Aryl amine substituted low energy gap carbazole polymers: Preparation and photovoltaic properties

Article abstract of DOI:10.1039/c0jm01335h

The preparation of a series of donor/acceptor copolymers comprising alternating 2,7-linked 3,6-dimethyl-9-arylamino-carbazole units and 5,7-bis-(thiophen-2-yl)-2,3-diphenyl-thieno[3,4-b]pyrazine units is presented. The triarylamine substituents are attach

Full text of DOI:10.1039/c0jm01335h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Aryl amine substituted low energy gap carbazole polymers: preparation and
photovoltaic properties†
David Mohamad,b Richard G. Johnson,a Dainius Janeliunas,a Mindaugas Kirkus,a Hunan Yi,a
b
David G. Lidzey and Ahmed Iraqi
a
*
*
Received 5th May 2010, Accepted 20th June 2010
DOI: 10.1039/c0jm01335h
The preparation of a series of donor/acceptor copolymers comprising alternating 2,7-linked 3,6-
dimethyl-9-arylamino-carbazole units and 5,7-bis-(thiophen-2-yl)-2,3-diphenyl-thieno[3,4-b]pyrazine
units is presented. The triarylamine substituents are attached to the carbazole repeat units either
through the para- or meta-positions. The effect of the linkage position of triaryl amine substituents in
this series of polymers as well as the effect of substitution of the phenyl groups at the 2,3-positions of
5,7-bis-(thiophen-2-yl)-2,3-diphenyl-thieno[3,4-b]pyrazine repeat units with alkoxy-substituents are
investigated in this work. The polymers were characterized by NMR spectroscopy, UV-Vis absorption
spectroscopy and fluorescence spectroscopy, and their molecular weights were estimated using gel
permeation chromatography. Polymers P1–P4 absorb light up to 900 nm and have energy gaps ranging
from 1.37 to 1.40 eV. Photovoltaic cells with ITO/PEDOT:PSS/P2:PC70BM(1/4, w/w)/Al showed an
open circuit voltage of 0.60 V under white light illumination, power conversion efficiency of 0.67% and
short circuit current of 3.6 mA cmꢀ2
.
ranging from 1.1 and 1.3 eV. 9 Photovoltaic devices with a power
conversion efficiency of 0.61% and short circuit current of 5.2
mA cmꢀ2 were obtained from one polymer in the series that had
an energy gap of 1.1 eV. Besides the magnitude of the energy gap
of the polymer donors, the design rules for the development of
efficient polymers for photovoltaic applications should also take
into account the relative positions of the HOMO and LUMO
energy levels with respect to those of the acceptor materials in
order to have efficient electron transfer as well as optimum open
Introduction
Research into the use of conjugated polymers for application in
bulk heterojunction solar cells has been the subject of much
interest in recent years in view of their potential technological
value for energy generation. Major advances have been achieved, 1
however, new polymer systems with high absorption coefficients
and extended absorption spectra are still being sought for use in
this area. The design of low energy-gap conjugated polymers can
be achieved by introduction of alternate electron donor and
10
circuit voltage in devices.
acceptor repeat units along the polymer chains which results in
2
intra-molecular charge transfer.
In this work, we report the preparation and characterization of
new classes of donor/acceptor alternating copolymers
comprising 2,7-linked carbazole repeat units and dithienyl thie-
nopyrazine alternate repeat units in which triarylamine substit-
uents are attached to the carbazole units either through para- or
meta-linkages. An investigation into the properties of a series of
carbazole-based copolymers is conducted in order to probe; (i)
the effects the linkage positions of aryl amine substituents to the
backbone of polymer chains, and (ii) the effect of attaching
electron-donating alkoxy substituents to acceptor repeat units
along the polymer chains. We also present an investigation into
the physical properties of the polymers including a study of their
performance in bulk heterojunction solar cells when blended
with two different fullerene electron-acceptors.
This approach proved
successful in the preparation of a range of narrow energy gap
3
polymers for application in bulk heterojunction solar cells
including low energy gap alternating fluorene copolymers with
4
2,1,3-benzothiadiazole units and lower energy gap fluorene
5
copolymers with thienopyrazine or thiadiazoloquinoxaline
6
alternate repeat units. Donor/acceptor systems comprising
7
8
alternating 2,7-dibenzosilole as well as 2,7-carbazole units
have also been described by Leclerc et al. and showed great
promise when used with PCBM in bulk heterojunction photo-
voltaic cells with good power conversion efficiencies. We have
reported recently the preparation of a series of donor/acceptor
alternating copolymers comprising dithienyl thiadiazoloqui-
noxaline and 2,7-linked carbazole repeat units with energy gaps
Results and discussion
aDepartment of Chemistry, University of Sheffield, Sheffield, S3 7HF, UK.
E-mail: a.iraqi@sheffield.ac.uk; Fax: +44 114 222 9303; Tel: +44 114 222
9566
bDepartment of Physics and Astronomy, University of Sheffield, Sheffield,
S3 7RH, UK. E-mail: d.g.lidzey@sheffield.ac.uk; Fax: +44 114 222 3555;
Tel: +44 (0)114 222 3501
In this study, a series of four donor acceptor conjugated poly-
mers comprising alternate 2,7-linked-3,6-dimethyl carbazole
repeat units bearing triaryl amine substituents at the 9-position
and dithienyl thienopyrazine alternate repeat units were
prepared. The use of triaryl amines as substituents in these
materials is aimed at obtaining materials with good hole mobil-
ities which is an important factor in solar cell applications. 11 The
† Electronic supplementary information (ESI) available: Device metrics
for annealed devices and for films from blends of P1–P4 with PC60BM
or PC70BM at different percentage ratios. See DOI: 10.1039/c0jm01335h
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different classes of donor/acceptor alternating copolymers
prepared consist of polymers where the triarylamine substituents
are either linked to the polymer backbone at the 9-position of
carbazole repeat units through the para-position (polymers P1
and P2) or through the meta-position (polymers P3 and P4).
Furthermore, introduction of electron-donating alkoxy substit-
uents on the acceptor repeat units in polymers P2 and P4 also
allows a comparison to be made with polymers P1 and P3 which
do not have such substituents.
Preparation of polymers P3 and P4 required the preparation
of the carbazole bis-boronic ester derivative 5 in which the tri-
arylamine substituent is meta-linked to the 9-position of the
carbazole ring. Intermediate 5 was prepared in four steps
according to Scheme 1, starting from 3,6-dimethyl-9H-carbazole
1 upon displacement of fluorine from 3-fluoro-nitrobenzene in
the presence of a base to afford 2 in good yields. Intermediate 2
was then reduced with tin(II) chloride to afford amino interme-
diate 3. An Ullmann-type condensation of 3 with 1-(2-butyl-
octyloxy)-4-iodobenzene in the presence of copper(I) iodide and
[1,10]phenanthroline, afforded 4 in good yield. Further reaction
of 4 with bis(pinacolato)-diboron in the presence of Pd(dppf)Cl2
and potassium acetate gave the bis-boronate ester derivative 5 in
reasonable yield.
Polymers P1–P4 were prepared using Suzuki cross-coupling
condensation polymerisation reactions according to Scheme 2
upon reaction of bis-boronate ester derivative 5 or the known
Scheme 2 (i) (a) Pd(II) acetate/P(p-tolyl)3 (1/2), NEt4OH, toluene/H2O.
9
5
bis-boronate ester derivative 6 with known dibromides 7 or
8.3 End-capping of the polymers with phenyl groups was then
undertaken upon addition of bromobenzene and then phe-
nylboronic acid at the end of the polymerisation reactions. All
polymers were obtained in good yields. Gel permeation chro-
matography results from the polymerisation reactions (using
polystyrene standards) have shown weight average molecular
weight values Mw ranging from 65 300 to 132 000 Da
(Table 1) with polydispersity index values ranging from 5.1 to
8.3. These polydispersity index values are rather large espe-
cially given the fact that all the materials were fractionated at
the work-up stage by Soxhlet extraction with acetone and
diethyl ether in order to remove oligomers and low molecular
weight chains.
(b) Bromobenzene. (c) Phenylboronic acid.
Solution and solid-state electronic and fluorescence spectra
Absorption spectra of polymers P1–P4 both in chloroform
solutions and as thin films are shown in Fig. 1. The polymers
have absorption maxima between 616 and 635 nm in solution
and between 638 and 661 nm in the solid state (Table 1). Poly-
mers where the triarylamine substituents are linked through the
meta-position to the polymer backbone (P3 and P4) display
a slight red shift in their absorbance maxima both in solution and
in the solid state when compared to the corresponding polymers
where the triarylamine substituents are linked through the para-
position to the polymer backbone (P1 and P2 respectively). The
optical energy gaps of polymers P3 and P4 (Table 1) as deter-
mined from the onset of their absorption in the solid state are
also slightly lower than those of corresponding polymers P1 and
P2. This finding indicates that the triarylamine substituents exert
different stereoelectronic effects on the polymer backbones when
they are linked through the meta-position or through the para-
position. A comparison of the absorption spectra of polymers P1
and P2 which only differ by the presence of electron-donating
alkoxy substituents on the acceptor repeat units on P2 show
similar absorption maxima both in solution and in the solid state.
They also show the same optical energy gap indicating little effect
resulting from substitution of acceptor repeat units with alkoxy
substituents. No major difference is shown between the optical
energy gaps of polymers P3 and P4 which have meta-linked
arylamine substituents and in which alkoxy substitution of
acceptor repeat units has no major bearing on their optical band
gaps.
Scheme 1 (i) 3-Fluoro-nitrobenzene, K2CO3, DMF, 85%, (ii) (a)
SnCl2$2H2O, EtOH, (b) NaOH, 84%, (iii) 1-(2-butyl- octyloxy)-4-iodo-
benzene Cu(I)I, [1,10]phenanthroline DMF, 84%, (iv) bis(pinacolato)di-
boron, KOAc, Pd(dppf)Cl2, DMF, 68%.
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Table 1 GPC data, UV-vis-near IR, fluorescence data and energy gaps of polymers P1–P4
Absorption
Yield
Emission
lmax/nm
(CHCl3)
lmax/nm
(film)
Energy gapa
Eg (op)/eV
Polymer
Mn
Mw
PDI
(%)
lmax/nm (CHCl3)
lmax/nm (film)
P1
P2
P3
P4
15 800
12 700
13 100
13 400
132 000
65 300
85 700
80 200
8.3
5.1
6.5
6.0
84
88
80
81
620
616
624
635
642
638
658
661
795
774
789
783
934
946
910
999
1.40
1.40
1.39
1.37
a
Eg (op), optical band gap determined from the onset position of the absorption band.
In our previous studies on 2,7-linked carbazole main chain
13
polymers with dialkylamino-phenyl N-functional groups
14
or
with triarylamino N-functional groups
linked through the
para-position to the polymer backbones, we have found that
while the solvent polarity did not affect their electronic spectra, it
did have a marked effect on their fluorescence spectra. Major
differences were observed in their spectral features in solvents
like toluene and dichloromethane. For instance, the 2,7-linked
carbazole main chain polymer with 4-dialkylamino-phenyl
N-functional groups displayed one main emission band at 417
nm with a shoulder at 441 nm in toluene, while it displayed
a weak emission band at 417 nm and a more prominent broad
emission band at 486 nm in dichloromethane. A possible expla-
nation for these observations was attributed to intramolecular
15
charge-transfer states
associating the strongly electron
donating 4-dialkylamino phenyl substituents and the carbazole
repeat units on the polymer chains. In other studies, Kijima et al.
16
have also found marked differences in the properties of 2,7-
linked carbazole polymers with triarylamino substituents linked
to the polymer chain either through the meta- or para-positions.
Photoluminescence studies were conducted on polymers P1–
P4 in solution in both toluene and chloroform as well as in thin
films to investigate the effect of the linkage position of arylamine
substituents on their physical properties (Fig. 1). For all poly-
mers, there was no clear difference as to the values of their
emission maxima in solution in toluene or in chloroform.
Emission maxima varied from 774 to 795 nm in chloroform
solutions and 910 and 999 nm in films (Table 1) with Stokes shifts
ranging from 148 to 175 nm in solution and 252 to 338 nm in
films indicating the large structural differences of the polymers in
their ground and excited states.
Fig. 1 Absorption spectra in chloroform solutions (solid lines), films
(dashed lines) and photoluminescence spectra in chloroform solutions
(dash–dot lines) and films (dash–dot–dot lines) of (a) P1, (b) P2, (c) P3
and (d) P4.
Electrochemical studies
The absorption maxima of polymers P1–P4 are similar to
those of a donor–acceptor alternating copolymer comprising
2,7-linked 9-alkyl carbazole units and bis-(3-octylthien-2-yl)-
Cyclic voltammetry measurements on drop-cast polymer films of
P1–P4 were conducted in acetonitrile with tetrabutylammonium
perchlorate as an electrolyte. The cyclic voltammograms of
polymers P1 and P3 are shown in Fig. 2a and those of P2 and P4
in Fig. 2b. The redox potentials of the various polymers as well as
their respective HOMO and LUMO levels (vs. vacuum) are
shown in Table 2. All polymers show reversible oxidation and
reduction waves. A comparison of the redox behaviour of
polymers P1 and P3 indicates that the oxidation potential of
polymer P1 which has arylamine substituents linked through the
para-position to the polymer backbone (+ 0.53 V vs. Ag/Ag+), is
lower than that of polymer P3 where the aryl amine substituents
12
2,3-diphenyl-thieno[3,4-b]pyrazine which showed an absorp-
tion maximum of 675 nm. However, the absorption maxima
of polymers P1–P4 are all red-shifted in comparison to those
of the alternating poly[9,90-dioctylfluorene-2,7-diyl-alt-5,7-di
5
(thien-2-yl)-2,3-diphenyl-thieno[3,4-b]pyrazine] APFO-Green2
(615 nm). Their band gap is also lower than that of APFO-
Green2 (1.5 eV) reflecting a higher electron-donating character of
alternate carbazole repeat units in P1–P4 in comparison to
alternate fluorene repeat units in APFO-Green2.
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the same LUMO level of ꢀ3.6 eV. The electrochemical band gaps
(Eg (ec)) of the polymers were all in good agreement with their
optical band gaps (Eg (op)) as shown in Table 1.
Photovoltaic device characterization
Various preparation and thermal annealing protocols were
explored in an attempt to find the processing conditions neces-
sary to fabricate optimized PV devices with active layers con-
sisting of polymer/fullerene blends. Device characterization
results are presented in Table 3. In an attempt to optimize effi-
ciency, active layers were thermally annealed after cathode
deposition at 110 ꢁC, 130 ꢁC and 150 ꢁC for set periods of 30 min
to encourage self-organization of donor–acceptor domains. The
fullerene derivative PC70BM was also used (at a relative
concentrations of 50, 66 and 80% w/w) to enhance light
absorption and improve charge transport It was found that
thermal annealing at 150 ꢁC and the use of PC70BM at a weight
concentration of 80% lead to optimal device performance.
EQE and J–V data from the most efficient device that was
prepared from polymer P2 are presented in Fig. 3(a) and (b),
respectively. A significant rectification is obtained in the J–V
curve recorded in the dark. However under illumination, the J–V
curve is symmetric around VOC, hence resulting in a relatively
low device low fill factor of 31%, a value that could not be
improved by annealing at higher temperatures. In fact, thermal
annealing at 180 ꢁC resulted in no improvement over an
untreated device owing to PCBM crystallization. Such low fill
factors are indicative of a large series resistance that most likely
adversely affects the charge transport and extraction thus
reducing short circuit current (JSC) and power conversion effi-
Fig. 2 Cyclic voltammetry curves of thin film of (a) P1(solid line), P3
(dashed line) and (b) P2 (solid line), P4 (dashed line) on platinum disc
electrodes (area 0.031 cm2) at a scan rate of 100 mV sꢀ1 in acetonitrile/
tetrabutyl ammonium perchlorate (0.1 mol dmꢀ3).
Table 2 Redox potentials and energy levels of polymers P1–P4
Polymer E0ox/V E0red/V HOMOa/eV LUMOa/eV Eg (ec)b/eV
P1
P2
P3
P4
0.53
0.57
0.59
0.64
ꢀ1.42
ꢀ1.44
ꢀ1.41
ꢀ1.42
ꢀ5.10
ꢀ5.10
ꢀ5.10
ꢀ5.00
ꢀ3.60
ꢀ3.60
ꢀ3.60
ꢀ3.60
1.50
1.50
1.50
1.40
1,17
ciency (PCE).
The efficiency of this series of polymers when fabricated into
a
HOMO and LUMO positions (vs. vacuum) determined from onset of
oxidation and reduction, respectively. Eg (ec), electrochemical band
b
devices compares well to those determined in other studies on
8,9,18
materials with similar energy gaps
particularly the analo-
gap determined from the difference of the onset of oxidation and
reduction of the polymers.
gous APFO-Green2:PC60BM blend which had JSC ¼ 1.6 mA
cmꢀ2 under 1 Sun AM1.5G simulated solar irradiation.5 Through
careful optimization of processing parameters, we determine
optimized device metrics of PCE ¼ 0.67%, JSC ¼ 3.6 mA cmꢀ2
,
are linked to the polymer backbone through the meta-position
(+ 0.59 V vs. Ag/Ag+).
VOC ¼ 0.60 V and FF ¼ 31% measured from a 1 : 4 P2/PC70BM
device processed from chlorobenzene. By contrast, the perfor-
mance metrics of a 1 : 4 P2/PC60BM device (fabricated for the
sake of comparison and again characterized under 1 Sun
The same finding is observed in respect to polymers P2 and P4
where the oxidation potential of polymer P2 is also lower than
that of polymer P4 (0.57 V and 0.64 V vs. Ag/Ag+, respectively).
Single oxidation waves are observed for both the arylamine
substituents and the polymer backbone for all this series of
polymers. The fact that the polymers with arylamine substituents
that are linked through the para-position to polymer backbones
(P1 and P3) have lower oxidation potentials than the corre-
sponding polymers with meta-linked aryl amine substituents (P2
and P4) could possibly arise as a result of the electronic delo-
calization between the arylamine substituents and the polymer
backbone when the arylamine substituents are linked through
the para-position. The potentials of the reduction waves of all
polymers are of comparable values (ꢀ1.41 to ꢀ1.44 V vs. Ag/
Ag+). The HOMO and LUMO levels (vs. vacuum) of polymers
P1–P4 were respectively estimated from the onset of their
oxidation and reduction waves from cyclic voltammetry experi-
ments. The HOMO levels varied from ꢀ5.00 eV for polymer P4
to ꢀ5.10 eV for polymers P1, P2 and P3 while all polymers had
AM1.5G illumination) were: PCE ¼ 0.34%, JSC ¼ 1.8 mA cmꢀ2
,
VOC ¼ 0.57 V and FF ¼ 32%. It is clear that this improved
Table 3 P1–P4 OPV device performance metrics as measured under 1
Sun simulated AM1.5G illumination
PCE
(%)
FF
(%)
Thickness/
ꢂ 5 nm
Material
JSC/mA cmꢀ2
VOC/V
P1
P2a
P3
P4
0.58
0.67
0.38
0.65
3.4
3.6
2.7
3.5
0.59
0.60
0.50
0.65
29
31
28
29
55
70
67
63
a
By contrast, the performance metrics of a 1 : 4 P2/PC60BM device
which was processed from chlorobenzene for comparison measured
under equivalent conditions are: PCE ¼ 0.34%, JSC ¼ 1.8 mA cmꢀ2
,
VOC ¼ 0.57 V and FF ¼ 32%.
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chloroform-d1 solutions with TMS as the internal standard. GPC
curves were recorded on the equipment consisting of a Waters
Model 515 HPLC Pump, GILSON Model 234 Autoinjector,
MILLIPORE Waters Lambda-Max Model 481 LC Spectrom-
eter, Erma ERC-7512 RI Detector, PLgel 5m 500A Column, and
PLgel 10m MIXED-B Column using chloroform as the eluent at
a rate of 1 cm3 minꢀ1. Polymer solutions in chloroform (2.5 mg
cmꢀ3) were used as samples for GPC analysis. The GPC curves
were obtained by the RI-detection method, which was calibrated
with a series of polystyrene narrow standards (Polymer Labo-
ratories). Elemental analyses were carried out with a Perkin
Elmer 2400 CHN Elemental Analyzer for CHN analysis and by
€
a Schoniger oxygen flask combustion method for anion analysis.
UV-visible absorption spectra were measured using a Hitachi
U-2010 Double Beam UV/Visible Spectrophotometer or a Per-
kin-Elmer Lambda 45 UV/Vis Spectrometer. The absorbance of
polymers was measured in solution in chloroform at ambient
temperature using rectangular quartz cuvettes (light pathlength
¼ 10 mm). Samples of pristine polymer thin films for UV-visible
absorption spectra measurements were prepared by dip coating
quartz plates into 1 mg cmꢀ3 polymer solutions in chloroform
with measurements carried out at ambient temperature.
Samples of polymer thin films for photoluminescence (PL)
spectroscopy measurements were prepared by either spin-coating
or drop casting onto glass substrates from polymer solutions in
chlorobenzene (HPLC grade). PL spectroscopy measurements
were performed by imaging the emission from the sample into
a JY SPEX 1/4m monochromator using a silicon photodiode to
detect light from 2.2 eV to 1 eV and thereafter with an InSb
detector. Samples were optically excited using the 488 nm line of
an Ar+ laser that was mechanically chopped. Photodiode signals
were amplified by an EG&G current voltage amplifier and
a Stanford Research Systems SR830 Lockin-Amplifier. Solutions
were prepared from polymers dissolved in chloroform or toluene
(HPLC grade) decanted into glass cuvettes (light pathlength ¼
10 mm). Absorption spectra were obtained using JY Fluoromax 4
Fluorimeter. Absorption and PL measurements were carried out
at ambient temperature in air.
Fig. 3 Device data from P2. (a) EQE and absorption spectra from
a thermally annealed 1 : 4 P2/PC70BM device; part (b): J–V spectra of the
same device in the dark (dashed line) and under 1 Sun simulated AM1.5
illumination (solid line).
efficiency partly results from improved light absorption that
occurs as a result of using PC70BM instead of PC60BM. It is
apparent however that low fill factors (suggestive of serial
resistive losses) play a significant role in limiting the efficiency of
1,17
our devices
and effectively preclude the use of thicker active
layers. It is presently unclear whether the reduced fill factor and
PCE in our devices is contact or transport related. Clearly,
studies addressing charge extraction through the ITO/
PEDOT:PSS/polymer interface and electron and hole transport
through our polymer/fullerene thin films would help to resolve
this issue.
Cyclic voltammograms were recorded using a Princeton
Applied Research Model 263A Potentiostat/Galvanostat.
Measurements were carried out under argon at 25 ꢂ 2 ꢁC. 10 cm3
of a tetrabutylammonium perchlorate (TBAClO4) solution in
acetonitrile (0.1 mol dmꢀ3) was used as the electrolyte solution. A
three-electrode system was used consisting of an Ag/Ag+ refer-
ence electrode (silver wire in 0.01 mol dmꢀ3 silver nitrate solution
in the electrolyte solution), a platinum working electrode (2 mm
diameter smooth platinum disc, area ¼ 3.14 ꢃ 10ꢀ2 cm2), and
a platinum counter electrode (platinum wire). Polymer thin films
were formed by drop casting 1.0 mm3 of polymer solutions in
dichloromethane (HPLC grade) (1 mg cmꢀ3) onto the working
electrode, then dried in air. Ferrocene was employed as a refer-
ence redox system according to the recommendation of
IUPAC.20
Experimental section
Materials
19
2,7-Dibromo-3,6-dimethyl-9H-carbazole 1,
N,N-bis-(4-(2-
butyloctyloxy)phenyl)-4-(3,6-dimethyl-2,7-bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-9H-carbazol-9-yl)aniline 6, 9
5,7-bis-(5-bromo-thiophen-2-yl)-2,3-diphenyl-thieno[3,4-b]pyr-
azine 7 5 and 5,7-bis-(5-bromo-thiophen-2-yl)-2,3-bis-[4-(2-ethyl-
3
hexyloxy)-phenyl]-thieno[3,4-b]pyrazine 8 were prepared
according to literature procedures. Toluene was distilled over
sodium under an inert nitrogen atmosphere. N,N-Dime-
thylformamide (DMF) was dried and distilled over calcium
hydride before use. Acetonitrile (HPLC grade) was dried and
distilled over phosphorus pentoxide under an inert argon
Photovoltaic devices were fabricated using the basic architec-
ture ITO(50 nm)/PEDOT:PSS(30 nm)/Active Layer(60–80 nm)/
ꢀ
atmosphere, then stored over molecular sieves 3 A.
Al(100 nm) where the active layer was
a thin film of
Measurements
polymer blended with 80% PC70BM by weight. To fabricate
OPV devices, a thin film of poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) was first coated on the ITO patterned
NMR spectra were recorded on Bruker 250 MHz, AMX400
400 MHz or DRX500 500MHz NMR spectrometers at 22 ꢁC in
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substrates. The active layer solutions and thin films were
prepared in an inert N2 atmosphere maintained in a glove box.
Solutions were prepared at a concentration of 20 mg mlꢀ1 in
chlorobenzene then heated overnight at 60 ꢁC. To ensure that
each material was dissolved, they were passed through a 0.45 mm
PTFE syringe mounted filter. All films were spin cast in a glove
box at 2000 rpm prior to the thermal deposition of the cathode.
toluene to give 3 as ivory crystals (5.01 g, 84%), mp 198–200 ꢁC.
Mass (EI); (m/z): 442, 444, 446 (Mꢄ+). 1H NMR (CDCl3) dH/ppm:
2.53 (s, 6H); 5.10 (br s, 2H); 6.77 (d, 1H, J ¼ 7.6Hz); 6.82 (m,
1H); 6.86 (m, 1H); 7.35 (m, 1H); 7.59 (s, 2H); 8.12 (s, 2H). 13C
NMR (CDCl3), dC/ppm: 23.09; 112.88; 113.66; 114.49; 116.65;
121.33; 122.06; 122.65; 128.93; 130.87; 137.93; 140.32; 148.09. IR
(KBr): 3402.8; 3316.6; 3208.4; 2969.7; 2491.2; 2857.5; 1907.9;
1602.1; 1498.1; 1451.6; 1361.5; 1323.9; 1302.8; 1235.8; 1125.6;
1041.1; 995.9; 956.7; 942.0; 855.2; 839.1 cmꢀ1. Calcd. for
C20H16N2Br2: C, 54.08; H, 3.63; N, 6.31; Br, 35.97. Found: C,
54.27; H, 3.38; N, 6.12; Br, 35.88.
ꢁ
Thermal annealing at 150 C for 30 min was carried out at this
point. Devices were then encapsulated in nitrogen using a UV
curable epoxy that was used to stick a glass slide to the device
surface. The device active area was 0.045 cm2 as defined by the
overlap of ITO anode and the evaporated cathode.
Finished devices were characterized under 1 Sun (100 mW
cmꢀ2) simulated AM1.5G solar radiation delivered by a Newport
Class-A solar simulator whose output was first checked with an
NREL calibrated silicon photodiode. Performance metrics were
calculated from a J–V sweep taken with a Keithley 237 Source-
Measure Unit. For EQE measurements, light from a home-built
setup consisting of a tungsten lamp coupled with a Spectral
Instruments DK240 monochromator was focused onto the
devices with a 10ꢃ objective. The incident power was first
measured with a calibrated reference Newport photodiode before
the test photodiode was characterized.
Bis-(4-(2-butyloctyloxy)phenyl)-3-(2,7-dibromo-3,6-dimethyl-
9H-carbazol-9-yl)aniline 4. A mixture of 2,7-dibromo-3,6-
dimethyl-9-(3-amino-phenyl)-9H-carbazole
3 (2.69 g, 6.09
mmol), 1-(2-butyl-octyloxy)-4-iodobenzene (7.21 g, 18.58 mmol),
[1,10]phenanthroline (0.06 g, 0.33 mmol), copper(I) chloride
(0.03 g, 0.30 mmol) and potassium hydroxide (2.88 g, 51.33
mmol) in toluene (20 cm3) was refluxed under an inert atmo-
sphere for 48 h. The reaction mixture was then cooled to ambient
temperature and poured onto water (200 cm3). The resulting
mixture was extracted with dichloromethane (3 ꢃ 200 cm3) and
the combined organic phases were washed with water (200 cm3),
dried over MgSO4 and the solvent removed in vacuo to give the
crude product as an oil. The product was purified by chroma-
tography on silica gel using petroleum ether in a first stage, then
a more polar solvent mixture petroleum ether/ethyl acetate (10/
1). Product 4 was obtained as a light brown solid (4.82 g, 84%),
Preparation of monomers
All reactions were carried out under an inert nitrogen atmo-
sphere.
mp 94–98 C. Mass (EI); (m/z): 963, 965, 967 (Mꢄ+). H NMR
(CDCl3) dH/ppm: 0.89 (m, 12H); 1.30 (m, 32H); 1.75 (m, 2H);
2.54 (s, 6H); 3.80 (d, 4H, J ¼ 6.4 Hz); 6.87 (d, 4H, J ¼ 9.15 Hz);
6.95 (dd, 1H, J ¼ 6.41Hz; 2.14 Hz); 7.15 (d, 4H, J ¼ 8.54 Hz);
7.28 (m, 1H); 7.32 (m, 1H); 7.35 (m, 1H); 7.60 (s, 2H); 7.87 (s,
2H). 13C NMR (CDCl3) dC/ppm: 14.15; 22.72; 23.06; 23.10;
26.86; 29.1; 29.73; 31.09; 31.39; 31.90; 38.01; 71.06; 113.71;
113.99; 115.55; 116.96; 117.17; 118.12; 121.29; 122.12; 122.66;
124.56; 126.92; 128.97; 130.25; 139.97; 150.68; 156.18. IR (KBr):
2923.7; 2855.6; 1592.9; 1505.4; 1469.3; 1447.2; 1362.7; 1301.8;
1278.5; 1236.8; 1167.8; 1125.3; 1104.1; 1031.8; 996.7; 951.0;
866.5; 831.6 cmꢀ1. Calcd. for C56H72Br2N2O2: C, 69.70; H, 7.52;
N, 2.90; Br, 16.56. Found: C, 69.66; H, 7.52; N, 2.76; Br, 16.60.
ꢁ
1
2,7-Dibromo-3,6-dimethyl-9-(3-nitro-phenyl)-9H-carbazole 2.
A mixture of 2,7-dibromo-3,6-dimethyl-9H-carbazole 1 (7 g,
19.95 mmol), anhydrous K2CO3, (13.76 g, 0.10 mol), 3-fluo-
ronitrobenzene (10 g, 70.87 mmol) and DMF (95 cm3) was
heated at reflux for 24 h under an inert atmosphere. After cooling
to room temperature the solution was poured onto warm (40 ꢁC)
water (500 cm3). The dark green solid was collected by filtration
andꢁthe crude material was washed with several portions of warm
(40 C) water (3 ꢃ 300 cm3), then recrystallised from toluene to
ꢁ
obtain 2 as bright green crystals (7.98 g, 85%), mp 266–269 C.
Mass (EI); (m/z): 472, 474, 476 (Mꢄ+). 1H NMR (CDCl3) dH/ppm:
2.56 (s, 6H); 7.51 (s, 2H); 7.82 (m, 1H); 7.85 (m, 1H); 7.92 (s, 2H);
8.35 (m, 1H); 8.40 (m, 1H). 13C NMR (CDCl3) dC/ppm: 23.10;
113.03; 121.70; 121.8; 122.56; 122.62; 123.10; 130.31; 131.19;
132.73; 138.44; 139.75; 149.52. IR (KBr): 3038.8; 2919.6; 2856.3;
1605.3; 1527.8; 1450.1; 1343.1; 1304.0; 1234.9; 1081.8; 1041.7;
1000.4; 957.4; 869.6 cmꢀ1. Calcd. for C20H14N2Br2O2: C, 50.66;
H, 2.98; N, 5.91; Br, 33.70. Found: C, 50.85; H, 2.69; N, 5.76; Br,
33.41.
N,N-Bis-(4-(2-butyloctyloxy)phenyl)-3-(3,6-dimethyl-2,
7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carba-
zol-9-yl)aniline 5. A mixture of bis-(4-(2-butyloctyloxy)phenyl)-
3-(2,7-dibromo-3,6-dimethyl-9H-carbazol-9-yl)aniline 4 (3.41 g,
3.60 mmol), bis(pinacolato)diboron (3.41 g, 13.43 mmol), potas-
sium acetate (2.22 g, 22.62 mmol) and Pd(dppf)Cl2 (0.19 g, 0.26
mmol) in DMF (100 cm3) was heated to 100 C for 24 h. The
ꢁ
2,7-Dibromo-3,6-dimethyl-9-(3-amino-phenyl)-9H-carbazole 3.
A mixture of 2,7-dibromo-3,6-dimethyl-9-(3-nitro-phenyl)-9H-
carbazole 2 (6.39 g, 13.54 mmol), SnCl2$2H2O (18.22 g, 80.75
mmol), and ethanol (250 cm3) was heated at reflux for 24 h and
then cooled to room temperature. The solvent was removed in
vacuo and the resulting residue washed with a 10% (w/w) aqueous
solution of sodium hydroxide (300 cm3). The organic material
was extracted with diethyl ether (3 ꢃ 300 cm3) and the combined
organic phases dried over MgSO4, filtered and the solvent was
removed in vacuo. The crude product was recrystallised from
reaction mixture was then cooled to room temperature, poured
into water (100 cm3) and extracted with diethyl ether (3 ꢃ 100
cm3). The organic phases were combined, then washed with
water (2 ꢃ 100 cm3) and dried over MgSO4. The crude product
was purified by chromatography on silica gel using petroleum
ether/dichloromethane/pyridine (30/30/1) as eluent. Product 5
was separated as a white solid (2.60 g, 68% yield), mp 81–83 ꢁC.
1H NMR (CDCl3) dH/ppm: 0.82 (m, 12H); 1.21 (m, 32H); 1.31 (s,
24H); 1.66 (m, 2H); 2.63 (s, 6H); 3.71 (d, 4H, J ¼ 5.34 Hz); 6.85
(m, 2H); 6.81 (d, 4H, J ¼ 8.17 Hz); 7.00 (s, 1H) 7.12 (d, 4H,
This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 6990–6997 | 6995

View Article Online
J ¼ 8.79 Hz); 7.24 (m, 1H); 7.78 (br s, 4H). 13C NMR (CDCl3) dC/
ppm: 14.15; 22.47; 22.71; 23.09; 24.96; 26.86; 29.09; 29.73; 31.08;
31.39; 31.89; 38.03; 71.02; 83.36; 115.46; 117.02; 117.38; 117.86;
121.11; 124.96; 127.29; 129.95; 133.74; 135.26; 139.17; 139.9;
143.99; 149.98; 151.36; 156.11. IR (KBr): 2924.4; 2854.9; 1740;
1593; 1505.5; 1485.4; 1447.1; 1372.5; 1362.8; 1302.4; 1278.9;
70.57, 71.04, 111.58, 114.12, 115.37, 119.97, 121.68, 122.59,
124.00, 124.50, 127.08, 127.18, 127.48, 127.72, 131.40, 132.30,
134.85, 137.54, 140.13, 140.56, 145.13, 148.32, 152.44, 156.09,
160.31. Calcd. for C98H118N4O4S3: C, 77.84; H, 7.87; N, 3.70.
Found: C, 77.96; H, 7.89; N, 3.51.
1237; 1167.9; 1125.3; 1104.6; 1031.5; 997.2; 951.1; 831.6 cmꢀ1
.
Polymer P3. P3 was prepared following the same experimental
procedure as that adopted for the preparation of P1 and on the
same scale using N,N-bis-(4-(2-butyloctyloxy)phenyl)-3-(3,6-
dimethyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
9H-carbazol-9-yl)aniline 5 and 5,7-bis-(5-bromo-thiophen-2-yl)-
2,3-diphenyl-thieno [3,4-b]pyrazine 7 as the coupling partners.
It was obtained as a deep dark green solid (0.38 g, 80%).
Calcd. for C68H96B2N2O6: C, 77.11; H, 9.14; N, 2.64. Found: C,
77.21; H, 9.21; N, 2.54.
Preparation of the polymers
Polymer P1. Toluene (11 cm3) was added to mixture of pal-
ladium(II) acetate (13.86 mg, 0.0617 mmol) and tri-p-tolyl-
phosphine (31.84 mg, 0.105 mmol) and the vessel was heated to
50 ꢁC for 30 min. The solution was then cooled to room
temperature and transferred via a syringe to a separate flask
containing N,N-bis-(4-(2-butyloctyloxy)phenyl)-4-(3,6-dimethyl-
2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-car-
bazol-9-yl)aniline 6 (0.40 g, 0.3776 mmol) and 5,7-bis-(5-bromo-
thiophen-2-yl)-2,3-diphenyl-thieno[3,4-b]pyrazine 7 (230.53 mg,
0.3776 mmol) under an inert atmosphere. A deoxygenated tet-
raethylammonium hydroxide solution (2 cm3, 20% w/w in water,
2.61 mmol) was then added to the solution and the resulting
mixture heated at 95 ꢁC for 43 h. The mixture was then cooled to
room temperature and bromobenzene (0.11 cm3, 0.49 mmol) was
added under an inert atmosphere and the mixture heated at 95 ꢁC
for 2 h. The mixture was then cooled to room temperature and
phenylboronic acid (0.10 g, 0.82 mmol) was added under an inert
atmosphere and the mixture heated at 95 ꢁC for 4 h, then cooled
to room temperature. The reaction mixture was then poured
onto a mixture of methanol/water (10/1, 250 cm3). The resulting
precipitate was collected by filtration and purified by Soxhlet
extraction with acetone for 24 h, diethyl ether for 24 h then
toluene. The toluene fraction was concentrated to ca. 50 cm3 and
the polymer precipitated from methanol (250 cm3), filtered and
dried to afford P1 as a deep dark green solid (0.40 g, 84%). Mn ¼
1
Mn ¼ 13 100, Mw ¼ 85 700, PDI ¼ 6.5. H NMR (CDCl3) dH/
ppm: 0.70–0.95 (m, 12H), 1.10–1.50 (m, 30H), 1.60 (m, 2H), 2.70
(s, 6H), 3.55–3.80 (m, 4H), 6.60–7.80 (m, 30H), 8.00 (m, 2H). 13
C
NMR (CDCl3) dC/ppm: 13.09, 20.71, 21.67, 22.04, 25.89, 28.11,
28.69, 30.06, 30.37, 37.08, 69.76, 110.57, 114.3, 116.05, 121.0,
121.80, 123.79, 124.04, 126.40, 126.73, 127.89, 128.12, 129.03,
131.12, 133.47, 136.68, 138.00, 138.80, 139.26, 144.68, 149.41,
151.84, 155.06. Calcd. for C82H86N4O2S3: C, 78.43; H, 6.90; N,
4.46. Found: C, 75.42; H, 6.55; N, 4.06.
Polymer P4. P4 was prepared following the same experimental
procedure as that adopted for the preparation of P1 and on the
same scale using N,N-bis-(4-(2-butyloctyloxy)phenyl)-3-(3,6-
dimethyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
9H-carbazol-9-yl)aniline 5 and 5,7-bis-(5-bromo-thiophen-2-yl)-
2,3-bis-[4-(2-ethyl-hexyloxy)-phenyl]-thieno[3,4-b]pyrazine 8 as
the coupling partners. It was obtained as a deep dark green solid
(0.46 g, 81%). Mn ¼ 13 400, Mw ¼ 80 200, PDI ¼ 6.0. 1H NMR
(CDCl3) dH/ppm:, 0.70–1.00 (m, 24H). 1.10–1.50 (m, 48H), 1.55
(s, 4H), 2.75 (s, 6H), 3.60–4.05 (m, 8H), 6.60–7.45 (m, 22H),
7.50–7.70 (m, 4H), 8.05 (s, 2H). 13C NMR (CDCl3) dC/ppm:
10.11, 13.09, 20.7, 22.03, 22.81, 22.88, 25.88, 28.08, 28.68, 29.55,
30.06, 30.37, 30.85, 37.09, 38.36, 70.57, 71.04, 110.58, 113.14,
114.33, 115.96, 121.68, 120.92, 121.76, 123.04, 123.69, 126.06,
126.72, 130.45, 131.23, 133.76, 136.68, 136.89, 138.78, 139.03,
144.31, 149.4, 151.59, 155.06, 159.32. Calcd. for C98H118N4O4S3:
C, 77.84; H, 7.87; N, 3.70. Found: C, 76.77; H, 7.61; N, 3.47.
1
15 800, Mw ¼ 132 000, PDI ¼ 8.3. H NMR (CDCl3) dH/ppm:
0.75–0.95 (m, 12H), 1.10–1.50 (m, 30H), 1.65 (m, 2H), 2.70 (s,
6H), 3.55–3.80 (m, 4H), 6.65–7.80 (m, 30H), 8.00 (m, 2H). 13C
NMR (CDCl3) dC/ppm: 13.09, 20.57, 21.64, 22.03, 25.81, 28.06,
28.65, 30.05, 30.82, 36.99, 70.08, 110.49, 114.4, 119.04, 120.84,
121.61, 123.89, 126.19, 126.46, 126.63, 127.06, 128.09, 129.00,
131.17, 133.51, 136.55, 137.98, 138.18, 139.61, 144.58, 147.30,
151.76, 155.10. Calcd. for C82H86N4O2S3: C, 78.43; H, 6.90; N,
4.46. Found: C, 77.22; H, 6.60; N, 4.13.
Conclusions
A new class of donor/acceptor conjugated polymers comprising
alternating 2,7-linked carbazole repeat units and 5,7-bis-(thio-
phen-2-yl)-2,3-diphenyl-thieno[3,4-b]pyrazine units have been
successfully prepared using Suzuki cross-coupling condensation
polymerisation reactions. Polymers P1–P4 were obtained in
good yields with weight average molecular weight values Mw
ranging from 65 300 to 132 000 Da. Polymers absorb light up to
900 nm and have energy gaps ranging from 1.37 to 1.40 eV.
Polymers where the triarylamine substituents are linked through
the meta-position to the polymer backbone (P3 and P4) display
a slight red shift in their absorbance maxima both in solution and
in the solid state when compared to the corresponding polymers
where the triarylamine substituents are linked through the para-
position to the polymer backbone (P1 and P2, respectively)
indicating that the triarylamine substituents exert different ster-
eoelectronic effects on the polymer backbones when they are
Polymer P2. P2 was prepared following the same experimental
procedure as that adopted for the preparation of P1 and on the
same scale using N,N-bis-(4-(2-butyloctyloxy)phenyl)-4-(3,6-
dimethyl-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
9H-carbazol-9-yl)aniline 6 and 5,7-bis-(5-bromo-thiophen-2-yl)-
2,3-bis-[4-(2-ethyl-hexyloxy)-phenyl]-thieno[3,4-b]pyrazine 8 as
the coupling partners. It was obtained as a deep dark green solid
(0.50 g, 88%). Mn ¼ 12 700, Mw ¼ 65 300, PDI ¼ 5.1. 1H NMR
(CDCl3) dH/ppm: 0.85–1.05 (m, 24H), 1.15–1.50 (m, 48H), 1.65
(s, 4H), 2.70 (s, 6H), 3.65–3.98 (m, 8H), 6.71–7.75 (m, 26H), 8.05
(s, 2H). 13C NMR (CDCl3) dC/ppm: 11.1, 14.0, 21.5, 22.61, 23.00,
23.85, 26.78, 29.03, 29.62, 30.45, 30.52, 30.87, 31.8, 39.40, 37.96,
6996 | J. Mater. Chem., 2010, 20, 6990–6997
This journal is ª The Royal Society of Chemistry 2010

View Article Online
3 A. P. Zoombelt, M. Fonrodona, M. G. R. Turbiez, M. M. Wienk and
R. A. J. Janssen, J. Mater. Chem., 2009, 19, 5336; M. M. Wienk,
M. P. Struijk and R. A. J. Janssen, Chem. Phys. Lett., 2006, 422,
linked through the meta-position or through the para-position.
Attachment of alkoxy substituents to the phenyl groups of
acceptor repeat units on these polymers does not affect their
absorption maxima and has no marked effect on the electronic
delocalisation along their backbones. In contrast to previous
studies on wide band gap carbazole-based polymers with triar-
ylamine substituents, the solvent polarity has little effect on the
photoluminescence of this series of low band gap polymers and
both polymers with meta and para-linked aryl amine substituents
display similar emission maxima in toluene and chloroform
solutions. The Stokes shifts in this class of polymers ranged from
252 to 338 nm in films indicating large structural differences of
the polymers in their ground and excited states.
€
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4 M. Svensson, F. Zhang, S. C. Veenstra, W. J. H. Verhees,
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J. C. Hummelen, J. M. Kroon, O. Inganas and M. R. Andersson,
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5 F. Zhang, E. Perzon, X. Wang, W. Mammo, M. R. Andersson and
€
O. Inganas, Adv. Funct. Mater., 2005, 15, 745; N.-K. Persson,
M. Sun, P. Kjellberg, T. Pullerits and O. Inganas, J. Chem. Phys.,
€
Cyclic voltammetry measurements indicate that the oxidation
potential of polymers which have arylamine substituents linked
through the para-position to the polymer backbone (P1 and P2)
are lower than those of polymers where the arylamine substitu-
ents are linked to the polymer backbone through the meta-
position (P3 and P4) possibly as a result of the electronic
delocalization between the arylamine substituents and the
polymer backbone when the arylamine substituents are linked
through the para-position.
2005, 123, 204718; E. Perzon, X. Wang, F. Zhang, W. Mammo,
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J. L. Delgado, P. de la Cruz, O. Inganas, F. Langa and
M. R. Andersson, Synth. Met., 2005, 154, 53; S. Janietz,
€
H. Krueger, H.-F. Schleiermacher, U. Wurfel and M. Niggemann,
Macromol. Chem. Phys., 2009, 210, 1493.
6 X. Wang, E. Perzon, J. L. Delgado, P. de la Cruz, F. Zhang, F. Langa,
€
M. Andersson and O. Inganas, Appl. Phys. Lett., 2004, 85, 5081;
E. Perzon, X. Wang, S. Admassie, O. Inganas and
€
M. R. Andersson, Polymer, 2006, 47, 4261.
7 P.-L. T. Boudreault, A. Michaud and M. Leclerc, Macromol. Rapid
Commun., 2007, 28, 2176.
8 N. Blouin, A. Michaud and M. Leclerc, Adv. Mater., 2007, 19, 2295;
N. Blouin, A. Michaud, D. Gendron, S. Wakim, E. Blair, R. Neagu-
^
Plesu, M. Belletete, G. Durocher, Y. Tao and M. Leclerc, J. Am.
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9 H. Yi, R. G. Johnson, A. Iraqi, D. Mohamad, R. Royce and
D. G. Lidzey, Macromol. Rapid Commun., 2008, 29, 1804.
Studies on photovoltaic cells using 1 : 4 (weight : weight)
blends of the polymers and PC60BM as active layers on films cast
from chlorobenzene afforded devices with power conversion
efficiencies ranging from 0.26 and 0.34%. An optimized device
was obtained for a 1 : 4 P2/PC70BM device processed from
€
10 M. C. Scharber, D. Muhlbacher, M. Koppe, P. Denk, C. Waldauf,
chlorobenzene having a PCE ¼ 0.67%, JSC ¼ 3.6 mA cmꢀ2
,
A. J. Heeger and C. J. Brabec, Adv. Mater., 2006, 18, 789.
11 R. Pacios, J. Nelson, D. D. C. Bradley and C. J. Brabec, Appl. Phys.
Lett., 2003, 83, 4764; C. Melzer, E. J. Koop, V. D. Mihailetchi and
P. W. M. Blom, Adv. Funct. Mater., 2004, 14, 865.
VOC ¼ 0.60 V and FF ¼ 31% under 1 Sun AM1.5G simulated
solar irradiation.
ꢁ
12 S. Beaupre, A.-C. Breton, J. Dumas and M. Leclerc, Chem. Mater.,
2009, 21, 1504.
13 A. Iraqi, T. G. Simmance, H. Yi, M. Stevenson and D. G. Lidzey,
Chem. Mater., 2006, 18, 5789.
14 H. Yi, A. Iraqi, M. Stevenson, C. J. Elliott and D. G. Lidzey,
Macromol. Rapid Commun., 2007, 28, 1155.
Acknowledgements
We would like to acknowledge EPSRC for financial support of
this work (Grant number EP/F056370/1).
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This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 6990–6997 | 6997