Hole transport materials (HTMs) have a significant impact on the effectiveness of organic electronic devices; therefore, we present a molecular architecture of pyrazino[2,3-g]quinoxaline (PQ10)-based room-temperature organic liquid crystalline semiconductor (OLCS) as an alternative HTM. The PQ10 compound exhibits three different rectangular columnar (Colr) phases offering an impressive hole mobility of 8.8 × 10−3 cm2V−1s−1 which is found to be dexterous than most of existing polymeric hole transport materials. The charge transport mechanism is governed by the hole polarons hopping through H-aggregates of the PQ10 molecules and the hole mobility remains nearly constant throughout the mesophase range, but it decreases with increasing applied electric field. The current-voltage characteristics of the PQ10 have also been investigated in all three Colr phases and explained via the Poole-Frenkel conduction mechanism. The dielectric spectroscopy has been eventually carried out to understand the nature of dielectric permittivity and conductivity as a function of temperature and a correlation is established between the molecular architecture of the Colr phases and aforementioned physical properties. Solar cell simulation has been additionally performed to demonstrate that the PQ10 material can be a better choice as HTM for organic electronics and photovoltaic applications.

1. Journal Pre-proof
Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor: Proficient hole
transporting material for optoelectronic devices
Asmita Shah, Vinod Kumar Vishwakarma, Neichoihoi Lhouvum, Achalkumar Ammathnadu Sudhakar,
Pawan Kumar et al.
PII: S0167-7322(23)02341-3
DOI: https://doi.org/10.1016/j.molliq.2023.123535
Reference: MOLLIQ 123535
To appear in: Journal of Molecular Liquids
Received date: 18 August 2023
Revised date: 17 October 2023
Accepted date: 7 November 2023
Please cite this article as: A. Shah, V.K. Vishwakarma, N. Lhouvum et al., Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor:
Proficient hole transporting material for optoelectronic devices, Journal of Molecular Liquids, 123535, doi: https://doi.org/10.1016/j.molliq.2023.123535.
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© 2023 Published by Elsevier.

2. Graphical abstract
Pyrazino[2,3-g]quinoxaline core-based organic liquid
crystalline semiconductor: Proficient hole transporting
material for optoelectronic devices
Asmita Shaha, Vinod Kumar Vishwakarmab, Neichoihoi Lhouvumb,
Achalkumar Ammathnadu Sudhakarb,c
, Pawan Kumard
, Abhishek Kumar Srivastavae
,
Frederic Duboisa
, Treerathat Chomchoka,f
, Nattaporn Chatthamf
, Dharmendra Pratap Singha,∗
a Université du Littoral Côte d’Opale, UR 4476, UDSMM, Unité de Dynamique et Structure des Matériaux
Moléculaires, F-62228 Calais, France
b Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781039, Assam, India
c Centre for Sustainable Polymers, Indian Institute of Technology Guwahati, Guwahati, 781039, Assam, India
d Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW,
Calgary, Alberta T2N 1N4, Canada
e State Key Laboratory of Advanced Displays and Optoelectronics Technologies, Department of Electronics
and Computer Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, 999077,
Hong Kong
f Department of Physics, Faculty of Science, Kasetsart University, Thailand
Journal of Molecular Liquids ••••, •••, •••

3. Highlights
• The Pyrazino[2,3-g]quinoxaline (PQ10) core-based OSC exhibits rectangular columnar phases.
• PQ10 renders p-type charge carrier mobility of the order of 10−3 cm2V−1s−1.
• The current conduction in PQ10 is attributed to Poole-Frenkel mechanism.
• Charge carrier mobility, dielectric permittivity and conductivity remarkably depend on the mesophase ordering.
• PQ10 can be designed as HTL in optoelectronic devices resulting in a better performance.

4. Graphical Abstract
Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor: Proficient hole
transporting material for optoelectronic devices
Asmita Shah,Vinod Kumar Vishwakarma,Neichoihoi Lhouvum,Achalkumar Ammathnadu Sudhakar,Pawan
Kumar,Abhishek Kumar Srivastava,Frederic Dubois,Treerathat Chomchok,Nattaporn Chattham,Dharmendra Pratap
Singh

5. Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline
semiconductor: Proficient hole transporting material for optoelectronic
devices
Asmita Shaha
, Vinod Kumar Vishwakarmab
, Neichoihoi Lhouvumb
, Achalkumar
Ammathnadu Sudhakarb,c
, Pawan Kumard
, Abhishek Kumar Srivastavae
, Frederic Duboisa
,
Treerathat Chomchoka,f
, Nattaporn Chatthamf
and Dharmendra Pratap Singha,∗
aUniversité du Littoral Côte d’Opale, UR 4476, UDSMM, Unité de Dynamique et Structure des Matériaux Moléculaires, F-62228 Calais, France
bDepartment of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781039, Assam, India
cCentre for Sustainable Polymers, Indian Institute of Technology Guwahati, Guwahati, 781039, Assam, India
dDepartment of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
eState Key Laboratory of Advanced Displays and Optoelectronics Technologies, Department of Electronics and Computer Engineering, The Hong Kong
University of Science and Technology, Clear Water Bay, 999077 Hong Kong
fDepartment of Physics, Faculty of Science, Kasetsart University, Thailand
A R T I C L E I N F O
Keywords:
Columnar liquid crystal
Hole transport layer
Charge carrier mobility
polarized optical microscopy
Dielectric permittivity
I-V curve
A B S T R A C T
Hole transport materials (HTMs) have a significant impact on the effectiveness of organic electronic
devices; therefore, we present a molecular architecture of pyrazino[2,3-g]quinoxaline (PQ10)-based
room-temperature organic liquid crystalline semiconductor (OLCS) as an alternative HTM. The PQ10
compound exhibits three different rectangular columnar (Col𝑟) phases offering an impressive hole
mobility of 8.8 × 10−3 cm2V−1s−1 which is found to be dexterous than most of existing polymeric hole
transport materials. The charge transport mechanism is governed by the hole polarons hopping through
H-aggregates of the PQ10 molecules and the hole mobility remains nearly constant throughout
the mesophase range, but it decreases with increasing applied electric field. The current-voltage
characteristics of the PQ10 have also been investigated in all three Col𝑟 phases and explained via the
Poole-Frenkel conduction mechanism. The dielectric spectroscopy has been eventually carried out to
understand the nature of dielectric permittivity and conductivity as a function of temperature and a
correlation is established between the molecular architecture of the Col𝑟 phases and aforementioned
physical properties. Solar cell simulation has been additionally performed to demonstrate that the
PQ10 material can be a better choice as HTM for organic electronics and photovoltaic applications.
1. Introduction
Liquid crystalline phase, exhibiting both the molecular
ordering and the fluidity, is termed as nature’s delicate phase
of matter. These materials have been established as basic
building blocks for numerous technologies such as displays,
light-emitting diodes [1, 2], field-effect transistors [3, 4], and
so on due to the combination of both intriguing features
[5, 6, 7, 8]. A new LC molecular architecture composed
of the disc-like molecules was identified in 1977 which
has been termed as discotic liquid crystals (DLCs) [9].
Usually, the DLCs are made of a central 𝜋-conjugated core
surrounded by a flexible peripheral alkyl chain and other
functional groups. Upon cooling from the isotropic phase,
disc-like molecules self-assemble into discotic nematic or
columnar phases via 𝜋-𝜋 stacking. Attributing to the molec-
ular architecture adopted at the phase transition, hexago-
nal (Colℎ), rectangular (Col𝑟) or oblique (Col𝑜𝑏) columnar
phases are most commenly observed [7, 10, 11, 12]. This
2D molecular arrangement in the columnar phases offers
1D charge transport via hopping mechanism, a conduction
∗Corresponding author
Dharmendra.Singh@univ-littoral.fr (D.P. Singh)
https://udsmm.univ-littoral.fr/ (D.P. Singh)
ORCID(s): 0000-0001-6949-6110 (D.P. Singh)
mechanism in which a single-valued activation energy con-
tributes to the carriers hopping at the adjacent discs along
the columns [13]. This 2D-molecule arrangement and 1D
charge transport in DLCs make them suitable for their spe-
cific applications in organic electronics and optoelectronics
[4]; however, charge transport is highly dependent on the
molecular ordering [14]. Pyrazino[2,3-g]quinoxaline core-
based organic liquid crystalline semiconductors (OLCSs)
have proven their utility for modern opto-electronic devices
due to high charge carrier mobility in the order of 10−3
cm2V−1s−1 [5, 15, 16]. These materials commonly exhibit
rectangular or hexagonal columnar (Col𝑟 or Colℎ) phases via
𝜋-𝜋 stacking of the molecules. In the Col𝑟 or Colℎ phases,
often molecules opt face-on orientation (homeotropic) onto
the common substrates such as ITO, FTO, etc. This self-
assembled homeotropic orientation made them ideal for
fabricating organic light-emitting diodes (OLEDs) [1, 2,
5, 11], organic photovoltaics (OPVs) [17, 18, 19, 20] and
sensing [21]; however, the fabrication of organic field-effect
transistors (OFETs) requires planar orientation of discotic
molecules [22].
The preparation of low bandgap organic materials has
attracted a great deal of attention for organic electronic
and optoelectronic devices [23]. The dearomatization of 𝜋-
conjugated structures via quinoid formation is one of the
D.P. Singh et al.: Preprint submitted to Elsevier Page 1 of 13

6. Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor
approaches for synthesizing such organic materials [24].
Pyrazino[2,3-g]quinoxaline is a known heteroaromatic core
that could be employed to make low bandgap organic ma-
terials. The strong 𝜋-𝜋 interactions in this core make it
suitable for rendering high charge carrier mobility. The self-
assembling behaviour of such a core leads to the formation of
H-aggregates or J-aggregates which helps in photophysical-
based applications (like OLEDs, bioimaging and photody-
namic therapy, waveguides, etc) of this material [25, 26, 27,
28]. By changing the attached functional moieties around
the pyrazino[2,3-g]quinoxaline core, several chemical archi-
tectures can be developed which evince strong structure-
property correlation [25]. This approach has shown vital im-
portance for the rational design of organic self-assemblies.
The use of charge transporting layers (i.e. hole transport-
ing layer (HTL) and electron transporting layer (ETL)) is
required in optoelectronic devices. For instance, to increase
external quantum efficiency (EQE), it is necessary to im-
prove the drawbacks of current HTLs, such as low charge
carrier mobility, grain boundary formation, and energy gap
(HOMO/LUMO levels). The EQE of LEDs can be expressed
by: 𝜂=𝛾 × 𝜙𝑃𝐿 × 𝜂𝑜𝑢𝑡 ; where, 𝛾, 𝜙𝑃𝐿, and 𝜂𝑜𝑢𝑡, are the
charge balancing factor, the photoluminescence quantum
yield and the light outcoupling efficiency, respectively [29].
If we use any organic material with higher hole mobility, we
can significantly improve the efficiency of LEDs. Another
issue with these devices is drooping, which occurs as a
result of the auger effect and higher leakage current, that
severely distorts the charge balancing factor and shifts the
recombination zone away from the emission zone in LED
devices [30]. This issue can be solved by increasing hole
transport or decreasing electron mobility. As a result, an
efficient HTM is required to achieve optimal EQE for such
devices by improving the value of 𝛾. Besides, a solar cell
device can be prepared into two configurations (viz. n-i-p
and p-i-n) depending on which layer to deposit on the trans-
parent substrates and the transparent conductive substrate
is coated with the ETL or HTL, respectively. This makes
HTL an essential part of a device. The ideal HTM should
be thermally and chemically robust, exhibiting high hole
mobility and electronic energy levels compatible with that
of the organic material [31, 32]. This concept can give us a
road map for enhancing solar cell device efficiency [33].
Herein, we have investigated the PQ10 compound for
using it as HTL in a solar cell device reported earlier and
pointed out the current challenges in OLED and organic
photovoltaic (OPV) technologies. The optical texture, hole
mobility, current-voltage characteristics, and dielectric pa-
rameters of the PQ10 compound are examined in this study
to gain insight into its HTM behaviour. In the end, solar cell
simulation has been additionally carried out to prove our
hypothesis to use PQ10 as HTL. This study will undoubtedly
help with the design of organic solar cells using QDs and
non-fullerene materials [34].
2. Materials and Experimental Section
2.1. Synthesis of Pyrazino[2,3-g]quinoxaline
(PQ10) organic liquid crystalline
semiconductor (OLCS)
The chemical structure and phase transitions of the
compound PQ10 are presented in figure 1. Following a
recent methodology reported by Vishwakarma et al.[5], the
PQ10 derivative of pyrazino[2,3-g]quinoxaline was synthe-
sized utilizing a synthetic process indicated in figure S1
(Electronic supplementary information). In the first step,
two equivalents of 1,2-dimethoxybenzene were reacted with
oxalyl chloride in the presence of anhydrous AlCl3 by
Friedel-Craft’s acylation to obtain the corresponding ben-
zil derivative. In the second step, this benzil derivative
was demethylated on refluxing in the presence of 47%
hydrobromic acid and glacial acetic acid in a 1:1 ratio
to obtain the tetrahydroxy derivative. In the third step,
this compound was alkylated with 1-bromodecane under
Williamson’s ether synthesis, in the presence of anhydrous
K2CO3 to obtain the tetradecyloxy derivative. Finally, the
tetradecyloxy 1,2-diketone was condensed with 1,2,4,5-
tetraaminobenzene tetrahydrochloride to get the PQ10-
derivative. All the measurements described in this article
were carried out in the cooling cycle. The compound PQ10
exhibited isotropic−Col𝑟1, Col𝑟1−Col𝑟2 and Col𝑟2−Col𝑟3
phase transitions at ≈ 95.1, 84.4 and 47.2 𝑜C, respectively
with the scan rate of 10 𝑜C/min (fig. 1(b)). The existence
of rectangular mesophase was confirmed by differential
scanning calorimetry (DSC) and X-ray diffraction (XRD) as
reported in ref.[5].
2.2. Polarized optical microscopy (POM)
The optical textures of the PQ10 compounds in Col𝑟1,
Col𝑟2 and Col𝑟3 phases were analyzed via polarized op-
tical microscopy (POM), using a Zeiss AXIO polarized
optical microscope with Infinity-2 software. Throughout the
POM study, the desired temperature was maintained using
a Linkam LTS 350 hotplate connected to Linkam TMS94
temperature controller.
2.3. Estimation of charge carrier mobility via
time-of-flight (ToF) technique
The charge carrier mobility is one of the crucial pa-
rameters for opto-electronic, organic-electronic and photo-
voltaic devices which can be measured using the time-of-
flight (ToF) technique [6]. It is the most suitable technique
to measure the hole and/or electron mobility in organic
materials. The ToF technique was equipped with a Nd:YAG
pulsed laser having an excitation wavelength of 355 nm and
5 ns pulse width. The PQ10 compound was first filled in
the LC cell of thickness 9.2 𝜇m in the isotropic phase and
subsequently cooled with a very slow cooling rate of 0.1
𝑜C/min to obtain the best molecular orientation. Thereafter,
the compound PQ10 was irradiated by the laser pulse and
displacement of holes has been recorded on a digital os-
cilloscope (Keysight, DSOX3022T) under the application
of an external positive voltage in the range of 30−50 V
D.P. Singh et al.: Preprint submitted to Elsevier Page 2 of 13

7. Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor
Figure 1: (a) Chemical structure of the pyrazino[2,3-
g]quinoxaline (PQ10) molecule and (b) DSC thermograph of
the PQ10 compound. Insets of (b) represent the bar graph
showing the thermal behavior of the PQ10 (based on the
cooling cycle in the POM with a scan rate of 10 𝑜
C/min) and
enlarge portion of DSC thermograph to show Col𝑟2-Col𝑟3 phase
transition.
with the help of Keithley 6487 voltage source. As, we per-
formed temperature-dependent ToF, therefore, during mea-
surements, the temperature was controlled by the Eurotherm
3204 temperature controller with an accuracy of ± 0.01
𝑜C. The output transient photocurrent curves were used to
calculate the hole mobility (𝜇ℎ) by using the relation:
𝜇ℎ =
𝑑2
𝜏𝑉
(1)
where, 𝜏 is the transit time obtained by the photocurrent
curves, V is the applied voltage and d is the thickness of
the LC cells. During the ToF measurements, only 3% laser
intensity was employed which is equivalent to ≈ 2 mW.
2.4. Current-voltage (I-V) characteristics
The PQ10 compound was filled in the ITO-coated LC
cell of thickness 9.2 𝜇m and current-voltage (I-V) charac-
teristics of the PQ10 compound have been carried out using
Keysight B2902A Precision Source/Measure Unit (SMU) as
a function of voltage into different phases, i.e. Col𝑟1 (90 𝑜C),
Col𝑟2 (80 𝑜C) and Col𝑟3 (43 𝑜C). During the measurement,
the temperature of the ITO-coated LC cell was maintained
with an accuracy of ± 0.1 𝑜C using the Linkam LTS 350
hotplate.
2.5. Dielectric spectroscopy
Broadband dielectric spectroscopy of the PQ10 com-
pound has been carried out by using an impedance analyzer
HP4284A to understand the nature of molecular dynamics
within the mesophase and also at the phase transitions. The
PQ10 compound was filled in the planar and homeotropi-
cally anchored cells of thickness 9.2 𝜇m. The temperature-
dependent dielectric measurements, in the frequency in-
terval of 20 Hz−1 MHz, have been performed with the
temperature precision of 0.1 𝑜C using a custom-made hot-
stage attached to a PID controller Eurotherm 3204. A rela-
tively tiny temperature step was taken to investigate in-depth
molecular dynamics near the phase transitions.
2.6. Device architecture and solar cell simulations
Polymeric HTLs are used in optoelectronic technology
at the moment, however they have issues with long-term
aggregation, poor mobility, and expensive cost. These poly-
meric HTLs can be replaced with supramolecular meso-
gens. The chemical architecture of PQ10 molecules allows
us to use it as HTL; therefore, we have employed PQ10
as HTL in quantum dots-based solar cells (QDSCs). The
simulation of solar cell devices has been performed via
Ansys Lumerical software. First of all, QDs-based solar cell
device with an active area of 0.25 × 0.20 cm2, consisting
of ITO/ZnO/InP/MoO3/Ag, has been designed as reported
by Crisp et al.[35] to prove our hypothesis for using the
PQ10 as HTL. The thicknesses of ITO, ZnO, InP, MoO3
and Ag were taken as 180, 40, 180, 10 and 200 nm, re-
spectively and simulation has been performed. In this device
structure, MoO3 acts as hole transport layer (HTL). Here-
after, MoO3 was replaced by PQ10 and the performance
of solar cell was simulated and compared with that of the
ITO/ZnO/InP/MoO3/Ag. The simulation theory for solar
cell devices has been described in detail and presented in
supplementary information (ESI).
Using Lumerical FDTD, different layers of solar cell
device were created. The details of layer thicknesses and
materials’ parameters used for simulating solar cell perfor-
mance are presented in ESI tables 1 and 2, respectively. The
optical properties of the device including absorption spectra
were determined (figure S2, ESI). AM 1.5 solar spectrum
with power equivalent to 100 mW/cm2 was used as the light
source in the simulation. The number of absorbed photons
per unit volume can be represented as the ratio of absorbed
power and the energy of a photon:
𝑔 =
𝑃𝑎𝑏𝑠
ℏ𝜔
=
−0.5|𝐸2|𝑖𝑚𝑎𝑔(𝜖)
ℏ
(2)
Here, the generation rate is the integration of g over the
simulation spectrum.
The quantum efficiency of a solar cell, QE(𝜆), is defined
as:
𝑄𝐸(𝜆) =
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑝ℎ𝑜𝑡𝑜𝑛𝑠
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑛𝑠
≅
𝑃𝑎𝑏𝑠(𝜆)
𝑃𝑖𝑛(𝜆)
(3)
D.P. Singh et al.: Preprint submitted to Elsevier Page 3 of 13

8. Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor
where, P𝑎𝑏𝑠(𝜆) and P𝑖𝑛(𝜆) are the powers of the absorbed
and incident light, respectively, within the solar cell at a
particular wavelength 𝜆. By using QE, the internal quantum
efficiency (IQE), has been determined using the relation:
𝐼𝑄𝐸(𝜆) =
∫ 𝜆
ℎ𝑐
𝑄𝐸(𝜆)𝐼𝐴𝑀1.5(𝜆)𝑑𝜆
∫ 𝜆
ℎ𝑐
𝐼𝐴𝑀1.5(𝜆)𝑑𝜆
(4)
where, h is Plank’s constant, c is the speed of light in the
free space and I𝐴𝑀1.5 is solar spectrum radiation which is
incident on solar cell device.
The external quantum efficiency (EQE) was calculated
using the following expression:
𝐸𝑄𝐸 = 𝐼𝑄𝐸 × (1 − 𝑅) (5)
where, R represents the reflection loss.
The charge transport in solar cell was estimated by using
electrostatic potential (Poisson’s equation) and the density
of free carriers (drift-diffusion equation). By assuming that
all electron-hole pairs have contributed to photocurrent, the
short circuit current density (J𝑆𝐶) can be represented as:
𝐽𝑆𝐶 = 𝑒
∫
𝜆
ℎ𝑐
𝑄𝐸(𝜆)𝐼𝐴𝑀1.5(𝜆)𝑑𝜆 (6)
where, e represents the electronic charge.
The photovoltaic energy conversion efficiency (𝜂) is
commonly measured to assess the performance of the solar
cell which can be written as:
𝜂 =
𝐹𝐹 × 𝑉𝑂𝐶 × 𝐽𝑆𝐶
𝑆𝐴𝑀1.5𝐺
=
𝑃𝑚𝑎𝑥
𝑆𝐴𝑀1.5𝐺
(7)
where, FF is the fill-factor, V𝑂𝐶 is the open-circuit voltage,
J𝑆𝐶 is the short-circuit current, and S𝐴𝑀1.5𝐺 is the incident
power from the AM1.5G solar radiation equivalent to 100
mW/cm2. The complete mathematical approach for calculat-
ing power conversion efficiency is described in supplemen-
tary information.
3. Results and discussion
Polarized optical microscopy (POM) is a preliminary
approach for determining the nature of mesophase; thus, we
performed POM on the PQ10 compound. The optical tex-
tures of the PQ10 compound at various temperatures were
recorded under the crossed polarizer/analyzer condition, as
shown in figure 2. In the PQ10 compound, the out-of-plane
conformal rotations of the phenyl moieties offer the self-
organization of molecules resulting in an improper face-on
orientation onto the ITO substrate. A perfect homeotropic
orientation of columns is turned into a black optical texture;
nevertheless, any mismatch in the homeotropically oriented
columns produces defects, which are typically seen as bright
domains (see enlarged regions in figures 2(b) and 2(c)). The
optical textures in Col𝑟3, Col𝑟2 and Col𝑟1 are almost identical
and only differ in the size of defect domains. The observed
optical textures in the Col𝑟1 (90 𝑜C), Col𝑟2 (80 𝑜C) and Col𝑟3
(40 𝑜C) phases render the classical texture of rectangular
columnar (Col𝑟) phase. All three Col𝑟 phases appear to be
similar at first glance, but they differ slightly in terms of their
transition enthalpies and lattice parameters. Furthermore,
the size of defect domains in these three Col𝑟 phases have
been slightly modified.
As described in the experimental section, the time-
of-flight (ToF) technique, (schematic presented in
figure 3(a)), can be extensively employed on organic
molecules/semiconductors to determine the electronic
and/or ionic charge transport including the evaluation of
charge carrier mobilities for electrons, holes and ions.
The compound PQ10 exhibits unipolar p-type charge
carrier (i.e. holes) mobility. We have investigated hole
mobility in the PQ10 compound by analyzing transient
photocurrent curves at different temperatures by applying
positive external voltages ranging from 30 to 50 V, followed
by determining the transit time of holes during their
propagation from top-to-bottom ITO electrodes (figure
3(b)). The intersection point of two plateau regions in the
photocurrent curve (corresponding to the movement of
holes and their arrival at the bottom electrode) provides the
value of transit time. The ToF signals are classified into two
types: non-dispersive and dispersive [36]. In the absence of
significant trapping and no space charge, a non-dispersive
photocurrent signal can be obtained. This non-dispersive
signal is analogous to the bulk photogeneration, where
two excitons interact and dissociate into free electrons and
holes. On the other hand, a dispersive photocurrent curve
can be observed under certain conditions. The presence
of impurities creates localized energy levels and may act
as traps during the charge carrier movement through the
material [37]. These traps either temporarily (in the case
of shallow traps) or permanently (in the case of deep
traps) ensnare the charge carriers. An additional thermal
energy and/or applied electric field can release the charge
carriers that are bound by shallow traps. The presence of
traps results in a dispersive photocurrent curve as well as
a decreased flight-time (transit time) due to the multiple
times trapping of charges. Usually, non-dispersive or
dispersive photocurrent curves are observed for OLCSs;
which generally depends upon the molecular geometry,
molecular packing of electron-rich or deficient moieties,
microscopic molecular ordering, formation of traps and
defects, etc.
For the compound PQ10, the transient photocurrent
curves plotted at different temperatures (i.e. 40−80 𝑜C)
by applying 45 V are shown in figure 3(b). The disper-
sive nature of curves has been observed at all tempera-
tures which might be attributed to the defects and traps in
the PQ10. In general, compound PQ10 shows 1D charge
transport owing to its Col𝑟 structure; however, the presence
of traps causes a longer transit time and thus lowers the
D.P. Singh et al.: Preprint submitted to Elsevier Page 4 of 13

9. Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor
Figure 2: Polarized optical micrographs (POMs) of compound
PQ10 recorded in cooling cycle at (a) 90 𝑜
C (Col𝑟1), (b) 80
𝑜
C (Col𝑟2) and (c) 40 𝑜
C (Col𝑟3) under the crossed polarizers
condition. The scale bar represents 100 𝜇m. Insets show the
enlarge regions of POMs.
charge carrier mobility. It is evident from figure 3(c) that
the PQ10 compound exhibits p-type (hole) mobility in the
range of 6.8−8.8 × 10−3 cm2V−1s−1. Besides, the hole
mobility remains almost same within the entire range of the
Col𝑟 mesophase. As far as charge transport in the PQ10
compound is concerned, it can be understood as follows:
the excitons and polarons are fundamentally important in
understanding the operation of optoelectronic devices such
as OPVs, OFETs, and OLEDs because they play a vital
role in the electronic and optical properties of OLCSs [38].
The formation of neutral electronic excitations in conjugated
organic molecules, such as pyrazino[2,3-g]quinoxaline and
its derivative, is associated with significant nuclear relax-
ation, the majority of which occurs along the 3,4-didecyloxy
phenyl−pyrazino[2,3-g]quinoxaline stretching mode. PQ10
molecules constitute H-aggregates in the thin film structure
[5], where nuclear relaxation competes with charge transfer,
which is mediated by the electronic interactions between
the PQ10 core and functional moieties attached to the core.
These inter-unit interactions in the PQ10 compound result
in delocalized hole polarons that travel in a distorted hop-
ping pattern via H-like aggregates. It must be noted that
the observed value of hole mobility (i.e. 6.8−8.8 × 10−3
cm2V−1s−1) is lower than that of their original values as
the presence of traps retards the velocity of charge carriers;
however, our values of hole mobility are superior by one
order as compared to that of a similar homolog[16]. The
preparation of a trap-free nano-dimensional thin film of this
compound is still a challenge. The field dependence of hole
mobility is depicted in the inset of figure 3(c). The observed
hole mobility is proportional to the square root of the applied
electric field, i.e. 𝜇ℎ ∝ exp(𝛽 E1∕2) resulting in a decrease
in the hole mobility with increasing value of the applied
field. This nature of mobility confirms the disordering with
shallow traps only, i.e., traps with an energetic distribution
not much larger than k𝐵T. Rybak et al.[16] have recently
investigated a similar compound exhibiting R = OC8H17
attached to the pyrazino[2,3-g]quinoxaline core and also
determined the hole mobility; however, their order of hole
mobility was limited to 10−4 cm2V−1s−1 which was sup-
posed to be higher than our case due to the presence of
shorter alkyl chain length. This is probably because of the
improper molecular ordering, the presence of shallow traps
and/or mis-match in the absorption band of the compound
with respect to the laser excitation wavelength used in the
ToF measurement.
The HOMO and LUMO energy levels of the PQ10
compound were determined by using cyclic voltammetry.
Ag/AgNO3, glassy carbon and platinum wire were used as
a reference, working and counter electrodes, respectively.
The cyclic voltammogram of the PQ10 compound has been
recorded in dichloromethane (DCM) solution of tetra-n-
butylammonium perchlorate (TBAP) (0.1 M), used as a
supporting electrolyte, at a scanning rate 0.1 mV s−1 by
taking ferrocene as a reference (figure 4(a) and (b)). The half-
wave potential of Fc/Fc+ was found to be 0.48 V relative
to Ag/Ag+ reference electrode. The HOMO and LUMO
of the PQ10 compound were estimated from the formula
by using: E𝐻𝑂𝑀𝑂= −(4.8−E1∕2,𝐹𝑐∕𝐹𝑐+ + E𝑜𝑥𝑑,𝑜𝑛𝑠𝑒𝑡) and
E𝐻𝑂𝑀𝑂=E𝐿𝑈𝑀𝑂−E𝑔,𝑜𝑝𝑡 eV, respectively. The HOMO and
LUMO values have been found to be −5.68 and −3.30 eV,
respectively. Figure 4(c) depicts the variation of conduction
current as a function of applied voltage in the range of −2V
to +2V at different temperatures. The insets of figure 4(c)
D.P. Singh et al.: Preprint submitted to Elsevier Page 5 of 13

10. Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor
Figure 3: (a) Schematic representation of the time-of-flight (ToF) technique, (b) Transient photocurrent curves for PQ10
compounds at different temperatures and (c) variation of hole mobility as a function of temperature at different voltages.
The insets of (b) and (c), respectively, represent the 1D propagation of holes and variation of hole mobility as a function of
applied field (E1∕2
) at 40 and 90 𝑜
C.
show the schematics of I-V measurement cell and charge
migration between the PQ10 and ITO electrodes.
The Poole-Frenkel conduction mechanism is usually ob-
served in disordered materials; therefore, it can be employed
to a thin film of the PQ10 material sandwiched between two
ITO electrodes (i.e., metal-insulator-metal thin film stack).
The current-voltage (I-V) equation can be represented as
[39, 40]:
𝐽 ≃ 𝑒
𝑉
𝑑
𝑛𝜇ℎ.
[
𝑒𝑥𝑝
−𝐸𝑡
k𝑇
]
(8)
where, J is the current density, n is the charge carrier
density, e is the elementary charge, 𝜇ℎ is the hole mobility,
V is the applied voltage, d is the sample thickness and E𝑡 is
the trap energy level.
At low voltage (V < 1.5 V), the I-V curves follow
the Ohm’s law; in which the growth of current is directly
proportional to the applied voltage; i.e., J = ne𝜇ℎ × (V/d).
This relation is obtained under a boundary condition of exp[-
E𝑡/kT] ≈ 1, in eqn. 8. The energy gap between the work
function of ITO and the HOMO of PQ10 is ≈ 1 eV (inset
of fig. 4(c)); therefore, charges can easily migrate between
the ITO electrode and the PQ10 material resulting in an
almost linear relationship between the current and voltage.
At higher voltages (≥ 2V), a deviation from the linear I-V
relationship has been noticed. The slope, m, of the I-V curve
when fitted linearly is equal to ne𝜇/d. Assuming that ne/d
remains the same, the ratio of the hole mobilities will be
equivalent to the ratio of slopes at the given temperatures.
We have found that m(80𝑜𝐶)/m(43𝑜𝐶) = 1.3 which is almost in
good agreement with 𝜇ℎ(80𝑜𝐶)/𝜇ℎ(43𝑜𝐶) = 1.2 (calculated at
35V). According to the I-V curves and their interpretation,
the conduction process in the PQ10 materials is attributed to
the Poole-Frenkel equation under a limiting condition which
D.P. Singh et al.: Preprint submitted to Elsevier Page 6 of 13

11. Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor
Figure 4: Cyclic voltammograms of (a) ferrocene (ref.) and (b) compound PQ10 in dichloromethane solution of tetra-n-
butylammonium perchlorate (TBAP) (0.1 M) at a scanning rate 0.1 mV s−1
; the half wave potential of Fc/Fc+
was found
to be 0.48 V relative to Ag/Ag+
reference electrode and (c) Current-Voltage (I-V) curves of compound PQ10 at 90 𝑜
C (Col𝑟1), 80
𝑜
C (Col𝑟2) and 43 𝑜
C (Col𝑟3). Insets depict the schematic of sample cell used for I-V measurement and charge migration between
PQ10 and ITO electrode.
is consistent with the mobility values of the p-type (i.e.,
holes) charge carriers.
The dielectric spectroscopy was carried out as a function
of frequency and temperature on the planar and homeotropic
anchored cells to observe the effect of the surface alignment
layer on the dielectric properties of the PQ10. For the PQ10
compound, the relative dielectric permittivity and conduc-
tivity, respectively, are plotted in figure 5 (a) and (b), as a
function of temperature. The complex dielectric permittivity
(𝜖*), can be expressed as [41]:
𝜖∗
= 𝜖′
− 𝑖𝜖′′
= 𝜖∞ +
(𝜖0 − 𝜖∞)
1 + (𝑖𝜔𝜏)1−𝛼
− 𝑖
𝜎
𝜔𝜖𝑠
(9)
where, 𝜖’ and 𝜖”, are the real and imaginary parts of the
complex dielectric permittivity, respectively. 𝛿𝜖= 𝜖0 − 𝜖∞
is the relaxation strength, 𝜖∞ is the high frequency limit of
dielectric permittivity, 𝜔=2.𝜋.f, 𝜏 is the relaxation time, 𝛼 is
the distribution parameter and 𝜎 is the conductivity.
Usually, the dielectric function is a combination of
static (i.e. 𝜖∞) and dynamic dipole contributions (i.e. (𝜖0-
𝜖∞)/[1+(i𝜔𝜏)1−𝛼]). The molecular ordering has a notable
impact on the value of dielectric permittivity. It is evident in
figure 5(a) that the value of dielectric permittivity depends
on the nature of the Col𝑟 phase and surface anchoring of
the alignment layer attributed to the dipole moments of
the pyrazino[2,3-g]quinoxaline core (𝜖𝑐) and 3,4 didecyloxy
phenyl terminal moieties (𝜖𝑇 ). In the Col𝑟1 phase, both pla-
nar and homeotropically anchored cells have nearly the same
dielectric permittivity. This nature may be explained by the
fact that the Col𝑟1 phase appears just after the isotropic phase
and the viscosity is relatively alike in both the Col𝑟1 and
isotropic phases, leading to nearly identical dielectric per-
mittivity values. However, the planar and homeotropic cells
have demonstrated different dielectric permittivity values in
the Col𝑟2 and Col𝑟3 phases. Due to the 3,4 didecyloxy phenyl
terminal moieties and the pyrazino[2,3-g]quinoxaline core’s
dipole moments not being in the same plane, this nature is
observed (see insets of figures 5(a) and (b)). Different values
of dielectric permittivity in the Col𝑟2 and Col𝑟3 phases are
resulted due to the difference in the resultant dipole moment
(𝜖𝑅) in the planar and homeotropically oriented PQ10 filled
LC cells. It is evinced in figure 5(b) that the value of
conductivity is temperature-dependent owing to the range
of Col𝑟1, Col𝑟2, Col𝑟3 phases; however, in a particular Col𝑟
phase, the average value of conductivity in the planar and
homeotropically anchored cells is almost same. It indicates
that the observed value of conductivity is < 𝜎𝑎𝑣𝑔 > and
has not been remarkably influenced by the nature of the
alignment layer.
The characteristics of the PQ10 compound evinced that
it can be used as HTL; therefore, a quantum dots (QDs)-
based solar cell device made of ITO/ZnO/InP/MoO3/Ag
has been chosen as a reference [35]. A 10 nm layer of
D.P. Singh et al.: Preprint submitted to Elsevier Page 7 of 13

12. Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor
Figure 5: Variation of (a) dielectric permittivity (taken at 1
kHz) and (b) conductivity as a function of temperature of the
compound PQ10 in the Col𝑟1, Col𝑟2, Col𝑟3 phases.
MoO3 has served as the HTL in this structure. To assess
the performance of solar cell using mesogenic HTL, MoO3
has been replaced by the PQ10 and device performance is
simulated. The experimental values of short circuit current
(J𝑆𝐶), open-circuit voltage (V𝑂𝐶), maximum power (P𝑚𝑎𝑥),
fill-factor (FF) and the power conversion efficiency (PCE)
of the ITO/ZnO/InP/MoO3/Ag device were reported to be
8.3 mA/cm2, 237 mV, 0.649 mW/cm2, 33% and 0.647%,
respectively [35]. The simulated values of the same structure
were found to be 8.59 mA/cm2, 234 mV, 0.654 mW/cm2,
33% and 0.654%, respectively, which are identical to the
experimental values. Henceforward, the solar cell simulation
has been performed by considering the PQ10 material as
HTL.
The simulated values of J𝑆𝐶, V𝑂𝐶, P𝑚𝑎𝑥, FF and PCE of
the ITO/ZnO/InP/PQ10 (10 nm)/Ag device were estimated
to be 9.071 mA/cm2, 208 mV, 0.584 mW/cm2, 31% and
0.584%, respectively. It is observed that the J𝑆𝐶 value for
the solar cell device consisting of the PQ10 material is
superior to that of the reference device which is attributed
to the 1D charge transport in the PQ10 that assists in charge
propagation along one dimension. Later on, we increased
the thickness of the PQ10 material and simulation has been
repeated for the ITO/ZnO/InP/PQ10 (25 nm)/Ag device. For
this device, the values of J𝑆𝐶, V𝑂𝐶, P𝑚𝑎𝑥, FF and PCE were
found to be 9.348 mA/cm2, 229 mV, 0.678 mW/cm2, 32%
and 0.678%, respectively. The use of PQ10 increases the
current density of solar cell devices by 12.6% as compared
to the reference device which is attributed to the reduction of
charge carrier recombination at the HTL/InP interface owing
to the 1D charge transport offered by the PQ10. The overall
performance of the PQ10-based solar cell device is superior
to the MoO3-based reference device; however, the value of
V𝑂𝐶 is slightly compromised. The performance of different
solar cell devices is tabulated in Table 1. Due to its attractive
hole mobility and 1D charge transport, the PQ10 has proved
its utility as hole transport material (HTM) and can be used
in different optoelectronic devices. The schematic of solar
cell devices and simulation results are presented in figure 6.
A detailed comparison of the properties of most com-
monly used HTMs is presented in Table 2 which clearly
indicates that the PQ10 compound is superior to other
HTMs owing to the higher value of hole mobility, suitable
HOMO/LUMO levels and visible range absorption charac-
teristics.
4. Conclusions
In summary, an ambient temperature p-type OLCS ma-
terial based on pyrazino[2,3-g]quinoxaline (PQ10) has been
prepared and evaluated as a HTL material. The rectangular
columnar (Col𝑟) phase of the PQ10 compound has a max-
imum hole mobility of 8.8 × 10−3 cm2V−1s−1 according
to time-of-flight calculations. The hole mobility exhibits
field-dependent behaviour throughout the mesophase and is
nearly temperature invariant. The current-voltage behavior
of the PQ10 has also been investigated on a 9.2 𝜇m thin
film of the PQ10 sandwiched between two ITO/glass elec-
trodes which act as a metal-insulator-metal thin film stack.
The current conduction in the PQ10 compound is mainly
governed by the Poole-Frenkel conduction mechanism under
boundary conditions. The temperature-dependent behaviour
of dielectric permittivity and conductivity has also been
carried out by performing dielectric spectroscopy which
revealed that the values of permittivity and conductiv-
ity remarkably depend on the molecular ordering of the
PQ10 material. A QDs-based solar cell device made of
ITO/ZnO/InP/MoO3/Ag has been theoretically investigated
and evaluated with the published experimental results. Here-
after, the PQ10 was used as HTL to replace MoO3, and
the performance of solar cell devices was examined. An
optimized J𝑆𝐶 of 9.348 mA/cm2 has been recorded for
the PQ10-based solar cell device, which is almost 12.6%
superior to the reference device that shows a J𝑆𝐶 of 8.3
mA/cm2. Finally, the performance of different HTLs has
been compared with that of the PQ10. The obtained results
have revealed that the PQ10 can be one of the better choices
as HTL material for optoelectronic devices.
D.P. Singh et al.: Preprint submitted to Elsevier Page 8 of 13

13. Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor
Figure 6: Schematics of solar cell device using (a) MoO3 and (b) PQ10 as HTL. (c) and (d), respectively, represent the variation
of current density (J𝑆𝐶 ) and power (P𝑚𝑎𝑥) as a function of voltage.
Device structure J𝑆𝐶 V𝑂𝐶 P𝑚𝑎𝑥 FF PCE (𝜂)
(mA/cm2
) (mV) (mW/cm2
) (%) (%)
ITO/ZnO/InP/MoO3/Ag (E) 8.3 237 0.649 33 0.647
ITO/ZnO/InP/MoO3/Ag (S) 8.592 233.92 0.653 33 0.663
ITO/ZnO/InP/PQ10/Ag (S1) 9.070 207.85 0.584 31 0.585
ITO/ZnO/InP/PQ10/Ag (S2) 9.347 228.77 0.677 32 0.685
Table 1
Comparison of the performance of solar cell devices exhibiting MoO3 and PQ10 as HTL. The E and S depict the experimental
and simulation data, respectively. S1 and S2 represent the simulation data having 10 and 25 nm thickness of the PQ10 layer,
respectively. The experimental data of solar cell device ITO/ZnO/InP/MoO3/Ag has been taken from Ref. [35].
Acknowledgements
DPS and AKS, respectively, are thankful to Cam-
pus France and Research Grants Council (RGC) Hong
Kong for financing the PHC PROCORE mobility project
46846TK (France)/F-HKUST602/20 (Hong Kong). DPS is
thankful to Samuel de Champlain project (0185-CAN-22-
0010) and also to the ULCO for providing financial as-
sistance via the BQR-2022 project. AAS thanks to SERB,
Govt. of India, and BRNS-DAE for funding this work
via project CRG/2018/000362 and 2012/34/31/BRNS/1039,
respectively. NC sincerely thanks to NSRF via the Pro-
gram Management Unit for Human Resources and Institu-
tional Development, Research and Innovation (grant number
B11F660024) and Kasetsart University Research and Devel-
opment Institute (KURDI) FF(KU)49.66 for funding.
Conflict of interest
The article is original and has been written by the stated
authors who are aware of its content and approve its submis-
sion. This article has neither been published previously nor
under consideration for the publication elsewhere. We also
declare "No conflict of interest exists".
Supplementary material
The synthesis protocol of the pyrazino[2,3-
g]quinoxaline (PQ10) organic liquid crystalline
semiconductor (OLCS) and normalized absorption
spectra of solar cell devices consisting of MoO3 and
the PQ10 as HTLs are provided as supplementary
figures S1 and S2, respectively. The combined curve of
cyclic voltammograms of reference ferrocene (Fc) and
compound PQ10 in a dichloromethane solution of tetra-n
D.P. Singh et al.: Preprint submitted to Elsevier Page 9 of 13

14. Pyrazino[2,3-g]quinoxaline core-based organic liquid crystalline semiconductor
HTL Material HOMO LUMO Hole mobility 𝜆𝑎𝑏𝑠 Reference
(in eV) (in eV) (in cm2
V−1
s−1
) (in nm)
Poly(triaryl amine) -5.20 -2.30 1 ×10−3
395 [42, 43, 44]
X44 -5.06 -2.12 9 ×10−4
381 [45]
X51 -5.14 -2.21 1.5 ×10−4
307, 365 [46]
X55 -5.23 -2.26 6.8 ×10−4
402 [47]
X59 -5.15 -2.10 5.5 ×10−5
387 [48]
X60 -5.15 -2.10 1.9 ×10−4
387 [49]
X62 -5.14 -2.22 7.9 ×10−5
383 [50]
TaTm -5.40 -2.35 4 ×10−3
350 [51]
Br-2PACz -6.10 -2.64 5.4 ×10−4
450 [52, 53]
Spiro-OMeTAD -5.13 -2.06 8.1 ×10−5
388 [54, 55]
F4TCNQ -8.34 -5.24 0.4 416, 768 [56]
F6-TCNNQ -9.32 -5.37 0.45 ≈ 500, 750 [57, 58]
TFB -5.3 -2.3 2 ×10−3
390 [59]
HMTPD -5.9 -2.6 - 302 [60, 61]
PEDOT:PSS -5.2 -2.4 0.75 220, 600 [62, 63]
MoO3 -6.6 -2.2 540 311, 750 [35, 64, 65]
PQ10 -5.68 -3.30 8.8 ×10−3
470 present study
Table 2
Comparison of the energy levels, hole mobility and absorbance of the different HTL materials.
butylammonium perchlorate (TBAP) is presented in figure
S3. The absorption and emission spectra of the compound
PQ10 have been given as figure S4. Along with the specifics
of layer thickness and material characteristics, the theory of
solar cell modeling and device performance is also covered
in ESI.
Data Availability Statement
The raw/processed data required to reproduce these find-
ings cannot be shared at this time; however, the data will be
made available on a justified demand.
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18. The article is original and has been written by the stated authors who are aware of its
content and approve its submission. This article has neither been published previously
nor under consideration for the publication elsewhere. We also declare "No conflict of
interest exists".