Introduction of perylene units for enhanced interchain interaction in conjugated polymers for organic photovoltaic devices

Article abstract of DOI:10.1021/ma202712r

A series of semiconducting copolymers, poly[2,7-(9,9′- dioctylfluorene)-alt-5,5′-(4′,7′-di-2-thienyl-2′, 1′,3′-benzothiadiazole)] (PFDTBT), poly[2,2′-(9,9-dioctyl-9H- fluorene-2,7-diyl)dithiophene-alt-5,5′-(4′,7′-di-2-thienyl- 2′,1′,3′-benzothiadiazole)] (PFD2TBT), and their ter-polymers containing perylene units were synthesized using Suzuki coupling polymerization. The perylene units were introduced to improve the charge-transport ability by enhancing the π-π interaction between polymer chains. The resulting polymers were characterized by ¹H NMR, elemental analysis, DSC, and TGA. The synthesized polymers were soluble in common organic solvents, and formed smooth and uniform spin-coated thin films. All of the polymers studied were found to exhibit good thermal stability, losing less than 5% of their weight upon heating to approximately 350 °C. Perylene- containing polymers showed higher field-effect mobilities than the corresponding PFDTBT or PFD2TBT polymers because of the enhanced π-π interaction between polymer chains upon the introduction of perylene units. Bulk heterojunction solar cells were fabricated with configuration of ITO/PEDOT:PSS/polymer:PC₇₁BM/ TiO_x/Al. The devices using the perylene-containing polymers showed higher short-circuit currents, and fill factors than the corresponding PFDTBT or PFD2TBT devices. One of the fabricated devices using a perylene-containing copolymer showed a maximum power conversion efficiency of 3.16%, with a short circuit current density of 9.61 mA/cm², open circuit voltage of 0.81 V, and fill factor of 41%.

Full text of DOI:10.1021/ma202712r

Article
pubs.acs.org/Macromolecules
Introduction of Perylene Units for Enhanced Interchain Interaction in
Conjugated Polymers for Organic Photovoltaic Devices
Ji-Hoon Kim,† Hee Un Kim,† Dongbo Mi,† Sung-Ho Jin,‡ Won Suk Shin,∥ Sung Cheol Yoon,∥
In-Nam Kang,§ and Do-Hoon Hwang*,†
†Department of Chemistry, and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Korea
‡Department of Chemistry Education and Interdisciplinary Program of Advanced Information and Display Materials, Pusan National
University, Busan 609-735, Korea
∥Korea Research Institute of Chemical Technology, 100 Jang-dong, Yuseong-gu, Daejeon 305-343, Korea
§Department of Chemistry, The Catholic University of Korea, Bucheon 420-743, Korea
ABSTRACT: A series of semiconducting copolymers, poly-
[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-
benzothiadiazole)] (PFDTBT), poly[2,2′-(9,9-dioctyl-9H-fluo-
rene-2,7-diyl)dithiophene-alt-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-
benzothiadiazole)] (PFD2TBT), and their ter-polymers
containing perylene units were synthesized using Suzuki
coupling polymerization. The perylene units were introduced
to improve the charge-transport ability by enhancing the π−π
interaction between polymer chains. The resulting polymers were characterized by 1H NMR, elemental analysis, DSC, and TGA.
The synthesized polymers were soluble in common organic solvents, and formed smooth and uniform spin-coated thin films. All
of the polymers studied were found to exhibit good thermal stability, losing less than 5% of their weight upon heating to
approximately 350 °C. Perylene- containing polymers showed higher field-effect mobilities than the corresponding PFDTBT or
PFD2TBT polymers because of the enhanced π−π interaction between polymer chains upon the introduction of perylene units.
Bulk heterojunction solar cells were fabricated with configuration of ITO/PEDOT:PSS/polymer:PC71BM/TiOx/Al. The devices
using the perylene-containing polymers showed higher short-circuit currents, and fill factors than the corresponding PFDTBT or
PFD2TBT devices. One of the fabricated devices using a perylene-containing copolymer showed a maximum power conversion
efficiency of 3.16%, with a short circuit current density of 9.61 mA/cm2, open circuit voltage of 0.81 V, and fill factor of 41%.
can lead to possible enhancements in the photocurrents of
OPVs.4 The most widely used approach for obtaining low-band
gap structures is to utilize internal charge transfer (ICT) from
an electron-donating to an electron-accepting moiety.5−7 Some
interesting structures using the ICT approach have been
reported. For example, Brabec et al. reported a low-band gap
polymer, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-
b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]
(PCPDTBT), which showed absorption in the range 380−900
nm in the AM 1.5 G spectrum, good hole mobility (0.01−0.1
cm2 V−1 s−1) and a high power conversion efficiency (PCE) of
3.2% with PC71BM as the electron acceptor.8,9 A 9.9-
dialkylfluorene unit has previously been used as a building
block for constructing donor−acceptor-type conjugated poly-
mers in OPVs such as poly[2,7-(9,9′-dialkylfluorene)-alt-5,5′-
(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PFDTBT).
PFDTBT showed a maximum PCE of 2.2%, and PFDTBT
derivatives containing branched alkyl chains (BisDMO-
PFDTBT) showed higher PCEs.10,11
1. INTRODUCTION
Extensive research has been carried out during the past decade
on bulk heterojunction solar cells that utilize organic semi-
conducting materials.1 The main advantages of organic
photovoltaic devices (OPVs) are their lightweight, flexible
nature, and their cost-effective manufacture by various printing
technologies. The photoconversion process in OPVs is
accomplished by efficient light absorption by conjugated
polymers and accepting materials, charge separation at the
interface of the donor polymer and the accepting materials, and
subsequent charge collection at the electrodes. Matching the
absorption of the conjugated polymer to the solar spectrum
determines the ultimate performance of the solar cell. Among
the variety of conjugated polymers that have been studied,
poly(p-phenylenevinylene) (PPV),2 poly(alkylfluorene) (PF),
and polythiophene (PT)3 derivatives are typical p-type electron
donor materials.
However, the performance of photovoltaic cells with these
polymers is limited by their relatively large band gap energies,
which are not yet optimized with respect to the solar spectrum.
Therefore, the design and synthesis of new low-band gap
polymers become very important to better harvest the solar
spectrum, especially in the red and near-infrared ranges, which
Received: December 15, 2011
Revised: February 18, 2012
Published: February 24, 2012
© 2012 American Chemical Society
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Scheme 1. Synthetic Routes of PFDTBT and the Perylene Terpolymers
Scheme 2. Synthetic Routes of PFD2TBT and the Perylene Terpolymers
Some molecules and polymers that have flat structures such
as fluorene,12−16 anthracene,17 and pyrene18,19 have been
reported to form excimers or aggregates by strong interchain
π−π interactions. Excimer formation could affect the emission
properties of molecules or polymers, therefore, the excimer or
exciplex formation has been used as a fluorescence probe for
molecular recognition.19 In organic photovoltaic devices, π−π
interactions between low-band gap donor polymers are
important for charge transportation. Enhanced π−π inter-
actions in p-type donor polymers could improve their charge-
transport ability, and thus increase short-circuit currents in solar
cells.
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the illumination of simulated solar light with 100 mW/cm2 (AM 1.5
G) by an Oriel 1000 W solar simulator. Electric data were recorded
using a Keithley 236 source-measure unit, with all characterizations
carried out in an ambient environment. The illumination intensity was
calibrated by a standard Si photodiode detector (PV Measurements .
Inc.) which had been calibrated at the NREL. The incident photon-to-
current conversion efficiency (IPCE) was measured as a function of
the wavelength in the range 360−800 nm by using a halogen lamp as
the light source, where the calibration was performed using a silicon
reference photodiode. Measurements were carried out after masking
the fabricated device except the active cell area. All the characterization
steps were carried out under ambient air conditions.
In this study we report on a new approach to improve PCE
of OPVs by enhancing the interchain π−π interaction. To do
this we introduced flat perylene units into the conjugated
polymers. We synthesized poly[2,7-(9,9′-dioctylfluorene)-alt-
5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PFDTBT),
poly[2,2′-(9,9-dioctylfluorene-2,7-diyl)dithiophene-alt-5,5′-
(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PFD2TBT), and
their ter-polymers with perylene units. Optical, electrical and
photovoltaic properties of the polymers were investigated.
Synthetic routes and polymer structures are shown in Schemes
1 and 2.
2.5. Synthesis of Monomers and Polymers. 2,7-Dibromo-9,9-
dioctyl-9H-fluorene and 2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) were synthesized according
to a previous report.20
2. EXPERIMENT
2.1. Materials. Fluorene, HBr, 2,1,3-benzothiadiazole, n-octyl
bromide, bromine, perylene, diethyl ether, N-bromosuccinimide,
tripropyl(thiophen-2-yl)stannane, CCl4, 2-isopropoxy-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane, tetrakis(triphenylphosphine)palladium
were all purchased from Aldrich. [6,6]-Phenyl C71-butyric acid methyl
ester (PC71BM) was purchased from Nano-C. All chemicals were used
without further purification.
Dibromo-9,9-dioctyl-9H-fluorene. 1H NMR (CDCl3, ppm): δ 7.52
(d, 1H). 7.50 (s, 1H), 7.44−7.47 (m, 3H), 7.43 (d, 1H), 1.99−1.95
(m, 4H), 1.31−1.29 (m, 24H), 0.98 (t, 6H). 13C NMR (CDCl3, ppm):
δ 151.1, 141.8, 134.1, 130.5, 127.4, 125.1, 50.8, 44.3, 33.1, 30.2, 29.9,
29.1, 22.4, 14.8.
2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-
1
2.2. Measurements. 1H and 13C NMR spectra were recorded
using a Bruker AM-400 spectrometer and absorption spectra were
measured using a JASCO JP/V-570 spectrometer. The molecular
weights of the polymers were determined by gel permeation
chromatography (GPC) analysis relative to a polystyrene standard
using a Waters high-pressure GPC assembly model M590. Thermal
analyses were carried out on a Mettler Toledo TGA/SDTA 851e, DSC
822 e analyzer under a N2 atmosphere with a heating and cooling rate
of 10 °C/min. Cyclic voltammetry (CV) measurements were
performed on an AUTOLAB/PGSTAT12 at room temperature with
a three-electrode cell in a solution of TBABF4 (0.10 M) in acetonitrile
and with a scan rate of 50 mV/s. Polymer films were prepared by
dipping platinum working electrodes into the polymer solutions and
then drying in air. A platinum wire was used as a counter electrode and
a Ag/Ag+ electrode was used as a reference electrode.
1,3,2-dioxaborolane) (1). H NMR (CDCl3, ppm): δ 7.77 (d, 2H),
7.72 (d, 2H), 7.69 (s, 2H), 1.99−1.95 (m, 4H), 1.37 (s, 24H), 1.09−
1.21 (m, 20H), 0.78 (t, 6H), 0.53 (m, 4H).13C NMR (CDCl3, ppm): δ
150.71, 144.14, 133.87, 130.90, 129.16, 119.58, 83.93, 55.40, 40.30,
32.00, 30.15, 29.41, 29.36, 25.16, 23.83, 22.80, 14.27. Anal. Calcd for
C41H64B2O4:C, 76.64; H, 10.04; B, 3.36; O, 9.96. Found: C, 76.61; H,
10.01; B, 3.32; O, 9.89.
Synthesis of 4,7-Dibromobenzo[c][1,2,5]thiadiazole. A solution of
bromine (35.21 g, 22.32 mmol) in 100 mL of HBr (48%) was slowly
added dropwise to a solution of 10 g (73.44 mmol) of
benzothiadiazole. After refluxing for 6 h, an orange solid precipitated
was formed. The mixture was allowed to cool to room temperature,
and a saturated solution of NaHSO3 was then added to neutralize the
residual Br2. The mixture was filtered and exhaustively washed with
water. The solid was then washed once with cold diethyl ether and
purified by flash chromatography to give 20.51 g (95%) of compound
3. 1H NMR (CDCl3, ppm): δ 7.73 (s, 2H). 13C NMR (CDCl3, ppm):
δ 152.6, 132.1. 113.6.
2.3. Fabrication of Organic Thin-Film Transistor Devices.
Organic thin-film transistor (OTFT) were fabricated using a bottom-
contact geometry device (channel length L = 12 μm, and width, W =
120 μm). The source and drain contacts consisted of gold (100 nm),
and the dielectric was silicon oxide (SiO2) with a thickness of 300 nm.
The SiO2 surface was cleaned, dried, and pretreated with a solution of
10.0 mM octyltrichlorosilane (OTS-8) in toluene at room temperature
for 2 h under a N2 atmosphere to produce nonpolar and smooth
surfaces onto which the polymers could be spin-coated. The polymers
were dissolved to a concentration of 0.5 wt % in chlorobenzene. Films
of the organic semiconductors were spin-coated at 1000 rpm for 50 s
to a thickness of 60 nm. All device fabrication procedures and
measurements were carried out in air at room temperature.
2.4. Fabrication of Photovoltaic Devices. Composite solutions
(1:5 wt % ratio) of polymers and PC71BM were prepared using 1,2-
dichlorobenzene as a solvent. The concentration was adequately
controlled in the range 1.0−2.0 wt %. For the measurement of the
optical absorption spectra of PCBM, the solutions were spun on
precleaned UV-grade silica substrates. The polymer photovoltaic
devices were fabricated with a typical sandwich structure of ITO/
PEDOT:PSS/active layer/LiF or TiOx/Al. The ITO coated glass
substrates were cleaned through a routine cleaning procedure,
including sonication in detergent followed by distilled water, acetone,
and 2-propanol. A 45 nm thick layer of PEDOT:PSS (Baytron P) was
spin-coated onto a cleaned ITO substrate after exposing the ITO
surface to ozone for 10 min. The PEDOT:PSS layer was then baked
on a hot plate at 140 °C for 10 min. The active layer was spin-coated
from the predissolved composite solution after filtering through a 0.45
μm (PTFE) syringe filter. The device structure was completed by
depositing 0.7 nm of LiF and a 100 nm Al cathode (to act as the top
electrode) onto the polymer active layer under vacuum (3 × 10−6
Torr) in a thermal evaporator. Current density−voltage (J−V)
characteristics of all polymer photovoltaic cells were measured under
Synthesis of 4,7-Di-2′-thienyl-2,1,3-benzothiadiazole. Tripropyl-
(thiophen-2-yl)stannane (1.5 g, 11.7 mmol) was added to a stirred
solution of 4,7-dibromo-2,1,3-benzothiadiazole (1.32 g, 4.5 mmol),
and bis(triphenylphosphine)palladium(II) dichloride (95 mg, 0.09
mmol) in THF (50 mL). The mixture was refluxed overnight. The
resulting mixture was extracted with ethyl acetate and brine. The
organic layer was separated, dried with anhydrous MgSO4 and the
resulting solution was then concentrated by evaporation. A red solid
(product yield 85%) was obtained by recrystallizing the crude product
1
in methanol. H NMR (CDCl3, ppm): δ 7.84 (s, 2H), 7.43 (d, 2H),
7.21 (t, 2H), 6.61 (d, 2H) 13C NMR (CDCl3, ppm): δ 155.1, 142.8,
135.2, 129.1, 128.0, 127.9, 127.1.
Synthesis of 4,7-Di-2′-(5′-bromo)-thienyl-2,1,3-benzothiadiazole
(2). N-Bromosuccinimide (2.05 g, 11.5 mmol) was added to a stirred
solution of 4,7-di-2′-thienyl-2,1,3-benzothiadiazole (1.44 g, 4.8 mmol)
in acetic acid (25 mL) in darkness. The mixture was stirred at room
temperature for 36 h, and 2 M HCl was added to this mixture. The
product was extracted with chloroform and the organic phase was
successively washed with 10% sodium bisulfate, 2 M HCl and water
before being dried over MgSO4. The solvent was removed to afford a
dark-red crystalline product (1.30 g, 59%) after recrystallization from
1
chloroform. H NMR (CDCl3, ppm): δ 7.80 (dd, 4H), 7.13 (d, 2H)
13C NMR (CDCl3, ppm): δ 154.2, 140.8, 131.1, 129.6, 129.1, 128.0,
111.6. Anal. Calcd for C14H6Br2N2S3: C, 36.70; H, 1.32; Br, 34.88; N,
6.11;S, 20.99. Found: C, 36.68; H, 1.28; Br, 34.78; N, 6.08; S, 20.88.
Synthesis of 3,9-Dibromoperylene (3). Bromine (8.81 g, 7.92
mmol) was added in darkness to a stirred solution of perylene (5 g,
19.81 mmol) in carbon tetrachloride (200 mL). The mixture was
stirred at 110 °C for 12 h and 2 M HCl was added to the mixture. The
product was extracted with chloroform and the organic phase was
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successively washed with 10% sodium bisulfate, 2 M HCl and water
before being dried over MgSO4. The solvent was removed to afford
dark-red crystals (2.81 g, 63%) that were recrystallized from hot
(CDCl3, ppm): δ 8.12−7.44 (12H), 1.96 (4H), 1.35−0.67 (30H).
Found: C, 70.98; H, 6.17; N, 4.86; S, 20.16.
Poly[2,7-(9,9′-dioctylfluorene)-co-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-
benzothiadiazole)-co-3″,9″-perylene] (PFDTBT_5Per). 2,2′-(9,9-Di-
octyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane) (0.5 g, 0.78 mmol) was mixed with 4,7-di-2′-(5′-bromo)thienyl-
2,1,3-benzothiadiazole (0.32 g, 0.691 mmol, 0.9 equiv), 3,6-
dibromoperylene(0.032 g, 0.078 mmol, 0.1 equiv), tetrakis-
(triphenylphosphine)palladium (3.0 mg, 2.6 μmol), and toluene (10
1
hexane. H NMR (CDCl3, ppm): δ 8.21 (dd, 2H), 7.83 (dd, 2H),
7.80(dd, 2H), 7.40−7.37(m, 4H) 13C NMR (CDCl3, ppm): δ 118.0,
121.3, 124.1, 125.9, 126.9, 127.1, 128.2, 128.9, 130.1, 135.8. Anal.
Calcd for C20H10Br2: C, 58.55; H, 2.42; Br, 38.94. Found: C, 58.30; H,
2.30; Br, 38.66.
Synthesis of 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)dithiophene
(4). Compound 1 (7.3 g, 13.31 mmol),5-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)thiophen-2-ylium (6.23 g, 29.28 mmol) and 50 mL
of toluene were placed in two neck flasks under a N2 atmosphere.
Then 32 mL of a 2 M K2CO3 solution was injected, and Pd(PPh3)4
(0.46 g, 0.39 mmol) and Aliquat 336 (0.53 g, 1.33 mmol) were added
to the solution. The solution was refluxed at 90 °C for 2 days. The
reaction mixture was them cooled to room temperature. The reaction
mixture was extracted with methylene chloride and brine and the
solvent was evaporated. The mixture was purified by column
chromatography using hexane to give a liquid product yield was of
1
mL) for this polymerization. H NMR (CDCl3, ppm): δ 8.22−7.44
(∼22H), 1.96 (4H), 1.35−0.67 (∼30H). Found: C, 75.47; H, 6.78; N,
3.41; S, 14.99.
Poly[2,7-(9,9′-dioctylfluorene)-co-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-
benzothiadiazole)-co-3″,9″-perylene] (PFDTBT_10Per). 2,2′-(9,9-Di-
octyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane) (0.5 g, 0.78 mmol) was mixed with 4,7-di-2′-(5′-bromo)thienyl-
2,1,3-benzothiadiazole (0.28 g, 0.62 mmol, 0.8 equiv), 3,6-
dibromoperylene(0.06 g, 0.14 mmol, 0.2 equiv), tetrakis-
(triphenylphosphine)palladium (3.0 mg, 2.6 μmol) and toluene (10
1
mL) for this polymerization. H NMR (CDCl3, ppm): δ 8.22−7.44
1
75%. H NMR (CDCl3, ppm): δ 7.65 (d, 2H), 7.60 (d, 2H), 7.55 (s,
(∼22H), 1.96 (4H), 1.35−0.67 (∼30H). Found: C, 76.29; H, 6.38; N,
3.78; S, 16.19.
2H), 7.37 (d, 2H), 7.28 (d, 2H), 7.09 (t, 2H), 2.01 (t, 4H), 1.20 (m,
20H), 0.77 (t, 6H), 0.67 (m, 4H). 13C NMR (CDCl3, ppm): δ 149.1,
141.0, 140.7, 133.5, 131.2, 129.0, 128.8, 128.2, 127.3, 125.1, 53.8, 44.1,
32.1, 30.0, 29.5, 29.3, 24.3, 22.5, 13.9.
Poly[2,7-(9,9′-dioctylfluorene)-co-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-
benzothiadiazole)-co-3″,9″-perylene] (PFDTBT_20Per). 2,2′-(9,9-Di-
octyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane) (0.5 g, 0.78 mmol) was mixed with 4,7-di-2′-(5′-bromo)thienyl-
2,1,3-benzothiadiazole (0.24 g, 0.35 mmol, 0.6 equiv), 3,6-
dibromoperylene(0.09 g, 0.31 mmol, 0.4 equiv), tetrakis-
(triphenylphosphine)palladium (3.0 mg, 2.6 μmol), and toluene (10
Synthesis of 2,2′-(5,5′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis-
(thiophene-5,2-diyl))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)
(5). A n-BuLi solution in hexanes (11.26 mL, 2.0 M) was added at −70
°C to a solution of compound 4 (5 g, 9.01 mmol) in dry THF (150
mL). The mixture was stirred for 30 min at −70 °C, and 2-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yloxy)propan-1-ylium (5.52 mL,
27.03 mmol) was then added at −70 °C. This mixture was allowed
to reach room temperature overnight. The resulting mixture was
extracted with methylene chloride and brine and then dried with
anhydrous MgSO4. After filtering, the solvent was evaporated, to
provide a product yield of 56%. 1H NMR (CDCl3, ppm): δ 7.43−7.63
(m, 10H), 1.96 (t, 4H), 1.35 (s, 24H), 1.20 (m, 20H), 0.77 (t, 6H),
0.67 (m, 4H). 13C NMR (CDCl3, ppm): δ 152.14, 151.93, 140.68,
138.30, 133.24, 128.20, 125.19, 124.42, 120.73, 120.31, 84.30, 55.37,
40.52, 31.89, 30.11, 29.34, 29.28, 24.92, 23.87, 22.71, 14.20. Anal.
Calcd for C49H68B2O4S2: C,72.94; H,8.50; B,2.68; O,7.93;S,7.95.
Found: C,72.89; H,8.42; B,2.55; O,7.89; S,7.81.
General Polymerization Procedure. Three copolymers were
synthesized by Suzuki coupling polymerization.21,22 The 2,2′-(9,9-
dioctyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane), 2,2′-(5,5′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-
diyl))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane), and 4,7-di-2′-(5′-
bromo)-thienyl-2,1,3-benzothiadiazole, 3,9-dibromoperylene mono-
mers were synthesized according to previous reports. The reaction
mixture of 2 M K2CO3 (8 mL), tetrakis(triphenylphosphine)palladium
(0.06 g), and Aliquat 336 (0.07 g) in 15 mL of anhydrous toluene was
stirred at 90 °C for 3 days. Following this an excess amount of 1-
bromobenzene and phenylboronic acid (the end-capper) was dissolved
in 1 mL of anhydrous toluene and stirred continuously for 12 h. The
reaction mixture was cooled to approximately 50 °C and 200 mL of
methanol was added slowly with vigorous stirring of the reaction
mixture. The polymer fibers were collected by filtration and
reprecipitation from methanol and acetone. The polymers were then
purified further by washing for 2 days in a Soxhlet apparatus, with
acetone used to remove oligomers and catalyst residues. Column
chromatography with a chloroform solution was then performed on
the polymer. The reprecipitation procedure in chloroform/methanol
was then repeated several times. The resulting polymers were soluble
in common organic solvents.
1
mL) for this polymerization. H NMR (CDCl3, ppm): δ 8.22−7.44
(∼22H), 1.96 (4H), 1.35−0.67 (∼30H). Found: C, 77.28; H, 5.98; N,
3.31; S, 13.65.
Poly[2,2′-(9,9-dioctylfluorene-2,7-diyl)dithiophene-alt-5,5′-(4′,7′-
di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PFD2TBT). 2,2′-(5,5′-(9,9-
Dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane) (0.5 g, 0.61 mmol) was mixed with 4,7-di-
2′-(5′-bromo)thienyl-2,1,3-benzothiadiazole (0.34 g, 0.74 mmol, 1.2
equiv) tetrakis(triphenylphosphine)palladium (3.0 mg, 2.6 μmol), and
1
toluene (10 mL) for this polymerization. H NMR (CDCl3, ppm): δ
8.18−7.44 (∼16H), 1.96 (4H), 1.35−0.67 (∼30H). Found: C, 66.82;
H, 5.08; N, 3.30; S, 22.76.
Poly(2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)dithiophene-co-5,5′-
(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)-co-3″,9″-(perylene))
(PFD2TBT_5Per). 2,2′-(5,5′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis-
(thiophene-5,2-diyl))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.5
g, 0.61 mmol) was mixed with 4,7-di-2′-(5′-bromo)thienyl-2,1,3-
benzothiadiazole (0.25 g, 0.54 mmol, 0.9 equiv), 3,6-dibromoperylene
(0.025 g, 0.06 mmol, 0.1 equiv), tetrakis(triphenylphosphine)
palladium (3.0 mg, 2.6 μmol), and toluene (10 mL) for this
1
polymerization. H NMR (CDCl3, ppm): δ 8.21−7.44 (∼26H), 1.96
(4H), 1.35−0.67 (∼30H). Found: C, 68.65; H, 5.46; N, 3.01; S, 23.12.
Poly[2,2′-(9,9-dioctylfluorene-2,7-diyl)dithiophene-co-5,5′-(4′,7′-
di-2-thienyl-2′,1′,3′-benzothiadiazole)-co-3″,9″-perylene]
(PFD2TBT_10Per). 2,2′-(5,5′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)bis-
(thiophene-5,2-diyl))bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.5
g, 0.61 mmol) was mixed with 4,7-di-2′-(5′-bromo)-thienyl-2,1,3-
benzothiadiazole (0.22 g, 0.49 mmol, 0.8 equiv), 3,6-dibromoperylene
(0.05 g, 0.12 mmol, 0.2 equiv), tetrakis(triphenylphosphine)palladium
1
(3.0 mg, 2.6 μmol), and toluene (10 mL) for this polymerization. H
NMR (CDCl3, ppm): δ 8.21−7.44 (∼26H), 1.96 (4H), 1.35−0.67
(∼30H). Found: C, 73.40; H, 5.94; N, 2.41; S, 20.91.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization of Polymers. The
synthesis of the polymers was carried out using palladium-
catalyzed Suzuki coupling reactions between dibromoaryl and
diborolanylaryl monomers (Schemes 1 and 2). The synthesized
polymers were soluble in common organic solvents such as
chloroform, chlorobenzene, and toluene, and the polymer
Poly[2,7-(9,9′-dioctylfluorene)-alt-5,5′-(4′,7′-di-2-thienyl-2′,1′,3′-
benzothiadiazole)] (PFDTBT). 2,2′-(9,9-Dioctyl-9H-fluorene-2,7-diyl)-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (0.5 g, 0.78 mmol) was
mixed with 4,7-di-2′-(5′-bromo)thienyl-2,1,3-benzothiadiazole (0.40 g,
0.87 mmol, 1.1 equiv), tetrakis(triphenylphosphine)palladium (3.0 mg,
1
2.6 μmol), and toluene (10 mL) for this polymerization. H NMR
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solutions formed uniform thin films after spin-coating. The
number-average molecular weights (Mn) of the synthesized
polymers were determined by GPC using polystyrene as a
standard. PFDTBT showed a relatively low molecular weight
but other synthesized polymers including PFD2TBT gave
number-average molecular weights higher than 10 000 g/mol
with polydispersity index (PDI = Mw/Mn) ranging from 1.86 to
3.49. The actual compositions of the polymers were determined
by elemental analysis using the sulfur contents as the standard.
The actual compositions of the copolymers were not the same
as the feed ratios but we found that the differences were not
significant.
3.2. Thermal Properties. The thermal properties of the
polymers were investigated by using thermo-gravimetric
analysis (TGA) and a differential scanning calorimeter
(DSC). PFDTBT-based polymers showed better thermal
stability than PFD2TBT-based polymers. Moreover,
PFDTBT-based polymers exhibited good thermal stability,
losing less than 5% of their weight upon heating to
temperatures greater than 400 °C. The thermally induced
phase-transition behavior of the polymers was investigated by
using a DSC under a N2 atmosphere. The glass transition
temperatures (Tg) of the polymers were in the range 105−161
°C and were found to be dependent on the polymer structure.
PFD2TBT-based polymers showed higher glass transition
temperatures than PFDTBT-based polymers, and as the
perylene contents in the copolymers increased, the polymer
gave a higher glass transition temperature. No synthesized
polymer showed any crystalline temperature (Tc), except
PFDTBT_20Per. The value of Tc for PFDTBT_20Per was
191 °C. TGA thermograms of the polymers and DSC curve of
PFDTBT_20Per are shown in Figure 1. Molecular weights and
thermal properties of the synthesized polymers are summarized
in Table 1.
main chain.21 Peak and edge positions were almost equivalent
for PFDTBT, PFDTBT_5Per and PFDTBT_10Per, but the
maximum absorption peak for PFDTBT_20Per moved to a
shorter wavelength region. This might be due to a decrease in
the internal charge- transfer (ICT) interaction cause by a
decreased content of electron-withdrawing DTBT monomer in
the copolymer. PFD2TBT and the ter-polymer solutions also
exhibited two absorption bands at around 410 and 520 nm. The
absorption maxima and edges of the polymer solutions showed
little dependence upon perylene contents. The UV−visible
absorption spectra of the synthesized polymers in a chloroform
solution are shown in Figure 2.
The polymer thin films showed a similar absorption trend;
however, the films showed increasingly larger redshifts in
absorption than those for the solution as shown in Figure 3. A
strong interchain interaction between polymer chains in the
solid state would result in the red-shifting of the absorption
peak and the onset. Optical band-gaps of the polymers were
obtained from the absorption onsets of the polymer films. The
measured optical band gaps of the copolymers were in the
range 1.95−1.82 eV.
3.4. Electrochemical Properties. Electrochemical cyclic
voltammetry (CV) was employed to investigate the redox
behavior of the polymers and to determine their HOMO and
LUMO energy levels.25 CV was performed under an Ar
atmosphere in a solution of TBABF4 (0.10 M) in acetonitrile at
a scan rate of 50 mV/s and room temperature. A platinum plate
was used as a working electrode. A platinum wire was used as
the counter electrode, and a Ag/AgNO3 electrode was used as
the reference electrode. The HOMO energy level was
determined by using oxidation onsets (Eox) of the polymer
films.26 The LUMO energy levels of the polymers were
determined by combining the HOMO energy levels obtained
from CV and the optical bandgap energies obtained from the
edges of absorption.26 To obtain the oxidation potentials of the
polymer films, the reference electrode was calibrated using a
ferrocene/ferrocenium (Fc/Fc+) salt couple, whose redox
potential is assumed to have an absolute energy level of
−4.80 eV in a vacuum.26 The potential of this external standard
determined under the same conditions was 0.4 eV. As a result,
the HOMO energy values were calculated using the equation
onset
onset
EHOMO = −(Eox
+ 4.4) eV, where Eox
is the onset
oxidation potential vs Ag/Ag+.
The HOMO energy levels of PFDTBT, PFD2TBT, and the
ter-polymers were determined to lie in the range from −5.58 to
−5.41 eV. Optical band gap energies show little change with by
changing perylene unit ratios, but the HOMO energy levels of
the polymers were moved slightly toward higher positions with
the introduction of perylene units, as illustrated in Figure 4.
This arises from the result of a decrease in the contents of high
electron affinity DTBT units as the number of perylene units in
the polymers increased.27 The UV−visible absorption spectra,
optical band-gaps, and HOMO/LUMO energy levels of the
polymers are summarized in Table 2.
3.5. TFT Characteristics of the Polymer Thin Films. To
measure the field effect mobility of the charge carriers, OTFT
were fabricated on a silicon wafer using a bottom contact
geometry (channel length L = 12 μm, and width, W = 120 μm)
under nitrogen. The TFT devices of the polymers exhibited
typical p-channel transistor characteristics. Figure 5 shows the
transfer curves of the devices fabricated using the polymers as
the active layer. The field-effect mobility was calculated in the
saturation regime using the following equation: Ids = (W/
Figure 1. TGA thermograms of polymers and DSC curve of
PFDTBT_20Per.
3.3. Optical Properties. The UV−visible absorption
spectra of the synthesized polymers were recorded in both
chloroform solution and as a solid thin film. PFDTBT and the
ter-polymer solutions exhibited two distinct absorption bands,
where the first absorption peak is at around 387 nm,
corresponding to the π−π* transition of the conjugated
polymer backbone,23,24 and the second absorption band in
the longer wavelength region at around 530 nm, can be
attributed to the strong internal charge-transfer (ICT)
interaction between donating and accepting groups in the
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Table 1. Average Molecular Weights and Thermal Properties of the Synthesized Polymers
ratio (perylene mol %)
a
a
a
b
c
d
polymer
polymer yield (%)
Mn (g/mol)
Mw (g/mol)
PDI
Td (°C)
Tg (°C)
feed ratio
actual ratio
PFDTBT
72
78
69
58
71
73
65
4500
14 000
12 000
11 000
16 000
12 000
11 000
7900
50 000
32 000
33 000
36 000
39 000
30 000
1.8
3.5
2.7
3.0
2.3
3.3
2.7
434
408
430
431
300
332
392
105
110
124
128
154
158
161
−
−
PFDTBT_5Per
PFDTBT_10Per
PFDTBT_20Per
PFD2TBT
5.0
10.0
20.0
−
2.8
11.4
17.3
−
PFD2TBT_5Per
PFD2TBT_10Per
5.0
6.1
10.0
13.3
a
b
Mn, Mw, and PDI of the polymers were determined by gel permeation chromatography using polystyrene standards in CHCl3. Temperature at 5%
c
d
weight loss by a heating rate of 10 °C/min under nitrogen. Determined by DSC at a heating rate of 10 °C/min under nitrogen. Calculated by
elemental analysis through the amount of sulfur contained in copolymers as a standard.
PFDTBT and PFD2TBT plays an important role in increasing
the carrier mobility, possibly due to the enhanced interchain
π−π interaction. The characteristics of the TFTs using the
polymers as the active layers are summarized in Table 3.
3.6. Photovoltaic Properties. Photovoltaic devices were
fabricated using PFDTBT, PFD2TBT and the ter-polymers as
p-type electron donors and by using [6,6]-phenyl C71-butyric
acid methyl ester (PC71BM) as an n-type acceptor. Two
different types of photovoltaic devices were fabricated and
characterized, with the device structures of ITO/PEDOT:PSS/
polymer:PC71BM/LiF/Al and ITO/PEDOT:PSS/poly-
mer:PC71BM/TiOx/Al shown in Figure 6. The solution-
processable titanium suboxide (TiOx) layer was employed as
an optical spacer30,31 and as a hole blocker27 between the bulk
heterojunction(BHJ) layer and the top metal electrode.
Figure 7 shows the current−voltage (J−V) characteristics of
Figure 2. UV−visible absorption spectra of the polymers in
chloroform solution.
the ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al devices. It
can be observed that the open circuit voltages of cells fabricated
using PFDTBT and its ter-polymers range from 0.70 to 0.77 V.
The device fabricated using PFDTBT showed higher open
circuit voltages than those of perylene-containing copolymers.
This coincides with the relative HOMO energy levels of the
polymers. As the perylene content in the copolymers increased,
the amount of DTBT contents decreased, and so the HOMO
of the polymers moved to higher energy levels. PFDTBT has
the deepest HOMO energy level, thus it showed the highest
open circuit voltage (0.77 V). It is also noteworthy that, the
short circuit current of the devices increased with an increase in
the number of perylene units in the polymer, except for the
case of PFDTBT_20Per. Even though PFDTBT_20Per
showed the highest hole mobility among the PFDTBT-based
copolymers, it is UV−visible absorption spectrum showed
Figure 3. UV−visible absorption spectra of the polymer thin films.
greater blue-shifting than PFDTBT, as shown in Figures 2 and
3, and it showed the widest band gap energy among all of the
polymers considered herein. This may be why the short circuit
current of the device fabricated using PFDTBT_20Per is lower
than that of PFDTBT, despite it higher mobility. The device
2L)μCi(Vgs − Vth)2, where Ids is the drain-source current in the
saturated region, W and L are the channel width and length,
respectively, μ is the field-effect mobility, Ci is the capacitance
per unit area of the insulating layer, and Vgs and Vth are the gate
fabricated using PFDTBT-10Per showed the best photovoltaic
and threshold voltages, respectively.28,29 Interestingly, the field-
performance among the PFDTBT-based polymers (Voc = 0.74
V, Jsc = 7.19 mA/cm2, PCE = 2.16%). For the photovoltaic
effect mobility increased with an increase in the number of
perylene units in the corresponding parent polymers. The field-
effect mobility for PFDTBT, PFDTBT-5Per, PFDTBT-10Per,
and PFDTBT-20Per was determined to be 1.0 × 10−4, 1.2 ×
10−4, 5.3 × 10−4, and 1.2 × 10−3 cm2/(V s), respectively.
Similarly, the measured field-effect mobility of PFD2TBT,
PFD2TBT-5Per, and PFD2TBT-10Per was 2.7 × 10−6, 1.1 ×
10−5, and 2.1 × 10−5 cm2/(V s), respectively. This result
suggests that the introduction of the flat perylene structure to
devices fabricated using PFD2TBT-based polymers, the short-
circuit currents of the devices also increased as the perylene
content in the copolymers increased; however, the PCEs of the
devices using PFD2TBT-based polymers were lower than those
of PFDTBT-based polymers because of lower open-circuit
voltages and short-circuit currents. The device fabricated using
PFD2TBT_10Per showed the highest PCE (1.51%) among the
PFD2TBT-based polymers. The characteristics of the ITO/
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Figure 4. Band diagram for PC71BM and the synthesized polymers.
Table 2. Optical and Electrochemical Properties of the Synthesized Polymers
λmax, abs (nm)
λonset (nm)
a
b
b
c
polymers
solution
film
film
optical Egopt (eV)
HOMO (eV)
LUMO (eV)
PFDTBT
387, 531
385, 534
387, 537
387, 487
412, 518
406, 524
409, 527
387, 543
387, 545
390, 558
385, 522
422, 547
416, 550
420, 560
652
650
645
642
675
679
682
1.90
1.90
1.92
1.93
1.83
1.82
1.81
−5.58
−5.56
−5.55
−5.51
−5.43
−5.41
−5.41
−3.68
−3.66
−3.63
−3.58
−3.60
−3.59
−3.60
PFDTBT_5Per
PFDTBT_10Per
PFDTBT_20Per
PFD2TBT
PFD2TBT_5Per
PFD2TBT_10Per
a
b
1 × 10−5 M in anhydrous chloroform. Polymer film on a quartz plate by spin-casting from a solution in chloroform at 1500 rpm for 30 s.
c
Calculated from the absorption band edge of the copolymer films.
PEDOT:PSS/polymer:PC71BM/LiF/Al devices are summar-
ized in Table 4.
The morphologies of the polymers and PC71BM blend films
were observed by tapping-mode atomic force microscopy
(AFM). Figure 10 shows the AFM images of the
polymer:PC71BM (1:5 wt %) films, which were taken after
removal of the Al electrode and after device fabrication. The Al
electrode was removed by dipping the devices for 1 min in a
mixed acid solution containing of 16 mL of concentrated
phosphoric acid, 1 mL of concentrated hydrochloric acid, 1 mL
of acetic acid, and 2 mL of D2O. The blend films showed
almost equivalently measured smooth surfaces (according to
their root-mean-square (rms) roughness) with small degrees of
phase separation. The rms roughness of the active layers were
0.49 nm for PFDTBT, 0.44 nm for PFDTBT-10Per, 0.51 nm
for PFD2TBT, and 0.50 nm for PFD2TBT-10Per, respectively.
This result suggests that the morphologies of all the active films
are very similar, so that such a property will not affect the Jsc of
the photovoltaic devices. Therefore, the improvement of PCE
and Jsc in the fabricated photovoltaic devices is derived from the
enhanced π−π interaction between polymers that results from
the introduction of perylene units.
Figure 8 shows that the J−V characteristics of ITO/
PEDOT:PSS/polymer:PC71BM/TiOx/Al devices provide a
similar trend to those of ITO/PEDOT:PSS/poly-
mer:PC71BM/LiF/Al devices but their PCEs increased mainly
due to increased short circuit currents. The increase in the short
circuit current originates from the introduction of the thin
titanium suboxide (TiOx) layer (ca. ∼10 nm). The titanium
suboxide layer could block holes to the aluminum electrode,
thus it could increase the short circuit current of the
photovoltaic device. The device fabricated using PFDTBT-
10Per showed the highest PCE of 3.16% with an open circuit
voltage of 0.81 V, a short circuit current of 9.61 mA/cm2 and
the highest fill factor (0.41) among the fabricated devices. The
characteristics of the ITO/PEDOT:PSS/polymer:PC71BM/
TiOx/Al devices are summarized in Table 5.
Figure 9 shows the incident photon-to-current conversion
efficiency (IPCE) spectra of the ITO/PEDOT:PSS/PFDTBT-
10Per + PC71BM/LiF and TiOx/Al devices. The absorption
onset corresponding to the photon-to-current conversion
efficiency was about 750 nm for these devices, and the
absorption onset values corresponding to the maximum
conversion efficiency values were 427 and 560 nm.
4. CONCLUSIONS
We introduced perylene units into PFDTBT and PFD2TBT to
enhance the interchain π−π interaction. The optical and
electrochemical properties of the synthesized copolymers
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Figure 5. Transfer characteristics of OTFTs fabricated using the copolymers as the active layer at a constant source-drain voltage of −80 V. (a)
PFDTBT, (b) PFDTBT-10Per, (c) PFDTBT-20Per, (d) PFD2TBT, and (e) PFD2TBT-10Per.
Table 3. Summary of the characteristics of OTFTs fabricated
using the polymers as the active layers
polymer
Mobility (cm2/(V s))
Ion/Ioff
PFDTBT
1.0 × 10−4
1.2 × 10−4
5.3 × 10−4
1.2 × 10−3
2.7 × 10−6
1.1 × 10−5
2.1 × 10−5
1 × 104
1 × 104
1 × 105
2 × 105
6 × 102
1 × 103
5 × 103
PFDTBT-5Per
PFDTBT-10Per
PFDTBT-20Per
PFD2TBT
PFD2TBT-5Per
PFDTBT-10Per
Figure 6. Structure of photovoltaic devices fabricated using the
polymers.
showed were slight modification upon perylene addition.
Interestingly, the field-effect mobility of the TFTs, using the
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Figure 7. J−V characteristics of the photovoltaic devices with ITO/
PEDOT:PSS/polymer+PC71BM (1:5)/LiF/Al configuration.
Figure 9. Incident photon-to-current efficiency (IPCE) spectra of the
two different structured devices using PFDTBT-10Per.
Table 4. Summary of the Characteristics of Photovoltaic
Devices with ITO/PEDOT:PSS/Polymer + PC71BM (1:5)/
LiF/Al Configuration
polymer
PFDTBT
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0.77
0.72
0.74
0.70
0.65
0.67
0.64
5.92
6.13
7.19
6.23
5.02
5.54
5.95
0.34
0.38
0.40
0.38
0.31
0.36
0.39
1.56
1.69
2.16
1.67
1.03
1.35
1.51
PFDTBT_5Per
PFDTBT_10Per
PFDTBT_20Per
PFD2TBT
PFD2TBT_5Per
PFD2TBT_10Per
Figure 10. AFM topography images (5 × 5 μm) of active composite
films of (a) PFDTBT:PC71BM(1:5), (b) PFDTBT-10Per:PC71BM-
(1:5), (c) PFD2TBT:PC71BM(1:5), and (d) PFD2TBT-
10Per:PC71BM(1:5).
Figure 8. J−V characteristics of the photovoltaic devices with ITO/
PEDOT:PSS/polymer + PC71BM(1:5)/TiOx/Al configuration.
electron donor, also increased as the perylene content in the
copolymers increased, which is consistent with the field-effect
mobility results of the polymers. We conclude that the
introduction of perylene to the conjugated polymer chain
enhances the π−π interaction between polymer chains and
improves its charge-transport ability, leading to increases short
circuit currents in the photovoltaic devices.
Table 5. Summary of the Characteristics of Photovoltaic
Devices with ITO/PEDOT:PSS/Polymer + PC71BM(1:5)/
TiOx/Al Configuration
polymer
PFDTBT
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
0.79
0.76
0.81
0.64
0.72
0.76
8.40
9.09
9.61
8.52
9.46
9.68
0.35
0.38
0.41
0.35
0.36
0.38
2.35
2.64
3.16
1.92
2.46
2.82
PFDTBT_5Per
PFDTBT_10Per
PFD2TBT
AUTHOR INFORMATION
PFD2TBT_5Per
PFD2TBT_10Per
■
Corresponding Author
*E-mail: dohoonhwang@pusan.ac.kr.
polymers as the active layer increased with an increasing
perylene content in the copolymers. The short circuit currents
of the photovoltaic devices fabricated using the polymers as an
Notes
The authors declare no competing financial interest.
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(31) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J.
S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009,
3, 297.
ACKNOWLEDGMENTS
■
This research was financially supported by the Ministry of
Knowledge Economy (MKE) under the New & Renewable
Energy Program through a KETEP grant (No.
20103020010050 and No. 2011T100200034), and by the
National Science Foundation (NRF) grant funded by the
Korean government (MEST) (No. 2011-0011122) through
GCRC-SOP (Grant 2011-0030668).
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