High Hole Mobility and Efficient Ambipolar Charge Transport in Heterocoronene-Based Ordered Columnar Discotics

Article abstract of DOI:10.1021/jacs.9b09126

Heterocoronene, a new redox-active core fragment, is utilized for the synthesis of room-temperature columnar discotic liquid crystals (DLCs). Three wedge-shaped side chains having different lengths of alkyl tails are introduced at the periphery of the het

Full text of DOI:10.1021/jacs.9b09126

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Article
High Hole Mobility and Efficient Ambipolar Charge Transport
in Heterocoronene-based Ordered Columnar Discotics
Joydip De, INDU BALA, Santosh Prasad Gupta, Upendra Kumar Pandey, and Santanu Kumar Pal
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b09126 • Publication Date (Web): 04 Nov 2019
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High Hole Mobility and Efficient Ambipolar Charge Transport in
Heterocoronene-based Ordered Columnar Discotics
Joydip De,† Indu Bala,† Santosh Prasad Gupta, ‡ Upendra Kumar Pandey,# $ * and Santanu Kumar Pal†*
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†Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, SAS.
Nagar, Knowledge City, Manauli-140306, India
‡ Department of Physics, Patna University, Patna-800005, India
# Interdisciplinary Centre for Energy Research (ICER), Indian Institute of Science (IISc) Bangalore, C. V. Raman Road, Ban-
galore 560012, Karnataka, India
$ Department of Electrical Engineering, School of Engineering, Shiv Nadar University, Gautam Buddha Nagar, Uttar Pra-
desh- 201314, India
ABSTRACT: Heterocoronene, a new redox-active core fragment is utilized for the synthesis of room temperature columnar discotic
liquid crystals (DLCs). Three wedge-shaped side chains having different lengths of alkyl tails are introduced at the periphery of the
heterocoronene core to prepare three kinds of discotic molecules 1 (R: C10H21), 2 (R: C12H25) and 3 (R: C14H29). XRD analysis con-
firmed the packing variation in the columnar lattices regulated by alkyl chains of discrete length and steric bulk. When used in SCLC
devices, compound 1 exhibits a high hole mobility value of 8.84 cm2/V.s at ambient temperature whereas compounds 2 and 3 show
efficient ambipolar charge transport behavior with maximum hole (h) and electron mobility (e) of 0.70 cm2/V.s and 3.59 cm2/V.s,
respectively, for compound 3. The mobility values (h = 8.84 cm2/V.s for 1 and e = 3.59 cm2/V.s for 3) are remarkable and highest
ever disclosed for any DLC based organic semiconductors, which are promising to make a good balance between mobility and pro-
cessability in devices. The grazing incidence small and wide-angle X-ray scattering (GI-SAXS/WAXS) experiments are employed
to quantify the extent of alignment in the film state which correlates with the observed trend of mobility values.
self-repairing from the structural defects (upon annealing) led
INTRODUCTION
to the formation of pin-hole free devices.19 Here, we show that
Intensive efforts for the rational design of new soft materials
the highly efficient charge transport can be obtained by choos-
are rigorously mandated for the production of highly efficient,
ing a novel planar discotic core through suitable alignment in
cheaply processable organic semiconductors. In particular, the
the devices. In this system, we report very high hole mobility
major attention is being paid to increase the charge transport
and efficient ambipolar charge carrier transporting capabilities
properties of these materials to achieve fast response in de-
(having a precise balance of electron and hole mobilities) which
vices.1 However, the crucial challenge is to find a subtle balance
are extremely useful for the device point of view.
between mobility and processability of these materials. For ex-
e-
ample, single crystals2 known for high mobilities, suffer from
inherent fragility and flexibility, which limit their usability in
devices. Similarly, in polymeric systems,3-6 inadequate solubil-
ity, low purity, structural and energetic disorder have curbed
their charge transport properties. To overcome these challenges,
recent research has focused on organic small molecules7 which
offer solution-processed defect-free films along with high
chemical purity. In this context, discotic liquid crystals (DLCs)8
comprising of central rigid aromatic core substituted with man-
tle of flexible alkyl chains are in spotlight of material scientists
in current years.9 Strong π-π interactions among rigid aromatic
cores arrange the discogens in columnar fashion and thus charge
transport in these materials is expected to be quasi-one-dimen-
sional. Besides that, the supramolecular columnar architecture
is advantageous in terms of providing the right balance between
mobility and processability.10-15 Several approaches have been
reported to achieve high charge carrier mobility in such systems
such as increasing the intracolumnar order within the discs, in-
clusion of directional H-bonding interactions in the cores, and
so on.16 An alternative strategy for the effective charge transport
is via aligning the columns macroscopically in one direction to
the substrate surface.17,18 In addition, the propensity of DLCs of
h+
Scheme 1. Design of heterocoronene-based ambipolar DLCs.
The hunt for condign ambipolar charge transport materials
remains a significant challenge over the past few years. These
bipolar transport materials would be highly cost-effective due
to the incorporation of a single layer of organic materials in de-
vice architecture.20, 21 The strategy to design ambipolar materi-
als is usually focused on molecules with the small HOMO-
LUMO energy gap following the common belief that electron-
donor systems should lead to hole-transport materials while the
electron-acceptor ones should give rise to electron transport ma-
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terials.22-24 However, this perception is claimed to be an over-
Page 2 of 7
efficient ambipolar charge transport behavior with the maxi-
mum hole (h) and electron mobility (e) of 0.70 cm2/V.s and
3.59 cm2/V.s, respectively, for compound 3. To date hole and
electron mobility of 8.84 cm2/V.s and 3.59 cm2/V.s, respec-
tively, are the highest values observed for any DLC materials
known so far.
1
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simplification, indeed, hole and electron conduction are inher-
ent characteristics of organic semiconductors.22,25,26 In fact, in
devices, by matching the electrodes work function with the
HOMO-LUMO energy levels of materials, charge injection of
both polarities is possible.22,23,25,26 We note that observation of
both hole and electron mobilities in a single discotic system is
rare and only a few of such ambipolar transport materials are
reported. For example, Imahori et al. reported ambipolar donor-
acceptor based discotics that exhibit maximum hole charge car-
rier mobility (μh) of 0.26 cm2/V.s and electron mobility (μe) of
0.11 cm2/V.s, measured by time-of-flight (TOF) technique.27
Using space charge limited current (SCLC) method, Sierra and
co-workers designed H-bonded donor-acceptor columnar as-
sembly that shows μh and μe in the order of 10-2 cm2/V.s.28 Other
groups have also reported the ambipolar mobilities up to 10-3 to
10-5 cm2/V.s for DLCs based on donor-acceptor systems.29,30
However, the molecular structures comprising electron-rich and
electron-deficient segment behaved well in this regard, though
the search for suitable ambipolar charge transport materials
comprising a single chromophore is a challenging task.20,31,32 In
general in DLCs the molecular electronic properties can be
tuned either by variation of substituents or by the central core
skeleton itself.
RESULTS AND DISCUSSION
The three heterocoronene based DLCs, 1-3 (Scheme 1) have
been designed by introducing wedge-shaped side groups having
different lengths of alkyl tails (1: R = C10H21, 2: R = C12H25, 3:
R = C14H29) at the periphery of heterocoronene core. The de-
tailed synthesis and characterization of 1-3 are provided in
Scheme S1 and Figure S1-S6. The heterocoronene derivatives
(1-3) showed high thermal stability as determined by thermo-
gravimetric analysis (TGA) (Figure S7, Table 1).
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Under polarising optical microscope (POM), compound 1 ex-
hibits two distinct textures (having variation in their growth do-
mains) at two different temperatures ranges (Figure 1a and 1b)
correspond to two types of mesophases which were confirmed
by the existence of two peaks observed in differential scanning
calorimetry (DSC). Unlike 1, 2 and 3 exhibited a single peak in
DSC that corresponds to mesophase to isotropic transition. De-
tails are reported in Table 1, Table S1, Figure S8a-c, and Figure
S9. However, no crystallisation peak was observed up to -40 °C
indicating that mesophase is stable down to room temperature.
In the present manuscript, the molecular structure comprises
the heterocoronene core (single chromophore) is obtained by
incorporating four electron-withdrawing carboxaimides groups
to the electron-rich coronene core as reported by Mullen et. al.33
In addition, the introduced electron-donating peripheral groups
tunes the electronic behavior apart from inducing room temper-
ature columnar phases. Therefore, it can be envisioned that the
heterocoronene DLCs (1-3) can have the appropriate hole and
electron charge transport properties which can be benefitted to-
wards achieving ambipolar materials by the proper charge in-
jecting electrodes. Moreover, these materials exhibit a high ten-
dency to align homeotropically over a macroscopic regime,
warranted for devices. When used in SCLC devices, only hole
mobility of 8.84 cm2/V.s was observed for compound 1 at am-
bient temperature. In contrast, compounds 2 and 3 exhibited
Table 1. Thermal properties of 1-3
Phase transitionsa
Colr 64.7 Colh 94.9 I
Colh 73.7 I
T5% (C)b
344
Compound
1
2
3
319
Colh 93.1 I
336
aTransition temperatures in C from DSC data on 1st heating at a
b
rate of 5 ℃/ min. Temperature at which 5% weight loss was de-
tected in TGA.
c
a
e
f
104
h
a
103
h
c
0.4
0.8
1.2
1.6
2.0
-1)
q (Å
d
b
g
h
103
h
a
102
0.4
0.8
1.2
1.6
2.0
-1)
q (Å
Figure 1. POM image of compound 1: (a) 25 °C and (b) 80 °C obtained on cooling. 1D & 2D (inset) XRD pattern of 1: (c) Colr at 25 ºC and
(d) Colh at 70 ºC. (e) GISAXS pattern of compound 1 in Colr phase and (f) azimuthal vs 2 plot of the corresponding wide-angle peak. EDM
of 1 in respective (g) Colr phase and (h) Colh phase. Red color represents the high electron density region and deep blue is the lowest.
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To investigate the nature of mesophases of 1-3, X-ray diffrac-
The DFT studies were carried out to get an insight into the
electron delocalization in molecular orbitals. The contours of
the HOMO and LUMO of compound 1-3 are displayed in Fig-
ure S15. The HOMOs showed a bonding character and are lo-
calized over the central heterocoronene core with partial orbital
contribution from the four N-subtituted trialkoxyphenyl periph-
eral groups. On the other hand, the electron density in LUMO
was mainly spread over the core itself and shows an antibonding
character. Hence, both the HOMO and LUMO frontier orbitals
were found to be highly delocalized over the central π-conju-
gated aromatic core. In such system, the charge transport is an-
ticipated to be high with an increasing overlap of the frontier
orbitals of the π-stacked discotic cores.36,37 In general, most of
the DLCs forming columnar mesophases, the molecules rotate
fast about their columnar or stacking axis.38,39 Therefore, when
the orbitals are located on the center part of the π-conjugated
system, as in the present case, the time-averaged overlap of the
frontier orbitals increases and enhances charge transport behav-
ior.36,38,39
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2
3
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tion (XRD) studies have been performed. The observed peaks
in XRD pattern of compound 1 indicated two types of
mesophases at higher and lower temperatures. The lower tem-
perature phase (at 25 ºC) can be indexed to rectangular lattice
(Figure 1c) with the sum of hk values found to be even (2n, n is
an integer) and hence the assembly is 2D centered rectangular
lattice with c2mm space group (Table S2). At higher tempera-
ture, 1 exhibit peaks in the ratio 1:1/ 3: 1/ 4: 1/ 7: 1/ 11 and
√
√
√
√
thus indicates hexagonal assembly (Figure 1d, Table S3). The
grazing incidence small and wide-angle X-ray scattering (GI-
SAXS/WAXS) pattern at room temperature (Figure 1e) dis-
plays peaks corresponding to Colr phase and the maximum in-
tensity lies along the meridian direction. The corresponding
wide-angle region showed two strong peaks at the azimuthal
angle 188 and 332 and one weak peak in between at 260
(Figure 1f). The variation of lattice parameters and correlation
length ()34 in different phases for 1 with increasing temperature
has been examined (Figure S11a & S11d). For 2 and 3, the XRD
analysis confirmed the Colh nature in their mesophase tempera-
ture range (Figures S10a & S10b, Table S4 & S5, and Figure
S11b-c & S11e-f). The constructed electron density maps
(EDM)34 in Colr (Figure 1g) and Colh (Figure 1h) phase for 1
and Colh phase for 2 and 3 (Figure S12) showed clear visuali-
zation of molecules in their respective mesophases. The highest
electron density region (red color) corresponds to the aromatic
part while the lower one (blue) represents the aliphatic region.
The clear contrast observed in electron density indicates the ex-
pected nanophase segregation which results in the formation of
Col assembly.
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The charge carrier mobilities for compounds 1-3 were meas-
ured by SCLC technique. In this technique, the material is
crammed between two electrodes, one being Ohmic Contact for
a given sign of charge under measurement. Ohmic contact en-
sures efficient injection of charges, ensuring SCLC regime will
be reached while performing current/voltage characteristics.
Mobility can be extracted from Mott Gurney equation40 given
by
2
퐽 = 9 휀0휀푟휇 푉
(1)
3
8
푑
where J is measured current density, 휀0 is the permittivity of
free space, 휀푟 is the relative dielectric constant of the material,
V is the applied voltage, d is the thickness of the sample and 휇
is the mobility. Charge mobility can be extracted from equation
1, once we have the measured values of 휀푟 and d.
UV-vis and photoluminescence (PL) spectra of compound 1-
3 were recorded in dilute chloroform solution (10-6 M). All
compounds (1-3) show identical absorption spectra and the data
is provided in Table S6. In absorption spectra (Figure S13) they
show three bands (, , p)33 with peak maxima at 282 nm for 
band, 392 nm for p band, 540 nm for  band and a weak band
at longer wavelength regions (~635 nm). The weak band in the
longer wavelength can be attributed to the intramolecular
charge transfer within the π-conjugated system.35 In the PL
spectra, three sub-peaks at 550 nm (peak maxima), 589 nm and
635 nm were observed (Figure S13).
In order to investigate the ambipolar charge transport behavior
of compound 1-3, two different SCLC cells are fabricated i.e
hole-only and electron-only for measuring the hole and electron
mobilities, respectively. The hole-only devices consist of gold
(Au) and indium tin oxide (ITO) with Au as a hole injecting
electrode whereas electron-only devices were fabricated by us-
ing Zinc oxide (ZnO) coated ITO substrates as an electron in-
jecting electrodes. SCLC mobility for 1-3, elucidated from the
current/voltage curve is summarized in Table 3 and Table S8.
A typical SCLC behavior showing Ohmic and SCLC regime for
compound 1-3 is shown in Figure 2d and Figure S16. An aver-
age hole mobility values of 7.81 cm2/V.s, 0.35 cm2/V.s and 0.46
cm2/V.s were obtained for compound 1, 2 and 3, respectively.
Since DLCs exhibit unidirectional charge transport, mobility
values in DLCs are highly dependent on their molecular align-
ment.41 Additionally, in SCLC, injected charges have to travel
through anisotropic material between the two electrodes and
macroscopic degree of order plays a major role in the extracted
charge mobility. In order to obtain homeotropic alignment dif-
ferent thermal treatments were performed for compounds 1-3.
POM images of compound 1 under cross polarizer with the an-
gle of 90 and 45° (Figure 2a-2c) exhibited homeotropic align-
ment (~ 98%) in the hole-only SCLC cell. This alignment has
been further confirmed by the presence of a conoscopic inter-
ference pattern under POM (Figure 2c inset).
In order to explore the viability of hole and electron injection
process in these materials, cyclic voltammetry (CV) was carried
out. Almost similar kinds of oxidation and reduction curves
(Figure S14) were observed for 1-3 as revealed from their an-
odic and cathodic peak potential responses (Table S7). For com-
pounds 1-3, the two peaks in the oxidation cycle while one peak
in reduction wave was observed. The HOMO and LUMO en-
ergy levels are summarized in Table 2. Both the energy levels
are favorable for the hole and electron charge carrier injections
from the respective suitable electrodes.26 These results indicat-
ing the ambipolar charge transport behavior of the heterocoro-
nene derivatives.
Table 2. Energy levels of 1-3a
Compound
EHOMO (eV)
-4.90
ELUMO (eV)
-3.51
Eg,CV (eV)
1.39
1
2
3
-4.89
-3.51
1.38
-4.88
-3.77
1.11
aDetailed methodology used for calculations are provided in Supporting
Information
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10-1
10-2
10-3
d
e
8
6
4
2
0
Colr
Colh
Iso
0.01
0.1
V (Volts)
1
20
40
60
80
100
120
Temperature (C)
Figure 2. POM image of homeotropic alignment (~ 98%) in the SCLC hole-only mobility cell (3.02 m thick sample) of compound 1 under
cross polarizer at (a) 90 and (b) 45. (c) Schematic representation of homeotropic alignment in the cell (Inset shows the conoscopic image
of the aligned sample) (d) J-V characteristics for 1 at 25 °C representing ideal Ohmic and SCLC regimes. (e) Temperature dependence of
charge carrier mobility (Hole) of 1 upon heating is shown for one set of measurement.
respectively, whereas we were not able to achieve SCLC regime
Table 3. Charge mobility values for compounds 1-3
Mobility (cm2/V.s)a
to extract electron mobility for 1. Under POM (Figure S21), al-
most complete homeotropic alignment was observed for 3 in
electron-only cells while compound 2 showed partial align-
ment. The observed one order of magnitude higher electron mo-
bility for compound 3 over 2 correlates well with the POM im-
ages as alignment can lead to mobility differences up to two to
three orders.16,41,42 Moreover, a relatively better match between
the ZnO work function with LUMO in electron-only device
structure for 3 over 2 will ensure an improved electron injection
effectiveness that can be attributed to higher mobility observed
for 3. However, we were not able to find any working device to
extract electron mobility for compound 1 and thus could not be
measured. Reported mobility values for both hole and electron
for compound 1-3 can be treated as their lower limit since SCLC
warrants molecular alignment and charge injection effective-
ness. Remarkably, reported compounds show ambipolar behav-
ior with high mobility values rarely observed for single mole-
cule. To the best of our knowledge, reported mobility values are
the highest among ever reported SCLC mobility for individ-
ual16,44-47 as well as ambipolar27-32 semiconductors based on
DLCs (Table S9).
6.1
7.81 (8.84) ± 0.75 0.35 (0.51) ± 0.02 0.46 (0.70) ± 0.16
(-)b
0.24 (0.36) ± 0.06 2.50 (3.59) ± 0.71
6.2
6.3
Hole
Electron
aMobility values are average from different sets of measure-
ments (5-6). Data in brackets indicate the highest measured value.
bcould not be measured.
However, compounds 2 and 3, resulted partial homeotropic
alignment even after several attempts of different thermal treat-
ment (Figure S20a-S20d). This preferable homeotropic colum-
nar self–assembly of 1 correlates well (~ one order of magni-
tude higher) with observed hole mobility values among 1 to 3.
To measure electron mobility for compound 1- 3, ZnO, whose
work function is ~ -3.9 eV to 4.2 eV42,43 was chosen as an elec-
trode as it was providing a close match with LUMO levels of
compounds 1-3 (-3.51eV for 1/-3.51 eV for 2/-3.77 eV for 3)
for electron-only devices. An average electron mobility of 0.24
cm2/V.s and 2.50 cm2/V.s was obtained for compound 2 and 3,
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(20)
c
f
a
(11)
(10)
b
(11)
(06)
(35)
(10)
(04)
(22)
(02)
(20)
Substrate
peak
Substrate
peak
Substrate
peak
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d 350
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90
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150
0
30
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90
120
150
Azimuthal angle ()
Azimuthal angle ()
Azimuthal angle ()
Figure 3. GISAXS profile with indexing and cake (shown by annular region bounded with black color) for (a) (02) peak of 1 in Colr phase,
(b) (11) peak of 2 and (c) for (11) peak of 3 in Colh phase. (d), (e), (f) Azimuthal plot (Intensity vs. azimuthal angle in blue color half-filled
circle with Lorentzian fit in red color solid line) of compound 1, 2 and 3, respectively
Temperature-dependent charge mobility for compound 1 was
further performed to evaluate mobilities in Colr, Colh and iso-
tropic phases upon heating from 25 C up to 105 C as shown
in Figure 2e. A slight decrease in the charge mobility was ob-
served from Colr-Colh–Iso upon heating. Nevertheless, within
the same phases, charge mobility was almost identical. This
kind of temperature dependence is desirable for organic elec-
tronic devices for uniform operation in different geographical
conditions.
phase & (10) peak of the Colh phase. The variation of the posi-
tional order in the different phases with temperature for com-
pounds 1, 2, & 3 are shown in Figure S11d-S11f. Interestingly,
the positional order at room temperature also exhibits the same
trend as that of the orientational order. Therefore, it can be in-
ferred that the trend of both positional and orientational order is
consistent with the trend of mobility values (both hole and elec-
tron) observed in the SCLC measurement for the compounds.
In order to get insight into the extent of alignment GI-
SAXS/WAXS experiments have been performed at room tem-
perature in the thin film of compounds 1-3. The GI-
SAXS/WAXS data indexing resembles the Colr assembly for
compound 1 (Figure 3a) and Colh arrangement for compound 2
& 3 (Figure 3b, 3c) as described above. In the Col assembly,
molecules have positional as well as orientational (degree of
alignment) order which is inversely proportional to FWHM
(full width at half maxima) of the radial and azimuthal plot of
the diffraction peak, respectively. Therefore, both the orders
(positional as well as orientational) could be deduced from the
first peak of the XRD pattern. In the present case, as the first
peak in the GISAXS contain also the main beam intensity, for
calculation of the orientational order, azimuthal plot of the sec-
ond peak [i.e. for Colr phase (in 1) (02) peak and for Colh phase
(in 2 & 3) (11) peak] has been analyzed (Figure 3a-3c). FWHM
of the azimuthal plot of (02) peak of Colr phase in 1 is found to
be 38o (Figure 3d) whereas this is 60o (Figure 3e) and 52o (Figure
3f) for (11) peak of Colh phase in 2 & 3, respectively. Therefore,
the extent of orientational order follows the following trend i.e.
Colr phase of 1 > Colh phase of 3 > Colh phase of 2 at room
temperature.
CONCLUSION
In conclusion, a new core fragment, heterocoronene, is real-
ized for the first time to synthesize room temperature DLCs.
The rational design comprising a single chromophoric moiety
result in the formation of nanosegregated columnar superstruc-
tures with homeotropically aligned SCLC devices exhibiting
high hole mobility and efficient ambipolar charge transport be-
havior. The h and e values of 8.84 cm2/V.s and 3.59 cm2/V.s
for 1 and 3, respectively, are remarkable and highest ever dis-
closed for any DLC based organic semiconductors with a good
balance between mobility and processability. These results are
comparable with the best amorphous as well as polycrystalline
organic semiconductors.48 The observation of such efficient am-
bipolar charge transport materials (showing balanced electron
and hole mobilities) is highly warranted to the realization of ef-
ficient semiconducting devices.
ASSOCIATED CONTENT
Supporting Information. Detailed characterizations of all the new
compounds. This material is available free of charge via the Inter-
net at http://pubs.acs.org.
Moreover, positional order (correlation length) which is in-
versely proportional to FWHM of radial plot of the peak in the
diffraction pattern, is calculated for the (20) peak of the Colr
AUTHOR INFORMATION
Corresponding Author
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ACKNOWLEDGMENT
SKP and IB acknowledges funding from CSIR (02(0311)/17/EMR-
II) & 09/947(0061)/2015-EMR-1, respectively. we thank NMR,
HRMS and SAXS/WAXS facility at IISER. UKP thank DST-
INSPIRE Faculty award No. IFA12-ENG-27.
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Table of Contents artwork
1 : h,max = 8.84 cm2/V.s
3 : e, max = 3.59 cm2/V.s
e-
h+
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