Influence of fluorination in π-extended backbone polydiketopyrrolopyrroles on charge carrier mobility and depth-dependent molecular alignment

Article abstract of DOI:10.1039/c5tc01948f

A series of π-extended polydiketopyrrolo[3,4-c]pyrroles with a varying degree of fluorination on the thiophene-phenyl-thiophene comonomer was synthesized by Stille polycondensation. The influence of the degree of fluorination was studied with regard to th

Full text of DOI:10.1039/c5tc01948f

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Influence of fluorination in ‐extended backbone
polydiketopyrrolopyrroles on charge carrier mobility and depth‐
dependent molecular alignment
C. J. Mueller,a E. Gann,b,c C. R. McNeillc and M. Thelakkata*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A series of ‐extended polydiketopyrrolo[3,4‐c]pyrroles with a varying degree of fluorination on the thiophene‐phenyl‐
thiophene comonomer was synthesized by Stille polycondensation. The influence of the degree of fluorination was studied
with regard to the polymer properties, such as absorption, electrochemical redox potentials, the solid state structure as well
as depth‐dependent molecular alignment in thin films. Additionally, their performance in organic field effect transistors was
evaluated. Whereas fluorination slightly increases the alkyl lamella and shrinks the  spacings, the coherence lengths
were found to improve significantly in both directions. All polymers were found to be p‐type materials when employed in
organic field effect transistors (OFET). These devices can be tuned towards ambipolarity in the case of the tetrafluorinated
copolymer upon thermal annealing.
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fluorine and the hydrogen‐oxygen interactions that help lock
the conformation of the polymer backbone into a more
planarized state. In low bandgap polymers, the influence of
Introduction
Diketopyrrolopyrrole (DPP) based materials are promising
candidates for organic electronics due to their high ambipolar
charge carrier mobilities, low optical gap and high absorption
coefficient. Especially their use in organic field effect transistors
(OFETs) has received considerable attention over the last few
years. This is mainly due to their remarkable structural order1
leading to high planarity of the DPP moieties arising from strong
intermolecular  interactions which promote charge
transport.2, 3
fluorination on regioselectivity of the resulting copolymer
structure has also been reported.12 Not only fluorinated
phenylene units have been shown to be valuable building blocks
for semiconducting copolymers,13 but recently it has been
shown that for the TPT motif, bifluorination or bialkoxylation of
the central phenyl unit facilitates charge transport and leads to
either preferred edge‐on alignment in the case of fluorination
or face‐on alignment in the case of alkoxylation.9 On the other
hand, a tetrafluorinated TPT derivative has been employed in
oligomers14, 15 as well as in DPP copolymers carrying furanyl
flanking units by Sonar et al.16 and has been found to provide
ambipolarity in OFET devices. It is hitherto unknown whether
this ambipolarity is caused by the furanyl‐flanking units or the
tetrafluorination of the TPT. Therefore it is interesting, to study
the effect of fluorination in the conventional DPP copolymers
with thienyl flanking units. The impact of tetrafluorination in a
TPT moiety on the rotational barrier has been studied by
computational methods.17
The  interactions in DPP copolymers can be increased by
side chain engineering4,
or by incorporating planarizing
5
aromatic units as comonomers into the backbone.6 Examples
are long non‐alkylated oligoaromatic units such as hexa‐ or
heptathiophenes,7
vinylene‐bridged
bithiophenes
or
biselenophenes,4 as well as highly

‐extended donating units
consisting of thiophene, selenophene and phenyl moieties.8
Very recently it has been shown that comonomers with the
thiophene‐phenyl‐thiophene (TPT) motif are especially
promising candidates to facilitate highly planar, crystalline and
aggregating DPP copolymers.8‐10 In order to further promote the
planarization of semiconducting polymers the exploitation of
diffusive nonbonding heteroatom interactions has been
employed.11 Two of those interactions are the hydrogen‐
In this contribution we report the synthesis of a series of three
extended
‐conjugated PDPPs by using a non‐alkylated
thiophene‐n‐fluorophenylene‐thiophene (n = 0, 2, 4) unit as the
comonomer. The crystalline depth‐dependant alignment in thin
films and the charge transport properties in OFET devices is
studied with respect to the degree of fluorination in order to
gain insight into the structure‐property relationship of these
extended systems and to evaluate the utility of fluorination in
semiconducting polymers as measure for improving
semiconducting material properties. We also evaluate the
various non‐covalent interactions possible in these derivatives
‐
a. Applied Functional Polymers, Macromolecular Chemistry I, University of Bayreuth,
95440 Bayreuth, Germany. *E‐Mail: mukundan.thelakkat@uni‐bayreuth.de
b. Australian Synchrotron, 800 Blackburn Road, Clayton, VIC, 3168, Australia.
c. Department of Materials Science and Engineering, Monash University, Wellington
Road, Clayton VIC, 3800, Australia.
† Electronic Supplementary Information (ESI) available: Synthesis and
characterization of all monomers, detailed characterization of polymers including
NMR, TGA, GPC, cyclic voltammetry and Flash‐DSC. See DOI: 10.1039/x0xx00000x
a
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based on experimental crystallographic data of the building fluorinated derivatives where the yield was 80‐95%. Lithiation
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blocks.
of 3a‐c in THF at ‐78 °C followed by quencDhOinI:g10w.1i0th39t/rCim5TeCt0h1y9l4t8inF
chloride gave the desired monomers 4a‐c. For the fluorinated
compounds 4b and 4c the stannylation was quantitative. For all
three monomers it should be noted that purification is feasible
by crystallization due to the high crystallinity of the obtained
compounds.
Results and Discussion
Synthesis
The synthesis of the stannylated TPT‐comonomers is
summarized in Figure 1. For the nonfluorinated TPT unit 4a
The thienyl‐DPP monomer
5 was synthesized following a
a
literature procedure,21 which was adapted to increase the yield
and purity. Polymerizations were carried out in chlorobenzene
at 180 °C under microwave conditions using Pd2(dba)3 with P(o‐
Tol)3 as the catalytic system.22 All polymers were endcapped
with thiophene on both ends. Detailed procedures are given in
the experimental section. All polymers were obtained in high
number average molecular weights (Mn) in the range of
20 400 – 38 100 kg mol‐1 and dispersities (Đ) between 1.9 and
2.3 (Table 1). The individual GPC traces are given in the
Supporting Information (Figures S1‐S3) and show a monomodal
distribution for every polymer. All polymerizations were carried
out using similar conditions regarding concentrations, degree of
monomer purities, temperatures and catalytic system. All
compounds were polymerized to the precipitation limit in
chlorobenzene at 180 °C. The molecular weight of the
fluorinated polymers is therefore smaller due to the lower
solubility of these polymers upon introduction of fluorine
groups.
synthesis route which has been published for the
bis(tributyltin)‐analogs18, 19 was adapted. For the difluorinated
compound 4b as well as the tetrafluorinated 4c, a direct
lithiation from 2b and 2c, respectively, has been reported.16, 20
We found that monostannylated compounds that are obtained
as byproducts from direct lithiation of 2a‐c are difficult to
separate even by multiple recrystallizations and sublimations.
Therefore we adopted a different synthetic route involving first
bromination followed by lithiation and stannylation to obtain
4b and 4c. The almost quantitative conversion achieved in this
bromination route employed here results in high monomer
purities. Therefore, this bromination route was employed for all
the three stannylated DPP derivatives. In detail, starting from
the 1,4‐dibrominated phenylene compounds 1a‐c a Stille
coupling was employed to obtain the TPT blocks 2a‐c
.
Subsequent bromination with NBS in DMF yielded the
dibrominated TPT blocks 3a‐c in good yields, especially for the
Figure 1: Syntheses of the thiophene‐phenyl‐thiophene monomers with different degrees of fluorination. Experimental details are given in the Supporting Information. Synthesis of
the extended PDPP‐TPFnT copolymers by Stille‐polycondensation. All yields are given in parentheses.
Figure 2: Structures of the new extended PDPP[T]2‐TPFnT polymers in order of the overall electron density. R = 2‐Decyltetradecyl (C10/C14). The systematic nomenclature of the
DPP copolymers is adopted from our earlier publication.23
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Table 1: Intrinsic properties of the synthesized DPP polymers.
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DOI: 10.1039/C5TC01948F
a
a
c
d
e
h
i
Polymer
Mn
Mw
Đb
DPn
T5%
Tm
Tce
IPf
[eV]
‐5.69
‐5.72
‐5.89
EAg
[eV]
‐3.61
‐3.60
‐3.97
ECV
Eopt
[kg mol‐1]
38.1
24.5
[kg mol‐1]
71.4
53.1
[°C]
389
406
394
[°C]
307
337
344
[°C]
253
289
303
[eV]
2.08
2.12
1.92
[eV]
1.51
1.46
1.47
PDPP[T]2‐TPT
PDPP[T]2‐TPF2T
PDPP[T]2‐TPF4T
1.9
2.2
2.3
31
20
16
20.4
47.2
aDetermined by GPC at 150 °C using 1,2,4‐trichlorobenzene as the eluent; bdispersity; cDegree of Polymerization calculated from Mn; ddecomposition onset (5%)
determined by TGA; eaverage Flash‐DSC peak melting temperature (Tm) and crystallization temperature (Tc) at ‐200 K s‐1; fIonisation potential (HOMO), gelectron
affinity (LUMO) and hband‐gap determined by cyclic voltammetry in thin film; ioptical gap determined from absorption onset in film.
Photon Energy [eV]
2
Photon Energy [eV]
2
(a)
(b)
3.5
3
2.5
1.5
3.5
3
2.5
1.5
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PDPP[T]2-TPT
PDPP[Py]2-TPF2T
PDPP[Ph]2-TPF4T
PDPP[T]2-TPT
Absorption,
PDPP[Py]2-TPF2T
PDPP[Ph]2-TPF4T
0.20
0.15
0.10
0.05
0.00
Solution:
Absorption,
Photoluminescence
Film:
Photoluminescence
0.6
0.4
0.2
0.0
400
600
800
1000
400
600
800
1000
Wavelength [nm]
Wavelength [nm]
(c)
Photon Energy [eV]
2.5
Photon Energy [eV]
2.5
Photon Energy [eV]
2.5 1.5
4 3.5
3
2
1.5
4 3.5
3
2
1.5
4 3.5
3
2
PDPP[T]2-TPT
PDPP[T]2-TPF2T
PDPP[T]2-TPF4T
1.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
solution
film
solution
film
solution
film
0.8
0.6
0.4
0.2
0.0
300 400 500 600 700 800 900 1000
Wavelength [nm]
300 400 500 600 700 800 900 1000
Wavelength [nm]
300 400 500 600 700 800 900 1000
Wavelength [nm]
Figure 3: Absorption and photoluminescence data for the PDPP[T]2‐TPFnT copolymers measured in (a) chloroform solution and in (b) film. A comparison of solution and film
absorption spectra is shown in (c).
This behaviour has been reported before and is ascribed to the heating rates from 50 K s‐1 to 1000 K s‐1. The representative
higher degree of aggregation occurring in fluorinated 1000 Ks‐1 heating and cooling curves are shown in Figure 4b and
conjugated polymers.24 However, all three polymers are soluble the individual heating and cooling curves for all scan rates are
and processable from chloroform. It has been shown earlier by shown in the Supporting Information (Figure S4). The respective
Janssen et al. that the aggregation behaviour in PDPPs plays a melting points determined from the average peak value for the
significant role in determining alignment in thin film and the different ramping speeds are summarized in Table 1. The
morphology of polymer/fullerene blends. Furthermore it was nonfluorinated PDPP[T]2‐TPT shows the lowest melting
shown that these effects can be influenced by either tuning the temperature (307 °C) whereas fluorination leads to increased
solubilizing alkyl sidechains5 or the aromatic units along the melting temperatures of 337 °C and 344 °C for the di‐ and
backbone.6
tetrafluorinated copolymers, respectively. This is in accordance
Thermogravimetric analysis (see Supporting Information, Figure with our previous reasoning on differences in molecular weight
S6) shows very high decomposition onsets T5% around 400 °C for due to increased aggregation in the fluorinated polymers.
all three derivatives, as is common for PDPPs.
Crystallization temperatures are rate‐dependant due to the
The melting points of PDPPs are usually very high and close to kinetic nature of crystallization. The crystallization
their respective decomposition onsets which makes them temperatures derived from the ‐200 K s‐1 cooling curve given in
difficult to observe using conventional dynamic scanning Table 1 are exemplary and show the same trend as the melting
calorimetry (DSC). We therefore evaluated the thermal temperatures.
properties of these three polymers using a Flash‐DSC setup with
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The optical properties of the three polymers were investigated measurements were carried out in solutions containing a 0.1
by absorption as well as photoluminescence measurements. tetra‐n‐butylammonium hexafluorophosphate / acetonitrile
The individual solution as well as solid‐state spectra are shown electrolyte solution at a scan rate of 100 mV s‐1. The values for
M
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DOI: 10.1039/C5TC01948F
in Figure 3.
the ionization potential (IP), which closely resembles the
All three polymers show similar absorption patterns and an highest occupied molecular orbital (HOMO) as well as the
almost identical optical gap of 1.5 eV (see Table 1). When electron affinity (EA), which resembles the lowest unoccupied
comparing the individual solution spectra (Figure 3a), a slight molecular orbital (LUMO) are summarized in Table 1 and
shift in peak positions for the visible absorption band becomes comparatively shown in Figure 4.26 Interestingly, when
observable, which is not apparent in the solid state spectra compared to the nonfluorinated PDPP[T]2‐TPT (IP: ‐5.69 eV, EA:
(Figure 3b). More specifically, when comparing the solution and 3.61 eV), difluorination in PDPP[T]2‐TPF2T only has a negligible
film absorption spectra of the three individual compounds effect on both, IP (‐5.72 eV) and EA (‐3.60 eV). The fundamental
(Figure 3c), a notable change in absorption behaviour is bandgap Ecv of 2.1 eV is therefore also identical for both
observed only by the non‐fluorinated PDPP[T]2‐TPT. Upon di‐ polymers. Upon introduction of two more fluorine atoms
fluorination in PDPP[T]2‐TPF2T, the solution and thin film yielding PDPP[T]2‐TPF4T a clear shift of both energy levels
spectra become much more similar. Finally, for the towards lower values can be observed, resulting in an IP
tetrafluorinated PDPP[T]2‐TPF4T both absorption curves are of ‐5.89 eV and an EA of ‐3.97 eV as well as a fundamental
more or less identical. We ascribe this behaviour to the strong bandgap of 1.92 eV. Compared to the optical gap these values
aggregation of the fluorinated polymers and the presence of are significantly larger, which can be taken as an indicator for
aggregates in the solutions that don’t differ significantly from large a large exciton binding energy Eb in these polymers.26
those finally formed in film. This finding is supported by the fact
Influence of fluorination on backbone planarization: Weak non‐
that the photoluminescence intensity for PDPP[T]2‐TPT in
covalent interactions.
solution is stronger by a factor of about 3 than those from the
fluorinated counterparts PDPP[T]2‐TPF2T and PDPP[T]2‐TPF4T. There are several contradictory reports in the recent literature
This photoluminescence quenching is again ascribed to the on the origin and the exact conformation that are caused by
strong aggregation of the fluorinated polymers in solutions. In heteroatom interactions. Whereas elaborate theoretical work
thin film, when all the three polymers show the absorption by Ratner et al. based on quantum chemical calculations mainly
spectrum of the aggregated species, the photoluminescence favours hydrogen‐heteroatom interactions such as H
⋯F, H⋯O,
intensity is comparable for all three polymers. S etc., several reports claim that S F and O F interactions
H
⋯
⋯
⋯
influence the planarization of semiconducting polymer
backbones.9, 27 It has also been reported, that the interaction
(b)
(a)
PDPP[T]2-TPT
PDPP[T]2-TPF2T
Cooling (-200 K/s)
PDPP[T]2-TPF4T
-3.5
-4.0
-4.5
-5.0
-5.5
-6.0
between heteroatoms such as S⋯F and O⋯
F are nearly zero.11
Heating (200 K/s)
-1.5
-1.0
-0.5
0.0
We like to emphasize, that for most of the motifs employed in
these polymers, experimental structural data for building blocks
is available from single crystal crystallography. We therefore
analysed the Cambridge Crystallographic Database Centre
(CCDC) library for the various low molecular weight TPT motifs
with different degrees of fluorination on the central phenyl
moiety. The experimental data supports the model that has
been theoretically established by Ratner et al., i.e. for the
difluorinated TPF2T unit the fluorine atoms on the phenyl ring
point towards the hydrogen atoms of the adjacent thiophene
units.14 This experimental evidence does not support the
0.5
1.0
P
P
P
D
D
D
P
P
P
P
P
P
50
100
150
200
250
300
350
[
[
[
T
]
T
T
]
]
-
-
-
T
P
T
T
P
P
T
F
F
T
Temperature [°C]
T
Figure 4: (a) Energy level comparison for the extended backbone DPP copolymers with
a varying degree of fluorination. Values are determined by Cyclic voltammetry in thin
film; (b) Flash‐DSC curves for 200 Ks‐1 heating / cooling rates. See Supporting
Information (Figure S5) for additional scan rates.
proposed S⋯
F interactions in TPF2T, as reported in literature.9
Oxidation and reduction potentials of the individual polymers
were studied by cyclic voltammetry. The individual cyclic
voltammograms are shown in Supporting Information (Figure
S5). The energy levels of the polymers were calculated from the
redox half step potentials calibrated against ferrocene in
acetonitrile, for which a workfunction of ‐5.23 eV for the
ferrocene/ferrocenium redox couple is taken from literature.25
All potentials were obtained using a standard three‐electrode
cyclic voltammetry setup. Polymers were spincoated onto ITO
glass slides which were used as the working electrode and
GIWAXS
Crystalline packing within the polymer thin films was
investigated with Grazing Incidence Wide Angle X‐ray Scattering
(GIWAXS), see Figure 5. Exposures were taken at incident angles
between 0.05 and 0.4 degrees at a resolution of 0.01 degrees
near the critical angle ~0.17 degrees, to capture the highest
intensity critical angle scattering, as well as some depth
sensitive information (as shown in Figure 6).
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DOI: 10.1039/C5TC01948F
Non-Annealed
Annealed
S-Parameter
Momentum Transfer Qxy [Å]
Momentum Transfer Qxy [Å]
Sample Omega [degrees]
0.20 0.30
0.10
0.40
0.0
0.4
0.8
1.2
1.6
0.0
0.4
0.8
1.2
1.6
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
1.0
0.8
0.6
0.4
0.2
0.0
a)
-0.2
-0.4
PDPP[T]2-TPT as cast
PDPP[T]2-TPT annealed
0.0
0.0
0.4
0.4
0.8
0.8
1.2
1.2
1.6
1.6
0.0
0.0
0.4
0.4
0.8
0.8
1.2
1.2
1.6
1.6
0.10
0.10
0.20
0.20
0.30
0.30
0.40
0.40
b)
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
PDPP[T]2-TPF2T as cast
PDPP[T]2-TPF2T annealed
0.0
0.0
0.4
0.4
0.8
0.8
1.2
1.2
1.6
1.6
0.0
0.0
0.4
0.4
0.8
0.8
1.2
1.2
1.6
1.6
0.10
0.10
0.20
0.20
0.30
0.30
0.40
0.40
c)
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
PDPP[T]2-TPF4T as cast
PDPP[T]2-TPF4T annealed
0.10
0.20
0.30
0.40
0.0
0.4
0.8
1.2
1.6
0.0
0.4
0.8
1.2
1.6
Sample Omega [degrees]
Momentum Transfer Qxy [Å]
Momentum Transfer Qxy [Å]
Out-of-Plane Contribution
In-Plane Contribution
d)
e)
108
PDPP[T]2-TPT
PDPP[T]2-TPF2T
PDPP[T]2-TPF4T
(010)
(100)
(100)
2
107
106
105
104
(200)
(300)
(400)
104
(010)
8
7
6
5
4
3
PDPP[T]2-TPT
PDPP[T]2-TPF4T
PDPP[T]2-TPF2T
2
4
6
8
10
12
14
16
18
20
2
4
6
8
10
12
14
16
18
qz [nm-1]
q
xy [nm-1]
Figure 5. From left to right, the unannealed, and annealed 2D GIWAXS Patterns followed by the Herman’s orientation parameter vs incident angle. From top to bottom: a)
PDPP[T]2‐TPT, b) PDPP[T]2‐TPF2T, and c) PDPP[T]2‐TPF4T; d) Out‐of‐Plane contribution for the annealed polymers; e) In‐plane contribution for the annealed polymers.
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All scattering patterns show clear alkyl lamella stacking, that is scattering originates from deeper within tDhOeIf:i1lm0.1.0F39in/Ca5llTyC, 0a1b9o4v8eF
backbone to backbone across the alkyl side chains, and π‐π the critical angle of the substrate, the electric field intensity
stacking, from one molecular face to the next. All polymers becomes much more even throughout the entire system,
exhibit liquid crystalline, semi‐crystalline diffraction pattern, averaging out to an intensity of 1 at very high angles. Thus
with little to no mixed index peaks (peaks away from purely out‐ looking at the angular dependence of the Herman’s orientation
of‐plane or in‐plane), indicating that the elements are largely parameter in Figure 5, we can conclude that for all of the
stacking one dimensionally along different directions, and not unannealed films, as well as for the annealed PDPP[T]2‐TPT and
forming three dimensional crystals. Additionally, all the PDPP[T]2‐TPF2T films, the surface region of the samples are less
polymers exhibit a generally edge‐on orientation, with alkyl well oriented than the bulk of the film. For PDPP[T]2‐TPF4T, on
stacking largely out‐of‐plane and π‐π stacking in‐plane.
the other hand, the exceptionally high degree of edge‐on
We can see that the effect of annealing in all cases is the alignment extends all through the surface upon annealing.
intensification and sharpening of the peaks, with several orders Sector averages of the out‐of‐plane (vertical, Qz) and in‐plane
of alkyl lamella stacking apparent, indicating increased (horizontal, Qxy) directions of the annealed polymer films are
crystallinity and larger crystalline domains present within the shown in Figure 5d, and the results of fitting the alkyl stacking
annealed films. The degree of orientational alignment is lamella peaks and  stacking peaks is shown in Table 2.
quantified by the Herman’s orientation parameter28 which runs
Table 2. Crystalline parameters found from the GIWAXS patterns. Uncertainties of the
from ‐0.5 (face‐on orientation) to 1 (edge on orientation), of the
final digit are indicated in parentheses after the value.
second order alkyl stacking peak, plotted in the right most
b
c,f
d
e,f
column of Figure 5(a‐c). We can see at once that the unannealed
films are less well oriented, however all films still have an
average edge‐on structure. Upon annealing, the orientation is
strengthened in all cases towards almost perfect edge on
orientation.
Polymer
Ta
dalkyl
alkyl
d

[nm]
2.3(1)
2.15(5)
2.2(1)
2.2(1)
2.22(5)
2.2(1)
[nm]
5.1(1)
[nm]
[nm]
2.6(5)
5.2(1)
5.4(1)
7.6(1)
5.3(1)
8.1(1)
PDPP[T]2‐TPT
PDPP[T]2‐TPT
PDPP[T]2‐TPF2T
PDPP[T]2‐TPF2T
PDPP[T]2‐TPF4T
PDPP[T]2‐TPF4T
as cast
250 °C
as cast
250 °C
as cast
250 °C
0.395(3)
0.392(1)
0.375(1)
0.381(1)
0.377(1)
0.378(1)
22.3(1)
8.25(5)
19.3(1)
7.82(3)
18.9(1)
a Annealing process; b alkyl spacing; c alkyl stacking coherence length; d π‐π
stacking spacing; e π‐π stacking coherence length; f the crystal coherence length
gives information about the distance over which order is maintained and is
defined as  = 2/FWHM, where FWHM is the full width at half maximum of the
first order lamellar stacking or  stacking peak, respectively. It is related to the
Scherrer equation, which connects the width of a peak to the crystal size.
We can see that the process of annealing increases the
coherence lengths of both types of stacking, but particularly in
the alkyl stacking direction, where the coherence length
increases by a factor of 4 in the case of PDPP[T]2‐TPT, whereas
the increase is only by a bit more than a factor of 2 for both
fluorinated polymers. It is clear that the addition of fluorine to
the backbone decreases the stacking distance, with the
annealed PDPP[T]2‐TPF4T film having the smallest of the
annealed π‐π stacking distances. In the case of the unannealed
films, fluorination yields distinctly smaller  stacking values.
With the alkyl lamella stacking stretching out at the same time
upon fluorination, a general flattening of the unit cell can be
deduced.
Figure 6. The calculated X‐ray electric field intensity for 100 nm thick film on Si. The
area between the dotted black lines represents the polymer film. The critical angle at
approximately 0.16‐0.17 degrees is seen by the sudden large enhancement of electric
field intensity within the polymer film. The critical angle of the lower Si substrate can
be seen at ~0.2 degrees, at which point the X‐ray electric field intensity becomes much
more uniform throughout the film and substrate. At angles below the critical angle, the
electric field intensity within the film is entirely at the surface, within the top ~10 nm.
To understand the angular dependence of the Hermans
parameter vs angle, the electric field intensity of the X‐ray beam
is calculated vs angle and depth within a 100 nm thick polymer
film as shown in Figure 6. Depths greater than 100 nm represent
the Silicon substrate, while depths less than 0 represent the
ambient environment. Scattering from any depth within the
film is proportional to the X‐ray electric field intensity at that
depth. Thus, at incident angles less than the critical angle (~0.16
degrees in the simulation) X‐rays scatter exclusively from the
upper ~10 nm of the surface of the polymer film, while between
the critical angle of the polymer film and the critical angle of the
silicon substrate (~0.2 degrees) the location and intensity of
Charge Carrier Mobilities
To investigate the influence of the degree of fluorination on the
charge carrier mobilities, the polymers were investigated in
organic field effect transistor (OFET) devices using a bottom
gate / bottom contact (BGBC) configuration. Details on the
device preparation and charge carrier mobility determination
are given in the experimental section.
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Journal of Materials Chemistry C
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View Article Online
DOI: 10.1039/C5TC01948F
output p-type
PDPP[T]2-TPT
output n-type
output p-type
PDPP[T]2-TPF4T
output n-type
0.15
0.00
-0.01
-0.02
-0.03
-0.04
0.06
0.04
0.02
0.00
0.00
-0.05
-0.10
-0.15
- 40 V
- 60 V
0 V
- 40 V
0 V
0.10
0 V
- 60 V
- 80 V
0.05
20 V
20 V
40 V
- 80 V
80 V
forward,
reverse
-40 -20
Vd [V]
forward,
reverse
-20
60 V
0.00
-80
-60
0
20
40
Vd [V]
60
80
-80
-60
-40
0
20
40
d [V]
60
80
Vd [V]
V
transfer p-type
transfer n-type
transfer p-type
PDPP[T]2-TPF4T
transfer n-type
PDPP[T]2-TPT
1E-3
1E-4
1E-5
1E-6
1E-7
1E-8
1E-9
1E-10
1E-3
1E-4
1E-5
1E-6
1E-7
1E-8
1E-9
1E-10
1.3E-2
1.0E-2
7.5E-3
5.0E-3
2.5E-3
0.0
7.5E-3
5.0E-3
2.5E-3
0.0
saturated (-80 V)
linear (-5 V)
saturated (-80 V)
linear (-5 V)
saturated (+80 V)
linear (+5 V)
saturated (+80 V)
linear (+5 V)
-80
-60
-40
-20
0
0
20
40
g [V]
60
80
-80
-60
-40
-20
0
0
20
40
g [V]
60
80
Vg [V]
V
Vg [V]
V
Figure 7.Representative OFET I‐V curves in p‐channel (blue output) and n‐channel (red output) operation for the non‐fluorinated PDPP[T]2‐TPT (left) and the tetrafluorinated
PDPP[T]2‐TPF4T (right) after annealing at 350 °C. Solid lines represent forward scans, dashed lines the reverse scans in all graphs. In transfer curves (bottom), the black and gray
plots indicate the transfer characteristics in the saturation and linear operation regime, respectively. I‐V curves for the difluorinated PDPP[T]2‐TPF2T as well as a scaled comparison
of the n‐output characteristics for all three polymers are given in the Supporting Information (Figure S9, Figure S10).
Table 3. Average OFET‐Mobilities of PDPP[Ar]2‐TPFnT copolymers in a bottom gate / bottom contact configuration in as cast and annealed films.
As Cast
Annealedb 200 °C
Annealedb 250 °C
Annealedb 300 °C
Annealedb 350 °C
a
a
a
a
a
a
Polymer
µh
µh
µh
µh
µh
µe
[cm2V‐1s‐1]
[cm2V‐1s‐1]
[cm2V‐1s‐1]
[cm2V‐1s‐1]
[cm2V‐1s‐1]
[cm2V‐1s‐1]
PDPP[T]2‐TPT
PDPP[T]2‐TPF2T
PDPP[T]2‐TPF4T
(2.3 ± 0.4) × 10‐2
(5.7 ± 0.9) × 10‐3
(2.6 ± 0.5) × 10‐3
(4.1 ± 1.2) × 10‐2
(2.0 ± 0.4) × 10‐2
(1.3 ± 0.2) × 10‐2
(2.6 ± 0.9) × 10‐2
(1.5 ± 0.5) × 10‐2
(1.1 ± 0.3) × 10‐2
(5.8 ± 0.7) × 10‐2
(1.9 ± 0.8) × 10‐2
(1.4 ± 0.4) × 10‐2
(3.7 ± 3.4) × 10‐3
(2.5 ± 0.9) × 10‐2
(1.5 ± 0.8) × 10‐2
(2.0 ± 0.9) × 10‐4
(8.0 ± 3.4) × 10‐3
(1.3 ± 0.5) × 10‐2
aHole (µh) / electron (µe) mobilities determined from saturated operation regime. All values were averaged over a minimum of 10 devices; bAnnealing was performed
for 15 min in a nitrogen atmosphere. See Supporting Information (Table S2) for threshold voltages and on/off ratios.
As typical examples, the I‐V transfer as well as output curves for Although surface alignment plays a crucial role in OFET devices,
the non‐fluorinated PDPP[T]2‐TPT and the tetrafluorinated it is important to note that high hole mobilities are influenced
PDPP[T]2‐TPF4T are shown in Figure 7 and the mobility values by the electron‐richness of the polymer as well as the quality of
are summarized for different annealing conditions in Table 3. the dielectric‐semiconductor interface. As already discussed,
The I‐V curves for PDPP[T]2‐TPF2T are given in the Supporting the S⋯F interaction in general is supposed to help planarization
Information (Figure S9). A comparison of the output curves for of the polymer backbone11 which is also in agreement with the
all polymers in n‐channel operation can also be found in the structural data from single crystals of the incorporated
Supporting Information (Figure S10), giving an overview of the oligomers.14, 15, 29‐31 In a series of fluorinated DPP copolymers,
output currents in different scales. For the as cast films as well we have earlier shown that fluorination mainly improves the n‐
as for annealed films up to 250 °C, no n‐channel behavior was type character of semiconducting polymers or leads to
observable. For the p‐type devices, the threshold voltage was ambipolarity in otherwise exclusively p‐type materials.23
0 V for all devices and the non‐fluorinated PDPP[T]2‐TPT shows Annealing leads to an increased hole mobility at the cost of
the highest hole mobility (µh = 2.3 × 10‐2 cm2V‐1s‐1). higher threshold voltages around 30 V for the non‐fluorinated
Difluorination leads to a fivefold decrease in hole mobility PDPP[T]2‐TPT reaching a maximum of 5.8 × 10‐2 cm2V‐1s‐1 after
whereas tetrafluorination in PDPP[T]2‐TPF4T results in a annealing at 300 °C. This increase can be explained by the higher
decrease of one order of magnitude in hole mobility degree of edge‐on alignment as evidenced by the GIWAXS
(2.6 × 10‐3 cm2V‐1s‐1). This decrease in µh stands in contrast to experiments and an S‐Parameter close to 0.9 after annealing. In
the alignment of the as cast films which is much higher for the the fluorinated polymer films, the hole mobility is increased by
fluorinated derivatives as discussed in the GIWAXS section. one order of magnitude upon annealing at temperatures
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between 200 °C and 300 °C to give values in the range of wire in an AgNO3/acetonitrile solution (0.1
1‐2 × 10‐2 cm2V‐1s‐1.
M
DOI: 10.1039/C5TC01948F
measurements were calibrated with an external
Upon annealing at 350 °C, the non‐fluorinated PDPP[T]2‐TPT ferrocence/ferrocenium standard, IP and EA values were
shows an onset of a weak n‐type character with a rather low calculated considering the solvent effects as per a published
electron mobility value of 2.0 × 10‐4 cm2V‐1s‐1 and a high procedure33 using Equation 1 and 2 where the workfunction of
threshold voltage of 54 V, leading to barely visible output even Fc/Fc+ is taken to be ‐5.23 eV. The reduction half‐step potential
red
1/2
when operated at a high gate voltage of 80 V. Even though it is
E
(vs Fc/Fc ) is negative whereas the oxidation half‐step
ox
a weak n‐type behaviour, it clearly indicates the onset of potential
ambipolar behaviour. On the other hand, for the fluorinated
derivatives, annealing at 350 °C improves the n‐channel
E
(vs Fc/Fc ) is positive.
1/2
EA  5.23 eV  Ered (vs Fc/Fc )
(1)
(2)
1/2
IP  5.23 eV  Eox (vs Fc/Fc )
1/2
performance considerably. Thus, electron mobilities of Absorption measurements were carried out on a JASCO V‐670
8.0 × 10‐3 cm2V‐1s‐1 for PDPP[T]2‐TPF2T and 1.3 × 10‐2 cm2V‐1s‐1 spectrophotometer. Photoluminescence measurements were
for PDPP[T]2‐TPF4T respectively, could be obtained. The carried out on a JASCO FP‐8600 spectrofluorometer, excitation
threshold voltage for n‐channel operation is also decreased wavelengths were chosen at the maximum of the transition
with respect to the non‐fluorinated polymer and remains at around 430 nm for all samples. Optical properties in solution
around 50 V. Additionally, in these polymers, the hole‐mobility were measured in chloroform at a concentration of 0.01 mg/mL
is not negatively influenced by this high‐temperature annealing and a path length of 10 mm, films were spin‐coated onto glass
step and remains in the range of 10‐2 cm2V‐1s‐1. The origin of the slides from a 7 mg/mL chloroform solution at 1500 rpm.
high threshold voltages could be related to electron trapping at Solutions for spin‐coating were prepared by dissolving the
the dielectric/polymer interface.32
polymer in chloroform at the given concentration and stirring at
55 °C for 8 h. Thermogravimetric analysis was performed on a
Netzsch STA 449 F3 Jupiter under N2 atmosphere at a heating
rate of 10 K min‐1. Temperature of decomposition (T5%) was
calculated from the 5% degradation step of the respective
curve. Flash‐DSC measurements have been performed on a
Mettler‐Toledo Flash DSC 1.34
GIWAXS measurements were conducted at the SAXS/WAXS
beamline of the Australian Synchrotron.35 Highly collimated 9
keV photons were aligned to within 0.01 degrees to parallel to
the surface of each sample by use of a Silicon crystal analyzer
and zeroing the sample tilt. A Dectris Pilatus 1M detector
collected 2D scattering patterns, including those shown in
Figure 5. Each scattering pattern was tiled together from three
1‐second images with the detector slightly moved between
exposures, such that the resulting image removes gaps between
the detector modules. The sample to detector distance was
measured using a Silver Behenate scattering standard. Data was
analysed using a modified version of the NIKA small angle
scattering analysis package.36
Experimental Section
Materials and methods
All reagents were used without further purification unless
otherwise noted. Microwave reactions were conducted in
sealed containers using a Biotage Initiator Eight+ microwave. All
1
polymer H and 19F NMR spectra were recorded on a Varian
INOVA 300 spectrometer at 393 K in 1,1,2,2‐tetrachloroethane
1
(TCE) as solvent. The H spectra were referenced internally by
using the residual solvent resonances. Deuterated solvents
were obtained from Deutero. Gel permeation chromatography
(GPC) analysis was carried out on an Agilent (Polymer
Laboratories Ltd.) PL‐GPC 220 high temperature
chromatographic unit equipped with DP, RI and LS (15° and 90°)
detectors and three linear mixed bed columns of PLgel 13 µm
(Olexis)
with
a
linear
MW
operating
range:
500 ‐ 15,000,000 g mol‐1. GPC analysis was performed at 150 °C
using 1,2,4‐trichlorobenzene as the mobile phase. The samples
were prepared by dissolving the polymer (0.1 wt‐%) in the
mobile phase solvent in an external oven and the solutions were
run without filtration. The molecular weights of the samples
Device preparation and characterization
Organic thin film transistors substrates in bottom gate / bottom
contact configuration were bought from Fraunhofer IPMS (OFET
Gen. 4). Heavily n‐doped silicon (doping at wafer surface:
n~3 x 1017 cm‐3) was used as substrate and gate electrode.
Thermally grown silicon oxide (230 nm ± 10 nm) was used as the
gate dielectric. Gold electrodes (30 nm on 10 nm ITO as
adhesion layer) were used as source and drain contacts. The
channel width was 10 mm for all devices and channel lengths of
10 µm and 20 µm were used. The substrates were cleaned
subsequently in acetone and 2‐propanol in an ultrasonic bath
for 10 minutes each. Treatment in an ozone oven at 50 °C for
20 min was followed by immersion in a 1 wt‐% solution of
octyltrichlorosilane in toluene at 60 °C for 60 min. After rinsing
with toluene and 2‐propanol the substrates were dried in a
nitrogen stream and the polymer was spincoated from a
6 mg/mL chloroform solution at 2000 rpm under ambient
were
referenced
to
linear
polystyrene
(Mw = 162 ‐ 6,000,000 g mol‐1, K = 12.100 and Alpha = 0.707) and
were not corrected with K and Alpha values for the measured
sample. Cyclic voltammetry was performed under moisture‐
and oxygen‐free conditions using
butylammonium hexafluorophosphate
a
0.1
in
M
tetra‐n‐
acetonitrile
electrolyte solution. A standard three‐electrode assembly
connected to a potentiostat (model 263A, EG&G Princeton
Applied Research) was used at a scanning rate of 100 mV s‐1.
The working electrode was a 10 □ ITO coated glass substrate.
The polymers were spincoated onto the ITO substrates from
chloroform at 3 mg/mL and 1500 rpm to obtain thicknesses of
10‐20 nm. A platinum wire in acetonitrile was used as counter
electrode and the quasi‐reference electrode consisted of an Ag
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conditions. Devices were measured in a nitrogen atmosphere the polymer was precipitated in 200 mL methanolVaienwdArftiicllterOantleinde
DOI: 10.1039/C5TC01948F
using an Agilent B1500 Semiconductor Parameter Analyzer. The into a soxhlet thimble. The crude polymer was subjected to
devices were annealed in a nitrogen atmosphere at a maximum soxhlet extraction using methanol, acetone, hexane,
of 0.9 ppm O2 for 15 minutes consecutively at the temperatures dichloromethane and chloroform. The chloroform fraction was
given in the main text. Mobilities were calculated from the concentrated to 4 mL under reduced pressure and precipitated
slopes in the (Id)0.5‐Vg plots in the saturation regime using in 200 mL methanol. Filtration over a 0.45 µm PTFE filter
Equation 3 where Id is the drain current, W the channel width, L followed by drying in high vacuum afforded PDPP[T]2‐TPF2T as a
1
the channel length, Ci the capacitance, Vg the gate voltage and dark purple solid (131 mg, 70%). H NMR (300 MHz, TCE):
VT the threshold voltage, respectively.

(ppm) 8.84 (br, s), 7.40‐7.24 (br, m), 4.11 (br, s, R2N‐CH2‐), 2.06
(br, s, ‐R2CH‐), 1.44‐1.34 (br, m, ‐CH2‐), 0.95 (br, s, ‐CH3); 19F
NMR (282 MHz, TCE) (ppm) ‐117.96.
W
Ci(Vg VT )2
(3)
Id

2L

PDPP[T]2‐TPF4T. An oven dried microwave vial was loaded with
DPP[T]2{2DTd)2Br2 (163.46 mg, 0.144 mmol), (5,5'‐(perfluoro‐
1,4‐phenylene)bis(thiophene‐5,2‐diyl))bis(trimethylstannane)
(93.37 mg, 0.144 mmol), tri‐o‐tolylphosphine (5.20 mg,
17 µmol) and finally tris(dibenzylideneacetone)dipalladium(0)
(3.92 mg, 4.3 µmol). Dry and degassed chlorobenzene (5 mL)
was added, the vial was sealed with a septum cap and
subsequently purged with argon for 15 min. The mixture was
stirred for 30 min at 180 °C in the microwave. After cooling to
50 °C 2‐tributyltinthiophene (25 µL, 79 µmol) was added to the
green gel using a microliter syringe. The mixture was again
heated at 180 °C for 5 min in the microwave, cooled to 50 °C
and finally 2‐bromothiophene (30 µL, 308 µmol) was added. The
mixture was heated to 180 °C for 5 min in the microwave. After
cooling to room temperature the polymer was precipitated in
250 mL methanol and filtrated over a 0.45 µm PTFE filter. The
crude polymer was subjected to soxhlet extraction using
methanol, acetone, hexane and dichloromethane. The solid
remains were refluxed in 150 mL chloroform for 2 h and
filtrated. The filtrate was then concentrated to 4 mL under
reduced pressure and precipitated in 200 mL methanol.
Filtration over a 0.45 µm PTFE filter followed by drying in high
vacuum afforded PDPP[T]2‐TPF4T as a dark purple solid (108 mg,
Synthesis procedures for the DPP compounds.
All syntheses were conducted under argon using a double
manifold schlenk line. Glasware was severely dried under high
vacuum. Solvents were degassed by three freeze‐pump‐thaw
cycles. Monomers used for polymerization were recrystallized
freshly on the same day.
PDPP[T]2‐TPT. An oven dried microwave vial was loaded with
DPP[T]2{2DTd)2Br2 (124.4 mg, 0.110 mmol), 1,4‐bis(5‐
(trimethylstannyl)thiophen‐2‐yl)benzene
0.111 mmol), tri‐o‐tolylphosphine (3.92 mg, 13 µmol) and finally
tris(dibenzylideneacetone)dipalladium(0) (3.15 mg, 3.4 µmol).
Dry and degassed chlorobenzene (3.0 mL) was added, the vial
was sealed with a septum cap and subsequently purged with
argon for 15 min. The mixture was stirred for 30 min at 180 °C
in the microwave. After cooling to 50 °C 2‐tributyltinthiophene
(25 µL, 79 µmol) was added to the green gel using a microliter
syringe. The mixture was again heated at 180 °C for 5 min in the
microwave, cooled to 50 °C and finally 2‐bromothiophene
(30 µL, 308 µmol) was added. The mixture was heated to 180 °C
for 5 min in the microwave. After cooling to room temperature
the polymer was precipitated in 200 mL methanol and filtrated
into a soxhlet thimble. The crude polymer was subjected to
soxhlet extraction using acetone, hexane, dichloromethane and
chloroform. The chloroform fraction was concentrated to 4 mL
under reduced pressure and precipitated in 200 mL methanol.
Filtration over a 0.45 µm PTFE filter followed by drying in high
(62.85
mg,
52%). 1H NMR (300 MHz, TCE):
 (ppm) 8.85 (br, s), 7.71‐7.15
(br, m), 4.10 (br, s, R2N‐CH2‐), 2.07 (br, s, ‐R2CH‐), 1.44‐1.33 (br,
m, ‐CH2‐), 0.95 (br, m, ‐CH3); 19F NMR (282 MHz, TCE)

(ppm) ‐139.97.
vacuum afforded PDPP[T]2‐TPT as a dark purple solid (91 mg,
1
68%). H NMR (300 MHz, TCE):
 (ppm) 5.85 (br, s), 7.69‐7.37
Conclusions
(br, m), 4.10 (br, s, R2N‐CH2‐), 2.08 (br, s, ‐R2CH‐), 1.44‐1.34 (br,
m, ‐CH2‐), 0.95 (m, ‐CH3).
Diketopyrrolo[3,4‐c]pyrrole copolymers incorporating an
extended thiophene‐phenylene‐thiophene moiety with varying
degrees of fluorination have been successfully synthesized. The
concept of fluorination was evaluated in terms of alignment and
solid state packing. As evident from GIWAXS, fluorination has a
small influence on the alkyl spacing distance as well as the 
stacking distance. However, the crystal coherence length can be
significantly improved upon fluorination. Furthermore, we
showed that upon gradual increase of fluorination and
annealing the edge‐on alignment of the polymer chains
improves significantly, resulting in almost perfect edge‐on
alignment throughout the complete bulk of the film in the
tetrafluorinated derivative. Whereas alignment is improved,
fluorination does not necessarily lead to improved hole
transport, but is a good approach to achieve ambipolar
character in otherwise p‐type materials. From our results we
PDPP[T]2‐TPF2T. An oven dried microwave vial was loaded with
DPP[T]2{2DTd)2Br2
(148.80 mg, 0.132 mmol), (5,5'‐(2,5‐
difluoro‐1,4‐phenylene)bis(thiophene‐5,2‐
diyl))bis(trimethylstannane) (79.33 mg, 0.131 mmol), tri‐o‐
tolylphosphine (5.43 mg, 18 µmol) and finally
tris(dibenzylideneacetone)dipalladium(0) (4.23 mg, 4.6 µmol).
Dry and degassed chlorobenzene (5.0 mL) was added, the vial
was sealed with a septum cap and subsequently purged with
argon for 15 min. The mixture was stirred for 30 min at 180 °C
in the microwave. After cooling to 50 °C 2‐tributyltinthiophene
(25 µL, 79 µmol) was added to the green gel using a microliter
syringe. The mixture was again heated at 180 °C for 5 min in the
microwave, cooled to 50 °C and finally 2‐bromothiophene
(30 µL, 308 µmol) was added. The mixture was heated to 180 °C
for 5 min in the microwave. After cooling to room temperature
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Acknowledgements
We acknowledge financial support from DFG (SFB 840) and the
Bavarian State Ministry of Education, Science and the Arts (Solar
technologies go hybrid). C.J.M. thanks the Fonds der
Chemischen Industrie for funding the PhD with a Kekulé
scholarship and the German National Academic Foundation for
other support during the PhD. Support from the Elitenetzwerk
Bayern (ENB), Macromolecular Science is also kindly
acknowledged. Work in Australia was supported by the
Australian Research Council (grant DP13012616). This research
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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC01948F
Table of Contents entry
Influence of fluorination in π-extended backbone polydiketopyrrolopyrroles on charge carrier
mobility and depth-dependent molecular alignment
Christian J. Mueller, Eliot Gann, Christopher R. McNeill and Mukundan Thelakkat
The degree of fluorination in π-extended polydiketopyrrolopyrroles is correlated with
semiconductor properties in transistors and an improved molecular alignment in thin films using
depth-dependent grazing incidence X-ray scattering.