Abstract
This study used natural extracts from Lawsonia inermis L. (Henna), Terminalia catappa L.
(Tropical Almond), and Mangifera indica L. (Mango) as photosensitizers for dye-sensitized
solar cells. The mango and tropical almond extracts were extracted using ethanol while the
henna extract was gotten using alkaline extraction. The choice of extraction solvent was
based on the ability to permeate the plant membranes and toxicity of the solvent. This study
used TiO2 as the semiconductor material, iodide/triiodide as the redox electrolyte, and
PANI/Graphite couple as the counter electrode. From the results gotten from the study, the
henna extract produced the highest voltage and current when tested under projector light
(57.6mV and 0.018mA) and sunlight (0.188V and 0.282mA). In contrast, the mango extract
produced the lowest results under the same conditions (no electricity under projector light;
0.892mV and 0.0164mA under sunlight). Additionally, the results showed that the v
Introduction
Electricity is a form of energy that improves the quality of human life to a great extent. Its
demand has increased exponentially over the years thereby causing a strain on its sources.
Currently, the major source of electricity is burning of fossil fuels. The usage of fossil fuels
as a source of electricity results in the emission of greenhouse gases such as carbon dioxide
(CO2) and methane (CH4). These gases are very harmful to the environment as they
contribute to global warming. Furthermore, during mining and exploration of fossil fuels, the
environment in which they are mined usually gets polluted or degraded. Research shows that
this may lead to acid rain, drastic climate change, pollution of aquatic life, spread of diseases
that can kill both humans and animals, and other unfavorable and uncontrollable conditions
that damage the ecosystems.1
There is also concern that fossil fuel reserves are quickly
depleting and may not actually be limitless.
Another energy source that is being used to generate electricity is nuclear energy. Nuclear
power is mostly used in developed countries. Nuclear energy is advantageous because it
generates electricity through a self-sustained reaction; however, disposal of nuclear waste is a
major challenge. Also, in the event of an accident at the power plant, radioactive components
are released to the environment. Nuclear radiation is very hazardous to humans, plants,
animals, and the environment at large. A good example is the Chernobyl accident of 1986 at
the Chernobyl nuclear power plant in Ukraine. The accident left many dead, thousands ended
up with thyroid cancer, and the environment was polluted.2
This knowledge has sparked
research into other sources of energy that are both renewable and relatively harmless to the
environment.
Renewable energy sources are those energy sources that are inexhaustible, such as wind,
solar, geothermal energy, biomass, tidal energy, hydropower and many more. A good
2
characteristic of most renewable energy sources that makes them very attractive is the fact
they are ecofriendly.3 Although renewable energy is a good alternative to the use of fossil
fuels and nuclear energy, its implementation is quite challenging. This owes to the fact that
some renewable energy sources are intermittent, for example solar energy can only be
harnessed in the day and its supply decreases with increasing latitude.
Additionally, some renewable energy sources are limited by geography, and so cannot be
implemented in certain areas. For example hydroelectric energy is not feasible in regions with
very dry climates. Another challenge with renewable energy is the heavy investment needed
for development and installation of infrastructure. This is often coupled with a certain level of
risk and uncertainty. This is especially a challenge for developing nations such as countries in
Africa.4,5 Currently, on a global scale, 16% of primary energy sources are accounted for by
renewable energy sources. With the increasing consumption of electricity, there is a heavy
demand for more renewable energy sources to be developed.
Among all the current renewable energy sources, solar energy can be considered as one with
many advantages. This is especially more advantageous for nations in Africa because they are
located in tropical areas that receive massive solar radiation year in year out. Solar energy is a
form of energy that is totally clean and free. It is estimated that the sun delivers 120,000TW
of energy to the earth per hour and the earth currently needs 13TW of energy per year (0.01%
of the energy the sun provides the earth per hour).1
This shows that the energy from the sun is
more than adequate to meet the global energy needs if well harnessed.
Solar energy can be harnessed using two categories of technology known as concentrating
solar power and solar photovoltaics (SPVs).
1 Concentrating solar power uses mirrors to focus
the sun’s thermal energy on a fluid that is capable of heat transfer. The fluid generates steam
which is then used to drive a turbine that generates electricity. In contrast, in SPVs, donor
3
molecules undergo electronic excitation when exposed to photons. The excited electrons
migrate to electrodes present in the system to generate electricity. SPVs are also known as
Solar Cell. 1,6,7
1.1 Historical Background of Solar Cells and Their Working Mechanism
Solar cell or photovoltaics was first discovered in 1839 by French scientist Alexandre
Edmond Becquerel. He made the discovery when he observed that on exposure to sunlight,
metal electrodes coated with copper oxide or silver electrode produced a voltage.3 Much later
in 1873, it was discovered by Hernan Vogel that some organic dyes when used in silver
halide photographic films enhanced the response of some colors. Subsequent research helped
explain that electron transfer from the chromophore of the organic dyes to the silver halide
was responsible for this mechanism.8
This discovery would then go on to shape modern
photography. In 1877, scientists Adams and Day were able to make the first solid-state
Photovoltaic cell from selenium.9 Between 1930-1933, scientists such as B. Lange and L. O.
Grondahl were able to develop oxide/copper cells that were used in photography and optical
instruments as light meters.10
Photovoltaic (PV) effect was applied for the first time in 1954 at Bell Labs in the United
States by D. Chapin and G. Pearson and also at RCA Laboratories by Paul Rappaport.8,11 In
the papers they submitted in 1954, Rappaport explored the electron-voltaic effect in p-n
junctions induced by beta bombardment, while Chapin and Pearson explored a new silicon pn junction photocell for converting solar radiation into electrical power. The common ground
between these papers was their description of how incident light can be converted to
electricity by some semiconductor p-n junction devices. This marked the beginning of the
modern PV age. The cuprous oxide/copper cells and selenium cells were then tested to see
4
how efficient they were in converting solar energy to electricity. Their efficiencies were
about 0.1-0.5%, and by far less than the 6% efficiencies of the PV cells developed in 1954.11
In these solar cells, holes were created whenever light was absorbed by the silicon wafer and
the p-n junction served as a barrier between the electron and holes conduction points.
The use of sensitization in solar cells was also explored by several scientists and in 1991,
there was a significant breakthrough in the use of sensitization in solar cells. A Swiss
research group headed by Michael Graetzel was able to develop a new low-cost solar which
was called Dye-Sensitized Solar Cell (DSSC). This solar cell was able to produce an
efficiency of 7%~8%. The DSSC was inspired by the principle of photosynthesis combined
with the idea from the working mechanism of dye-sensitized silver halide emulsions that is
used in photography.2,12
The working mechanism of a solar cell is quite simple. When light is incident on a solar cell,
electrons gain energy and are excited from the semiconductor material. A circuit can be
created by connecting conductors to the positive and negative ends of the cell. These
conductors then capture the excited electrons thereby forming a flow of electric current which
can then be used to power up a device if the current is large enough.
1.2 Dye Sensitized Solar Cell (DSSC)
Dye-Sensitized Solar Cells was first researched in the year 1991 by scientist Michael
Graetzel. He proposed DSSC as an alternative to silicon solar cell. Compared with silicon cell
solar cells, DSSCs have a lot of advantages. It is very cost-effective and its performance is
not as affected as silicon cells in low light situations.13 A DSSC consists of a nanocrystalline
metal oxide semiconducting layer deposited on transparent fluorine doped tin oxide (FTO)
5
conductive glass, adsorbed dye on the semiconducting material, counter electrode, and an
electrolyte containing iodide and triiodide ions. The dye is the sensitizer and helps in
absorbing light. The counter electrode plays a vital role in collecting electrons and
regeneration of redox species used as a mediator. The entire process of converting solar
energy to electricity is based on charge separation. In a nutshell, the dyes absorb photon
which leads to electron excitation. The excited electrons transfer to the TiO2 conductive
electrode and through the electrolyte where redox reaction occurs. The electrons then return
to the dye through the counter electrode.13
In DSSCs, semiconductors that have wide bandgaps are sensitized with dyes so as to make
them convert visible light to electricity. Natural dyes are very advantageous because they are
ecofriendly and cost-effective. DSSCs also use nanostructured electrodes alongside the dye
for efficient charge injection by photoelectron. Conducting polymers show promise for use as
counter electrode materials in DSSCs, for example Polyaniline (PANI).14 It is very attractive
because it is cheap and can be easily synthesized, it exhibits high conductivity and is very
stable, and it has good redox properties for I3
–
reduction. The DSSC includes an anode which
is fluorine doped tin oxide (FTO) glass coated with TiO2 nanoparticles with dye adsorbed on
it, a cathode which is FTO glass with Polyaniline deposit on it and a redox I-
/I3
–
couple
electrolyte.13,14
1.3 How DSSCs Work
The working principle of DSSC is very similar to the process of photosynthesis. In DSSC, the
dye absorbs light and electrons gain energy and undergo excitation. In photosynthesis, CO2
accepts the excited electrons whereas in DSSC, titanium dioxide (TiO2) functions as the
electron acceptor. While water and oxygen play the role of redox electrolyte in
6
photosynthesis, this role is being played by iodide/triiodide (I?
/I3
?
). DSSC is structured in
multilayers to boost absorption of light as well as optimize electron collection and transfer.
This structure and its function can be likened to that of the thylakoid membrane in plants.12
Figure 1: Structure and Mechanism of a DSSC12
Below is a summary of the schematic of the DSSC structure in Fig. 1:
1. When light is incident on the solar cell, the dye molecules (S) absorb light and the
electrons in the dye gain energy. This energy causes the electrons to move from the
highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular
orbital (LUMO). At the LUMO, the dye is in a state (S*) where it is electronically
excited. In that state, electron is then injected into the conduction band of the TiO2
semiconductor.
15
2. The electron then diffuses through the TiO2 until it gets to the FTO anode surface.
3. After injecting electron into the conduction band of the semiconductor, the dye
molecule becomes positively charged (S+
). The positively charged dye molecule then
reacts with iodide ion in the redox electrolyte. In this reaction, the iodide undergoes
7
oxidation by giving electrons to the dye cation in order to regenerate the dye. The
oxidized form of the electrolyte is the triiodide ion.
4. The I3
–
then migrates to the cathode and gets reduced by accepting the electrons
excited from the dye in the first place. This reduction process regenerates the
electrolyte to iodide form.15
Using equations, the process can be summarized into three steps:
? Absorption
? Electron Injection
? Regeneration
In the operation of the cell, no chemicals are used up and no new chemicals are formed. This
means that DSSC undergoes a regenerative process.12
1.4 Counter Electrode: Polyaniline (PANI) Coupled with Graphite
The counter electrode completes the circuit of a DSSC. Its function is to intercept the excited
electron and redirect it to the electrolyte solution. There are certain criteria a material must
8
meet to be considered for use as a counter electrode for DSSC. The first and most important
criterion is conductivity. A counter electrode must be conductive so that electrons can flow
with very little resistance. It should also possess good redox property for the reduction of the
redox couple.12
Platinum remains the choice counter electrode because of its catalytic
property and it has produced the best efficiency so far, however, platinum is very expensive.
This has caused researchers to devote time into discovering alternatives that are capable of
replacing platinum as counter electrode. So far, carbonaceous materials like graphite and
carbon black have been explored with a certain level of success. Some conducting polymers
have also been explored because of their ease of production, high conductivity and stability,
as well their redox abilities for redox electrolytes like iodide/triiodide.
16,17
Polyaniline is a conducting polymer which has been experimented on as a counter electrode
for DSSC. It possesses some qualities and characteristics that make it advantageous over
many existing counter electrodes. Conducting polyaniline is very cheap and easy to
synthesize, highly conductive and has a high level of stability. PANI also possesses a very
good redox property which makes it advantageous to the redox couple used in the DSSC.
Graphite is coupled with PANI for this research so as to induce catalytic ability.16
Figure 2: Chemical structures of different forms of polyaniline.
9
1.5 Natural Dyes
Natural dyes have been used as an alternative to organic dyes as photosensitizers in DSSCs.
The function of the natural dyes is to absorb light and excite electrons as a result of the light
it has absorbed. Natural dyes are more advantageous than organic dyes for so many reasons.
For one, natural dyes are inexpensive as they can be easily gotten from different plants and
they are abundantly available in nature. Furthermore, natural dyes are environmentally
friendly and contain no carcinogens. Unlike organic dyes, natural dyes can be used directly
after extraction without the need for purification.
12
For this study, natural dyes from Terminalia catappa (tropical almond) leaves, Mangifera
indica (mango) leaves, and Lawsonia inermis (Henna) leaves will be used. Mango belongs to
the family of evergreen trees and has a genus of Anacardiaceae. The major components of
mango dye pigment are anthraquinones and flavonoids. These components also contain
lupeol, tannins, and saponnins.18 Henna dye is used in body painting and cosmetics in the
Northern part of Nigeria. It belongs to the Lawsonia genus and it is a flowering plant. It has a
reddish brown pigment that is solely made up of hennotanic acid or Lawsone.19 Tropical
almond is a tree that belongs to the family of Combretaceae. It has a lot of medicinal uses
such as anti-diabetic, antibacterial, anti-inflammatory, anti-fungal and even anti-HIV uses. It
has components such as cyclic triterpenes, flavonoids, and tannins among others.20
1.6 Problem Statement
Constant supply of electricity is one of the major challenges facing Nigeria. There are a lot of
industries in Nigeria that can function better and cut their costs if there were constant power
supply. The use of solar energy in Nigeria will not only improve the power supply in the
10
country, but also reduce our greenhouse gases emission by reducing our dependence on fossil
fuels like coal, and hydrothermal energy as a source of electricity. Dye Sensitized Solar Cell
is a form of solar cell that will be very advantageous for Nigeria as it is highly budgetfriendly. The materials required to build DSSCs can be easily sourced within the country as
well.
1.7 Aims and Objectives
The aims of this research are:
? to extract natural dyes from Terminalia catappa (tropical almond) leaves, Mangifera
indica (mango) leaves, and Lawsonia inermis (Henna) leaves
? to build a dye sensitized solar cell with PANI/Graphite as counter electrode
? to determine and compare the efficiencies of the dyes used in the research
1.8 Motivation and Hypothesis
A lot of research needs to be done to improve DSSCs and make it commercially available. I
propose that all chromophore can be used as dyes for DSSC. Furthermore, I hypothesize that
the dyes used in this research will produce a high efficiency in a DSSC where PANI coupled
with graphite is the counter electrode.
1.9 Significance
Dye sensitized solar cell is very economical and ecofriendly. The raw materials are readily
and abundantly available in Nigeria. If the efficiency is improved, it will help solve the
11
problem of intermittent power supply in Nigeria and also reduce our dependence on
hydrothermal energy and fossil fuel power plants as sources of electricity. This also applies to
the world at large. One of DSSCs greatest achievement in the world would be reduction of
greenhouse gases that are emitted into the atmosphere through burning of fossil fuels to
generate electricity.
1.10 Scope of Project
The following scopes of study were covered in this research:
Chapter 1: This chapter looks into introduction, aims and objectives of the research, as well
as the significance of the research. This chapter also looks into some background information
on the area of research.
Chapter 2: This chapter reviews some of the work done on DSSCs by various researchers. It
looks into reviews on the components of a DSSC such as photosensitizer, semiconductor
electrode, redox electrolyte, and counter electrode. It also covers the role of transparent
conducting oxides in DSSC, natural dyes and various techniques used to extract them,
titanium (IV) oxide and various methods of preparing its paste.
Chapter 3: This chapter covers the materials used throughout the research and the methods
used to carry out the research. It provides thorough information on the methods of extraction
used to extract natural dyes from the Terminalia catappa L., Mangifera indica L., and
Lawsonia inermis L. This chapter also provides information on the every step involved in
assembling the DSSC. These steps are preparation of TiO2 paste, preparation of photoanode,
synthesis of polyaniline emeraldine salt and iodide/triiodide redox electrolyte, preparation of
12
counter electrode, and the assembling of the DSSC. Finally, the chapter provides information
on how the DSSC was tested.
Chapter 4: This chapter looks into the results gotten from the IR and UV/Vis analysis of the
natural dyes extracted from the plants. The results gotten after measuring the conductivity of
the polyaniline are discussed in this chapter as well. Further, this chapter covers all the results
pertaining to the performance of the DSSC of each dye under projector light and sunlight.
These results are also discussed thoroughly in this chapter.
Chapter 5: This chapter summarizes the entire research and provides a conclusion based on
the results obtained from the research. Some of the challenges faced during the course of the
research and suggestions on ways to improve further research in this area are also provided in
this chapter