The
energy levels of the highest (HOMO) and lowest unoccupied molecular orbital
(LUMO) are then calculated according to the following equation.
HOMO = - (φox + 4.4 ) (eV)
LUMO = HOMO + Eg (eV)
where φox is the onset oxidation potential of the second scan from C-V data the Eg is
obtained from the UV-Vis absorption spectra.
X-ray Diffraction Measurement The films used for X-ray diffraction (XRD)
measurement are drop-cast on glass plates, and the baseline of the diffraction is
corrected by subtracting the diffraction from glass. The diffractometer is an automated
Philips Norelco powder diffractometer with a graphite monochromator mounted on
the detector arm. The data is collected using Cu radiation, 2-theta range of 2.0 to 35.0,
a step size 0.05 degree and a step count time of 3 seconds.
Hole Mobility Measure (SCLC)
Hole mobility is evaluated with a configuration of ITO/PEDOT:PSS/polymer/Au by
taking the current-voltage current in the range of 0-7 V and using the space charge
limited current (SCLC) model (Eqn. 1), where ε0 is the permittivity of free space, εr is
the dielectric constant of the polymer, µ is the hole mobility, V is the voltage drop
across the device, and L is the polymer thickness.
J = 9ε0εrµV2/8L3
Device Fabrication
Eqn. (1 )
PCEs are measured under an AM1.5G solar simulator (Sciencetech SS-0.5K Solar
Simulator). The current density-voltage (J-V) characteristics are recorded with a
Keithley 2400 source meter. The spectral response is measured with a commercial
photo modulation spectroscopic setup (Oriel). A calibrated Si photodiode is used to
determine the photosensitivity. Patterned ITO-glass substrates are used as the anode in
the polymer solar cells. The ITO coated glass substrates are cleaned by sonication in
detergent, deionized water, acetone, and isopropyl alcohol and dried in a nitrogen
stream, followed by an oxygen plasma treatment. Then the surface of the ITO
substrate is modified by spin-coating the conducting PEDOT:PSS layer, followed by
baking at 140 °C for 20 minutes under ambient conditions. The substrates are then
transferred into nitrogen-filled glove box. The polymers are blended with PC61BM
and dissolved in chlorobenzene (CB). All blended ratios of polymer/PCBM are by
weight. The solutions are then spin-coated onto the PEDOT:PSS layer at 800-1000
rpm. Then the devices are put on the heater for annealing or not. Subsequently, LiF
(0.8 nm) and Al (100 nm) electrodes are deposited via thermal evaporation in vacuum
(<10-6 Torr). The thicknesses of all the films are measured by a Dektak profiler. The
effective area is measured to be 0.04 cm2.
References:
[1] Hou L. J., Zhang S.Q., Guo X., Xu. F.; Li Y.F.; Hou J. H., Angew. Chem. Int. Ed.,
2011, 50, 9697.
[2] Zhou, H. X.; Yang, L. Q.; Stuart, A. C.; Price, S. C.; Liu, S. B.; You, W., Angew.
Chem. Int. Ed. 2011, 50, 2995.