New fused bis-thienobenzothienothiophene copolymers and their use in organic solar cells and transistors

Article abstract of DOI:10.1021/ma302390z

A new tetradodecyl-substituted DTBTBT donor unit is synthesized by a specific bis-annulation via Suzuki-Miyaura coupling and successfully incorporated into light absorbing electron donor copolymers for OPV and hole and electron transport OFET polymer devices. All copolymers (DTBTBT-co- benzothiadiazole (Bz), DTBTBT-co-thiophene (T) and DTBTBT-co-thienothiophene (TT)) show fully coplanar backbones and strong intermolecular interactions. The DTBTBT-Bz copolymer led to a deep HOMO level (-5.2 eV) and thus a large V _oc value of 0.92 V with PC₇₁BM as electron acceptor and a PCE of 3.7% with a J_sc of 6.78 mA/cm² could be obtained. A hole mobility of 0.1 cm²/(V s) has been observed for the highly coplanar and more crystalline DTBTBT-T copolymer.

Full text of DOI:10.1021/ma302390z

Article
pubs.acs.org/Macromolecules
New Fused Bis-Thienobenzothienothiophene Copolymers and Their
Use in Organic Solar Cells and Transistors
Laure Biniek,†,§,* Bob C. Schroeder,† Jenny E. Donaghey,† Nir Yaacobi-Gross,‡ Raja Shahid Ashraf,†
Ying W. Soon,† Christian B. Nielsen,† James R. Durrant,† Thomas D. Anthopoulos,‡ and Iain McCulloch†
†Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, United Kingdom
‡Department of Physics and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, United Kingdom
§Institut Charles Sadron, CNRS-Universite
́
de Strasbourg, 23 rue du Loess, 67034 Strasbourg, France
S
* Supporting Information
ABSTRACT: A new tetradodecyl-substituted DTBTBT
donor unit is synthesized by a specific bis-annulation via
Suzuki−Miyaura coupling and successfully incorporated into
light absorbing electron donor copolymers for OPV and hole
and electron transport OFET polymer devices. All copolymers
(DTBTBT-co-benzothiadiazole (Bz), DTBTBT-co-thiophene
(T) and DTBTBT-co-thienothiophene (TT)) show fully
coplanar backbones and strong intermolecular interactions.
The DTBTBT-Bz copolymer led to a deep HOMO level
(−5.2 eV) and thus a large Voc value of 0.92 V with PC71BM as
electron acceptor and a PCE of 3.7% with a Jsc of 6.78 mA/cm2 could be obtained. A hole mobility of 0.1 cm2/(V s) has been
observed for the highly coplanar and more crystalline DTBTBT-T copolymer.
good solubility and processability. The alkyl chains are attached
to the aromatic ring system via a sp2-hybridized carbon, thus
project coplanar to the polymer backbone, in contrast to the
case of sp3-hybridized carbons for which the alkyl chains
protrude out of the plane of the polymer backbone11 which
should be advantageous for intermolecular interactions.
The new electron-rich monomer was copolymerized with
three different comonomers, the electron-deficient 2,1,3-
benzothiadiazole (Bz) and two electron-rich units, thiophene
(T), and thieno[3,2-b]thiophene (TT) respectively. The choice
of electron-deficient and -rich comonomers allows us to
evaluate the potential of the new DTBTBT unit in both
organic photovoltaic and transistor devices.
INTRODUCTION
■
Molecular engineering of organic semiconductors has played an
essential role in the improvement of organic electronic devices
performances. In particular, the design and development of a
large range of different conjugated polymers, focused on
improving the optical absorption, planarizing the aromatic
backbone, and therefore adjusting the molecular orbital energy
levels, has led to an improved understanding of the structure-
properties relationships.1−5 Most of the highest performing
polymers show short π-stacking distances between polymer
backbones with wave functions often widely delocalized over
the polymer conjugated aromatic system.6 These properties,
leading to high charge carrier mobilities, have often been
obtained with ring fusion of heteroarenes.7−10
Herein we have designed and synthesized an extended and
soluble tetradodecylthieno[3′,2′:6,7][1]benzothieno[3,2-b]-
thieno[3,2-g][1]benzothiophene core unit (DTBTBT, Scheme
1) which was incorporated into light absorbing electron donor
copolymers for OPV and hole and electron transport OFET
polymer devices. The two outer thiophene rings whose acidic 2-
positions are easy to functionalize, facilitate the incorporation
of the heteroarene unit into polymers via various cross-coupling
schemes. Additionally, these thiophene rings offer low steric
distortion to the backbone planarity, i.e., a small dihedral angle,
when they are coupled into a polymer chain, which should be
advantageous for close packing in the solid state and provide
the basis for interesting electronic properties.5 The proposed
synthesis pathway allows the introduction of four lateral alkyl
side chains onto the ladder-type monomer, which promotes
In the following, the synthesis and optical, electrochemical
and morphological properties are presented and the perform-
ance of organic solar cells and organic transistors devices are
discussed.
EXPERIMENTAL SECTION
■
Materials. All chemicals were purchased from commercial
suppliers unless otherwise specified. The synthesis of the 2,2′-(13Z)-
hexacos-13-ene-13,14-diylbis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)
(2) and 2,8-dibromo-4,5,10,11-tetradodecylanthra[1,2-b:5,6-b′]-
bisthiophene (6) are described in the Supporting Information.
4,5,10,11-Tetradodecylthieno[3′,2′:6,7][1]benzothieno[3,2-b]-
thieno[3,2-g][1]benzothiophene (3). 3,6-Dibromo-2,5-bis(3-bromo-
Received: November 20, 2012
Revised: January 16, 2013
Published: January 31, 2013
© 2013 American Chemical Society
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Scheme 1. Synthesis Pathway toward DTBTBT Unit 3 and the Three New Copolymers
thiophen-2-yl)thieno[3,2-b]thiophene (1) (2.0 g, 3.22 mmol) and (Z)-
1,2-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,2-didodecyle-
thene (2) (3.97 g, 6.45 mmol) were dissolved in deoxygenated THF
(60 mL). The mixture was then degassed by bubbling nitrogen for 1.5
h at room temperature before the addition of Pd(dppf)Cl2 (165 mg,
0.22 mmol), (further degassing for 5 min). After degassing for
additional 5 min, degassed K3PO4(aq) (3 M, 10.7 mL) was added. The
reaction mixture was refluxed at 80 °C for 2 h and then at 65 °C
overnight. The reaction mixture was quenched with water, and
extracted with diethyl ether before being washed with water. The
collected organic layer was dried over MgSO4. After removal of the
solvent under reduced pressure, the residue was purified by column
chromatography on silica gel (hexane) to give a pale yellow solid (762
mg, 21.9%). 1H NMR (CDCl3, 400 MHz, ppm): δ 7.49 (d, J = 5.4 Hz,
2 H), 7.52 (d, J = 5.4 Hz, 2 H) 3.25 (m, 4 H), 3.08 (m, 4 H), 1.73−
1.68 (m, 4 H), 1.47−1.29 (m, 4 H), 1.28 (m, 72H), 0.88 (m, 12 H),
13C NMR (100 MHz, CDCl3): δ 138.05, 133.92, 132.88, 131.98,
29.68, 29.62, 29.55, 22.86, 14.28. MS (MALDI−TOF LD+,
C66H102Br2S4): calcd, 1182.5; found, 1182.8. mp (°C): 125−128.
DTBTBT-Bz and DTAT-Bz by Suzuki polymerization. A 20 mL
microwave vial was charged with monomer 4 or 6 (0.160 g, 1.0 equiv),
1.0 equiv of 2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester)
monomer, 2.2 mol % of tris(dibenzylideneacetone)dipalladium(0),
and 8.8 mol % of tri(o-tolyl) phosphine, a drop of Aliquat 336 and
toluene (0.03 M). This mixture was degassed with argon for 1 h,
followed by the addition of degassed Na2CO3 solution (1.0 M). The
resultant mixture was degassed for another 10 min then sealed and
heated at 120 °C for 48 h. The polymer was precipitated from acidic
methanol (HCl + CH3OH), and collected by filtration and washed
with methanol. The polymer was further purified by washing via
Soxhlet extraction with methanol (24 h), acetone (24 h), and hexane
(24 h) and then was dissolved in chloroform. The remaining palladium
residues were removed by treating a polymeric chloroform solution
with an aqueous sodium diethyldithiocarbamate solution for 2 h at 50
°C under vigorous stirring. Afterward, the organic phase was separated
from the aqueous phase and washed several times with water. The
polymeric solution was concentrated under reduced pressure and
precipitated into cold methanol. The polymer was filtered off and
dried under high vacuum for at least 24 h providing deep blue solids.
DTBTBT-Bz (95 mg, 59%). Mn = 19 kg mol−1, Mw = 132 kg mol−1.
DTAT-Bz. (132 mg, 82%) Mn = 19 kg mol−1, Mw = 46 kg mol−1.
DTBTBT-T and DTBTBT-TT by Stille Polymerization. An oven-
dried microwave vial was charged with 2,8-dibromo-4,5,10,11-
tetradodecylthieno[3′,2′:6,7][1]benzothieno[3,2-b]thieno[3,2-g][1]-
benzothiophene (4) (90 mg, 0.076 mmol, 1 equiv), and 1 equiv of 2,5-
di(trimethyltin)thiophene (31 mg, 0.076 mmol) or 2,5-di-
(trimethyltin)thieno(3,2-b)thiophene (35 mg, 0.076 mmol), tris-
(dibenzylideneacetone)dipalladium(0) (1.53 mg, 1.67 μmol, 2.2 mol
%) and tri-o-tolylphosphine (2.04 mg, 6.69 μmol, 8.8 mol %). The vial
was sealed, and dry chlorobenzene (1.1 mL) was added. The reaction
mixture was degassed with argon for 30 min before being placed in the
microwave reactor and subjected to the following heating conditions:
120 °C for 2 min, 140 °C for 2 min, 160 °C for 2 min, and 180 °C for
40 min. Once the reaction had cooled, the viscous red liquid was
precipitated in methanol and filtered into a Soxhlet thimble. The
131.47, 131.09, 130.55, 124.15, 123.83, 32.09, 31.85, 31.18, 30.58,
30.39, 30.30, 29.87, 29.81, 29.71, 29.54, 22.86, 14.28. MS (MALDI−
TOF LD+, C66H104S4): calcd, 1024.7; found, 1025. Mp (°C): 113−
115.
2,8-Dibromo-4,5,10,11-tetradodecylthieno[3′,2′:6,7][1]-
benzothieno[3,2-b]thieno[3,2-g][1]benzothiophene (4). 4,5,10,11-
Tetradodecylthieno[3′,2′:6,7][1]benzothieno[3,2-b]thieno[3,2-g][1]-
benzothiophene (3) (470 mg, 0.46 mmol) was dissolved in CHCl3/
acetic acid (40 mL/4 mL) under argon in the dark. N-
Bromosuccinimide (173 mg, 0.96 mmol) was added portionwise
over 15 min. The reaction mixture was stirred overnight at room
temperature then quenched with water, extracted with chloroform and
washed with water. The collected organic layer was dried over MgSO4.
After removal of the solvent under reduced pressure, the residue was
recrystallized in a mixture of ethanol and dichloromethane to give a
pale yellow solid (530 mg, 97%). 1H NMR (CDCl3, 300 MHz, ppm),
δ: 7.43 (s, 2H), 3.15 (m, 4H), 2.96 (m, 4H), 1.68 (m, 8H), 1.51−1.28
(m, 72H), 0.87 (m, 12H). 13C NMR (100 MHz, CDCl3, ppm), δ:
137.81, 133.01, 132.89, 132.17, 131.65, 131.38, 130.48, 126.58, 113.00,
32.10, 31.78, 31.71, 31.19, 30.51, 30.33, 30.27, 29.88, 29.82, 29.76,
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Scheme 2. Synthesis of the Anthradithiophene-Based Polymer DTAT-Bz
precipitates were purified by Soxhlet extraction with acetone, hexane,
and chloroform (24 h each). The chloroform fraction was stirred with
aqueous sodium diethyldithiocarbamate tetrahydrate at 50 °C for 1 h,
then the organic layer was separated and washed with water. The
chloroform fraction was further purified by preparative scale size
exclusion chromatography to remove the low molecular weight
fractions. The recovered polymer solution was concentrated under
reduced pressure and precipitated into methanol. After filtration the
polymer was isolated as a dark red solid (42 mg, 49%). DBTBT-T (42
mg, 49%): Mn = 29 kg mol−1, Mw = 77 kg mol−1. DBTBT-TT (33 mg,
37%): Mn = 9.5 kg mol, Mw = 17 kg mol−1.
Photocurrent was measured using a Keithley 2400 source meter;
Measurement duration for a given wavelength was sufficient to ensure
the current had stabilized (up to around 5 s under low or zero bias
light conditions).
Fabrication and Characterization of OFETs. Top-gate, bottom-
contacts (TG-BC) organic field-effect transistors (OFETs) were
fabricated onto glass substrate using evaporated bilayer source/drain
(S/D) electrodes. These electrodes were made using a 20 nm layer of
Al covered with a sequentially deposited 25 nm thick layer of Au. As
deposited electrode structures were then treated with the contact work
function modifier monolayer agent pentafluorobenzenethiol
(PFBT).12 The glass/electrodes substrates were then transferred into
a nitrogen glovebox in order to complete the remaining device
fabrication and electrical characterization stages. Polymer layers were
spin-cast from hot (100 °C) ODCB on top of the substrates and
annealed at 120 °C for 15 min. The fluoropolymer CYTOP (Ashai
Glass) was used as the gate dielectric layer. Device fabrication was
completed with the evaporation of the top Al gate electrode by
thermal evaporation under high vacuum (10−6 mbar).
1
Characterizations. H NMR and 13C NMR spectra were recorded
on a Bruker 400 spectrometer in CDCl3 solution at 298 K unless,
1
polymers H NMR were recorded on a Bruker DRX400. Number-
average (Mn) and weight-average (Mw) molecular weights were
determined with an Agilent Technologies 1200 series GPC in
chlorobenzene at 80 °C, using two PL mixed B columns in series,
and calibrated against narrow polydispersity polystyrene standards.
The polymers were additionally purified on a Shimadzu preparative
scale size exclusion chromatography system using chlorobenzene as
eluent and an Agilent PLgel 10 μm MIXED-D column at 80 °C. Mass
spectra were recorded by the Imperial College London Department of
Chemistry, Mass Spectrometry Service, on a Micromass Platform II or
AutoSpec-Q spectrometer. UV−vis absorption spectra were recorded
on a UV-1601 Shimadzu UV−vis spectrometer. Photoelectron
spectroscopy in air (PESA) measurements were recorded with a
Riken Keiki AC-2 PESA spectrometer with a power setting of 5 nW
and a power number of 0.5. X-ray diffraction patterns of thin films
deposited on the Si/SiO2 substrate were obtained with a
PANALYTICAL X’PERT−PRO MRD diffractometer equipped with
a nickel-filtered Cu Kα1 beam and a X′ CELERATOR detector, using a
current of 40 mA and an accelerating voltage of 40 kV. DSC
experiments were carried out with a TA Instruments DSC Q20 and
TGA plots were obtained with a Perkin-Elmer Pyris 1 TGA. The
morphology of polymer: PC71BM blend was observed by TEM. The
blend films on ITO/PEDOT:PPS substrate were floated on a 10 wt %
aq HF solution and recovered on TEM copper grids. TEM was
performed in bright field and diffraction mode using a Philips CM12
microscope equipped with a MVII CCD camera (Soft Imaging
System; CDD = charge-coupled device).
Fabrication and Characterization of Solar Cells. All organic
photovoltaic devices have a conventional device architecture, ITO/
PEDOT:PSS/polymer:PC71BM/LiF/Al. The precoated ITO glass
substrates were cleaned with acetone and isopropyl alcohol under
sonification, followed by drying and oxygen plasma treatment. A 30
nm layer of PEDOT:PSS was spin-coated onto the plasma-treated
ITO substrate and baked at 150 °C for 20 min. An 80 nm active layer
consisting of blend of polymer and PC71BM dissolved in o-
dichlorobenzene (ODCB) was spin-coated on the PEDOT:PSS
layer and then LiF (2 nm)/Al (120 nm) cathode was finally deposited
by thermal evaporation under high vacuum (10−6 mbar) through a
shadow mask. The pixel size, defined by the spatial overlap of the ITO
anode and Li/Al cathode, was 0.045 cm2. The device characteristics
were obtained using a xenon lamp at AM1.5 solar illumination (Oriel
Instruments). The photon flux of light incident on the samples was
calibrated using a UV-enhanced silicon photodiode. A 590 nm long-
pass glass filter was inserted into the beam at illumination wavelengths
longer than 620 nm to remove light from second-order diffraction.
RESULTS AND DISCUSSION
■
1. Synthesis of Monomers and Polymers. Scheme 1
illustrates the synthetic route toward the new DTBTBT unit 3
and the copolymers.
In order to get the desired functionalized sp2-carbon bond
formation, a specific bis-annulation by Suzuki−Miyaura
coupling13 was used. Indeed, DTBTBT was obtained by a
palladium-catalyzed double cross-coupling reaction of the
previously reported tetrabromo-tetraaryl 14 and the vic-
diborylated dodecene 2. This convenient one-pot reaction
afforded the new ladder-type monomer 3 in a reasonable yield.
The synthesis of the vic-diborylated dodecene 2, described in
the Supporting Information, could be easily adapted for
different kinds of substituents, thus providing a large range of
alkylated derivatives.
A subsequent bromination of 3 with N-bromosuccinimide
provided the final monomer 4 in good yield. Polymer
DTBTBT-Bz was synthesized via a Suzuki coupling between
monomer 4 and 2,1,3-benzothiadiazole-4,7-bis(boronic acid
pinacol ester), whereas polymers DTBTBT-T and DTBTBT-
TT were obtained via Stille coupling between monomer 4 and
2,5-bis(trimethylstannyl)-thiophene or 2,7-bis-
(trimethylstannyl)-thieno[3,2-b]thiophene, respectively.
For comparison, an angular fused anthradithiophene
(DTAT) unit was also prepared, which had been independently
reported by Wu et al.14 The desired 4,5,10,11-
tetradodecylanthra[1,2-b:5,6-b′]bisthiophene was synthesized
in good yield (43%, Scheme 2) by using tetrakis-
(triphenylphosphine)palladium as catalyst in combination
with potassium carbonate. After bromination of the ring-closed
monomer, DTAT-Bz polymer was synthesized using the same
polymerization condition as DTBTBT-Bz to ensure a valid
comparison.
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Table 1. Properties of the Polymers
a
b
c
d
e
sol
film
polymer
Mn/Mw (kDa)
λmax (nm)
λmax (nm)
Egopt (eV)
EHOMO (eV)
ELUMO (eV)
Td (°C)
DTBTBT-Bz
DTBTBT-T
DTBTBT-TT
DTAT-Bz
19/132
29/77
9/17
403/613
497
403/621
496
1.8
2.2
2.2
1.8
−5.2
−4.9
−4.9
−5.3
−3.4
−2.7
−2.7
−3.5
c
450
435
430
440
494
494
19/46
355/582
356/634
b
a
Values extracted from temperature dependent studies at 85 °C. Estimated from the optical absorption edge in thin films. Measured by UV-PESA.
Values deducted from EHOMO and optical band gap. Decomposition temperature (5% weight loss) determined by thermal gravimetric analysis
d
e
under nitrogen.
Figure 1. Normalized UV−vis absorption spectra of polymers in CB at 25 °C (dashed line) and in thin films (plain line) of (a) donor−acceptor
alternated copolymers and (b) thiophene and thienothiophene based polymers.
Figure 2. Minimum energy conformations of trimers of DTBTBT-Bz and DTAT-Bz. Gaussian optimized at the B3LYP/6-31G* level with
visualization of the corresponding HOMO and LUMO energy distributions.
All polymers were purified by precipitation from methanol
followed by Soxhlet extraction using acetone, hexane and
chloroform. The latter fraction was stirred with aqueous sodium
diethyldithiocarbamate to remove residual catalytic impurities.
Lower molecular weight fractions of DTBTBT-T and
DTBTBT-TT were removed by recycling gel permeation
chromatography (GPC) in chlorobenzene. The number-
average molecular weights (Mn) of the polymers were evaluated
by GPC in chlorobenzene at 80 °C to be between 20 to 30
kDa, except for the thieno[3,2-b]thiophene derivative
(DTBTBT-TT) which has a lower Mn of 9 kDa (See Table
1). This lower value might be due to the more planar backbone
of the polymer which can easily lead to strong intermolecular
interaction and aggregation, thus limiting the polymer solubility
and therefore chain growth. All polymers are soluble in hot
chlorinated solvent such as chloroform, chlorobenzene (CB)
and ortho-dichlorobenzene (ODCB), whereas at room temper-
ature they tend to aggregate and gel in solution.
alternating copolymers in CB at room temperature and in
thin films are shown in Figure 1a. The benzothiadiazole based
copolymers show two distinct features, a π−π* transition at
higher energy and an internal charge transfer (ICT) transition
at lower energy. Interestingly the anthradithiophene DTAT-Bz
based polymer shows a higher intensity for the π−π* transition,
than for the ICT transition, in contrast to the DTBTBT-based
polymer. Furthermore, the bandwidth of the transition at lower
energy is larger for DTBTBT-Bz. This might be explained by a
larger overlap of π orbitals due to a better packing.15 In both
cases, almost no absorption maximum shift from solution to the
solid state is observed, which corroborates the strong
aggregation of the polymers in solution. The optical bandgaps
extracted from the absorption edges are 1.8 eV for both
DTBTBT-Bz and DTAT-Bz.
The absorption spectra of the electron rich thiophene and
thieno[3,2-b]thiophene based DTBTBT-T and DTBTBT-TT
polymers are presented in Figure 1b. Their optical properties
are similar due to their structural similarity. Again, no shifts of
the absorption maxima, around 495 nm, can be observed from
Optical Properties and Polymer Chain Conformation.
The UV−vis absorption spectra of both donor−acceptor
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Figure 3. Minimum energy conformations of trimers of DTBTBT-T and DTBTBT-TT with their calculated torsion angles.
Figure 4. X-ray diffraction (XRD) patterns of drop-casted films of polymers on Si/SiO2 substrates as-cast, annealed at 100 and 200 °C.
solution to thin film corroborating the assumption of strong
aggregation of the polymer chains even in dilute solution.
However, we note a slight broadening of the absorption toward
the lower wavelengths in the case of the TT based polymer
probably due to a lower molecular weight. The optical
bandgaps extracted from the absorption edges are 2.2 eV for
both DTBTBT-T and DTBTBT-TT.
The aggregation in solution has been studied by in situ
temperature-controlled UV−vis absorption measurements
(Figure I, Supporting Information). Upon heating, a change
of the absorption spectra is observed most notably for the
DTAT-Bz polymer with a blue shift of the absorption
maximum and a disappearance of the second shoulder at low
energy. A clear isosbestic point supports the idea of an
aggregate -to -solute conversion upon heating whereas only a
slight decrease of the absorption intensity is observed for
DTBTBT-Bz. It seems more difficult to break up the
aggregation in solution for the DTBTBT-Bz polymer than
for DTAT-Bz which confirms a stronger intermolecular
interaction of the bis(thienobenzothienothiophene) unit than
the anthradithiophene unit.
2. Even if both polymers are fully coplanar (torsion angles of 0°
and 2° for DTBTBT-Bz and DTAT-Bz, respectively) we note a
less pronounced backbone curvature for DTBTBT-Bz polymer
compared to the DTAT-Bz. This geometrical difference might
explain the strongest tendency of DTBTBT based polymer to
aggregate in solution16 and is also leading to a larger
delocalization of the HOMO energy distribution over the
backbone. In this system, the sulfur atoms of the central
thienothiophene ring contribute to the highest occupied
molecular orbital (HOMO) energy distribution, which should
enhance the probability of close orbital interactions between
cofacial polymer chains in the solid state, thereby facilitating
charge carrier interchain hopping. The anthradithiophene unit
presents a slightly disrupted HOMO energy delocalization
probably due to the fact that a fusion of aromatic benzene rings
is leading to a decrease of the benzenoid character.17
Meanwhile the lowest unoccupied molecular orbital (LUMO)
energy distribution seems more localized on the benzothiadia-
zole acceptor unit for DTBTBT-Bz than for DTAT-Bz
supporting the idea of the stronger ICT behavior observed
by UV−vis spectroscopy.
The minimum energy conformations of the Bz based
polymers were calculated via density functional theory (DFT)
using the B3LYP/6-31G* model and are represented in Figure
The copolymerization of the DTBTBT unit with thiophene
(T) or thienothiophene (TT) lead to slightly different polymer
backbone conformations as shown in Figure 3. While the TT
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based trimer is almost fully coplanar, the DTBTBT-T shows a
slight backbone curvature. However in both cases the very small
torsion angles highlight the coplanarity of these new ladder
type polymers. The calculated HOMOs and LUMOs (Figure
II, Supporting Information), are delocalized over the entire
conjugated backbone for both systems. The HOMOs are
predominantly aromatic in nature, whereas the LUMOs for
both polymers are mainly of a quinoidal character. The good
wave function delocalization and the coplanarity should
facilitate both intra- and intermolecular charge transport.
Polymer Energy Levels. The HOMO energy levels of the
polymers were measured by photoelectron spectroscopy in air
(UV-PESA; see Table 1) and values of −5.2 and −5.3 eV were
found for DTBTBT-Bz and DTAT-Bz, respectively. The
slightly higher value for the DTBTBT based polymer might be
due to the more electron rich thienothiophene inner ring. The
LUMO energy levels were subsequently estimated from the
optical band gaps and the HOMO levels; values of −3.4 eV for
DTBTBT-Bz and −3.5 eV for DTAT-Bz were obtained. In the
case of the DTBTBT-T and DTBTBT-TT polymers, higher
HOMO and LUMO levels at −4.9 and −2.7 eV respectively
have been measured. These values are in good agreement with
the calculated ones, −4.8 eV for the HOMO levels and −2.6 eV
for the LUMO levels of both polymers. We stress that the
observed tendencies for aggregation imply that the thin-film
morphology of these materials can vary greatly with processing
conditions (choice of casting technique, solvent, temperature,
and concentration). Since the morphology can affect both UV-
PESA and UV−vis measurements, the values obtained for the
frontier-orbital energy levels must be regarded as estimated
values.
Photovoltaic Results. The photovoltaic performances of
all polymers were evaluated in bulk heterojunction solar cells
with a conventional device architecture, ITO/PEDOT:PSS/
polymer:PC71BM/LiF/Al and are summarized in Table 3. The
Table 3. Photovoltaic properties of DTAT and DTBTBT
containing polymers
P (ratio PC71BM)
DTAT-Bz (1:3.5)
Jsc (mA/cm2) Voc (V)
FF
PCE (%)
4.55
6.00
7.15
6.78
8.10
6.41
0.94
0.93
0.89
0.92
0.70
0.66
0.64
0.57
0.57
0.60
0.59
0.56
2.7
3.2
3.6
3.7
3.3
2.3
DTBTBT-Bz (1:3.5)
DTBTBT-Bz (1:2, 1% CN)
DTBTBT-Bz (1:3, 1% CN)
DTBTBT-T (1:2)
DTBTBT-TT (1:2)
highest open circuit voltage (Voc) values were obtained with
both low-bandgap polymers, DTAT-Bz and DTBTBT-Bz,
which also have the lowest lying HOMO energy levels of all
polymers. Despite the high Voc value of 0.94 V, the power
conversion efficiency (PCE) of DTAT-Bz was limited by the
low current (2.7%). In the case of DTBTBT-Bz the current was
increased to 6.0 mA·cm−2 leading to a higher PCE of 3.2%. We
believe that the better packing and higher absorption range of
the DTBTBT unit are responsible for the current increase
compared to DTAT. The external quantum efficiency (EQE)
spectrum represented in Figure 5 also supports this hypothesis
and confirms that more charges are generated in the case of
DTBTBT-Bz. In order to get an insight into the blend
morphology, both polymer:PC71BM blends were investigated
by transmission electron microscopy (TEM). Figure III, parts a
and c (see Supporting Information), depicts the typical blend
morphologies in bright field for DTBTBT-Bz:PC71BM (ratio
1:3.5) and DTAT-Bz:PC71BM (ratio 1:3.5). For the DTBTBT
based system, a clear phase separated morphology is observed
with darker features (∼150 nm in diameter) ascribed to PCBM
domains, which are surrounded by brighter areas attributed to
the polymer phase.18 In contrast, DTAT based blends display a
very different morphology without clear phase separation
between PC71BM and polymer. Instead, a regular distribution
of holes (∼100 nm in diameter) is observed; the origin of
which is yet not clear. However, this lack of phase separation
might be the origin of the lowest devices performances.
Regarding electron diffraction, none of the films showed any
reflection from the polymers but only Debye−Scherrer ring at
0.46 nm characteristic of PCBM domains.19 This observation
underlines the poor in-plane order of both polymers in these
devices. In order to get a more finely intermixed morphology
(shown in Figure IIIb, Supporting Information), the PC71BM
content was reduced and 1% of chloronaphthalene (CN) was
added to the active layer blend. The Voc was barely influenced
by those modifications; however the short circuit current
density (Jsc) and fill factor (FF) were increased, leading to
device efficiencies of 3.7%. In that case, the DTBTBT-
Bz:PC71BM blend films display more continuous domains,
even though the domain sizes are still rather large (∼100 nm)
which could potentially lead to exciton recombination, thus
limiting device performance.
Thermal Properties. All polymers show good thermal
stability with decomposition temperatures above 400 °C. No
phase transitions were observed by DSC in the temperature
range between 60 and 295 °C at a heating rate of 20 °C/min
for all polymers. However X-ray diffraction measurements
revealed out of plane reflections attributed to lamellar type
crystallinity for all films drop-casted on silicon substrates as
illustrated in Figure 4. The lamellar and the π−π stacking
distances, summarized in Table 2, are slightly shorter in the
Table 2. Lamellar (dL) and π-Stacking (dπ) Distances
Determined by XRD
as cast
100 °C
200 °C
polymers
dL (Å) dπ (Å) dL (Å) dπ (Å) dL (Å) dπ (Å)
DTBTBT-Bz
DTBTBT-T
DTBTBT-TT
DTAT-Bz
22.1
23.5
22.7
22.5
3.7
3.6
3.6
3.8
22.3
23.9
23.1
23.7
3.7
3.6
3.6
3.8
22.7
24.2
23.3
23.1
3.7
3.6
3.6
3.9
case of bis(thienobenzothienothiophene) than for the
anthradithiophene based polymer supporting the idea that the
DTBTBT unit is favoring stronger intermolecular interaction
than the DTAT moiety.
A higher degree of crystallinity and a shorter π−π stacking
distance (3.6 Å) can be observed for the thiophene and
thienothiophene based polymers, as compared to the Bz
copolymers, probably due to a better organization of the
polymer chains due to their smallest backbone curvature, which
should be beneficial for the charge transport properties. An
increase in crystallinity upon annealing can be observed in
DTBTBT-T and DTBTBT-TT.
Both the thiophene and the thienothiophene copolymers
have much higher HOMO energy levels, which is reflected in
their lower Voc values. Despite the larger bandgap of these
materials and a more limited absorption range, good Jsc values
and high FFs could be obtained, leading to device efficiencies
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Figure 5. J−V curves of the solar cells based on polymers/PC71BM under the illumination of AM 1.5, 100 mW/cm2, and the associated EQE.
Figure 6. (a) Transfer and (b) output characteristics for the DTBTBT-T OFET. The channel dimensions are length L = 20 μm and width W = 1000
μm.
Figure 7. (a) Transfer and (b) output characteristics for the DTBTBT-TT OFET. The channel dimensions are length L = 20 μm and width W =
1000 μm.
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Figure 8. Transfer characteristics for the DTBTBT-Bz (a) and DTAT-Bz OFETs (b). Both devices have channel dimensions of length L = 20 μm
and width W = 1000 μm.
exceeding 3.3% for DTBTBT-T (Table 3). It is important to
note that this value has been obtained without further
optimization of the blend ratio or the active layer morphology.
The addition of chloronaphthalene in this case did not improve
the performances possibly because of the more crystalline
nature of DTBTBT-T compared to DTBTBT-Bz. The higher
backbone coplanarity and higher molecular weight of the
thiophene based polymer might be at the origin of the higher
charge generation observed in the EQE and the better charge
transport properties leading to better OPV performances.
Field-Effect Transistor Measurements. The charge
carrier mobilities were evaluated using top-gate, bottom-
contacts (TG-BC) organic field-effect transistors (OFETs)
after annealing at 120 °C for 15 min. Figure 6a and Figure 7a
show the transfer characteristics measured for, respectively,
DTBTBT-T and DTBTBT-TT OFET devices. The device
based on the thiophene unit exhibit high p-type mobility (∼0.1
cm2/(V s)). Comparing the thiophene and thienothiophene
based polymers reveals a large difference in turn on voltages
and while the on/off ratio is comparable (∼104) the hole
mobility of the thiophene based polymer is an order of
magnitude higher than the thienothiophene one (∼0.01 cm2/
(V s)). The higher hole mobility of DTBTBT-T might be
explained by its higher molecular weight and slightly lower
crystallinity.20 Close examination of the output curves (Figure
6b and Figure 7b) at low drain voltages (VD), obtained from
the two devices reveals significantly different hole injection
characteristics. A significant injection barrier to holes can be
seen in DTBTBT-T devices. The latter is manifested as a
superlinear dependence of drain current (ID) as a function of
VD at low fields. This feature is absent in DTBTBT-TT devices,
where an Ohmic-like linear dependence of ID on VD can be
observed. This finding is rather puzzling since photoelectron
spectroscopy measurements suggest similar HOMO energy
levels for the two polymers (Table 1). Therefore, the apparent
injection barrier seen in DTBTBT-T devices cannot attributed
solely to an energy offset between the electrode work function
and the HOMO level, but other factors (e.g., film morphology
at the metal and dielectric interfaces) may be responsible.
Figure 8a displays a typical set of the transfer characteristics
for the DTBTBT-Bz transistor. The device exhibits ambipolar
characteristic with hole and electron mobilities of ∼0.01 cm2/
(V s) and ∼0.005 cm2/(V s), respectively. The on/off ratio is
relatively low and on the order of 103. The transfer
characteristics for the DTAT-Bz based transistor are shown
in Figure 8b. Again, clear ambipolar charge transport
characteristic are observed although the overall performance
of the device appears to be lower than the DTBTBT-Bz based
one probably due to the more pronounced backbone curvature
of the DTAT based material. A maximum hole and electron
mobilities around ∼0.005 cm2/(V s) and ∼0.001 cm2/(V s),
respectively and a channel current on/off ratio in the order of
102 is obtained for DTAT-Bz. The low hole mobility might also
be at the origin of the low short circuit current observed in
DTAT-Bz:PC71BM solar cells. The ability to inject and
accumulate both type of carriers (i.e., holes and electrons)
within the transistor channel is attributed to both, the relatively
narrow band gap of the polymers as well as to the use of bilayer
S/D electrodes that consist of a sequentially deposited low
workfunction (Al ∼ 4.3 eV) and a high workfunction (Au ∼ 5
eV) metals. These simple but rather versatile bilayer S/D
electrodes enable the fabrication of devices with ambipolar
injection characteristics. We do note however that further
improvement in electron injection and transport would be
anticipated through the use of S/D electrodes with even lower
work functions (e.g., Ca, Al/LiF). This however is beyond the
scope of this work and will be addressed in future
investigations.
CONCLUSIONS
■
We have successfully synthesized three copolymers of a new
tetradodecyl-substituted DTBTBT donor unit and different
monomers such as Bz, T, and TT by Stille or Suzuki
polymerization. A tetradodecyl-substituted DTAT unit copoly-
merised with Bz was also synthesized and used as a comparison.
The introduction of the alkyl chains in the 2 and 7 positions
and the two outer thiophene ring allowed us to incorporate the
DTBTBT unit into soluble polymers with good planarity,
which were demonstrated to be suitable for both OFET and
OPV applications. The DTBTBT-Bz copolymer with its
electron poor benzothiadiazole unit led to a deep HOMO
level and thus a large Voc value of 0.92 V with PC71BM as
electron acceptor and a PCE of 3.7% with a Jsc of 6.78 mA/cm2
could be obtained. A hole mobility of 0.1 cm2/(V s) has been
observed for the highly coplanar DTBTBT-T copolymer. The
results indicate that the copolymers based on this new
DTBTBT donor unit with optimized alkyl side chains21 are
promising semiconducting polymers for efficient OFET and
OPV devices.
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Article
(16) Rieger, R.; Beckmann, D.; Mavrinskiy, A.; Kastler, M.; Mullen,
K. Chem. Mater. 2010, 22, 5314−5318.
̈
ASSOCIATED CONTENT
* Supporting Information
Experimental part of intermediates 2, 5, and 6, NMR spectra of
monomer 4 and copolymers, temperature controlled UV−vis
absorption of four copolymers in solution, energy distribution
of DTBTBT-T and DTBTBT-TT, and bright field TEM
images of typical blends. This material is available free of charge
via the Internet at http://pubs.acs.org/.
■
S
(17) Suresh, C. H.; Gadre, S. R. J. Org. Chem. 1999, 64, 2505−2512.
(18) Beal, R. M.; Stavrinadis, A.; Warner, J. H.; Smith, J. M.;
Assender, H. E.; Watt, A. A. R. Macromolecules 2010, 43, 2343−2348.
(19) Yang, X.; Van Duren, J. K. J.; Rispens, M. T.; Hummelen, J. C.;
Janssen, R. A. J.; Michels, M. A. J.; Loos, J. Adv. Mater. 2004, 16, 802−
806.
(20) Kline, R. J.; McGehee, M. D. Polym. Rev. 2006, 46, 27−45.
(21) Bronstein, H.; Leem, D. S.; Hamilton, R.; Woebkenberg, P.;
King, S.; Zhang, W.; Ashraf, R. S.; Heeney, M.; Anthopoulos, T. D.; de
Mello, J.; McCulloch, I. Macromolecules 2011, 44, 6649−6652.
AUTHOR INFORMATION
Corresponding Author
*E-mail: laure.biniek@ics-cnrs.unistra.fr.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was in part carried out with financial support from
Solvay, BASF, SUPERGEN, EC FP7 Project X10D and EC
FP7 Project ONE-P, the Centre for Plastic Electronics at
Imperial College, Doctoral Training Centre EP/G037515/1,
the National Research Fund of Luxembourg and the Interna-
tional Collaborative Research Program of Gyeonggi-do, Korea.
We are grateful to Scott E. Watkins (CSIRO Materials Science
and Engineering, Victoria, Australia) for his contribution to the
polymer energy levels determination by UV-PESA measure-
ments and to Martin Brinkmann (Institut Charles Sadron,
Strasbourg, France) for his great help and constructive
discussions on TEM experiments.
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