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Studying discotic liquid crystalline physical gel formation and
their applications in solar cells
by
Sehrish Iqbal
2016-12-0013
Supervisor: Dr. Ammar Ahmad Khan
Co-supervisor: Dr. Habib ur Rehman
Department of Physics
Syed Baber Ali School of Science and Engineering
Lahore University of Management Science
This dissertation is submitted for the degree of
MS Physics
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CERTIFICATE
I hereby recommend that the thesis prepared under my supervision by: Sehrish
Iqbal on title: Triphenylene Discotic Liquid Crystals Physical Gel formation and their
application in Solar Cells of the requirements for the MS degree.
Dr. Ammar Ahmed khan
Advisor (Chairperson of Defense Committee)
Recommendation of Thesis Defense Committee:
Co supervisor: Dr. Habib ur Rahman
Name Signature Date
Name Signature Date
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I would like to dedicate this thesis to my parents and respected teachers.
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ACKNOWLEDGMENT
I would never be able to complete my dissertation without the guidance of my advisor, help
from friends, and support of my family. I would like to express my sincere gratitude to my
supervisors Dr. Ammar Ahmed Khan and Dr. Habib ur Rehman, for his excellent guidance,
encouragement, support and providing me an opportunity to do my research work under their
supervision.
I would also like to thank my Parents and friends for their constant love and support.
Sehrish Iqbal
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Abstract
Liquid crystals form ordered mesophases that undergo phase transitions as a function of
temperature. Semiconducting triphenylene discotic liquid crystals can be used to form self-
assembled physical gels that are of great interest because of the physical and electronic properties
of the fibrous network of low molecular organo-gelators (LMOGs) that forms in particular
solvents. In this research we show that gel formation is strongly affected by modulating the solvent
(physical medium of gelation). Solvent properties such as polarity and dielectric permittivity play
an essential role in gel formation as well as fiber formation in triphenylene discotic liquid crystals.
We apply the Hansen solubility parameter approach to understand the solvent space for gelation
and use our results as a predictive model for untested solvents. Finally, as an application of the
gels, dye sensitized solar cells utilizing three different liquid crystal gelators are fabricated, and
physical properties of the gels are correlated with photovoltaic performance.
In this work Differential scanning calorimetry (DSC) is used for the study of phase-transition
temperatures of pure liquid crystalline materials, and polarizing optical microscopy is used to
determine the texture of the self-assembled fibrillar network in gel formation. I-V measurements,
X-Ray diffraction spectroscopy (XRD) and U-V visible spectrum are used for the study of gel
formation lead to conductivity and phase identification. In addition, Hansen solubility parameters
shows that within the Hansen space particular solvents forms gelation that highly depends on the
solubility of the solvents.
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Table of Contents
Chapter 1: Liquid Crystals ........................................................................................................................ 8
1.1 Introduction ......................................................................................................................................... 8
1.2 Solubility ....................................................................................................................................... 9
1.3 Gelation ....................................................................................................................................... 10
1.4 Intermolecular interactions of solvents ....................................................................................... 11
I. Dielectric constant (ϵ) ................................................................................................................. 11
II. Single and multi-component solubility parameters (δ) ........................................................... 11
III. Kamlet-Taft solvent parameters .............................................................................................. 11
IV. Hildebrand parameter .............................................................................................................. 12
V. Hansen Solubility Parameters ..................................................................................................... 12
1.5 Dye-Sensitized Solar Cells.......................................................................................................... 15
Chapter 2: Experimental Methods and Procedures .............................................................................. 17
2.1 Gel Formation ............................................................................................................................. 17
I. Material Required ....................................................................................................................... 17
II. Procedure ................................................................................................................................ 18
III. Cell Fabrication ....................................................................................................................... 18
IV. Lithography ............................................................................................................................. 19
2.2 Fabrication of DSSCs ................................................................................................................. 20
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2.3 Experimental Methods ................................................................................................................ 21
Chapter 3: Results and Discussion .......................................................................................................... 22
3.1 Phase transition ........................................................................................................................... 22
3.2 Conductivity measurements ........................................................................................................ 25
3.3 Behavior of Different Solvents in DLCs ..................................................................................... 29
3.4 Hansen solubility parameters ...................................................................................................... 30
Chapter 4: Conclusion .............................................................................................................................. 37
Future work ................................................................................................................................................. 38
Bibliography ............................................................................................................................................... 39
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Chapter 1: Liquid Crystals
1.1 Introduction
Discotic liquid crystals are ordered mesophase that are formed from disc-shaped molecules known
as “discotic mesogens”[1]. Discotic mesogens are typically composed of an aromatic core
surrounded by alkyl chains[2]. The aromatic core transfers charge through π-conjugated system
that allows the discotic liquid crystals to be electrically semi conductive along π -tacking direction.
Liquid crystals are also known as noncolumnar phases such as nematic phase, due to the substance
interaction between the phase and composition of distinct molecule. Liquid crystalline materials
can be applied in a wide range of applications. e.g. the optoelectronic applications of Discotic
liquid crystals ‘in photovoltaic devices as organic semiconductors[3], optical compensation layers
in LCDs, and as LMOGs[4].
Low molecular mass organogelators (LMOGs) are comparatively new materials that have many
applications. Low molecular mass organogelators are monomeric subunits, that have strong non-
covalent interactions “which ” form “self-assembled” fibrillar “network” (SAFINs), take “solvent” between
“constituents[5]”. The “LMOGs” explained the “structural” “features” that are “responsible” for “gelation”.
As “SAFINs” are “formed’ the ‘elongated ’ fibers ‘that’ becomes ‘entangled ’ and ‘captured ’ the ‘solvent’s ’
molecule. When the solvent’s molecule is ‘captured’ ‘by’ the “‘network”’, which is restrained by “surface”
tension effect. The gel solubility “is” ‘depending’ “on” the “equilibrium” among the ‘assembled ’ network
and “‘liquified”’ ‘gelators ’. The main ‘feature’ of ‘LMOGs’ is their “ ‘ability ” ’ to ‘maintain’ that “‘organic” ’’ solvent
‘at ’ its ‘boiling’’ point ‘due’ to ‘’general ’’ solvent ‘fiber’ interaction[6]. “Gels” are “self-assembled”’ “by the “‘non-
covalent” “interaction such “as Ven “‘der' Waals” “interactions,” “‘π-stacking”’ and “‘hydrogen’ bonding”. “The
“key” feature of “gel” formation is its self-assembly and depends on the adjustable bond formation[7].
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Many organic liquids such as paraffins, alcohols, aromatic and chlorinated molecules, nematic and
sematic liquid crystalline materials, electrolytes, polymerizable liquid and other numerous range
of functional group have been gelled by these LMOGs.
1.2 Solubility
Solubility is the property of a constituent solute that soluble in a solvent, depends on the physical
and chemical properties of the solvent and the solute. The intermolecular forces between the solute
and the solvent are used to determine the solubility of a solute into the solvent. Solubility
parameters are used for the choice of solvent that is mostly based on the rule “Like dissolves like”.
A solvent can be classified as a good solvent or a bad solvent. If the solute is soluble in the given
solvent, it is called a good solvent because in between solute and solvent there are strong
intermolecular forces[7]. If the solute is not soluble in the given solvent, it is called a bad solvent
because within solute and solvent there are weak intermolecular forces. Solvent properties
demonstrate an imperative part in intermediating the collection and self-assembly of atomic
gelators and their progress into fibers. To relate the solubility parameters and gelation capacities
of atomic gelators diverse solubility parameters are utilized.
To determine the range of liquids that are probably going to be gelled by any specified gelators,
solubility parameters are used[7]. Infact, we discuss the gelation domain and propose an enhanced
system for the solubility tests, and a definite strategy for the assurance of the gelation space[8].
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1.3 Gelation
The workings of the gelation process as well as its chemistry are the two main factors on which
gel formation mostly depends. To form a network those gelators that are created by covalent bond
are known as “chemical-gelators or gel”, while those gelators that are created by the formation of
non-covalent bond, like “hydrogen bonding or π-π interactions” are known as physical gels. If we
talk about specifically, so those gelators that are depends on π-systems interaction are known as
“π-gel”[9].
To limit the substance flow, an organized network of a gelators is used macroscopically, within
the gel. Condensed columns of triphenylene, HAT5 and HAT6 molecules are used in the form of
fibers for the formation of gels described in the paper. Whereas, Fibers are made of π-stack
molecules. The four flat fused rings in Triphenylene derivatives are the reason behind an extended
conjugated π-electron system. symmetric and antisymmetric substitutions in the alkyl chains are
used to make Triphenylene gels. A relevant method of LC gel fabrication is used in the mechanism
of gelation, gelators are of LC molecules used in a solvent medium. The selection of solvent is
very critical in the formation of gel for the reason that the substance LCs phase-splits from the
solvent. Besides, phase separation, intermolecular non-covalent interactions results into the
formation of long-term fibrous collections which eventually results in an adjustable temperature
dependent Liquid-Crystal network that maintains the solvent[9].
Different techniques are used in triphenylene discotic liquid crystals molecules to understand the
π-organogels. The functional groups of different solvents such as alcohol, imidazole react with the
alkyl chain of the symmetric DLCs. The properties of the functional group have durable effect on
gel formation. The interactions between molecules and solvation properties of functional groups
are the main characteristics for the gel formation.
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1.4 Intermolecular interactions of solvents
Different parameters are used to compute the solvent interaction in gel formation.
Dielectric constant (ϵ)
Single and multi-component solubility parameters (δ)
Dimroth-Reichardt parameter (ET (30))
Kamlet-Taft solvent parameters
Hildebrand parameter
Hansen Solubility Parameters
I. Dielectric constant (ϵ)
Dielectric constant tells us about the polarity of the solvents[10]. The solvents with high dielectric
constant will have high value of polarity.
II. Single and multi-component solubility parameters (δ)
This parameter tells us that how compatible is the solvent for “gelation”, “depending” on the
gelators/solvent system, a low/high “solubility” parameter “can” indicate “low/ high” “thermal” solubility
of the gels respectively[11].
III. Kamlet-Taft solvent parameters
These parameters are based on the solvatochromic relationship which measures distinctly, the
hydrogen donor bond (α), hydrogen acceptor bond (β) and polarizability (π*) of the solvents[12].
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IV. Hildebrand parameter
These parameters measure “the” energy of vaporization that proceeds to form the donor/acceptor
hole (p+) within the solvent, which is “further” modified as Hansen solubility parameters[12].
V. Hansen Solubility Parameters
Hansen dissolvability parameters (HSPs) are generally utilized for the selection of reasonable
solvents for indicated solutes. Basically, this idea was introduced to guess the solubility of different
polymers in various solvents. This method is totally based on the idea “like dissolves like” which can
only be possible if both solute and solvent have similar Hansen solubility parameters (HSPs). These
parameters reveal the information about the liquid’s total energy of vaporization which consists of
various individual parts.[13]
In particular, each molecule is given three Hansen parameters, and all of them normally calculated
in MPa0.5.
• δd dispersion forces
• δp dipolar intermolecular force
• δh hydrogen bonds
A Hansen space is formed; if we take these parameters as coordinates for a single point in three
dimensions (3-D). In this three-dimensional space, two molecules which are closer to each other
will dissolve into each other. Interaction / Solubility Radius (R0) is a value assumed to the
substance being soluble (solute), it controls the radius of circle in Hansen space and its inside is
the three Hansen parameters. Distance (Ra) between solubility directions and midpoint of the
solute circle in Hansen space can be computed by using the following formula
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𝐑𝐚 = √𝟒(𝛅𝐝 − 𝛅𝐝𝐬 )𝟐 + (𝛅𝐩 − 𝛅𝐩
𝐬 )𝟐 + (𝛅𝐡 − 𝛅𝐡𝐬 )𝟐 (1)
∗ The HSPs for the solvents are δds, δp
s, and δhs
∗ The HSPs for the solute are δd, δp and δh
The distance (Ra) can be compared with the interaction / Solubility radius (R0). If Ra < R0 then
there is a high probability of the solvent to dissolve the solute.
The ratio of distance (Ra) and interaction radius (R0) is termed as relative energy difference
(RED)
𝐑𝐄𝐃 =𝐑𝐚
𝐑𝐨 (2)
If
• RED < 1, system is similar and dissolve
• RED > 1, the system is non-solvent
• RED = 1, the system is partially dissolve
If triphenylene (HAT6) is considered as an example, we normally compare the HSPs for solvents
with the HSPs for triphenylene (HAT6). On the off chance that the distance (Ra) between the two
points in Hansen space is small with a particular distance (Ro), at that point triphenylene (HAT6)
and given solvent have high possibility of being dissolve. The estimation of HSPs of solvents can
be looked in the literature and the estimation of HSPs for the triphenylene (HAT6) are controlled
by testing the solubility of triphenylene (HAT6) in the solvent and plotting the outcome by using
the values of δd, δp, δh and Ro. A MATLAB program is used to find the HSPs for the triphenylene
(HAT6) and radius Ro.[13]
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The calculation is comparable as Hansen's calculation for processing HSPs. As indicated by the
solubility of the solvents the information goes into the computer program such 1 demonstrates a
good solvent, however 0 demonstrates a bad solvent as shown in the figure 1[13] The program
logically estimates the given data information with a nature of-fit function called the Desirability
Function to fit the given data information
𝐃𝐚𝐭𝐚 𝐟𝐢𝐭 = (𝐀𝟏 ∗ 𝐀𝟐 ∗ 𝐀𝟑 ∗ . . . . . . .∗ 𝐀𝐧)𝟏
𝐧 (3)
Where n = Number of solvents
As the fit enhances during an improvement, given data information fit approaches 1.0. At the point
when all the good solvents are included inside the circle and all the bad ones are outside of it:
𝐀𝐢 = 𝐞−(𝐞𝐫𝐫𝐨𝐫 𝐝𝐢𝐬𝐭𝐚𝐧𝐜𝐞) (4)
*Ai for a given good solvent inside the circle and for a given bad solvent outside the circle is 1.0.
“The error distance is the distance to the sphere boundary for the solvent in error either as being
good and outside the sphere or as being bad and inside the sphere”.
Ro is one of the objective of our program that is calculated.
Figure 1 Illustrates the Hansen space in which Ra is the distance of the solvent from the center of the
sphere while Ro is the radius of the sphere in Hansen space
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For good solvent outside the circle, Eq. 4 is changed to Eq. 5, for bad solvent inside the circle, the
formation turns to Eq. 6.
𝐀𝐢 = 𝐞−(𝑹𝒂−𝑹𝒐) (5)
𝐀𝐢 = 𝐞−(𝑹𝒐−𝑹𝒂) (6)
The relative energy difference (RED) value has been taken according to Eq. 2
Our objective in this investigation is the calculation of Ro for the triphenylene (HAT6), for which
every good solvent has Ra value lower than this Ro value and every bad solvent have Ra value
higher than this Ro value. The estimation of Data Fit in eq. (4) is a vital perspective for deciding
the HSPs of the triphenylene (HAT6).
The results of our program by the value of 1 for Data Fit, as shown in Table 5.
1.5 Dye-Sensitized Solar Cells
The electron transport, hole transport and light absorption[14] are the properties of different photo-
optical devices in which Dye-sensitized solar cells (DSCs) are supposed to be the most important
type of solar cells. A broad-band gap semiconductor such as TiO2 is attached with sensitizing dye
in DSSCs as shown in the Figure 2 .
The photo-excited electrons suddenly jump to the conduction band that transfers electron to one
of the electrodes when the dye absorbs light. A redox reaction[15] takes place in the electrolyte
that usually consists of iodide/triiodide (I-/I3-), the dye gets oxidized and transports the hole to the
counter-electrode and become neutral. The high efficiency dye combined with the mesoporous
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TiO2 that results in maximum light absorption and charge injection. The life time of the devices is
more effected by the evaporating nature of solvents. Mostly liquid acetonitrile is used as a redox
electrolyte in DSSCs[16].Which “includes” the use of “solid” hole transport “layers, electrolytes, ionic
“liquids”, and alternative “low” vapor “pressure[17]" solvents.
In DSSCs the self-assembled triphenylene discotic liquid crystals physical gels can be used as an
electrolyte. At the TiO2 interface the physical gel reduces the electron recombination.
In addition, the life time of DSSCs can be improved by using the gel electrolyte as compared to
the standard electrolyte, because the gel network maintains the solvent for a long range of time.
(b)
Figure 2 (a) Schematic representation of DSSC (b) Sequence of events in DSSC
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Chapter 2: Experimental Methods and Procedures
The experimental methods and procedures that are used in the gel formation with different
solvents along with the fabrication of dye sensitized solar cells are discussed in this chapter
2.1 Gel Formation
In this section we introduce materials and procedure that is used in gel formation.
I. Material Required
2,3,6,7,10,11-Hexakis(hexyloxy)triphenylene (HAT6) Discotic liquid crystals
2,3,6,7,10,11-hexakis(pentyloxy)triphenylene (HAT5) Discotic Liquid crystals
50% HAT5 + 50% HAT6 (Mixture)
To prepare the homogeneous mixture of HAT5 and HAT6, both samples were
combined by the ratio of 50:50 in chlorobenzene. The homogeneous mixture of
HAT5 and HAT6 was obtained by evaporating chlorobenzene through the
rotavapor[18]
Acetonitrile
Acetonitrile Based Electrolyte
To prepare the Acetonitrile based electrolyte, add 0.05M iodine (I2),0.5M 4-
tertbutylpyridine, 0.5M Li I in 10ml Acetonitrile. Further, increase the iodide ion
add 0.3M EMITCB in it.
Glass slides
25 μm spacer film
FTO glass slides
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II. Procedure
To prepare the discotic liquid crystal HAT6 physical gels, 5mg HAT6 was added in 1ml pure
acetonitrile/ acetonitrile-based electrolyte”. Which is then heated at about 60 oC “to melt” the
“crystalline” form of HAT6 “into the” liquid solution. After that “the” solution “was” placed in sonicator,
the sample was heated to melt at 60oC once again. Later the sample was cooled down at room
temperature to get the gels. It is a repeatable process, by cooling and heating, we can change the
phase transition between solution and gel form. Repeating the same process for HAT5 and their
Mixture to obtain their gels respectively as shown in Figure 3
(a) (b)
Figure 3 Illustration of gel formation in glass bottles. (a) Gel formation in HAT6 before and
after inversion. (b) Another illustration of gel formation [9]
III. Cell Fabrication
Simple sandwich cells were prepared by using two glass substrates, joined together by the help
of 25 μ m sealing and spacer film to keep a “fixed gap” between “the substrates”. Subsequently,
the connections were developed at the end of sandwich substrates. Fill the cell with the gel
electrolyte for further characterizations as shown in the Figure.4 (a) . For the conductivity
measurements, a sandwich cell is prepared, in which one side has a pattern using the photo-
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lithography technique and the other side is an FTO slide that are joined together by using the
25 μm spacer film. In Addition, the connections are made on the both sides of the sandwich
cell by using the thin copper wire as shown in Figure 4 (b). After that the cell is filled with
liquid electrolytes and gel electrolytes respectively and connect with the source meter to
measure the I_V characterization.
(a) (b)
Figure 4 (a) Illustration of simple cell filled with the liquid crystals (b) The Illusion of a cell
prepared for conductivity measurements of the liquid electrolytes.
IV. Lithography
To make connections for I-V measurements on the glass slide, lithography technique is used. In
which first spin coat the washed glass slide with the photo resist AZ1512 at 2800 rpm for 45 sec,
followed by heating at 100 oC for 1 min. After spin coating, deposit the patterns for 2 secs on the
glass slides using the mask liner through photolithography. Moreover, Pt is deposit using the
magnetron sputtering.
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2.2 Fabrication of DSSCs
There are two electrodes that are commonly used in photo-optical devices, photo-anode and a
counter cathode electrode. In DSSCs photo-anode was prepared by first spin coated with
compact “TiO2 layer solution “that is prepared by taking 4.75g anhydrous IPA and 0.25g titanium
butoxide mixed with 2 drops of HCl, followed by filtration by using 0.2 micrometer filter, at
3000 “rpm” for 30s.The sample was annealed by 500ºC for 60 “minutes”. “ Later”, using the circular
pinching mask that controls the active area of device of about 0.28 cm2 a mesoporous “TiO2 paste”
was “coated” and annealed at 500 ºC to give an approximately 8 μ m “thick” mesoporous “layer”. For
dye absorption the substrate is then” placed in a dye ” N719 solution” for 24 hours’. sol is used to
prepare the counter electrode through spin coating at 3000 “rpm” for “30 s that is further annealed
at 500 ºC. Both electrodes are sealed by using 25 μ m “Spacer” film. Subsequently, after preparing
the DSSCs, cells are filled with liquid electrolyte and electrolyte gels of HAT5, HAT6 and their
Mixture respectively as shown in Figure 5
Figure 5: (a) Illustrates the photo-anode (b) The Pt coated counter electrode (C) DSSC
prepared by joining both electrodes using the 25 μm spacer film
(a) (b) (c)
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2.3 Experimental Methods
Polarizing optical microscope (POM)
UV-Vis spectrometer
X ray “powder” diffraction (XRD)
Differential “Scanning” Calorimetry (DSC)
Source meter (IV measurements) Keithley 2400
Solar simulator
MATLAB 2017a Software
A Polarized optical microscope is “used” to visualize the” gel formation “inside” the sandwich “cells.
POM is used to observe optically due to its anisotropic character. It has both polarizer and analyzer
that are adjusted perpendicular to each other. “The image is “obtained” by the “interaction” of plane-
polarized “light” with the “birefringent” specimen to “produce” two individuals “wave” components that
“are” each polarized in “mutually” perpendicular planes”. A UV-Vis (300-1100 nm) spectrometer was
used to record spectra in solution. The crystalline nature of the gels was studied using x-ray
diffraction technique (XRD) λ=1.54 Å. Symmetric -2θ scans of the samples over the range from
2θ = 15o to 2θ =25o were used to study the crystalline behavior of gels. To measure the phase
transition of the gels “DSC is “used. A Keithley 2400 “parameter” analyzer is used “to” take “I-V”
measurements on FTO-FTO “packed sandwich cells “filled” with the “gels. A solar simulator is used
to test the “DSSC” devices. A MATLAB 2017a Software was used to find the HSP parameters of
the triphenylene discotic liquid crystals.
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Chapter 3: Results and Discussion
The characterization of liquid electrolyte in gel formation, the solubility parameters that effects on
gelation and the application of gel electrolyte in dye sensitized solar cells are discussed in this
chapter.
3.1 Phase transition
Triphenylene discotic liquid crystals have thermotropic properties. Differential scanning
calorimetry (DSC) is used to determine the phase transition of DLCs by heating. There are an
exothermic and endothermic reactions take place by heating and cooling the discotic liquid
crystals[19]. DLCs change their phase by heating from liquid crystals to isotropic and by
cooling liquid crystals to crystalline Shown in the Fig.4., a very wide transition was observed
by both heating and cooling the DLCs.
(a) (b) (c)
Figure 6: (a) Illustrates the DSC for DLCs HAT5 in which the phase change by heating and
cooling. (b) Shows DSC for HAT6 and (c) Illustrates the DSC for the mixture of HAT5 and HAT6
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POM was used to measure the phase change into the gel. The cell shown in Figure.4 (a) was filled
with liquid electrolyte physical gel of Triphenylene discotic liquid crystals that was analyzed by
using polarized optical microscope (POM) as shown in Figure.7. The formation of birefringent,
interconnected triphenylene fibers shows that it is the pack of molecular columns of DLCs. The
formation of gel is basically the phase transition. By heat the DLCs with the solvent the LCs melts
and change their phase from liquid crystals to isotropic and by cooling the liquid crystals fibers
entangled with the molecules of the solvent and form the complete physical gel.
(a) (c)
(b) (d)
Figure 7 (a),(b) Shows the micrographs of gel of DLC HAT5 E with Acetonitrile based electrolyte
and (c) ,(d) Shows the micrographs of gel formation of HAT5 E+ with Acetonitrile based
electrolyte
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UV visible spectrum and XRD is used to figure out the band gap and the crystalline nature of
DLCs as shown in the Figure.8 (a) and (b) respectively.
(a)
Table 1: Shows d-spacing of DLCs
(b)
HAT 5 HAT 6 Mixture
0.466 nm 0.489 nm 0.478nm
0.441 nm 0.426 nm 0.427 nm
Eg
= 𝒉𝒗
λ
Eg =
𝟏𝟐𝟒𝟎
350
Eg
= 3.5 eV
Figure 8(a) UV visible spectrum ,Illustrates the band gap for DLCs. (b) XRD, Illustrates the
crystalline behavior of DLCs
Using Bragg's equation
2 d Sin ϑ = n λ
λ = 1.54 Å
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3.2 Conductivity measurements
For the conductivity measurements, a sandwich cell, whose one side has a definite Pt coated area
(deposit by lithography) and the other side has FTO slide, joined by using 25 μm spacer film was
filled with triphenylene discotic liquid crystal physical gel. Appling the connections, measured the
conductivity of HAT5, HAT6 and Mixture gel with different electrolytes the results are shown in
the Figure 9. The results show that HAT5 liquid electrolyte gels are more conductive as compared
to HAT6 gels.
(a) (c)
(b) (d)
Figure 9(a) Illustrates the conductivity comparison b/w HAT5,HAT6 and Liquid electrolyte (b)
Illustrates the conductivity comparison b/w HAT5,HAT6 and Liquid electrolyte in which EMITCB
is added (c) Illustration of two liquid electrolyte conductivity comparisons (d) Illustrates the (a)-
(c) all gel electrolyte conductivity comparison.
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All of the gels prepared with HAT5, HAT6 and mixture with two different electrolytes was used
in DSSCs as an electrolyte and calculated the corresponding efficiency in the solar cells with the
measure of Jsc and Voc. the I_V measurements are taken by using the solar simulator as shown in
the Figure.10 and the corresponding values of efficiencies are shown in Table 2.
(a) (c)
(b) (d)
Figure 10(a) Illustrates the comparison b/w I_V characteristics of HAT5 with two different by
fixing the area of 0.28 cm2 (b) Illustrates the comparison of HAT6 with two different liquid
electrolytes (c) Illustrates the I_V comparison b/w two liquid electrolytes (d) Illustrates the I_V
comparison of Mixture with different electrolytes.
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100 data points are taken through the solar simulator to calculate the efficiency, which was not
enough to reach the value of Voc . To calculate the value of Voc, extrapolate the data using Origin
software. Then the plotted graphs are shown as in Figure 11
(a) (c)
(b) (d)
Figure 11(a)(b)(c) and (d) Illustrates the extrapolated DSSCs I_V curves comparisons b/w
different Liquid electrolytes
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Efficiencies of DSSCs
Table 2: Shows the efficiencies of different electrolytes in DSSCs
The efficiencies of DSSCs are calculated by using different Liquid electrolytes and DLCs gel
electrolytes. The DLC HAT6 Gel electrolyte gives the maximum efficiency of 4.65 % .
Electrolytes Isc(mA) Voc(mV) FF Efficiency (%)
HAT5GelElectrolyte 1.879 764.60 0.72 4.27
HAT5 Gel Electrolyte + 1.675 747.75 0.72 3.81
HAT6 Gel Electrolyte 2.017 776.02 0.72 4.55
HAT6 Gel Electrolyte + 2.065 756 0.72 4.65
Mixture Gel Electrolyte 2.035 732.62 0.66 4.21
Mixture Gel Electrolyte + 2.224 718.85 0.56 3.79
Liquid Electrolyte 1.211 784.17 0.75 2.82
Liquid Electrolyte E+ 1.280 820.65 0.76 3.18
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Figure 12: Shows the efficiencies bar_graphs of different electrolytes
3.3 Behavior of Different Solvents in DLCs
Different solvents are experimentally tested with triphenylene discotic liquid crystals such as
Ethanol, Methanol, DMSO, Acetone, Acetonitrile, DMF, Hexane, DCM, Toluene, IPA,
Chlorobenzene, Dichlorobenzene, Chloroform, n-butanol, Ethyl-acetate, some solvents are
soluble in DLCs, some solvents form precipitates while some solvents form gelators in
Discotic liquid crystals. . Table 2. Shows the behavior of different solvents in triphenylene
discotic liquid crystals.
# Solvents Gelation/Precipitate/Liquid
1 Ethanol Gelation
2 Methanol Gelation
3 Acetonitrile Gelation
4 n-butanol Gelation
5 IPA Gelation
6 Water Precipitates
7 DMSO Precipitates
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8 Acetone Liquid
9 DCM Liquid
10 Toluene Liquid
11 DMF Liquid
12 Chlorobenzene Liquid
13 Dichlorobenzene Liquid
14 Chloroform Liquid
15 Hexane Liquid
16 Ethyl-acetate Liquid
Table 3: Illustrates the behavior of different solvents in Discotic Liquid crystals.
3.4 Hansen solubility parameters
Hansen solubility parameters tell us about the solvent which are soluble in the given solute by
using the Hansen solubility parameters of the solvent. A MATLAB code is used to measure the
HSPs for the solvent. To measure these values, take the HSPs for the solvent tested experimentally
with the tested solubility.1 shows the solvent is soluble with given solute and 0 shows that the
solvent is not soluble with the given solute. By using this information, calculate the relative energy
difference (RED) and plot it to check whether which solvents points lie within the circle and which
solvents points lies outside the circle those points which lies inside the circle will participate in gel
formation and gives the values of HSPs parameters for the solute.
Solvents δd δp δh solubility RED
Ethanol 15.8 8.8 19.4 0 1.2103
Methanol 15.1 12.3 22.3 0 1.3967
31
DMSO 18.4 16.4 10.2 0 1.0005
Acetone 15.5 10.4 7 1 0.9960
Acetonitrile 16 12.8 6.8 0 1.0091
DMF 17.4 13.7 11.3 1 0.9994
Hexane 14.9 0 0 1 0.9328
DCM 18.2 6.3 6.1 1 0.6893
Toluene 18 1.4 2 1 0.6391
IPA 15.8 6.1 16.4 0 1.0910
Chlorobenzene 19 4.3 2 1 0.5669
Dichlorobenzene 19.2 6.3 3.3 1 0.5831
Chloroform 17.8 3.1 5.7 1 0.6826
n-butanol 16 5.7 15.8 0 1.0562
Ethyl-acetate 15.8 5.3 7.2 1 0.8944
Table 4 : Shows the Relative energy difference (RED) for different solvents by using their solubility
and HSPs parameters
32
Solvents δd δp δh gelation RED(G)
Ethanol 15.8 8.8 19.4 1 0.9898
Methanol 15.1 12.3 22.3 1 0.9711
DMSO 18.4 16.4 10.2 0 1.0642
Acetone 15.5 10.4 7 0 1.0000
Acetonitrile 16 12.8 6.8 1 0.9999
DMF 17.4 13.7 11.3 0 1.0370
Hexane 14.9 0 0 0 1.0164
DCM 18.2 6.3 6.1 0 1.0688
Toluene 18 1.4 2 0 1.0803
IPA 15.8 6.1 16.4 1 0.9939
Chlorobenzene 19 4.3 2 0 1.1002
Dichlorobenzene 19.2 6.3 3.3 0 1.0996
Chloroform 17.8 3.1 5.7 0 1.0639
n-butanol 16 5.7 15.8 1 0.9998
Ethyl-acetate 15.8 5.3 7.2 0 1.0099
Table 5: Shows the Relative energy difference (RED) by using the HSPs for the solvents and their
gelation ability.
33
Delta_d Delta_p Delta_h R_o
Solubility 24.905 10.344 22.895 21.620 Data fit = 1
Gelation 7.0747 19.896 19.699 23.138 Data fit = 1
Table 6:Illustrates the calculated HSPs for the Discotic Liquid Crystals
(a) (b)
RED >1 Non-soluble
RED <1 soluble
Poor solvent
Good solvent
Del
ta_d
Del
ta_d
Delta_p Delta_p
Figure 13: (a) shows the Hansen space, the solvent points lies inside the circle are soluble in DLCs
(b) the solvent points lie inside the circle form gelators with DLCs.
34
In addition, using the RED formula, substitutes the values of HSPs for the solvents and HSPs for
the solute that is calculated using the MATLAB computer program and determine the possibility
of the solvent of being soluble in discotic liquid crystals. Also, the possibility of the solvent to
form precipitate/gelation in DLCs. In this way, we can determine the solvent whether it has ability
to form gelators without doing any experiment.
# solvents δd δp δh Solubility
(RED)
Gelation
(RED G)
1 Methyl-2
pyrrolidone
18.0000 12.3000 7.2000 0.8550 1.1364
2 Acetophenone 19.6000 8.6000 3.7000 0.6062 1.3742
3 Methylene
dichloride
18.2000 6.3000 6.1000 0.6893 1.2710
4 g-Butyrolactone 19.0000 16.6000 7.4000 0.9341 1.1685
5 Ethylene
dichloride
19.0000 7.4000 4.1000 0.6262 1.3449
6 Isophorone 16.6000 8.2000 7.4000 0.8694 1.1027
7 o-
Dichlorobenzene
19.2000 6.3000 3.3000 0.5831 1.3950
8 Tetrahydrofuran 16.8000 5.7000 8.0000 0.8237 1.1570
9 Diacetone
alcohol
15.8000 8.2000 10.8000 0.9870 0.9860
35
10 Methylethyl
ketone
16.0000 9.0000 5.1000 0.9117 1.1023
11 2-Nitropropane 16.2000 12.1000 4.1000 0.9578 1.0909
12 Ethylene glycol
monoethyl ether
16.2000 9.2000 14.3000 1.0487 0.9435
13 Propylene
carbonate
20.0000 18.0000 4.1000 0.9103 1.3074
14 Cyclohexanol 20.0000 18.0000 4.1000 0.9103 1.3074
15 Trichloroethylene 18.0000 3.1000 5.3000 0.6607 1.3439
16 1,4- Dioxane 19.0000 1.8000 7.4000 0.5962 1.3988
17 Ethylene glycol
monobutyl ether
16.0000 5.1000 12.3000 0.9635 1.0518
18 Nitroethane 16.0000 15.5000 4.5000 1.0662 1.0309
19 Ethylene glycol
monomethylether
16.2000 9.2000 16.4000 1.1032 0.9253
20 Butyl acetate 15.8000 3.7000 6.3000 0.8712 1.1807
21 Methyl isobutyl
ketone
15.3000 6.1000 4.1000 0.9227 1.1469
22 Nitromethane 15.8000 18.8000 5.1000 1.1839 0.9844
23 Diethylene glycol 16.6000 12.0000 20.7000 1.2540 0.8923
24 Benzene 18.4000 0 2.0000 0.6038 1.5109
36
25 Diethyl ether 14.5000 2.9000 5.1000 0.9751 1.1617
26 Ethanol amine 17.0000 15.5000 21.2000 1.3219 0.8811
27 Carbon
tetrachloride
17.8000 0 0.6000 0.6636 1.5100
28 Propylene glycol 16.8000 9.4000 23.3000 1.2870 0.9678
29 Ethylene glycol 17.0000 11.0000 26.0000 1.3964 0.9788
30 Formamide 17.2000 26.2000 19.0000 1.5685 0.9171
Table 7: Illustrates that which solvents have probability to being soluble and form gelation in
Discotic Liquid Crystals without experimentation
Poor solvent
Good solvent
Del
ta_d
Delta_p Delta_p
(a) (b)
Non-soluble
soluble
Figure 14: (b) shows the Hansen space, the solvent point’s lies inside the circle are soluble in DLCs(a) The solvent points lie inside the circle form gelators with DLCs.
37
Chapter 4: Conclusion
Triphenylene discotic liquid crystals are used to form physical gel in Acetonitrile/Alcohol based
electrolyte because of the π-stacking non-covalent intermolecular interactions within the
molecules. Polarized optical micrographs exhibit the birefringent fibers of DLCs that represents
the columnar[20] mesophase of Discotic Liquid Crystals. DLCs are used to increase the life time
of photo-optical devices by using the gel as an electrolyte. To improve the efficiencies of DSSCs
liquid gel electrolytes can be used. The HAT6 gel electrolyte shows the maximum efficiency of
4.65% in DSSCs.
EMITCB was added to improve the efficiency of the liquid electrolyte but it was not properly
soluble with Li I and Iodine in Acetonitrile. Therefore, the conductivity instead of increasing it
decreased in I_V characteristics of liquid electrolyte comparison.
The HSPs parameters of the solvents from literature and the experimentally determine solubilities
are added in data fit model by using Nelder-Mead algorithm to determined the HSPs parameters
for the solute. Moreover, by using the HSPs for the solute and the solvent determine the Relative
energy difference (RED) between the solute and the solvent. Furthermore, using the relation of
RED with Ra and Ro we can predict that which solvents have ability to be soluble in triphenylene
discotic liquid crystals. The same process is repeated for experimentally calculated gelation with
HSPs of the solvents and theoretically we can predict that which solvents have ability to form
gelation.
38
Future work
Optimization of Liquid electrolyte
Preparation of DSSCs
Characterization of the DSSCs more precisely
39
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