Fluorination in thieno[3,4-: C] pyrrole-4,6-dione copolymers leading to electron transport, high crystallinity and end-on alignment

Article abstract of DOI:10.1039/c7tc01999h

A series of copolymers based on thieno[3,4-c]pyrrole-4,6-dione and thiophene-phenyl-thiophene with varying degrees of fluorination on the phenyl unit was synthesized by Stille polycondensation. The influence of the degree of fluorination on the optical, thermal and electrochemical properties of these polymers is systematically studied. Additionally, the charge transport in organic field effect transistors (OFETs) as well as the thin film alignment are investigated. After thermal annealing the non-fluorinated as well as difluorinated polymers show ambipolar charge transport in OFETs. In contrast, tetrafluorination results in exclusively n-type behaviour with an electron mobility of 3.7 × 10^-4 cm² V^-1 s^-1. GIWAXS measurements of as-cast and annealed films reveal a very interesting structural alignment in thin films. All polymers show the rarely observed "end-on" orientation, where the polymer chains stand on the substrate. This may be explained by the low molecular weights of these polymers in relation to the film thickness. Fluorination does not influence the microstructural properties, only the propensity of crystallization is increased.

Full text of DOI:10.1039/c7tc01999h

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Fluorination in Thieno[3,4-c]pyrrole-4,6-idone Copolymers leading
to Electron Transport, high Crystallinity and End-on Alignment
T. Weller,a M. Breuniga, C. J. Muellera, E. Gannc, C. R. McNeillb and M. Thelakkata*
Received 00th January 20xx,
Accepted 00th January 20xx
A series of copolymers based on thieno[3,4-c]pyrrole-4,6-dione and thiophene-phenyl-thiophene with a varying degree of
fluorination on the phenyl unit was synthesized by Stille polycondensation. The influence of the degree of fluorination on
the optical, thermal and electrochemical properties of these polymers is systematically studied. Additionally, the charge
transport in organic field effect transistors (OFETs) as well as the thin film alignment are investigated. After thermal
annealing the non-fluorinated as well as difluorinated polymers show ambipolar charge transport in OFETs. In contrast,
tetrafluorination results in exclusively n-type behaviour with an electron mobility of 3.7 x 10-4 cm2 V-1 s-1. GIWAXS
measurements of as-cast and annealed films reveal a very interesting structural alignment in thin films. All polymers show
the rarely observed “end-on” orientation, where the polymer chains stand on the substrate. This may be explained by the
low molecular weights of these polymers in relation to the film thickness. Fluorination does not influence the
microstructural properties, only the propensity of crystallization is increased.
DOI: 10.1039/x0xx00000x
www.rsc.org/
combination with fullerenes in organic solar cells.11 Also, some
of these materials exhibit very high hole mobilites of up to 1.3
cm2 V-1 s-1 in OFET devices.12 Regarding electron transport, only
Introduction
In the past, research in the field of polymeric semiconductors
has mainly focused on hole-conducting materials synthesized
according to the alternating donor-acceptor concept. Recently,
there is more interest in electron-conducting materials, also as
alternative for fullerene derivatives for applications. In
particular naphthalene diimide (NDI) containing copolymers
have attracted appreciable attention and have been
successfully applied in all-polymer solar cells with efficiencies
exceeding 8%.1 Moreover, it has been repeatedly shown that
in all-polymer solar cells, the alignment of polymer chains with
their planar π-faces of their backbone, either in an edge-on or
face-on manner, considerably affects the device
performance.2,3
a few examples have been published to date to the best of our
knowledge.13–16 The highest mobility for unipolar n-type
transport was reported for copolymers consisting of TPD and
isoindigo resulting in a field-effect mobility of 0.01 cm2 V-1 s-1.14
Marks et al. showed ambipolar charge transport in TPD
containing copolymers with thiophene as comonomer, where
the n-type field effect mobility even exceeds the
corresponding hole mobility.17 Very recently, TPD containing
copolymers were applied as acceptor polymers in
polymer/polymer blend solar cells with appreciable
performance.18,19
As comonomer, we selected thiophene-phenyl-thiophene
(TPT), which is a rigid building block leading to highly
crystalline materials.20,21 Intrachain nonbonding interactions
such as H-S or H-F can improve the planarity and therefore, the
crystallinity and stacking behaviour. This is the case for di- and
tetrasubstitution of the phenyl protons with small and strongly
electronegative fluorine atoms. Crystal structures of the
difluorinated small molecules gave evidence that the structure
is locked by H-F interactions20 contrary to some reports where
S-F interactions were claimed.22 Especially Ratner et al.
showed that the contribution of S-F interactions is nearly zero
as determined by theoretical calculations.23 Beside the
influence of fluorination on the crystalline structure, it is also a
powerful tool to improve electron affinity resulting in higher
electron transport and in some cases ambipolar charge
transport. In the literature, several reports have been
Thieno[3,4-c]pyrrole-4,6-dione (TPD) is an interesting electron-
deficient building block unit due to its high planarity and easy
synthetic
accessibility.4,5
Additionally,
copolymers
incorporating this acceptor unit show high long-term thermal
stability, which is also an important and desirable property for
application in optoelectronic devices.6 In general, the TPD
moiety has been incorporated as an electron-withdrawing unit
in classical low bandgap copolymers to obtain donor
materials7–10, which achieved efficiencies of over 9% in
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Journal Name
published, which show this concept, mainly based on either in chloroform solutions (0.01 mg/mL) using quartz
diketopyrrolopyrrole (DPP)24–28 and naphthalene diimide cuvettes with an internal diameter of 10 mm or in thin films
(NDI)29,30 copolymers.
spin coated on glass slides from chloroform solutions
In this contribution, we report a series of TPD containing (5 mg/mL) at 1500 rpm. For thermogravimetric analysis (TGA)
copolymers with different degrees of fluorination of the TPT a Netzsch STA 449 F3 Jupiter was used at a scanning rate of
unit. The synthesis and detailed structural characterization of 10 K min-1 under nitrogen atmosphere. The temperature of
the copolymers containing TPD and different TPT units are decomposition, T5% was determined at 5% mass loss. Flash DSC
described. To guarantee sufficient solubility, the TPD unit was measurements were conducted on a Mettler-Toledo Flash DSC
decorated with a branched octyldodecyl substituent. The 1 at scanning rates between 50 and 1000 K min-1 under
comonomer TPT was di- and tetrafluorinated (TPF2T and nitrogen. The organic thin film transistor substrates with a
TPF4T) in order to gradually increase the degree of bottom gate, bottom contact architecture were purchased
fluorination. The systematic study of the influence of form Fraunhofer IPMS (OFET Gen. 4). As substrate and gate
fluorination on the properties regarding structure formation electrode a heavily n-doped silicon wafer (doping at wafer
and OFET mobilities are given here. Especially, we were surface: n ~ 3 x 1017 cm-3) was used. The gate electrode is
interested to elucidate the influence of fluorination on separated from the source and drain electrodes by thermal
crystallinity and consequently on charge transport. Very silicon dioxide (Standard 90/230 nm) as dielectric. Source and
surprisingly, the GIWAXS studies show end-on alignment of drain electrodes are made from gold with a thickness of 30 nm
polymer chains, which is usually rare in the case of conjugated adhered to the dielectric by a layer of 10 nm ITO. One chip
polymers without any reactive grafting of the chain to a carries four groups with four identical transistors with a
substrate.
channel length of 2.5, 5, 10 and 20 μm, respectively, and a
channel width of 10 mm. The substrates were rinsed with
acetone and purified by ultasonification in acetone and 2-
propanol for 10 min each. After plasma treatment for 15 min
at 50 °C, silanisation with octadecyltrichlorosilane in toluene (1
vol%) at 60 °C for 1 h was conducted. The devices were rinsed
with toluene and stored in 2-propanol until spin coating. Films
were spin coated from chloroform solutions (10 mg/mL) at
5000 rpm under ambient conditions and directly transferred
into the glove box. Current-voltage characteristics were
measured under nitrogen atmosphere using an Agilent B1500
Semiconductor Parameter Analyzer. Annealing was performed
at 250 °C for 15 min under nitrogen atmosphere. The charge
carrier mobility was calculated using equation (1), where IDS is
the drain-source current, VG the gate voltage, L the channel
length, W the channel width and Ci the capacitance. Therefore,
Experimental Part
Materials and Methods
All commercially available reagents were used as received
unless otherwise noted. Reactions under microwave
irradiation were conducted in a Biotage Initiator Eight+
Microwave. Size exclusion chromatography (SEC) was carried
out on an Agilent (Polymer Laboratories Ltd.) PL-SEC 220 high
temperature chromatographic unit equipped with three linear
mixed bed columns (PSS Polefin linear XL) and DP and RI
detectors. Analysis was performed at 160 °C using 1,2,4-
trichlorobenzene as the eluent. The samples were prepared by
dissolving the polymer (0.1 wt-%) in the solvent in an external
oven and the solutions were injected and eluted without
filtration. The molecular weights of the samples were
referenced to polystyrene standards (Mw = 518-2,600,000 g
mol-1, K = 12.100 and Alpha = 0.707). Cyclic voltammetric
measurements were carried out in thin films under moisture-
and oxygen-free conditions using a three-electrode assembly
connected to a potentiostat (model 263A, EG&G Princeton
Applied Research) at a scanning rate of 100 m V s-1. As
1/2
the square root of the drain-source current IDS was plotted
against the gate voltage VG in the saturation regime and the
slope of the resulting curve was determined to calculate the
mobility.
2
ꢅ
ꢇ
2ꢍ
ꢆ ꢈꢉ
ꢀꢁꢂꢃ
=
ꢄ
ꢌ
∙
(1)
ꢅꢊꢋ
ꢎꢏꢐ
Synthesis procedures
General procedure for the Stille polycondensation
microwave vial was loaded with the monomers 1,3-dibromo-5-
(2-octyldodecyl)-4H-thieno[3,4-c]pyrrol-4,6(5H)-dione (1 eq)
and the different TPTs (1 eq) and tri(o-tolyl)phosphine (0.12
electrolyte
solution
tetra-n-butylammonium
hexafluorophosphate in acetonitrile with a concentration of
0.1 M was used. Glass substrates coated with ITO (10 Ω/□)
were used as working electrode, on which thin films from
CHCl3 solutions with a concentration of 5 mg/mL were spin
coated at 1500 rpm. A platinum wire in the respective
electrolyte solution and Ag/AgNO3 in acetonitrile (0.1 M) were
used as counter and reference electrode, respectively. Each
measurement was calibrated by the internal standard
ferrocene/ferrocenium. UV/Vis spectra were recorded on a
JASCO V-670 spectrophotometer. Fluorescence spectroscopy
was performed on a JASCO FP-8600 spectrofluorometer using
the wavelength maximum of transition for absorption as
excitation wavelength. The optical properties were analysed
:
A
1
2
eq) and chlorobenzene (77 mM) was added. After degassing by
argon bubbling for 10 min, tris(dibenzylideneacetone)di-
palladium(0) (0.03 eq) was added. The microwave vial was
sealed and purged with argon once more. The polymerization
reaction was conducted at 180 °C for 1 h under microwave
irradiation. After cooling to room temperature, 2-
tributyltinthiophene was added to end-cap the polymers and
the reaction mixture was stirred for 5 min at 180 °C. The end-
capping step was, subsequently, repeated with 2-
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bromothiophene for 10 min. The polymer solution was syntheses of the stannylated TPT comonomers have already
precipitated in methanol (300 mL) and filtered. Further been reported by our group.27 The dibrominated phenyl
purification was carried out by sequential Soxhlet extraction derivatives are coupled with thiophene in a Stille reaction
using methanol, acetone and hexane. Finally, the polymer was followed by bromination and stannylation. For the stannylated
collected by chloroform extraction and the solution was TPF4T monomer with tetrafluorinated phenyl unit, we chose an
concentrated under reduced pressure. Precipitation in alternative synthetic procedure. The tetrafluorinated phenyl
methanol, filtration and drying in vacuum yielded the desired group was flanked with thiophene by direct heteroarylation
polymer.
P(TPD-TPT): Following the general procedure, a solution of reported in the literature.32 With palladium acetate as catalyst
1,3-dibromo-5-(2-octyldodecyl)-4H-thieno[3,4-c]pyrrol- and potassium pivalate as base in dimethylacetamide, the
4,6(5H)-dione (183 mg, 0.309 mmol, 1 eq), 1,4-bis(5- precursor TPF4T was synthesized in 47% yield. Direct arylation
starting from the corresponding brominated compound as
1
(trimethyl-stannyl)thiophen-2-yl)benzene (176 mg, 0.309 is a powerful tool to combine aromatic units without the need
mmol, 1 eq) 2a, tri(o-tolyl)phosphine (11.3 mg, 0.037 mmol, of additional functionalization of one of the coupling
0.12 eq) and tris(dibenzylideneacetone)dipalladium(0) compounds with, for example, boronic acid ester or tin
(8.50 mg, 9.27 μmol, 0.03 eq) in 4 mL chlorobenzene was compounds. This compound was directly stannylated using n-
stirred for 1 h at 180 °C yielding P(TPD-TPT) (120 mg, 55%) as a butyl lithium and trimethyltin chloride without prior
black solid. 1H NMR (300 MHz, CDCl3): δ = 5.06-8.28 (8H), 2.90- bromination achieving the stannylated monomer in high purity
3.88 (2H), 0.31-2.16 (39H) ppm.
P(TPD-TPF2T): Following the general procedure, a solution of Stille polycondensation was applied to polymerize the TPD
1,3-dibromo-5-(2-octyldodecyl)-4H-thieno[3,4-c]pyrrol- monomer with the three different TPT monomers carrying
4,6(5H)-dione (183 mg, 0.309 mmol, 1 eq), (5,5’-(2,5- different numbers of fluorine atoms. The polymerization was
difluoro-1,4-phenylene)bis(thiophene-5,2-diyl))bis(trimethyl- conducted with tris(dibenzylideneacetone)dipalladium(0) as
following a literature described procedure.33
1
stannane) 2b (187 mg, 0.309 mmol, 1 eq), tri(o-tolyl)phosphine catalyst, tri(o-tolylphosphine) as ligand in chlorobenzene for
(11.3 mg, 0.037 mmol, 0.12 eq) and tris(dibenzylidene- one hour at 180 °C under microwave irradiation. After
acetone)dipalladium(0) (8.50 mg, 9.27 μmol, 0.03 eq) in 4 mL polymerization, the polymer chains were end-capped using
chlorobenzene was stirred for 1 h at 180 °C yielding P(TPD- thiophene and were purified by Soxhlet extraction. All the
TPF2T) (100 mg, 44%) as a black solid. 1H NMR (300 MHz, three polymers are well soluble in common organic solvents
CDCl3): δ = 5.06-8.63 (6H), 2.90-3.94 (2H), 0.50-2.37 (39H) such as chloroform and toluene. Size exclusion
ppm. 19F NMR (300 MHz, CDCl3): δ = -(114-120) ppm.
P(TPD-TPF4T):Following the general procedure, a solution of due to the strong aggregation of polymer chains at room
1,3-dibromo-5-(2-octyldodecyl)-4H-thieno[3,4-c]pyrrol-4,6 temperature, especially with increasing degree of fluorination
(5H)-dione (139 mg, 0.234 mmol, 1 eq), (5,5’-(perfluoro-1,4- (Fig. 1). The molecular weights (PS calibration) as well as the
chromatography was conducted in trichlorobenzene at 160 °C
1
phenylene)bis(thiophene-5,2-diyl))bis(trimethylstannane) 2c dispersities are summarized in Table 1. In general, the relative
(150 mg, 0.234 mmol, 1 eq), tri(o-tolyl)phosphine (8.57 mg, molecular weights obtained for semicrystalline polymers with
0.028 mmol, 0.12 eq) and tris(dibenzylideneacetone) PS calibration need to be cautiously interpreted in terms of the
dipalladium(0) (6.45 mg, 7.03 μmol, 0.03 eq) in 3 mL real molecular weights. However, a comparison among the
chlorobenzene was stirred for 1 h at 180 °C yielding P(TPD- three polymers provides some indications. With the degree of
TPF4T) (145 mg, 80%) as a black solid. 1H NMR (300 MHz, fluorination, the number-average molecular weights (Mn)
CDCl3): δ = 5.10-8.29 (4H), 2.93-3.96 (2H), 0.53-2.44 (39H) increase from 7.90 kg mol-1 for P(TPD-TPT) to 11.0 kg mol-1 for
ppm. 19F NMR (300 MHz, CDCl3): δ = -(135-142) ppm.
P(TPD-TPF2T) and to 13.6 kg mol-1 for P(TPD-TPF4T). These
relative molecular weights (from SEC) do not constitute very
high molecular weight. However, for a comparison of the
influence of structural changes on crystallinity, orientation and
charge transport, these are sufficient, since we have
eliminated the influence of end-groups by end-capping. The
dispersities for all three polymers are in the range of 1.1-1.3.
Even after taking into account the fact that the polymers were
purified by Soxhlet extraction, these dispersities are very low
for polycondensates. Additionally, the polymers were analysed
by
Results and discussion
Synthesis
The dibromo TPD monomer carrying octyldodecyl solubilizing
side chains was synthesized following procedures described in
the literature.31 These long, asymmetric branched side chains
were chosen to maintain solubility of the polymer due to the
less soluble TPT comonomer. Detailed synthesis procedures of
all monomers can be found in the Supporting Information. The
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Scheme 1
Synthesis of TPD containing polymers via Stille polymerization in a microwave reactor.
Table 1 SEC, absorption, electrochemical and thermal properties of the TPD containing polymers.
a
Mn
a
b
T5%
c
Tm
e
Mw
Ð
Tcc
absorbance peaks [nm]d
Eopt
IPf
EAf
[kg/mol]
7.85
[kg/mol]
8.87
[°C]
441
435
442
[°C]
400
417
420
[°C]
364
394
393
solution
452
film
475
460
453
[eV]
1.97
1.92
1.95
[eV]
[eV]
P(TPD-TPT)
P(TPD-TPF2T)
P(TPD-TPF4T)
1.13
1.14
1.28
-5.46
-5.75
-6.03
-3.56
-3.64
-3.90
12.5
12.5
435
13.6
17.6
427
a
b
c
Determined by SEC in 1,2,4-trichlorobenzene at 160 °C using PS calibration; decomposition at 5% mass loss determined by TGA at 10 K/min under Ne2; melt and
d
crystallization temperature from Flash DSC measurements at 1000 K/min under N2; maximum absorption peaks from CHCl3 solution and in thin film; optical gap
determined by absorption edge in thin films; f ionisation potential (IP) and electron affinity (EA) determined form cyclic voltammetry in thin films.
measurement difficult. The melting and crystallization
temperatures for the di- and tetra-fluorinated polymers are
very similar and in the range of 420 °C and 390 °C, respectively.
By fluorination, the melting as well as the crystallization
temperature is shifted to higher values due to the stronger
intermolecular interactions in fluorinated polymers. However,
the degree of fluorination does not influence the melting and
crystallization temperatures.
(a)
(b)
P(TPD-TPT)
P(TPD-TPT)
P(TPD-TPF2T)
P(TPD-TPF2T)
P(TPD-TPF4T)
P(TPD-TPF4T)
Optical properties
5 100 150 200 250 300 350 400 450
Temperature [°C]
103
104
Molecular Weight [g/mol]
10
In order to study the influence of fluorination on the optical
properties, UV/Vis as well as fluorescence measurements were
performed in solution and in the solid state. The spectra are
shown in Fig. 2 and the absorption peaks in solution and thin
films as well as the optical gap obtained from the absorption
edge of thin film spectra are summarized in Table 1. In
solution, all the three polymers feature a single broad
absorption band, which is slightly blue shifted upon
fluorination due to the electron-withdrawing properties of the
fluorine atoms. Besides this broad absorption band centered
at around 420-450 nm, a shoulder at around 550 nm is visible,
which arises from an aggregated character of chains in
solution. In the solid state, the absorption is red-shifted as
compared to their solution spectra due to aggregation; the
corresponding spectra can be found in the Supporting
Information (see Fig. S3, ESI†). Additionally, the shoulder of
aggregated species becomes more prominent. The optical gap
was calculated from the absorption edge in thin films resulting
in similar values of just below 2 eV for all polymers. In
conclusion, fluorination has almost no effect on the optical
gap. Excitation at the absorption maximum results in three
emission peaks in solution. The intensity of the
photoluminescence decreases possibly caused by the stronger
aggregation of the fluorine containing polymers in solution.
Fig. 1 (a) SEC measurements in trichlorobenzene at 160 °C calibrated with PS
standards. (b) Flash DSC traces with a scanning rate of 1000 K/min.
1H- and 19F-NMR and the spectra are depicted in the
Supporting Information. In the 19F-NMR spectra, the peaks
arising from the fluorine can be assigned. All the three
polymers show high thermal stability as shown in TGA
measurements. The decomposition temperatures at 5% mass
loss are in the range of 435 to 442 °C (Table 1). The
crystallization behaviour was investigated with Flash DSC
measurements, where the melting and crystallization near the
decomposition can be studied due to high scanning rates in
the range from 50 to 1000 K/min. In Fig. 1b, the region of
interest is shown for a scanning rate of 1000 K/min and the
corresponding melting and crystallization temperatures are
summarized in Table 1. The complete spectra with all scanning
rates are shown in the Supporting Information (see Fig. S2,
ESI†). The polymer P(TPD-TPT) shows a clear melting as well as
a recrystallization peak at 400 °C and 364 °C, respectively. For
the fluorinated polymers, melting and recrystallization are not
so pronounced as is the case for the non-fluorinated sample.
Due to the fluorination, the adhesion of the polymers to the
sample chip in Flash DCS is reduced making the exact thermal
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solution:
thin films:
1.0
0.8
0.6
0.4
0.2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
P(TPD-TPT)
P(TPD-TPT)
P(TPD-TPF2T)
P(TPD-TPF2T)
P(TPD-TPF4T)
P(TPD-TPF4T)
0.8
0.6
0.4
0.2
0.0
0.0
300
400
500
600
700
Wavelength [nm]
800
900
1000
300
400
500
600
700
800
900
1000
Wavelength[nm]
Fig. 2
(a)
Comparison of absorption and fluorescence measurements of the three TPD containing polymers in (a) CHCl3 solution and (b) thin films.
results in a lowering of the IP by 0.3 eV, whereas the EA is
almost not affected. By tetrafluorination the IP and EA are
further lowered by 0.3 eV, respectively. The lowering of energy
levels upon fluorination agrees with similar studies reported in
the literature for other low band-gap polymers.28,35,36 The
band-gap determined by cyclic voltammetry is increased by
fluorination and remains constant for di- and tetra-fluorinated
polymers while both energy levels are lowered. Cyclic
voltammetry measurements clearly show that the fluorination
has a strong impact on the energy levels, which are gradually
lowered upon fluorination.
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
-6.0
-6.5
(b)
P(TPD-TPT)
P(TPD-TPF2T)
P(TPD-TPF4T)
P(TPD-TPT) P(TPD-TPF2T)
P(TPD-TPF4T)
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Potential vs Fc/Fc+ [V]
OFET
Fig. 3 (a) Cyclic voltammetry measurements in thin films and (b) energy levels
calculated from the onset of oxidation and reduction potential.
The charge carrier mobilities of the polymers were measured
using OFETs with a bottom gate, bottom contact architecture.
Here, the charge transport occurs in the plane of the thin film
at the dielectric interface. Procedures for the device
preparation as well as the mobility determination can be found
in the experimental section. Measurements were performed
on both as-cast films and films annealed at 250 °C for 15 min
under nitrogen. In Table 2, the average values for hole and
Moreover, the relative intensities of the vibronic peaks change
drastically upon fluorination, which gives a measure for the quality
and nature of aggregates.34 In comparison, the photoluminescence
in thin films shift to red and show only the two main emission
peaks, whereas the third one becomes weak.
Electrochemical properties
electron mobilities of
a minimum of six devices are
As fluorination is known to have a significant effect on the
energy levels of conjugated polymers, the electrochemical
properties of thin films of the polymers were analysed by cyclic
voltammetry measurements. A three-electrode setup with ITO
as working electrode in acetonitrile was used. The cyclic
voltammograms are shown in Fig. 3a. A reversible oxidation
peak is observed for all the three polymers. Upon fluorination,
the oxidation waves are shifted to higher voltages and
therefore, the polymers become less oxidizable due to the
decreased electron density along the polymer backbone.
Additionally, the reduction potential is affected by fluorination
and is shifted to lower voltages. In comparison to the
oxidation, the reduction seems to be irreversible under our
conditions of measurement. The ionization potentials and
electron affinities were calculated from the onsets of the
reduction and oxidation waves, respectively. These values are
only an approximate estimate of the energy levels of thin films
in polar solvents and should not be considered as absolute
values. In Table 1 the results are summarized and the energy
levels are graphically illustrated in Fig. 3b. Difluorination
summarized. The output and transfer characteristics of as-cast
and annealed P(TPD-TPT) films are depicted in Fig. 4. In the
case of the as-cast film, exclusively p-channel behaviour
(Fig. 4a) is observed, whereas the material shows ambipolar
character after annealing (Fig. 4b). The hole mobility reaches a
value of 1.1 x 10-4 cm2 V-1 s-1 and the electron mobility 3.4 x 10-
4 cm2 V-1 s-1. Herein, the hole mobility is improved by one order
of magnitude after annealing. Interestingly, the polarity of the
charge carriers is ambipolar even without fluorination. The
same observation was made for the difluorinated polymer
P(TPD-TPF2T) but having slightly higher hole and electron
mobilities. The corresponding characteristics can be found in
the Supporting Information. No clear transistor behaviour was
observed for the tetrafluorinated polymer in the as-cast state.
After annealing, exclusively n-channel behaviour (Fig. 4c) was
observed with an electron mobility of 3.7 x 10-4 cm2 V-1 s-1,
which is in the same range as the difluorinated polymer. The
observed high threshold voltage (see Table S1 in the ESI†) can
be ascribed to electron trapping at the dielectric/polymer
interface. These measurements give a good insight into the
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nature of charge carrier polarity but can be of course shifted to
higher values by extensive device optimisation, which is not
the main question of this work. After annealing the non-
fluorinated as well as the difluorinated polymer shows
ambipolar character, whereas by fluorination the hole as well
as the electron mobility is increased. In contrast,
tetrafluorination generates exclusively n-type behaviour with
comparable electron mobilites as for the diflourinated
polymers.
Structural analysis - GIWAXS
In order to understand the influence of structural order and
alignment in this series of copolymers, grazing incidence wide-
angle X-ray scattering (GIWAXS) was performed at the
SAXS/WAXS beamline at the Australian Synchrotron.37 11 keV
electrons were used with scattering patterns recorded on a
Dectris Pilatus 1M detector, following procedures detailed
elsewhere.38 Fig. 5 presents the two-dimensional scattering
patterns taken of as-cast and annealed films. The images
presented in Fig. 5 were taken close to the critical angle
(~ 0.12°) which was identified as the angle of incidence that
Table 2 OFET hole and electron mobilites for as-cast films and after thermal
annealing at 250 °C for 15 min under nitrogen.
as-cast
annealed
a
a
a
μh
μh
[cm2V-1s-1]
μe
[cm2V-1s-1]
[cm2V-1s-1]
P(TPD-TPT)
P(TPD-TPF2T)
P(TPD-TPF4T)
(2.5 ± 0.8) x 10-5 (0.4 ± 0.1) x 10-4
(5.0 ± 1.9) x 10-5 (1.1 ± 0.5) x 10-4
(1.1 ± 0.1) x 10-4
(3.4 ± 1.5) x 10-4
(3.7 ± 1.6) x 10-4
-
-
aHole (μh) and electron (μe) mobilites calculated from saturation regime. All values
are averaged for a minimum of six devices.
6 | J. Name., 2012, 00, 1-3
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as-cast
output p-tpye
P(TPD-TPT)
output p-tpye
P(TPD-TPT)
annealed
ouput n-type
forward
reverse
annealed
P(TPD-TPT)
0 V
(a)
(b)
ouput n-type
forward
reverse
P(TPD-TPF4T)
+80 V
annealed
0.5
0.4
0.3
0.2
0.1
0.0
(c)
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-20 V
0.00
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-20 V
-40 V
-40 V
-0.05
-60 V
-80 V
-60 V
+80 V
-0.10
+60 V
+20 V
+60 V
-0.15
-80 V
0 V
+40 V
-0.20
+40 V
0 V
forward
reverse
forward
reverse
+20 V
0 V
-0.25
-80 -70 -60 -50 -40 -30 -20 -10
Vds [V]
0
-80 -70 -60 -50 -40 -30 -20 -10
0
0
10 20 30 40 50 60 70 80
Vds [V]
0
10 20 30 40 50 60 70 80
Vds [V]
Vds [V]
annealed
transfer n-tpye
annealed
P(TPD-TPT)
transfer p-tpye
P(TPD-TPT)
Vds = -75 V
transfer n-type
annealed
P(TPD-TPF4T)
transfer p-tpye
as-cast
P(TPD-TPT)
Vds = -75 V
1E-3
1E-3
9E-4
8E-4
7E-4
6E-4
5E-4
4E-4
3E-4
2E-4
1E-4
1E-6
1E-7
8E-4
1E-5
1E-6
1E-7
1E-8
1E-9
3E-3
2E-3
2E-3
1E-3
5E-4
0
Vds = +75 V
Vds = +75 V
forward
reverse
1E-6
1E-7
1E-8
7E-4
6E-4
5E-4
4E-4
3E-4
2E-4
1E-4
0
1E-8
1E-9
1E-10
forward
reverse
forward
reverse
-80 -70 -60 -50 -40 -30 -20 -10
Vg [V]
0
0
10 20 30 40 50 60 70 80
Vg [V]
0
10 20 30 40 50 60 70 80
Vg [V]
-80 -70 -60 -50 -40 -30 -20 -10
Vg [V]
0
Fig. 4 Representative OFET output (top) and transfer (bottom) characteristics. Solid lines represent forward scans and dashed line reversed scans. (a) p-channel
operation of P(TPD-TPT) as-cast and (b) p- and n-channel operation of P(TPD-TPT) after annealing at 250 °C. (c) n-channel operation of P(TPD-TPF4T) after annealing at
250 °C.
gave the maximum scattering intensity. One-dimensional in- plane. For both edge-on and end-on oriented chains the
plane and out-of-plane scattering profiles are also provided in stacking peak is expected to be found along Qxy.
π-
Fig. 6. All polymers exhibit multiple orders of alkyl stacking, Despite having different degrees of fluorination, the polymers
indexed as (k00), even in as-cast films indicating a high all possess similar crystallographic parameters (see Fig. S8 in
propensity of these polymers to crystallise. The alkyl stacking the ESI†). A π-π stacking distance of 0.38 ± 0.01 nm is observed
scattering intensity is mainly observed along Qz for the for all polymers with there being more variation with
polymers indicating a predominantly “edge-on” stacking of annealing than from polymer to polymer (the effect of
crystallites. Prominent π-stacking peaks, indexed as (010), are annealing of the π-π stacking however is not systematic).
observed in-plane consistent with this molecular orientation. Fluorination does seem to lead to a tighter alkyl stacking
Alkyl stacking scattering intensity is also observed from distance, decreasing from 3.4 nm to 3.3 nm going from P(TPD-
crystallites at other azimuthal angles indicating the presence of TPT) to P(TPD-TPF4T), although a slight increase is observed for
crystallites with orientation other than edge-on. A backbone P(TPD-TPF2T) (~ 3.45 nm). Judging by the increase in scattering
stacking peak, indexed as (001), is also seen at a Q value of ~ intensity of the alkyl stacking peaks, there appears to be an
3.2 nm-1. Curiously this backbone stacking peak is observed increase in the degree of crystallinity with fluorination,
out-of-plane (along Qz, just below the second order alkyl particularly in the as-cast films. This would suggest that while
stacking peak) as well as along Qxy. Generally, for conjugated fluorination has not affected the molecular packing of the
polymers the backbone is thought to lie flat in the plane of the polymers, it has increased the propensity of chains to
film which is likely to be the case for thin films where the film crystallise during solution deposition.
thickness is smaller than the contour length of the polymer. Relating the microstructural data to the OFET data, the similar
Here, however, a significant (001) peak intensity is observed charge carrier mobilities achieved for the different polymers
out-of-plane indicating the “end-on” orientation of some can be seen as being consistent with the similar
polymer chains. This unusual observation is thought to be due microstructural
to the relatively low molecular weight of these polymers and
the relatively large film thickness of these films (around
120 nm) relative to the contour length. The series of alkyl
stacking peaks along Qz and the presence of a backbone
stacking peak along Qz indicates separate populations of
polymer chains with distinct orientation. The lamellar stacking
peak associated with end-on oriented chains is expected to be
found along Qxy suggesting that the in-plane (100) peak is
associated with end-on oriented chains rather than face-on
oriented crystallites. Such an assignment is consistent with the
fact that only a very weak π-stacking peak is observed out-of-
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of the chains to the substrate. This may be explained by the
relatively low molecular weights and therefore, small contour
length of these polymer in comparison to the film thickness.
Acknowledgements
We acknowledge finance support from Bavarian State Ministry
for Education, Science and the Arts under the SolTech project.
T.W. thanks the Elite Network Bavaria Macromolecular Science
programme at the University of Bayreuth. This research was
undertaken in part on the SAXS/WAXS beamline at the
Australian
Synchrotron,
Victoria,
Australia.
C.R.M.
acknowledges support from the Australian Research Council
(DP130102616).
Fig. 5
Two-dimensional GIWAXS patterns of P(TPD-TPT) (a, d) P(TPD-TPF2T) (b, e)
and P(TPD-TPF4T) (c, f). (a-c) show as-cast films while (d-f) show films annealed at
References
250 °C.
1.
2.
3.
4.
L. Gao, Z.-G. Zhang, L. Xue, J. Min, J. Zhang, Z. Wei and Y.
(a)
Li, Adv. Mater., 2016, 28(9), 1884.
P(TPD-TPT)
P(TPD-TPF2T)
P(TPD-TPF4T)
as-cast
annealed
1014
1013
1012
1011
1010
J. W. Jo, J. W. Jung, H. Ahn, M. J. Ko, A. K.-Y. Jen and H. J.
Son, Adv. Energy Mater., 2017,
K. Zhou, R. Zhang, J. Liu, M. Li, X. Yu, R. Xing and Y. Han,
ACS applied materials & interfaces, 2015, (45), 25352.
7(1), 1601365.
7
N. Zhou, X. Guo, R. Ponce Ortiz, T. Harschneck, E. F.
Manley, S. J. Lou, P. E. Hartnett, X. Yu, N. E. Horwitz, P.
Mayorga Burrezo, T. J. Aldrich, J. T. Lopez Navarrete, M.
R. Wasielewski, L. X. Chen, R. P. H. Chang, A. Facchetti
and T. J. Marks, J. Am. Chem. Soc., 2015, 137(39), 12565.
J. Warnan, C. Cabanetos, A. El Labban, M. R. Hansen, C.
Tassone, M. F. Toney and P. M. Beaujuge, Adv. Mater.,
2014, 26(25), 4357.
1
10
Momentum Transfer Qxy [nm-1]
(b)
P(TPD-TPT)
P(TPD-TPF2T)
P(TPD-TPF4T)
as-cast
1015
1014
1013
1012
1011
1010
5.
6.
annealed
A. Tournebize, J.-L. Gardette, C. Taviot-Guého, D. Bégué,
M. A. Arnaud, C. Dagron-Lartigau, H. Medlej, R. C. Hiorns,
S. Beaupré, M. Leclerc and A. Rivaton, Polym. Degrad.
Stab., 2015, 112, 175.
7.
8.
M.-C. Yuan, M.-Y. Chiu, S.-P. Liu, C.-M. Chen and K.-H.
Wei, Macromolecules, 2010, 43(17), 6936.
1
10
Momentum Transfer Qz [nm-1]
Z. Li, S.-W. Tsang, X. Du, L. Scoles, G. Robertson, Y. Zhang,
F. Toll, Y. Tao, J. Lu and J. Ding, Adv. Funct. Mater., 2011,
21(17), 3331.
Fig. 6
One-dimensional GIWAXS scattering profiles of P(TPD-TPT), P(TPD-TPF2T) and
P(TPD-TPF4T) taken along the in-plane (a) and out-of-plane (b) directions.
9.
Y. Zou, A. Najari, P. Berrouard, S. Beaupré, B. R. Aïch, Y.
Tao and M. Leclerc, Journal of the American Chemical
Society, 2010, 132(15), 5330.
properties. The fact that hole conduction was not observed in
the case of P(TPD-TPF4T) is then likely due to difficulties in
injecting holes into the deep HOMO of this polymer.
10. Y. Zhang, S. K. Hau, H.-L. Yip, Y. Sun, O. Acton and A. K.-Y.
Jen, Chem. Mater., 2010, 22(9), 2696.
Conclusions
11. J.-H. Kim, J. B. Park, I. H. Jung, A. C. Grimsdale, S. C. Yoon,
H. Yang and D.-H. Hwang, Energy Environ. Sci., 2015,
We have demonstrated that fluorination is a powerful tool to
generate exclusively n-type behaviour in thieno[3,4-c]pyrrole-
4,6-dione based polymers. For this, thieno[3,4-c]pyrrole-4,6-
dione was successfully copolymerized with thiophene-phenyl-
thiophene moieties with different degrees of fluorination.
Whereas the optical properties are almost not affected by
fluorination, the energy levels, both IP and EA, are lowered. In
GIWAXS measurements the thin film alignment of these
polymers surprisingly show end-on alignment, which is rarely
observed in conjugated polymers without any reactive grafting
8
(8), 2352.
12. Q. Wu, M. Wang, X. Qiao, Y. Xiong, Y. Huang, X. Gao and
H. Li, Macromolecules, 2013, 46(10), 3887.
13. F. Grenier, P. Berrouard, J.-R. Pouliot, H.-R. Tseng, A. J.
Heeger and M. Leclerc, Polym. Chem., 2013, 4(6), 1836.
14. G. Kim, A.-R. Han, H. R. Lee, J. Lee, J. H. Oh and C. Yang,
Chem. Commun., 2014, 50(17), 2180.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Journal of Materials Chemistry C
Page 9 of 10
View Article Online
DOI: 10.1039/C7TC01999H
Journal Name
ARTICLE
15. W. Wang, S. Yan, W. Lv, Y. Zhao, M. Sun, M. Zhou and Q.
Ling, Journal of Macromolecular Science, Part A, 2015,
52(11), 892.
35. Z. Li, J. Lu, S.-C. Tse, J. Zhou, X. Du, Y. Tao and J. Ding, J.
Mater. Chem., 2011, 21(9), 3226.
36. K. Kawashima, T. Fukuhara, Y. Suda, Y. Suzuki, T.
Koganezawa, H. Yoshida, H. Ohkita, I. Osaka and K.
Takimiya, Journal of the American Chemical Society,
2016, 138(32), 10265.
16. Z. A. Wang, J. Kuwabara, A. Ichige, T. Yasuda and T.
Kanbara, Synthetic Metals, 2016, 222, 383.
17. X. Guo, R. P. Ortiz, Y. Zheng, M.-G. Kim, S. Zhang, Y. Hu,
G. Lu, A. Facchetti and T. J. Marks, J. Am. Chem. Soc.,
2011, 133(34), 13685.
37. N. M. Kirby, S. T. Mudie, A. M. Hawley, D. J. Cookson, H.
D. T. Mertens, N. Cowieson and V. Samardzic-Boban, J
Appl Crystallogr, 2013, 46(6), 1670.
18. S. Liu, Z. Kan, S. Thomas, F. Cruciani, J.-L. Bredas and P.
M. Beaujuge, Angew. Chem., Int. Ed., 2016, 55(42),
12996.
38. T. T. Do, K. Rundel, Q. Gu, E. Gann, S. Manzhos, K. Feron,
J. Bell, C. R. McNeill and P. Sonar, New J. Chem., 2017,
41(8), 2899.
19. S. Liu, X. Song, S. Thomas, Z. Kan, F. Cruciani, F. Laquai, J.-
L. Bredas and P. M. Beaujuge, Adv. Energy Mater., 2017,
376, 1602574.
20. D. J. Crouch, P. J. Skabara, J. E. Lohr, J. J. W. McDouall, M.
Heeney, I. McCulloch, D. Sparrowe, M. Shkunov, S. J.
Coles, P. N. Horton and M. B. Hursthouse, Chem. Mater.,
2005, 17(26), 6567.
21. D. J. Crouch, P. J. Skabara, M. Heeney, I. McCulloch, S. J.
Coles and M. B. Hursthouse, Chem. Commun., 2005(11),
1465.
22. H. G. Kim, B. Kang, H. Ko, J. Lee, J. Shin and K. Cho, Chem.
Mater., 2015, 27(3), 829.
23. N. E. Jackson, B. M. Savoie, K. L. Kohlstedt, M. La Olvera
de Cruz, G. C. Schatz, L. X. Chen and M. A. Ratner, J. Am.
Chem. Soc., 2013, 135(28), 10475.
24. P. Sonar, J. Chang, Z. Shi, J. Wu and J. Li, J. Mater. Chem.
C, 2015, 3(9), 2080.
25. J. Y. Lee, J. W. Jo and W. H. Jo, Org. Electron., 2015, 20
125.
,
26. C. J. Mueller, C. R. Singh, M. Fried, S. Huettner and M.
Thelakkat, Adv. Funct. Mater., 2015, 25(18), 2725.
27. C. J. Mueller, E. Gann, C. R. McNeill and M. Thelakkat, J.
Mater. Chem. C, 2015, (34), 8916.
3
28. J. H. Park, E. H. Jung, J. W. Jung and W. H. Jo, Advanced
materials (Deerfield Beach, Fla.), 2013, 25(18), 2583.
29. X. Li, X. Liu, P. Sun, Y. Feng, H. Shan, X. Wu, J. Xu, C.
Huang, Z.-K. Chen and Z.-X. Xu, RSC Adv, 2017, 7(28),
17076.
30. A. Luzio, D. Fazzi, F. Nübling, R. Matsidik, A. Straub, H.
Komber, E. Giussani, S. E. Watkins, M. Barbatti, W. Thiel,
E. Gann, L. Thomsen, C. R. McNeill, M. Caironi and M.
Sommer, Chem. Mater., 2014, 26(21), 6233.
31. A. Najari, S. Beaupré, P. Berrouard, Y. Zou, J.-R. Pouliot,
C. Lepage-Pérusse and M. Leclerc, Adv. Funct. Mater.,
2011, 21(4), 718.
32. R. Matsidik, J. Martin, S. Schmidt, J. Obermayer, F.
Lombeck, F. Nubling, H. Komber, D. Fazzi and M.
Sommer, J. Org. Chem., 2015, 80(2), 980.
33. N. J. Findlay, J. Bruckbauer, A. R. Inigo, B. Breig, S.
Arumugam, D. J. Wallis, R. W. Martin and P. J. Skabara,
Adv. Mater., 2014, 26(43), 7290.
34. F. C. Spano, J. Clark, C. Silva and R. H. Friend, The Journal
of chemical physics, 2009, 130(7), 74904.
This journal is © The Royal Society of Chemistry 20xx
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GIWAXS measurements confirm end-on alignment for TPD-copolymers obtained from Stille
polycondensation using fluorinated and non-fluorinated comonomers.