N -Alkyl substituted 1 H -benzimidazoles as improved n-type dopants for a naphthalene-diimide based copolymer

Article abstract of DOI:10.1039/c8ta04901g

Doped polymer semiconductors are actively studied for opto- and micro-electronic applications including thermoelectric generators, where a high electrical conductivity is a key factor. In general, n-type doping is more challenging to achieve than p-type doping. Here we study n-type doping of a commonly used electron transporting naphthalene-diimide bithiophene copolymer with a series of air-stable and solution-processable benzimidazole dopants. To understand the role of dopant structure on miscibility and the resulting conductivity, benzimidazoles with different linear and branched alkyl substituents were synthesized, and their doping efficacy compared through combined morphological, electrical and thermoelectric characterization. We observe a clear dependence of the nature of the alkyl substituent on dopant intercalation into the semicrystalline morphology. By increasing the length or the steric hindrance of the alkyl substituents, the miscibility between dopant and copolymer is enhanced leading to optimized electrical conductivity.

Full text of DOI:10.1039/c8ta04901g

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and C. BERTARELLI, J. Mater. Chem. A, 2018, DOI: 10.1039/C8TA04901G.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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JPolueransael odfoMnaotetraiadljsuCsht emmairsgtriynsA
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DOI: 10.1039/C8TA04901G
Journal Name
ARTICLE
N-alkyl substituted 1H-benzimidazoles as improved n-type
dopants for a naphthalene-diimide based copolymer
B. Saglioab, M. Muraa, M. Massettibc, F. Scurattibd, D. Berettab, X. Jiaoe, C. R. McNeille, M. Sommerf,
Received 00th January 20xx,
Accepted 00th January 20xx
A. Famularia, G. Lanzanibc, M. Caironib* and C. Bertarelliab*
DOI: 10.1039/x0xx00000x
Doped polymer semiconductors are actively studied for opto- and micro-electronic applications including thermoelectric
generators, where a high electrical conductivity is a key factor. In general, n-type doping is more challenging to achieve than
p-type doping. Here we study n-type doping of a commonly used electron transporting naphthalene-diimide bithiophene
copolymer with a series of air-stable and solution-processable benzimidazole dopants. To understand the role of dopant
structure on miscibility and the resulting conductivity, benzimidazoles with different linear and branched alkyl substituents
were synthesized, and their doping efficacy compared through combined morphological, electrical and thermoelectric
characterization. We observe a clear dependence of the nature of the alkyl substituent on dopant intercalation into the
semicrystalline morphology. By increasing the length or the steric hindrance of the alkyl substituents, the miscibility between
dopant and copolymer is enhanced leading to optimized electrical conductivity.
www.rsc.org/
P-type conducting polymers and related dopants have been
deeply investigated in past years and impressive conductivity
values have been recently achieved.14–16 On the contrary, for
Introduction
Conjugated polymers have found application in various
electronic devices, including transistors and circuits,1–3 solar
cells4,5 and light emitting diodes.6,7 Charge transport in polymer
films is largely governed by the polymer chemical structure and
solid-state packing of interacting conjugated segments8.
Research in this field has led to a rationalization of the
molecular design and a better understanding of the charge
transport mechanism. Through the establishment of structure-
property relationships,9–11 tailored structures and optimized
processing have led to enhanced electronic performance.
For most electronic and opto-electronic devices, it is desirable
to tune the electrical conductivity (σ) through doping in order
to enable optimized devices. More recently, the increased
interest in polymer thermoelectrics12,13 as cost-effective waste
heat energy converters has re-activated research in molecular
doping to achieve high and controllable values of σ. Doping of
conjugated polymers is typically achieved by the addition of
molecular dopants which produce an increase in the number of
free charge carriers and hence of σ.
the case of the n-type counterparts, which are required to
enable thermocouples for efficient thermoelectric generators,
examples are still limited. Issues limiting n-type conducting
polymers are related to ambient stability: while stable electron
transporting materials are known, their corresponding radical
anions that form upon doping are prone to undergo re-
oxidation to the neutral form if exposed to e.g. humidity or
oxygen.17 The extent of the instability of the radical anion to
ambient conditions strongly depends on the position of the
lowest unoccupied molecular orbital (LUMO) level, as
demonstrated for example in rylene diimides.18 N-type dopants
strongly suffer from air-instability, as well.
N-type doping of polyacetylene was first performed under inert
atmosphere using metal complexes, such as sodium and
potassium naphthalene,19 with elemental dopants such as Na,
K, Li and Cs,20–23 or alternatively, inorganic salts as Li2CO3,
Na2CO3, K2CO3, Cs2CO3, and LiNH224–28 have also been proposed.
However, such doping could only be carried out by evaporation
at high temperatures, which is less attractive than solution-
based techniques; moreover, inorganic ions often affect the
device operation lifetime since they diffuse through the organic
layer. Aromatic donor compounds, e.g. TTF29,30 or BEDT-TTF,31
form charge transfer complexes and can be processed from
solution, while being unstable in air. This last issue can be
overcome by stable precursors, which can be in situ converted
into the dopant upon thermal or light activation, e.g. 1H-
benzimidazole derivatives such as DMBI-POH32 or the organic
salt o-MeO-DMBI-I33. The latter can only be processed by
evaporation.
a.Dipartimento di Chimica, Materiali e Ing. Chimica “G. Natta , Politecnico di
Milano, Piazza L. Da Vinci 32, 20133 Milano, Italy
b.Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia,
Via Pascoli 70/3, 20133 Milano, Italy *Email - chiara.bertarelli@polimi.it
c. Dipartimento di Fisica, Politecnico di Milano, Piazza L. Da Vinci 32, 20133 Milano,
Italy
d.Dipartimento di Elettronica, Informazione e Bioingegneria, Politecnico di Milano,
Piazza L. Da Vinci 32, 20133 Milano, Italy
e. Department of Materials Science and Engineering, Monash University Wellington
Road, Clayton, Victoria, 3800, Australia.
f. Chemnitz University of Technology, Chemistry Department, Strasse der Nationen
62, 09111 Chemnitz, Germany.
This journal is © The Royal Society of Chemistry 20xx
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4-Bromo-N,N-dipropylaniline (2d): N,N
-
DipropylanilVinieew A(1rticgle,O5n.l6in4e
In 2010, Bao et al. first reported n-type dopant 1,3-dimethyl-2-
2-phenyl-2,3-dihydro-1H-benzoimidazole (DMBI) – which had
been previously reported as a reagent for the reduction of
organic compounds34 – as an effective, solution processable and
air-stable dopant to significantly increase the conductivity of
phenyl-C61-butyric acid methyl ester (PCBM).35 Later, Schlitz et
al.36 used the same compound to dope poly(N,N′-bis-2-
octyldodecylnaphthalene-1,4,5,8-bis-dicarboximide-2,6-diyl-
alt-5,5′-2,2′-bithiophene) P(NDI2OD-T2), obtaining an increase
of the electrical conductivity of about five orders of magnitude,
reaching 5 × 10-4 S/cm. In the same study a poor miscibility of
the dopant and P(NDI2OD-T2) was reported, i.e. phase
segregation occurred at high dopant concentration, limiting the
effectiveness of the doping and the maximum conductivity that
could be achieved. Another limitation relates to the strong
charge localization in the polymer, as evidenced by Fabiano et
al.37
DOI: 10.1039/C8TA04901G
mmol, 1 eq.) was dissolved in 17 ml dry DMF, and the solution
was stirred at -18°C under argon. NBS (1.05 g, 5.92 mmol, 1.05
eq.), dissolved in 5 ml dry DMF, was dropwise added over a
period of 1.5 hours. Then the reaction mixture was stirred at -
18 °C for 3.5 hours and at room temperature overnight. The
reaction mixture was separated between dichloromethane
(DCM)/water, the water phase extracted with DCM, the DCM
layers combined, dried over Na2SO4 and concentrated under
reduced pressure. A suspension of the residue in Et2O was
filtered, and the filtrate was washed with 3M NaOH aqueous
solution and water, dried over Na2SO4, and concentrated under
reduced pressure. The crude product was purified by flash
chromatography with silica gel and hexane:ethyl acetate (9:1)
to give the product as an orange oil in 83% yield.
1H NMR (d6-benzene, 400MHz): ppm 7.29 (dt, J=9.17 Hz, J=3.42
Hz, J=2.26 Hz, 2H H-Ph), 6.27 (dt, J=9.17 Hz, J=3.42 Hz, J=2.26,
2H, H-Ph), 2.79 (t, J=7.58, 4H, N-CH2), 1.29 (m, J=7.58, J=7.40,
4H, -CH2-), 0.65 (t, J=7.40, 6H, CH3).
Here we address the miscibility issue by studying the effect of
the benzimidazole alkyl substituents on the doping efficiency.
With electronic effects of the alkyl substituents on electron
transfer being negligible, we have designed and synthesized a
series of 1-H benzimidazoles in which the N-alkyl substituents
have been systematically varied (Fig. 1). This strategy is aimed
at increasing dopant solubility and miscibility with P(NDI2OD-
T2) through an enhanced interaction with its alkyl side chains.
In contrast to recent work that has focused on dopant/
copolymer miscibility by controlling backbone backbone
curvature38 or varying chain length or chain branching,39,40 we
show here that a tailored modification of the dopant structure
4-Bromo-N,N-diisopropylaniline (2e): N,N-Diisopropylaniline (546
mg, 3.08 mmol, 1 eq.) was dissolved in 3 ml anhydrous
acetonitrile under argon. NBS (548mg, 3.08 mmol, 1 eq.),
dissolved in 3ml anhydrous acetonitrile, was added dropwise at
0 °C over 30 minutes. The reaction mixture was stirred at 0 °C
for 1 hour and then at room temperature overnight. Water was
added, the reaction mixture was extracted with hexane, the
organic phase dried over Na2SO4 and the solvent removed
towards longer and bulkier alkyl substituents has a strong effect under reduced pressure. The crude product was purified by
miscibility and doping efficiency.
flash chromatography with hexane:ethyl acetate (8:2) as eluent,
obtaining the pure product as an orange oil with 48% yield. H
1
NMR (d6-benzene, 400MHz): ppm 7.25 (dt, J=9.17 Hz, J=3.42 Hz,
J=2.26, 2H H-Ph), 6.52 (dt, J=9.17 Hz, J=3.42 Hz, J=2.26Hz, 2H, H-
Ph), 3.33 (sestet, J=6.72 Hz, 2H, N-CH), 0.89 (d, J=6.72 Hz, 12H, -CH3).
Experimental
Dopant synthesis
Unless otherwise stated, all chemicals and solvents were
purchased from Sigma Aldrich and used as received. Reactions
of air- and water-sensitive reagents and intermediates were
carried out in dried glassware under argon. Thin layer
chromatography was performed by using silica gel on aluminum
foil. NMR spectra were acquired on a Bruker ARX400. Mass
spectroscopy was carried out with a Bruker Esquire 3000 plus.
4-Bromo-N,N-diisobutylaniline (2f): The reaction was carried out
according to the procedure of 2e. The crude product was purified by
flash chromatography using hexane:chloroform 9:1 as eluent to give
2f as an orange oil in 21% yield. 1H NMR (d6-benzene, 400MHz): ppm
7.28 (m, J=9.17 Hz, J=3.42 Hz, J=2.26, 2H H-Ph), 6.32 (m, J=9.17 Hz,
J=3.42 Hz, J=2.26, 2H, H-Ph), 2.79 (d, J=7.34 Hz, 4H, N-CH2), 1.84 (m,
J=6.72 Hz, 2H, -CH), 0.68 (d, J=6.72 Hz, 12H, -CH3).
4-(Dipropylamino)benzaldehyde (3d): Compound 2d (1.20 g, 4.68
mmol, 1 eq.) was dissolved in 9 ml anhydrous THF and the
solution was cooled to -78° C under an argon atmosphere. 2.5M
n-BuLi (in n-hexane, 4.77 mmol, 1.02 eq.) was added dropwise
and the reaction mixture was stirred for 30 minutes at the same
temperature. Anhydrous DMF (9.4 ml) was added and the
solution was kept stirring at -78° C for 30 minutes. Then the
mixture was warmed up to room temperature, quenched with
70 ml of water and extracted with DCM (3x). The combined
organic layers were dried with Na2SO4. After solvent removal,
the crude product was purified by flash chromatography with
Fig. 1 Molecular structure of N-alkyl substituted 1H- benzimidazoles.
2 | J. Name., 2012, 00, 1-3
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Fig. 2 Synthetic routes for the realization of N-alkyl substituted 1H- benzimidazoles.
hexane:ethylacetate (8:2) to give a yellow oil in 54% yield. 1H
NMR (d6-benzene, 400MHz): ppm 9.85 (s, 1H, CHO) 7.70 (d, J=8.92
Hz 2H H-Ph), 6.35 (d, J=8.92 Hz, 2H, H-Ph), 2.77 (t, J=7.64 Hz, 4H, N-
CH2), 1.24 (m, J=7.46 Hz, J=7.70 Hz, 4H, CH2-CH3), 0.60 (t, J=7.40 Hz
6H, CH2-CH3).
(m, J=3.18 Hz, J=2.01 Hz, 2H, H-Ph), 6.39 (m, J=3.18 Hz, J=2.01
Hz, 2H, H-Ph), 4.71 (s, 1H, N-H-N), 3.35 (m, J=7.27 Hz, J=7.95 Hz,
4H, N-CH2), 2.45 (s, 6H, N-CH3), 1.11 (t, J=6.85 Hz, 6H, CH2-CH3).
(Yield: 42%).
4-(1,3-Dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)-N,N-
dipropylaniline (DPrBI): The reaction was carried out according
to the procedure of DMBI. 1H NMR (d6-DMSO, 400MHz): ppm
7.27 (d, J=8.74 Hz, 2H, H-Ph), 6.65 (d, J=8.74 Hz, 2H, H-Ph), 6.58
(m, J=3.18 Hz, J=2.01 Hz, 2H, H-Ph), 6.39 (m, J=3.18 Hz, J=2.01
Hz, 2H, H-Ph), 4.67 (s, 1H, N-H-N), 3.24 (m, J=7.46 Hz, J=7,58,
4H, N-CH2), 2.44 (s, 6H, N-CH3), 1.54 (m, J=7.58 Hz, 4H, CH2-CH2-
CH3), 0.89 (m, J=9.00 Hz, J=7.40 Hz, 6H, CH2-CH3). (Yield: 17%).
4-(Diisopropylamino)benzaldehyde (3e): The reaction was
carried out according to the procedure of 3d
benzene, 400MHz): ppm 9.60 (s, 1H, CHO) 7.56 (d, J=8.07 Hz, 2H
H-Ph), 6.73 (d, J=8.07 Hz, 2H, H-Ph), 3.89 (m, J=6.48, J=6.66 Hz,
J=6.91 Hz 2H, N-CH), 1.23 (d, J=6.72 Hz, 12H, CH2-CH3). (Yield:
50%).
.
1H NMR (d6-
4-(Isobutyl(isopropyl)amino)benzaldehyde (3f): The reaction
was carried out according to the procedure of 3d.
1H NMR (d6-
benzene, 400MHz): ppm 9.60 (s, 1H, CHO) 7.59 (d, J=8.92 Hz 2H
H-Ph), 6.59 (d, J=8.92 Hz, 2H, H-Ph), 3.14 (d, J=7.34 Hz, 4H, N-
CH2), 2.00 (m, J=6.85 Hz, J=6.78 Hz, 2H, CH2-CH), 0.81 (d, J=6.60
Hz, 12H, CH-CH3). (Yield: 67%).
N,N-Dibutyl-4-(1,3-dimethyl-2,3-dihydro-1H-
benzo[d]imidazol-2-yl)aniline (DBuBI)
carried out according to the procedure of DMBI
:
The reaction was
1H NMR (d6-
.
DMSO, 400MHz): ppm 7.28 (d, J=8.74 Hz, 2H, H-Ph) 6.66 (d,
J=8.74 Hz, 2H, H-Ph), 6.58 (m, 3.18 Hz, J=2.01 Hz, 2H, H-Ph), 6.39
(m, J=3.18Hz, J=2.01 Hz, 2H, H-Ph), 4.70 (s, 1H, N-H-N), 3.27 (q,
J=7.24 Hz, 4H, N-CH2), 2.45 (s, 6H, N-CH3), 1.52 (m, J=8.07 Hz,
J=6.91 Hz, 4H, CH2-CH2-CH2), 1.34 (m, J=7.58 Hz, J=7.35 Hz, 4H,
CH2-CH2-CH2), 0.93 (t, J=7.34 Hz, 6H, CH2-CH3). (Yield: 31%).
4-(1,3-Dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)-N,N-
dimethylaniline (DMBI): The synthetic procedure of this
compound was previously reported by Naab.41 N,N’-dimethyl-
o-phenylenediamine (1.00 mmol, 1 eq.), N,N′-dimethyl-1,2-
phenylenediamine (1.00 mmol, 1 eq.) and a drop of glacial acetic
acid were dissolved in 2 ml of methanol. The mixture was
sonicated for 5 hours, followed by storage in the fridge
overnight. The precipitate was filtered and washed with
methanol to obtain DMBI as white powder in 26% yield. 1H NMR
(d6-DMSO, 400MHz): ppm 7.33 (d, J=8.74 Hz, 2H, H-Ph), 5.25 (d,
J=8.74 Hz, 2H, H-Ph), 6.58 (m, J=3.18 Hz, J=2.01 Hz, 2H, H-Ph),
6.39 (m, J=3.18, J=2.20, 2H, H-Ph), 4.73 (s, 1H, N-H-N), 2.93 (s,
6H, N-CH3), 2.44 (s, 6H, N-CH3).
4-(1,3-Dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)-N,N-
diisopropylaniline (DiPrBI): The reaction was carried out
according to the procedure of DMBI.
1H NMR (d6-DMSO,
400MHz): ppm 7.27 (d, J=8.74 Hz, 2H, H-Ph), 6.84 (d, J=8.74 Hz,
2H, H-Ph), 6.59 (m, J=3.18 Hz, J=2.01 Hz, 2H, H-Ph), 6.40 (m,
J=3.18 Hz, J=2.01 Hz, 2H, H-Ph), 4.69 (s, 1H, N-H-N), 3.84 (m,
J=6.78 Hz, J=6.85, 2H, N-CH), 2.45 (s, 6H, N-CH3), 1.21 (d, J=6.85
Hz, 12H, CH-CH3). (Yield: 31%).
4-(1,3-Dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)-N,N-
diisobutylaniline (DiBuBI): The reaction was carried out
according to the procedure of DMBI.
1H NMR (d6-DMSO,
400MHz): ppm 7.26 (d, J=8.74 Hz, 2H, H-Ph), 6.67 (d, J=8.74 Hz,
2H, H-Ph), 6.58 (m, J=3.18 Hz, J=2.01 Hz, 2H, H-Ph), 6.39 (m,
4-(1,3-Dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)-N,N-
diethylaniline (DEtBI): The reaction was carried out according
to the procedure of DMBI. 1H NMR (d6-DMSO, 400MHz): ppm
7.29 (d, J=8.74 Hz, 2H, H-Ph), 6.69 (d, J=8.74 Hz, 2H, H-Ph), 6.58
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J=3.18 Hz, J=2.01 Hz, 2H, H-Ph), 4.67 (s, 1H, N-H-N), 3.16 (d, measured with Agilent 5500 Atomic Force Microscope working
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DOI: 10.1039/C8TA04901G
J=7.09 Hz, 4H, N-CH2), 2.44 (s, 6H, N-CH3), 1.99 (m, J=6.72 Hz, in Acoustic Mode. All images are presented with the same
J=6.97 Hz, 2H, N-CH2-CH) 0.87 (d, J=6.60 Hz, 12H, CH-CH3). colour scale.
(Yield: 55%).
Grazing incidence wide angle X-ray scattering. GIWAXS
measurements were performed at the SAXS/WAXS beamline at
Sample preparation and doping procedure. The samples were the Australian Synchrotron43. 11 keV photons were used with
prepared starting from a solution of P(NDI2OD-T2) (Polyera scattering patterns recorded on a Dectris Pilatus 1M detector.
ActivInk N2200, Mn = 29 kDa, purchased from Flexterra Inc.), in Images shown were acquired at an incident angle close to the
1,2-dichlorobenzene (DCB) that was heated to 80 °C and stirred critical angle. Such images were chosen from a series of images
for 1 hour. The concentration of the solutions used for films for taken with incident X-ray angle varying from 0.05
 to 0.25 in
variable temperature electrical conductivity, room temperature steps of 0.01 with the chosen image showing the highest

conductivity and Seebeck coefficient measurements were 5 scattering intensity. The X-ray exposure time was 3 s such that
mg/ml, 10 mg/ml and 20 mg/ml, respectively. The solution was no film damage was identified. The sample-to-detector distance
filtered with a 0.45 µm pore size polytetrafluoroethylene (PTFE) was calibrated using a silver behenate sample. The results were
filter. The dopant was dissolved in DCB at a concentration of 10 analyzed by an altered version of the NIKA 2D44 based in
mg/ml. Aliquots of dopant were added to 100
l polymer IgorPro.
solution and mixed at room temperature. Dopant
concentrations between 10 % to 50 % were tested. Except for
Seebeck coefficient measurements (see below), films were spin
coated in an N2 glovebox at 1000 rpm for 90 s and then at 3000
rpm for 10 s onto glass substrates, previously cleaned with
acetone and iso-propanol and treated with an oxygen plasma
for 10 minutes. Films were annealed on a hot plate at ≈150 °C
for six hours in N2 glovebox.
Differential scanning calorimetry. Pristine dopants were
measured on a Perkin-Elmer DSC 8500 instrument equipped
with a liquid nitrogen cooling system (Perkin-Elmer CLN2) under
N2 at 10 K/min. Blend samples were prepared from o-DCB
solution, thoroughly dried and placed into the heating pans.
Measurements were acquired on a NETZSCH DSC 204 F1
Phoenix® under N2 at 10 K/min.
Thermogravimetry. TGA was performed on a Perkin Elmer TGA
Electrical conductivity measurements. Two 50 nm thick gold 4000 between 50−650 °C under N2 at 10 K min‐1.
contacts were evaporated (MB-ProVap-3) on the films leading
to devices with an inter-electrode distance of 7 mm and width
of 12 mm. Prior to measurement, samples were annealed at ≈80
Results and discussion
°C for 10 minutes under N2. Measurements were taken under
The six dopant molecules synthesised are shown in Fig. 1. DMBI
has been studied before and features methyl substituents. 4-
(1,3-Dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)-N,N-
diethylaniline (DEtBI) has ethyl substituents, 4-(1,3-dimethyl-
N2 atmosphere and at ambient temperature with a 2-point
method with a Keysight Technologies semiconductor parameter
analyzer (SPA).
2,3-dihydro-1H-benzo[d]imidazol-2-yl)-N,N-dipropylaniline
(DPrBI) has n-propyl substituents and N,N-dibutyl-4-(1,3-
dimethyl-2,3-dihydro-1Hbenzo[d]imidazol-2-yl)aniline (DBuBI)
has n-butyl substituents. DMBI, DEtBI DMPrBI and DBuBI thus
have linear chains of increasing length. In contrast, 4-(1,3-
dimethyl-2,3-dihydro-1H-benzo[d]imidazol-2-yl)-N,N-
diisopropylaniline (DiPrBI) and 4-(1,3-dimethyl-2,3-dihydro-1H-
benzo[d]imidazol-2-yl)-N,N-diisobutylaniline (DiBuBI) have
branched alkyl substituents of different size. The general
synthetic scheme consists of the ultrasound–assisted coupling
between the N,N′-dimethyl-1,2-benzendiamine and the N-alkyl
substituted p-aminobenzaldehyde (Fig. 2). For the synthesis of
DMBI, DEtBI and DBuBI the procedure previously reported for
DMBI41 was followed, starting from the commercially available
N,N-dialkyl aminobenzaldehydes. The products were
precipitated in the reaction mixture at low temperature,
separated by filtration and washed with small portions of
methanol. For the synthesis of the other dopants (DPrBI, DiPrBI,
DiBuBI), the starting compound was the N,N-dialkyl substituted
aniline, which was first brominated in p-position with N-
bromosuccinimide (NBS), lithiated with n-butyllithium and
Seebeck coefficient measurements
The polymer/dopant solutions were deposited by spin coating
(1000 rpm for 60 s) onto 2 cm x 1 cm glass substrates and then
annealed at 120
Seebeck measurements were carried out in vacuum at 90
after an overnight annealing treatment at 130 C in order to

C for 30 minutes in a nitrogen glovebox. The

C,

avoid any thermal effect during the measurement, using the
system described in Beretta et al.42
Temperature-dependent
electrical
conductivity
measurements. Four 50 nm thick gold contacts were
evaporated on the films leading to devices with an inter-
electrodes distance of 3 mm and width of 5 mm. Prior to four-
point conductivity measurement, samples were annealed at
≈150 °C overnight in vacuum. The four-point probes
measurements were taken in vacuum in the temperature range
between 200 K and 300 K, with a ∆T of 10 K between each step.
Downward and upward scans were performed in order to check
the absence of hysteresis during the measurements.
Atomic force microscopy. Sample preparation was identical as
for the electrical characterization. Topographies were
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DOI: 10.1039/C8TA04901G
Fig. 4 Temperature dependence of electrical conductivity between 200 and 300 K for
the polymer doped with three representative dopants at 20% concentration.
Fig. 3 Electrical conductivity σ vs. dopant concentration (from 10 to 50 %) of all N-alkyl
substituted 1H- benzimidazoles synthetized.
showed a conductivity in the same range of the dopant with n-
butyl substituents.
quenching
with
DMF
afforded
the
N,N-
dialkylaminobenzaldehyde (Fig. 2). The resulting compound was
finally coupled with N,N’-dimethyl-1,2-phenylenediamine and
products (2d-f) were precipitated and separated by filtration.
We have also tried to synthesize 1H-benzimidazoles with longer
alkyl chains (i.e. hexyl), but we could not isolate the pure
product by precipitation according to the procedure described
for all the other compounds.
The temperature stability and phase transitions of the dopants
were investigated using thermogravimetry (TGA) and
differential scanning calorimetry (DSC). From TGA, onset
We also performed measurements to determine the Seebeck
coefficients (α) of three representative polymer/dopant blends,
namely the reference 1H-benzimidazole with methyl (DMBI),
the dopant with linear chains giving the most effecting doping
(DBuBI) and the dopant with branched chains giving the most
effective doping (DiPrBI). The measured values of α at 363 K are
always negative, as expected for a Seebeck effect dominated by
electrons, with values of -44 μV/K for the DMBI-doped sample,
-35 μV/K for the DBuBi-doped sample and -32 μV/K for the
DiPrBi-doped sample. These values are extracted with an error
bar of ± 15 μV/K, owing to the high sample electrical resistance.
For the DiPrBi-doped sample which achieved the highest
electrical conductivity, we also estimated the power factor (PF)
temperatures for degradation under N2 of
~ 200 °C can be
extracted (Figure SI 1). Melting points Tm between 60 °C and 120
°C were observed for all dopants, with longer alkyl substituents
leading to lower Tm values (Figures SI 2-7).
As a general procedure to test the n-doping ability of these new
N-alkyl substituted 1H-benzimidazoles, aliquots of each dopant
dissolved in anhydrous DCB were separately added to
P(NDI2OD-T2) solutions with a dopant concentration varying
from 10 to 50 wt.-%. The room temperature conductivity of thin
films spin-coated as a function of dopant concentration is
shown in Fig. 3. Conductivity is found to increase by orders of
at 363 K to be ~
1.1 × 10-3 μW/mK.
The temperature dependence of the electrical conductivity was
measured for the same samples in order to characterize the
activation energy (Ea) of σ (Fig. 4). In all cases, σ decreases with
magnitude with respect to undoped P(NDI2OD-T2) (4.6
1.7
benzimidazoles are effective dopants. Conductivity as

10-7


10-7 S/cm), thus indicating that all N-alkyl substituted 1H-
a
function of dopant concentration follows the general trend
previously observed for the case of DMBI and is similar for all
dopants: σ first increases with concentration, reaching a
maximum in between 15 % and 30 % dopant concentration, and
then decreases for higher concentrations. This decrease at high
concentration was ascribed to phase segregation in the case of
DMBI36. For films doped with dopants with linear alkyl chains, σ
increases with increasing chain length. The maximum σ values
are 1.8


10-3

1.9

10-4 S/cm, 3.4
10-3


10-3
6.6


9.7
 
10-4 S/cm, 4.0 10-
3

1.6
10-4 S/cm and 4.1

10-4 S/cm, for DMeBI,
DEtBI, DPrBI and DBuBI, respectively. The maximum σ values for
samples doped with the propyl and butyl derivatives are very
close, thus suggesting that the effect of increasing the alkyl
chain length is approaching a plateau. When doping P(NDI2OD-
T2) with e N-alkyl-1H-benzimidazoles bearing branched chains,
the highest conductivity value of 7.2 1.4
10-3 10-4 S/cm was
achieved with DiPrBI at 25 % doping, which is almost one order
of magnitude higher than that obtained with DMBI. The DiBuBI
Fig. 5 Atomic Force Microscopy images of: a) undoped P(NDI2OD-T2) film, b)
P(NDI2OD-T2) film doped with DMBI, c) P(NDI2OD-T2) film doped with DBuBI, d)
P(NDI2OD-T2) film doped with DiPrBI. Dopant concentration is 20 % for all samples.
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decreasing temperature, with the best fit achieved with a
More quantitative structural information can be extracted from
sector-averaged 1D GIWAXS profiles along OOP and IP,
View Article Online
DOI: 10.1039/C8TA04901G
temperature dependence of ln 휎(푇) ∝ 푇−1/2(see SI for details),
indicating a variable range hopping (VRH) transport regime with
dimensionality of d = 1 as already reported previously. 37 Values
respectively47. The OOP profiles from Figure 6 c) clearly show
higher order (h00)’ diffraction peaks from all P(NDI2OD-
T2):dopant blends, suggesting that all dopants, regardless of the
nature of the alkyl side chain, are capable of inducing edge-on
P(NDI2OD-T2) crystallites in the blends. Intriguingly, the
incorporation of dopant molecules has a distinct effect on
P(NDI2OD-T2) crystallites along different crystallographic axis.
As shown in Figure 6 d), the IP P(NDI2OD-T2) (h00) lamellar
stacking peaks of residual face-on crystallites in the blends shift
toward higher values of q (magnitude of the scattering vector)
after the blending with dopants, while the P(NDI2OD-T2) (00l)
backbone stacking peaks retain a constant q value. This
observation implies that the dopants reduce the lamellar
spacing of P(NDI2OD-T2) the face-on crystallites, without
disrupting the originally well-ordered backbone arrangement.
Although IP lamellar stacking direction does not contribute to
charge transport, the obvious changes of lamellar spacing with
the addition of dopants unambiguously point out that strong
molecular interaction between P(NDI2OD-T2) and dopants
exists. Further details can be obtained from the 1D GIWAXS
profiles around the OOP (200)’ peaks and IP (004) peaks, which
are chosen for closer investigation since they do not show
overlap with other diffraction peaks. As plotted in Figure 6 e),
different dopants induce different populations of edge-on
P(NDI2OD-T2) crystallites, evidenced by the varying OOP (200)’
diffraction intensity in thin films with similar film thickness. The
dopant-induced edge-on P(NDI2OD-T2) crystallites may be one
of the contributors for the enhanced σ compared to the
undoped P(NDI2OD-T2) thin film. However, the fact that the
best performing P(NDI2OD-T2):DiPrBI thin film only exhibits an
intermediate level of edge-on crystallinity as indicated by the
plots around OOP (200)’ region, implies that other factors are
contributing to the improvement of σ. In addition, the impact of
the dopants on the P(NDI2OD-T2) backbone stacking has been
investigated by examining the IP P(NDI2OD-T2) (004) peaks as
shown in Figure 6 f). It is apparent that the incorporation of all
dopants, except for DMBI, slightly reduces the crystallinity of
P(NDI2OD-T2) backbone stacking, evidenced by the moderate
reduction of the IP P(NDI2OD-T2) (004) peak intensity. The
DMBI, on the other hand, almost eliminates the IP P(NDI2OD-
T2) (004) peak, suggesting that the DMBI dopant molecules
tend to disrupt the ordered arrangement of P(NDI2OD-T2)
molecules along the backbone direction. This observation
matches well with the AFM images where the largest surface
roughness and the lowest σ is observed for P(NDI2OD-
T2):DMBI. Lastly, the crystallite orientation distribution (COD)
profiles of the dopant-induced edge-on P(NDI2OD-T2)
crystallites have also been examined. Although no linear
correlation between COD and conductivity was found (see
Figure SI 12), the different COD extracted from the P(NDI2OD-
T2) thin films with various dopants provides direct evidence that
different molecular structures of dopants impact the
preferential orientation of P(NDI2OD-T2) crystallites
distinctively. Notably the P(NDI2OD-T2):DMBI has the broadest
for Ea of 330meV were determined for each system. Therefore,
the differences in σ cannot be ascribed to a modification of the
transport properties, i.e. a difference in the energetic barriers
experienced by charge carriers, but rather to the number of
molecules effectively active in the doping process. This evidence
suggests that the different alkyl chains play a role in the
polymer/dopant interaction and on the doping efficiency.
The microstructure of the different films was investigated first
by acquiring the topography of the respective surfaces with
AFM. Four types of exemplary samples were chosen: undoped
P(NDI2OD-T2)
, P(NDI2OD-T2):DMBI, P(NDI2OD-T2):DBuBI and
P(NDI2OD-T2):DiPrBI; in other words the blend with the lowest
σ, the blend with the dopant with the longest linear alkyl chains
and the blend with the highest σ, respectively. The undoped
P(NDI2OD-T2) film surface shows a typical fiber-like morphology
(Fig. 5). In all other cases, round features of varying number and
size appear as well. In particular, the surface of P(NDI2OD-
T2):DiPrBI film is similar to the undoped film, and it shows also
a similar r.m.s. roughness (Rrms
= 0.76 nm; Rrms
P(NDI2OD-T2)
= 0.86 nm). The doped sample with the lowest
P(NDI2OD-T2):DiPrBI
σ, instead, showed the highest surface roughness (Rrms
P(NDI2OD-
:DMBI = 2.55 nm), whereas the sample with longest linear alkyl
T2)
chains, characterized by an intermediate σ, has a roughness in
between the above mentioned cases.
To obtain further insights into the aggregation/crystallization
behaviour of P(NDI2OD-T2) in the presence of the various n-
type dopants, synchrotron-based grazing-incidence wide-angle
scattering (GIWAXS)45 measurements were conducted. Two of
the representative 2D GIWAXS patterns are displayed in Fig. 6
a) and b). As seen from Fig. 6 a), the simultaneous appearance
of in-plane (IP) (h00) and (00l) peaks, along with out-of-plane
(OOP) (010) peak indicates that in undoped films P(NDI2OD-T2)
crystallites adopt preferentially a face-on orientation with
respect to the substrate46. Interestingly, well-defined OOP
P(NDI2OD-T2) (h00)’ diffraction peaks occur with the
incorporation of the dopants. Since these out-of-plane
P(NDI2OD-T2) (h00)’ lamellar peaks are absent in the undoped
P(NDI2OD-T2) thin films, incorporation of the dopant molecules
induces an edge-on orientation of the crystallites. The 2D
GIWAXS patterns of the remaining P(NDI2OD-T2):dopant films
show a similar behaviour of varying extent (Fig. SI 11). In
addition, the absence of IP π-π stacking (0k0)’ with respect to
OOP lamellar (h00)’ suggests that the dopant molecules may be
disrupting the π-π stacking of the edge-on oriented P(NDI2OD-
T2) chains. Diffraction peaks related to a dopant phase are
absent, indicating either good intermixing of the dopant within
the polymer matrix, or that the dopant molecules are unable to
crystallize in the blend.
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COD supporting the higher degree of disorder in this blend to contraction in (h00)’ direction. Also, the less prevalent π-π
View Article Online
DOI: 10.1039/C8TA04901G
stacking peak can be rationalized by dopant intercalation.
which may contribute to its low σ.
Fig. 6 2D grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns for a) pure P(NDI2OD-T2) and b) P(NDI2OD-T2): DiBuBI, plotted with identical intensity scale. The
sector-averaged 1D profiles along out-of-plane (OOP) and in-plane are plotted in c) and d), respectively. Note the sharp peaks around 0.6 Å-1<q<0.7 Å-1 in P(NDI2OD-T2):DiPrBI
along OOP is parasitic scattering from upstream beamline. The close-up profiles for dopant induced P(NDI2OD-T2) (200)’ diffraction along OOP and P(NDI2OD-T2) (004) along IP
are also demonstrated in e) and f), respectively.
We interpret these observations by intercalation of dopants in These results are finally corroborated by differential scanning
between P(NDI2OD-T2) backbones. Such a picture is in calorimetry measurements on blends, where a new endotherm
agreement with the higher q values of the (h00)’ reflections in is observed at 30-40 K higher than Tm,dopant, that neither arises
the blend. Due to intercalation, the average distance between from the pristine dopant nor from P(NDI2OD-T2) (Figure SI 8).
two P(NDI2OD-T2) backbones increases generating additional Although a reliable quantification of the corresponding melting
space for the amorphous alkyl chain layers, which finally leads
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strategy allows to maintain the electron transfer process, while
DOI: 10.1039/C8TA04901G
optimizing the conductivity thanks to an improved doping
enthalpy ΔHm is not possible, on a qualitative basis ΔHm
increases with increasing doping concentration (Fig. S9).
View Article Online
A chemical reaction of the dopant and the copolymer matrix as
an alternative origin for the new endotherm can be excluded as
samples were fully soluble after the DSC measurements and the
corresponding NMR spectra did not show other signals than
those of dopant and copolymer (data not shown).
efficiency. Such a result, which is particularly relevant for n-type
doping, is important for the development of advanced opto-
electronic organic devices and efficient organic thermoelectric
generators.
The observation of intercalation of DMBI and its derivatives is
different compared to the findings of Schlitz et al.36 and also to
the miscibility behaviour of P(NDI2OD-T2) and DPBI, which is
the phenyl-derivative of DMBI. Shin et al. reported segregation
and crystallization of DPBI in P(NDI2OD-T2):DPBI blends
processed from chlorobenzene, leading to dopant melting in as-
prepared blends.38 While the different structure of DPBI
compared to DMBI may be one of the reasons for the different
behaviour, it is also possible that the solvent has a prominent
effect. Dichlorobenzene, as used here, is a better solvent for
P(NDI2OD-T2) compared to chlorobenzene, which was used by
Shin et al.48 Thus, the presence of a larger amount of P(NDI2OD-
T2) aggregates may promote the segregation of dopant already
in chlorobenzene solution, while dopant intercalation may be
favoured for processing from dichlorobenzene where less
aggregated, i.e. more molecularly dissolved chains, are present.
Hence, solvent quality may be important for controlling
intercalation, which will be the subject of further investigations.
Acknowledgements
The authors are greateful to C. Müller, Chalmers University of
Technology, Göteborg, for fruitful discussions, to A. Warmbold
and F. Nübling for TGA and DSC measurements on dopants and
blends. This work was performed in part at the SAXS/WAXS
beamline at the Australian Synchrotron, part of ANSTO. C.R.M.
acknowledges support from the Australian Research Council
(DP170102145).
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DOI: 10.1039/C8TA04901G
N-alkyl substituted 1H-benzimidazoles as improved n-type dopants of a
naphthalene-diimide based copolymer
B. Saglioab, M. Muraa, M. Massettibc, F. Scurattibd, X. Jiaoe, C. R. McNeille, M. Sommerf, A. Famularia, G. Lanzanibc, M.
Caironib* and C. Bertarelliab*
Alkyls in 1H-benzimidazoles influence their intercalation within the naphthalene diimide-based copolymer, thus affecting the
electrical conductivity.