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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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