CHAPTER-I
Introduction to Solar Energy and TMDC
Part-A
1.A.1. Introduction to Solar Energy
01
1. A.2. Utilization of Solar Energy
05
1. A.3. Conversion of Solar Energy
07
1. A.4. Conventional Sources of Energy
12
1. A.5. Non Conventional Sources of Energy
14
Part-B
1. B.1. Introduction to TMDC
27
1. B.2. Literature Survey of
33
Mo, W-Dichalcogenide thin films
1. B.3. Aim and Objectives of Research Work.
References-
39
41
1
Chapter-I
Part-A
Introduction to Solar Energy and TMDC
1. A.1. Introduction to Solar Energy:
Survival has always been the main preoccupation of mankind. In the
development of civilization man has adopted a more complex life style. The per
capita consumption of energy rose to two orders of magnitude larger than that of
primitive man. The demand of energy is ever increasing. Energy has joined the
ranks of food, shelters and clothing- three essential commodities of mankind. Till
the middle of the 19th century, wood and coal constituted the main source of
energy with smaller contribution from petroleum. In the end of the 20th century
dominant energy sources switched to petroleum, natural gas, hydropower and
nuclear energy. Since the reserves of fossil fuel are very limited and are being
depleted very fast, search for alternate sources of energy are being very much
important. The world is looking towards natural resources such as solar energy,
wind energy, tidal energy, biofuel-energy, ocean energy and geothermal energy
etc., out of these energy sources solar energy has greatest potential. In the last
few decades’ lot of efforts has been already gone in utilizing and converting
solar energy in different forms.
All the solar energy convertors utilize the radiations received from the sun.
Hence an understanding of the nature of these radiations is imperative.
1. A.1.1.The Nature of Solar Energy:
Solar radiation approximately resembles the radiation by an ideal black body at
5,762K. The solar energy received on the surface of the earth is subject to
number of processes such as
a) Atmospheric absorption
b) Reflection from earth’s surface
2
c) Scattering in the atmosphere and
d) Sun’s position relative to the earth
The solar energy input on to the earth’s surface therefore exhibits a wide
variation, however the energy received just outside the earth’s atmosphere is
constant, which is known as ‘solar constant’, defined as ‘’the energy received
from the sun per unit area, perpendicular to the incident solar radiations, in the
absence of earth’s atmosphere per unit time, at the earth’s mean distance from
the sun”. Recently the value of solar constant is estimated between 1368 and
1377 W.m-2 (Sayigh, 1978)[1]. The extraterrestrial solar spectrum referred to as
Air Mass Zero (AMO), extends approximately from 0.115m in the ultraviolet
region to about 1000m in the far infrared region, as shown in figure. The
energy carried by the UV, Visible and IR region of the solar spectrum is given in
the table 1.1, the visible region carries about 41% of the energy useful for
photovoltaic solar energy convertors. The IR region carries about 49% of the
total energy useful for solar thermal energy conversion.
1. A.1.2. Solar Radiations at Earth’s Surface:
All of the solar radiations that reaches the earth’s atmosphere will not arrive at
its surface, principally due to the absorption in the ozone layer and by the
atmospheric gases O2, H2O, CO2 etc., in the atmosphere. Scattering due to
gaseous molecules, dust particles, aerosols etc., also contributes to the loss of
terrestrial solar radiations. A typical spectral distribution of solar radiations at the
earth’s surface is given in figure-1.1. The loss in energy of the solar beams will
depend on the length of the air path or air mass traversed. A quantity used to
describe the relative energy density is known as Air Mass, as given below.
When the sun is at the zenith (at position S1), the light travels a vertical path
length S1O as shown in figure-1.2. For sun (at position S2) at another zenith
angle, the longer will be the path traversed is S2O. The grater the zenith angle,
the longer will be the path traversed in the atmosphere (or air mass covered).
The air mass m is defined as the ratio of the actual length traversed by the solar
3
radiations in the atmosphere to that traversed with the sun at the zenith. From
the figure 1.2.
---------------------- (1.1)
Where ‘z’ is the zenith angle and ‘A’ is the altitude of the sun.
Equation 1 has been derived from small zenith angles only for large value of ‘z’
the curvature of the earth’s surface becomes significant. The refraction due to
increasingly denser atmosphere should also be considered in this case.
(a)
(Air Mass Zero, AM0) ------No path is traversed in atmosphere.
(b)
(Air Mass One, AM1)
------Sun at zenith
(c)
(Air Mass Two, AM 2)
------Zenith angle 600C
(d)
(Air Mass Three, AM 3) ------Zenith angle 70032’.
4
TABLE-1.1 Energy distribution in the extraterrestrial solar radiations.
Region
Wavelength (m)
Percentage of Solar
Radiations
Ultraviolet
0.115-0.405
9.293
Visible
0.405-0.740
41.476
Infrared
0.740-5.000
5.000-1000
48.753
0.488
‘w’ is the Another quantity denoting the weather conditions and proportional to
the amount of water vapour in the atmosphere generally used to describe the
variations in solar radiations is ‘w’. The energy of solar radiations for some
typical values of ‘m’ and ‘w’ are given in table No-1.2.,
5
Table-1.2. Parameters of the solar spectrum as a function of absorption
conditions.
m
w
0
1
2
1
0
0
0
2
3
5
Comments
Outside atmosphere
Sea level, sun at the zenith
Sea level, Sun at Z = 60o
Sun at Zenith relative humidity
50%
Most Extreme conditions
Power (W.m-2)
1370
1075
890
900
600
Therefore amount of solar energy reaching any point on the earth depends upon
its latitude apart from the altitude of the sun (or Air Mass) and weather
conditions as discussed above.
The total incident solar radiation consist of both the “direct” and the “diffused”
components. The contribution from the later depends on the presence of
scattering dust particles, clouds, ecological and environmental conditions. For
example, the intensity of solar radiation for a typical clear day with the sun at the
zenith is 80% direct and the rest is diffused. On the other hand, for a much
cloudy day, most of the incoming radiation is diffused. It is very important to
know the externally diffused and the direct radiations for a particular place to
exploit it for energy conversion. For a place where a maximum portion of the
solar energy is generally diffused, any focusing device will not be of much use.
1. A.2. Utilization of Solar Energy:
A large range of power technologies exist which use the solar energy reaching
earth and converting it into different forms of energy, its utilization can be
classified into two as direct solar power and indirect solar power.
1. A.2.1.Direct Solar Power
It involves only one step transformation of solar energy into a usable form eg.
1) Sunlight hits a photovoltaic cell generating electricity.
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2) Sunlight hits a dark absorber surface of a solar thermal collector. The heat
energy may be carried away by fluid circuit.
1. A.2.2.Indirect Solar Power
It involves number of transformations to reach a usable form of energy as1) Vegetation utilize solar energy and convert into chemical energy by
photosynthesis which can later be burned as fuel to generate electricity (Biofuel ) methane (natural gas ) may be derived from the bio-fuel.
2) Wind turbines, hydroelectric dams are powered by solar energy through its
interaction with the earth’s atmosphere and the resulting weather phenomena.
3) Energy obtained from oil, coal etc., originated as solar energy captured by
vegetation in the remote geological past and fossilized.
4) Oceans thermal energy production uses the thermal gradient at different
depths to generate powers these temperature differences are due to the energy
of the sun.
Thus the solar energy is much competitive to overcome the increasing demand
of energy because of the following aspects,
1) Solar power is relatively pollution free
2) Its facilities can be operating with little maintenance after initial setup.
3) It can be viewed as a local source because of regional variances.
4) It is becoming more and more economical as production cost decreases and
more efficient technologies are coming in use. Some states and countries are
harvesting solar power as a viable energy source than purchasing it from other
costly sources.
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Sunlight is available in most locations, and it provides such an enormous
supply of renewable energy that if the whole global electricity demand would be
covered exclusively by photovoltaic, the total land area needed for light
collection would be only a few percent of the world’s desert area [2]. Solar cells
are easy to install and use, and their operational lifetimes are long, which
eliminates the need for continuous maintenance. Since photovoltaic systems are
modular, they are equally well suited for both centralized and non-centralized
electricity production. Therefore their potential uses range from consumer
electronics (pocket calculators, wristwatches etc.) to large power plants. Due to
its reliability and stability, solar energy is a good choice in applications where
power failures or shortages cannot be tolerated, for example in hospitals and
certain production plants. Photovoltaic systems can be installed on rooftops and
facades of buildings, and they can be combined with solar water heating
systems. The power generated by rooftop solar cells can be used locally, and
the surplus can be exported to the commercial grid if there is one in the region
[2, 3]. The possibility for local electricity production offers consumers more
freedom by reducing their dependence on the availability and price of
commercial electricity. This is a crucial feature especially in remote areas that
lack the infrastructure of electrification. It is actually more cost effective to install
a photovoltaic system than to extend the grid, if the power requirement lies more
than about half a kilometer away from the electrical line [4]. Rooftop photovoltaic
installations, both by public institutions and by individual citizens, are becoming
more and more common worldwide [2].
Binary and multinary compound thin films of chalcogenides of II-VI, IV-VI, III-V,
VI-VI groups have already emerged as proven and potential candidates with low
production cost and good performance in the field of solar energy conversion [5].
1. A.3. Conversion of Solar Energy:
The modern technologies enable us to harness solar energy to convert it in two
ways for electrical energy as well as for thermal energy. There are two distinct
types of solar energy conversion devices or systems, have been emerged over
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the years- one principally uses the infrared part of the solar spectrum and the
other is based on visible / UV region. The two categories are –
1. A.3.1. Solar Thermal Conversion Systems:
In the first category, the incident radiation is converted in to thermal energy
which is subsequently used for domestic water heating, (solar cookers, solar
heaters), refrigeration, driving mechanical pumps, thermoelectric generators,
etc.
Solar Cookers are used to convert the solar energy in to heat energy. The solar
cooker consists of an insulated box with the interior walls coated black. A glass
lid or a simple plane mirror reflector is attached to the box which serves as
passive solar collector. When solar radiations fall on solar collectors, they reflect
the radiations to the black surface where solar radiatios are trapped. In this
process the temperature reaches approximately 1200C,where cooking takes
longer time than an ordinary oven.
Solar water heater is a device used to supply hot water by converting solar
energy to heat energy. The solar water heater consists of a spherical reflector
which directs the solar rays to a focal point at which water is heated. A solar
water heater can be installed on top of the roof to provide domestic hot water.
1. A.3.2.Solar Photochemical Conversion Systems:
The second category includes the age old energy conversion processes,
photosynthesis and it includes the more recent developments based on
photochemical conversion in non-biological systems and photovoltaic effect or
photo excitation of semiconductors. Following is the table1.3, summarizes some
of these devices. The photochemical conversion of solar energy is attractive,
since it offers an opportunity of solar energy storage. It can be stored in the form
of a “fuel” which is easily transportable. The concerned photochemical
conversion implies light induced chemical changes in a system yielding products
capable of supplying energy. Such a general definition encompasses a wide
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range of reactions induced as result of direct quantum excitation or solar thermal
energy. We shall limit ourselves to the former category of reactions only. The
photochemical conversion process may be either i) Biological or ii) Nonbiological.
An ideal photochemical conversion
Where A and A* represents respectively the ground and the excited states of a
system undergoing a photon induced chemical change to B. Then B act as fuel
and supplies energy. It is obvious from the above that the reaction 1.2 a) should
store energy (endoergic reaction) while the reaction in 1.2 b) should be capable
of delivering energy (exoergic reaction). The energetic of the above reactions
can be represented as the energy profile of A, A* and B plotted for an endoergic
photochemical reaction. Photochemical systems are threshold device where
direct conversion of solar energy in to fuel can be activated above a certain
minimum amount of photon energy. A viable photochemical solar energy
convertor should satisfy the following requirements (Bolton, 1978, 1977; Davis et
al., 1977; Moggi, 1977):
a)
The photochemical reaction AB generating fuel B should be energetic (i.e.
H0 for thermal end use processes and G0 for electrical end use processes).
b)
The reaction B  A should be exoergetic.
c)
The reaction should yield a kinetically stable photoproduct. For storage of end
product B as fuel, the back reaction rate B  A should be extremely slow under
ambient conditions. On the other hand, the generation of energy accompanied by
reaction (1.3) in the presence of catalyst or temperature should be fast. These
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two criteria look contradictory. The photoproducts should be, therefore, rapidly
removed from the reactants.
d)
e)
The fuel B should be easily transportable.
In order to function as an energy storage device, the photochemical
conversion system should be such that the original material (A) is recovered
completely without any side reactions from the fuel B. Any side reactions will
lead to loss of the photoactive material. This makes their periodic replacement
and the energy conversion device gets costly. This is a very stringent condition
because the complete conversion of fuel in to the starting material is practically
impossible.
f)
For an efficient energy conversion, the photochemical reaction should make
use of a large portion of the solar spectrum; The threshold wavelength should lie
preferably in the IR region of the solar spectrum.
g)
h)
The quantum yield should be high.
Finally, the components of a photochemical energy convertor should be
cheap, non-toxic and easy to maintain. With the exception of photosynthesis, no
satisfactory photochemical conversion system is known. However, certain
systems, when developed, have a potential to meet the above requirements.
1. A.3.3. Why Solar Energy?
Solar energy is any form of energy that is radiated from the sun. It includes light,
radio waves and also X-rays, but the term is more associated with visible light.
Solar energy is needed by green plants for the process of photosynthesis, which
is the ultimate source of all food. The energy in fossil fuels (e.g., coal and oil)
and other organic fuels (e.g., wood) is derived from solar energy.
Using solar energy instead of the usual conventional energy is better.
However, in what way is solar energy better than conventional energy? The
Solar energy has the following advantages over conventional energy [6]:
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 Solar Energy does not require money (The sun is free).
 Depending on the amount of it used, the amount needed to pay can be very little.
 Solar (and other renewable) energy does not require connection to other
buildings or things; it can be used as a stand-alone.
 Solar Energy is unlimited (Until the day when the Sun dies out, which is veryvery long later...)
 Using solar energy leads to lesser greenhouse emissions compared to
conventional energy
 Thus, buildings and other industrial companies should change from using
conventional energy to solar energy because it helps to conserve our nonrenewable resources. Solar energy is renewable so why not use it instead?
 However, solar energy does have a few disadvantages.
 Different parts of the world receives different amount of sunlight at different times
of the day due to the rotation of the Earth on its own axis.
 There are causes like climatic conditions that affect the amount of light received.
 The solar panels cost a lot of money too. Since a vast amount of area is needed
in order to generate enough electricity, some countries do not have the money
to afford them.
Other facts
There are many ways that solar energy can be used effectively. Applications of
solar energy use can be grouped into three primary categories: i)
Heating/Cooling, ii)Electricity production, and iii)Chemical processes. The most
common uses of the solar energy are for water and space heating. There are
also uses such as ventilating solar air and these uses are growing in popularity.
Due to recent advances in solar detoxification technologies that help to clean the
water and air, there are a few applications which use solar energy that are able
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to be competitive with conventional technologies. In short, it means that the
solar energy from the sun is either converted to electricity, or converted to
thermal energy to heat up a fluid to produce steam that will turn the turbine,
introducing electricity as a result.
1. A.4. Conventional Sources of Energy:
They are also called as non-renewable forms of energy. They are fossil fuelsCoal, Petroleum and natural gas. The fossil fuels are the forms of stored energy.
The fossil fuels are derived from incomplete decomposition of dead plants and
animals. These organic matters can be converted into hydrocarbons and finally
to fossil fuels through complex chemical reactions. It takes million years to
generate a fossil fuel reserve. The fossil fuels are non renewable sources and
may be exhausted as a there is a over consumption of that, at present nearly
90% of the total commercial energies are from fossil fuels.
Coal- it is a fossilized plant material preserved by burial in sediments. The plant
materials undergo complex reactions by geological forces that compact and
condense it to a carbon rich fuel called Coal. It is a prime source of industrial
energy. Coal is classified according to its carbon and sulphur content.
The lowest energy content is there in lignite coal and greatest in anthracite coal.
Primarily it was used for generation of electrical energy. Most of the coal mines
in India are strip mining for surfaces mining. Mining causes lot of environmental
problems like pollution of air water and soil. It is responsible for many respiratory
diseases.
Petroleum and its derivatives- the petroleum oils are derived from organic
materials that were buried in deep earth. Million years ago at that depth, the high
pressure and temperature concentrate and transformed the organic to energy
rich materials. The containt of petroleum deposit will have varying mixtures of oil
gas and solid materials. The places in India from where the petroleum oil is
extracted are Mumbai High, Gujarat and Assam, as well as offshore basin of
river Godavari and Kaveri. The petroleum oils are mainly used for transportation,
13
generation of electricity and to run industries. However it causes a lot of harm to
the environment by pollution. The petroleum is obtained in the form of crud oil
which is further processed by petroleum refining i.e. fractional distillation which
gives usable products like petrol, diesel, kerosene and petroleum gas. In this
petroleum gas is released first. It is a mixture of hydrocarbons, namely Ethane
Propane and Butane. Butane is the main product. The petroleum gas can be
liquefied as liquefied petroleum gas (LPG), which is a cooking gas it is also used
for industries and transports etc. After petroleum gas there obtain petrol
(gasoline), kerosene, diesel then fuel oil at different temperatures and finally the
residue containing lubricating oil, paraffin wax and asphalt are also obtain.
Natural gas is the world’s third largest commercial fuel. It is the most rapidly
growing energy resource since it is convenient, cheap burns cleanly.
Comparatively it causes fewer environmental problems. It mainly consists about
95% Methane. It is found along with petroleum oil. It is used for domestic and
industrial purposes it can be compressed easily hence called compressed
Natural Gas (CNG).
The use of conventional technology to produce electrical power normally results
in pollution that affects everyone. It often relies on the burning of fossil fuels that
produce dangerous gases that often end up in the atmosphere. People and
animals breathe in the polluted air and plants absorb the pollution. Never the
less the conventional sources of energy are being depleted fastly the present
situation is well explained by the following graph.
14
Fig. 1.3 Graph of Conventional Oil Discoveries Vs. Production
1.A.5 Non - Conventional / Renewable Sources of Energy:
i. Nuclear Energy:
It is one of the important Non Conventional Source of energy. The Nuclear
power is generally the Electrical energy produced from controlled (i.e. non
explosive) nuclear reactions. Commercial plants in use to date use nuclear
fission reactions. Electric utility reactors heat water to produce steam, which is
then used to generate electricity. In 2007, 14% of the worlds electricity came
from nuclear power, despites concerns about safety and radioactive waste
management, more than 150 naval vessels using nuclear propulsions have
been built. Electricity demand in India has been increasing rapidly, and the534
billion kilowatt hours produced in 2002 was almost double the 1990 output,
though still represented only 505kwh per capita for the year. In 2006,744 billion
kWh gross was produced, but with huge transmission losses this resulted in only
505 billion kWh consumption. The percapita figure is expected to almost triple by
2020, with 6.3% annual growth, coal provides 68% of the electricity at present
but reserves are limited. The gas provides 8% and hydroelectric provides 15%.
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India’s fuel situation, with shortage of fossil fuels, is driving the nuclear
investment for electricity. 25% nuclear contribution is foreseen by year 2050, it
will be one hundred times the capacity
in year 2002. Almost as much
investment in the grid system as in power plants is necessary. Because India is
outside the nuclear Non-proliferation treaty due to its weapon programme, it has
been 34years largely excluded from tread in nuclear plant or materials, which
has hampered its development of civil nuclear energy until 2009. Due to these
tread bans and lack of indigenous uranium, there is considerable difficulty in use
of nuclear power for the Indians electricity demand.
ii. Wind Energy:
The evolution of windmills into wind turbines did not happen overnight and
attempts to produce electricity with windmills date back to the beginning of the
century. It was Denmark which erected the first batch of steel windmills specially
built for generation of electricity. After World War II, the development of wind
turbines was totally hampered due to the installation of massive conventional
power stations using fossil fuels available at low cost. But the oil crisis of 1973
heralded a definite breakthrough in harnessing wind energy.
Many European countries started pursuing the development of wind
turbine technology seriously and their development efforts are continuing even
today. The technology involves generation of electricity using turbines, which
converts mechanical energy created by the rotation of blades into electrical
energy, sometimes the mechanical energy from the mills is directly used for
pumping water from well also. The estimates of
wind potential in India ,is
expected to be sizable considering India’s coastline of about 7000 k.m. and the
extensive mountain ranges[7]. The wind power programme in India was started
during 1983-84 with the efforts of the Ministry of Non-Conventional Energy
Sources.
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Iii. Tidal Energy:
It is another form of energy known as tidal energy or tidal power. It is
derived
from ocean tides. Ocean tides and waves contain enormous amounts of energy
that can be harnessed to generate electrical energy. Tides are formed due to
the gravitational effects of the Sun and the Moon on the Earth. The tide moves a
huge amount of water twice each day. To harness tidal power, dams are built
across the entrance to a bay creating a reservoir. When the water passes
through the dam it moves the turbines of the generator and produce the
electrical energy. Though it looks easy, in fact it is difficult as it require a
construction that will sustain the effect of huge force of water. The another
arrangement can be done in which the off shore under water arrangement can
be made, so turbins can rotate as well as a vertical axes turbines can be
arranged. In Maharashtra in district Ratnagiri, near tehsil Guhagar and village
Dabhol,in 2009 the non conventional energy resources dept. of Govt.of
Maharashtra has set up first tidal power station at experimental level.
iv. Biomass Energy
Biomass is yet another important source of energy with potential to generate
power to the extent of more than 50% of the country’s requirements. India is
predominantly an agricultural economy, with huge quantity of biomass available
in the form of husk, straw, shells of coconuts wild bushes etc. With an estimated
production of 350 million tons of agricultural waste every year, biomass is
capable of supplementing coal to the tune of about 200 million tonnes producing
17,000 MW of power and resulting in a saving of about Rs.20,000 crores every
year. Biomass available in India comprises of rice husk, rice straw, bagasse,
coconut shell, jute, cotton, husk etc. Biomass can be obtained by raising energy
farms or may be obtained from organic waste.
The biomass resources including large quantities of cattle dung can be used in
bio-energy
technologies
viz.,
biogas,
gasifier,
biomass
combustion,
cogeneration etc., to produce energy-thermal or electricity. Biomass can be
17
used in three ways – one in the form of gas through gasifiers for thermal
applications, second in the form of methane gas to run gas engines and produce
power and the third through combustion to produce steam and thereby power.
v. Solar Energy
Solar Power was once considered, like nuclear power, ‘too cheap to meter’ but
this proved illusory because of the high cost of photovoltaic cells and due to
limited demand. Experts however believe that with mass production and
improvement in technology, the unit price would drop and this would make it
attractive for the consumers in relation to thermal or hydro power. The Solar
Photo Voltaic (SPV) technology which enables the direct conversion of sun light
into electricity can be used to run pumps, lights, refrigerators, TV sets, etc., and
it has several distinct advantages, since it does not have moving parts, produces
no noise or pollution, requires very little maintenance and can be installed
anywhere. These advantages make them an ideal power source for use
especially in remote and isolated areas which are not served by conventional
electricity making use of ample sunshine available in India, for nearly 300 days
in a year.
A Solar Thermal Device on the other hand captures and transfers the heat
energy available in solar radiation. The energy generated can be used for
thermal applications in different temperature ranges. The heat can be used
directly or further converted into mechanical or electrical energy.
vi. Other Sources
The other sources of renewable energy are geothermal, ocean, hydrogen and
fuel cells. These have immense energy potential, though tapping this potential
for power generation and other applications calls for development of suitable
technologies.
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a .Geo-Thermal Energy
Geo-Thermal energy is renewable heat energy from underneath the earth. Heat
is brought to near surface by thermal conduction and by intrusion into the earth’s
crust. It can be utilized for power generation and direct heat applications.
Potential sites for geo-thermal power generation have been identified mainly in
central and northern regions of the country. Suitable technologies are under
development to make its exploitation viable.
b. Ocean Thermal Energy
The vast potential of energy of the seas and oceans which cover about three
fourth of our planet, can make a significant contribution to meet the energy
needs. Ocean contains energy in the form of temperature gradients, waves and
tides and ocean current, which can be used to generate electricity in an
environment-friendly manner. Technologies to harness tidal power, wave power
and ocean thermal energy is being developed, to make it commercially viable.
c. Hydrogen and Fuel Cells
In both Hydrogen and Fuel Cells electricity is produced through an electrochemical reaction between hydrogen and oxygen gases. The fuel cells are
efficient, compact and reliable for automotive applications. Hydrogen gas is the
primary fuel for fuel cells also. Hydrogen can be produced from the electrolysis
of water using solar energy. It can also be extracted from sewage gas, natural
gas, naphtha or biogas. Fuel cells can be very widely used once they become
commercially viable.
d. Bio Fuels
In view of worldwide demand for energy and concern for environmental safety
there is needed to search for alternatives to petrol and diesel for use in
automobiles. The Government of India has now permitted the use of 5% ethanol
blended petrol. Tamilnadu is one of the nine States in the country where this
programme will commence from January 2003. Ethanol produced from
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molasses/ cane juice, when used as fuel will reduce the dependence on crude
oil and help contain pollution. Further, technology is also being developed to
convert different vegetable oils especially non-edible oils as bio-diesel for use in
the transport sector. They are however, in R & D stage only.
1.A.5.1. Potential and Exploitation of Renewable Energy Sources
India ranks fifth in the world in Wind power with installed capacity of 1612 MW
out of an estimated potential of 45,000 MW. Tamilnadu ranks first in the country
in Wind power with a capacity of 858 MW out of an estimated potential of 3050
MW. In biomass power the country has an installed capacity of 381 MW out of
total potential of 19500 MW. In Tamilnadu the installed capacity is 142 MW
against the potential of 1000 MW. The potential available under solar
photovoltaic energy is 20 MW per Sq.Km. But in view of high cost and heavy
investment involved the progress is rather slow. In Solar thermal energy (Solar
Water Heater system) 15 lakh M2 collector area has been installed in the country
against a potential of 1400 lakh M2. In Tamilnadu, 20084 M2 area has been
installed. There is considerable scope for expanding this activity with suitable
incentives. The most note-worthy achievement of Tamilnadu has been in
creating an installed capacity of about 1000 MW from the non-conventional
energy sources alone in the State, i.e., 13% of the total TNEB grid capacity
against 3.2% only for the country. The major component of this has come from
Wind Energy (858 MW) followed by co-generation in sugar industries (142 MW).
Further, this has largely come about through private investment due to attractive
policy initiatives of the State and Central Governments. It may be worthwhile to
offer various incentives to enhance its share further in view of the vast potential
available.
1.A.5.2. Solar Cells
The Becquerel effect was discovered in 1839 which further reflect in to origin of
solar cell. Then different types of solar cells were developed.They can be
20
classified, based on utilization of solar energy. Depending on the built in
potential, due to difference in Fermi energy level, solar energy utilization can be
classified as shown in the table 1.1
Table-1.1. Utilization of Solar Energy
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In semiconductor – semiconductor (S-S) junction cells, two types of junction are
occurred
a) Homojunction
b)Heterojunction
A homojunction cell consists of shallow p-n junction formed either by diffusion of
dopant into a monocrystalline semiconductor substrate or by growth of an epitaxial
layer onto the substrate. A heterojunction is the interference between two dissimilar
materials.
The semiconductor –metal (S-M) junction cells are commonly known as schottky
barrier cell. It is fabricated by depositing a semitransparent metal film on the
semiconductor surface. The transparent metal film is normally evaporated on a
carefully processed semiconductor surface.
The MIS and SIS junction cells have a thin interfacial layer of an insulator
between a top inducing contact (metal or semiconductor) and a base semiconductor.
The interfacial layer is generally an oxide or the compound which is always an
insulator.
The
semiconductor-
liquid
(SL)
junction
cells
consist
of
a
semiconductor
photoelectrode dipped into an electrolyte solution along with counter electrode. The
counter electrode is a suitable metal or graphite. The charge transfer process at the
semiconductor/ electrolyte interface results in band bending establishing a potential
barrier at the interface.
These cells are further classified in five groups:

Photoelectrolytic cell

Rechargeable cells

Photogalvanic cells

Photoelectrocatalytic cells

Photoelectrochemical cells
22
Out of these groups of cells the photoelectrochemical system is easiest one. A
PEC effect is defined as one in which irradiation of
electrode/ electrolyte system
produce a change in electric potential (on open circuit) or in the current flowing in
external
circuit
(under
short
circuit
conditions)
[8].
Early
semiconductor
electrochemistry studies [9], have shown that the distribution of potential at the
semiconductor – electrolyte interface is almost similar to that at a simple p-n junction. A
direct conversion of solar energy into electrical energy using a semiconductor
electrolyte interface was demonstrated by Gerischer [10].
Thus, photoelectrochemical (PEC) solar cells in principle can be much cheaper than
the traditional solid state photovoltaic cells. This is particularly important because of the
comparatively low solar radiation power density requiring the use of large area
converters. The future prospects of photoelectrochemical solar energy conversion
method depend on how completely its potential advantage can be realized in practice.
Secondly, the photoelectrochemical method is convenient in that, one of its versions photoelectrolysis enables light energy to be directly converted into chemical energy of
the photoelectrochemical reaction products and thus permits the energy storage
problem to be solved along with proper energy conversion.
During the last two-three decades much work has been done on photoelectrochemical
(PEC) system [11] in search of suitable liquid junction photovoltaic solar cells.
1.A.5.2.1 Basic Theory of Semiconductor-Liquid Junction.
a. Oxidation and Reduction Process:
The charge transfer across the semiconductor/ electrolyte interface in dark as well as
in light results in flow of current through the junction. This is the key concepts for
working in photoelectrochemical solar cells and reports are available which predict the
analogy between the semiconductor and electrolyte [5]
A process in which substance gains an electron is called a reduction reaction [12].
OX +
e-
↔ Red Eo
1.4
23
Where, OX and Red are oxidized and reduced species and Eo is the standard
electrochemical potential. The reverse process of losing an electron is called an
oxidation reaction. In a system, where one species loses an electron and other species
gains an electron is called redox system. If Eo is positive, the reaction proceeds from
left to right and when it is negative, the reaction proceeds from right to left.
a. The oxidized and reduced species in an electrolyte are analogous to
the conduction, valance bands respectively.
b. The term Efredox can also be explained in a similar manner as that of
the semiconductor Fermi level, Ef
c. The energy necessary to transfer an electron from the reduced
species to the oxidize species is analogous to the band gap; Eg, of
a semiconductor
d. The redox potentials is potential required to transfer an electron
from redox species to the vacuum level or vice-versa [13].
b. Semiconductor-Electrolyte Interface
The semiconductor/electrolyte interface was firstly studied and reported by
Brattain and Garret [9]. When semiconductor is dipped in redox solution, as its
chemical potential is different from the redox potential (Eredox), a new equilibrium is
established between the semiconductor and electrolyte solution by rearrangement of
charges. This results in a strong field near the junction. When the semiconductorelectrolyte junction is illuminated with light having energy greater than the band gap
energy of semiconductor, electron-hole pairs are produced in the depletion layer.
Charge separation takes place due to the local field present at the interface. The
probability of annihilation of a hole with an electron is reduced by this field. This
condition will be optimum when the light penetration depth is equal to the depletion
24
layer width. So that all the light is absorbed in the depletion layer and maximum
number of electron-hole pairs are produced in it. These separated charges produce a
counter charge and under open circuit conduction this counter field is maximum. This is
the open circuit voltage. The conduction band and valance band get shifted due to the
counter voltage. The photovoltage is given by the change in the Fermi level as shown
in Fig.1.4. When a counter electrode is immersed in the electrolyte and connected
externally to the semiconductor, the photo generated electron moves into the bulk of
semiconductor through the external circuit; it reaches the counter electrode to reduce
an oxidized species in the electrolyte. The hole is pushed to the electrode surface
where it oxidizes a species in the electrolyte. The photocurrent depends on the
absorption coefficient of the semiconductor, width of the space charge region, hole
diffusion length, area of illuminated electrode, photon energy and radiation intensity.
Under short circuit conditions, the Fermi levels of the semiconductor and the potential
of the redox couple of the solution are equalized and a net charge flows during the
illumination.
E
Semiconductor
(Indark)
Ec
e– 
Electrolyte
Ec*
Ef *
nEf *
WOx
Ef
Ef , redox
pEf *
(Indark)
Ev
WRed
Ev *
+
h+
h
Fig. 1.4. The position of bands under illumination responsible for photo
induced charge transfer
25
c. Classification of the Photoelectrochemical (PEC) Solar Cells
The PEC cells are very similar the Schottky type solid state solar cells. The principle of
charge transfer reaction at the semiconductor – electrolyte interface forms the basis of
various types, of photoelectrochemical solar cells. Depending on the net free energy
change (G) in the overall system, PEC cells can be divided into three categories.
i. Electrochemical Photovoltaic Cells (G = 0)
These cells consists of such a redox couple that the total cathodic and anodic
reactions do not lead to net chemical change i. e. a change in the net free energy, G
= 0. The electrodes do not participate in the chemical reaction, they only serve as a
“shuttle” for the charge transfer mechanism. At the semiconductor electrode,
(Red)solv. + h+
↔ (OX) solv.
1.6
At the metal counter electrode,
(OX) solv. + e- ↔ (Red)solv
1.7
The above cell is the regenerative type PEC cell used for direct production of
electricity.
ii. Photoelectrolysis Cells (∆G > 0)
Effectively two redox couples are present and a net chemical change takes place
in the system by converting the optical energy in to chemical energy. The
photoelectrolysis cells and some electrochemical storage cells belong to this category.
Some examples of reaction for the above type are
H2O +
hν
CO2 + H2O + hν
H2 + ½ O2
1.8
(CH2O)x + O2
Chemical energy
1.9
26
iii. Photocatalytic Cells (∆G < 0) :
In these cells, similar to above two redox couples are present such that a net chemical
changes take place. Hence ∆G < 0 and the optical energy provide the activation energy
for the chemical reaction. Example for photocatalytic cell is
N2 + 3H2 +
hν
2NH3
1.9
d. Conversion Efficiency and Fill Factor:
Photoelectrochemical cells for solar photon conversion are usually deigned to produce
electric power. Power-producing solar cells are designed to be operated at their
maximum-power point to produce electric power at the energy conversion efficiency
mp.
 mp 
i mpVmp
S
1.10
E
O
Where E
S
is the incident solar irradiance, imp is the maximum-power photocurrent
O
density and Vmp is the maximum power voltage. The ratio between the maximum power
generated and the product of the short-circuit photocurrent density isc and the opencircuit voltage Voc is known as the fill factor, fill. The higher the value of fill, the better
the quality of the device.
fill 
impVmp
I scVoc
1.11
27
Part-B
1. B.1. Introduction to Transition Metal Dichalcogenide (TMDC) :
Since the early 60’s the transition metal dichalcogenide materials have
received rapidly growing interest. About two thirds of the compounds of this
family assume layered structures. These compounds are commonly referred as
TCh2 compounds where T is the transition metal and Ch is the chalcogens S, Se
or Te. These compounds form structurally and chemically well defined family of
compounds having basic structure of sandwich type loosely coupled Ch-T-Ch
sheets as shown in figure 1.5, which makes these materials specific and highly
interesting. Within a layer, the bonds are strong, while between adjacent layers
they are remarkably weak. The weak Van der Waal’s forces are holding these
sheets together. As a consequence the crystals have facile basic cleavage,
lubricity and marked anisotropy in many physical property which accounts for
the interesting properties of these materials. Electrically, however they cover a
wide spectrum of properties. There are insulators like HfS2, semiconductors like
MoS2, semimetals like WTe2 and TcS2 and true metals like NbS2, VSe2, there
are
also
superconductors
like
NbCh2
and
TaCh2
which
show
anti-
ferromagnetism below about 150K [14].
Fig. 1.5 Crystal Structure of Transition Metal Dichalcogenide compound
These compounds contain quite wide non bonding d bands (1eV), the
extent to which these bands are filled, results in the diversity in properties of
these compounds. The anisotropy in chemical behavior due to the formation of
intercalation compounds has also attracted attention. The intercalation
28
compounds are formed by insertion of many atoms and molecules between the
adjacent layers.Stoichiometry and crystalline order of compounds are important.
The occurrence of superconductivity in the Nb and Ta compounds was found to
be strongly dependent on stoichiometry. Except some difficulties in few cases
the preparation of stoichiometric compounds is usually possible. In some
systems the stoichiometric dichalcogenides represents the boundary of a
continuum of materials can be mentioned as T1+XCh2, where x varies
continuously from zero to upward. This is especially severe in the group IV and
V dichalcogenides. In the Nb and Ta disulphides and diselenides metal rich
regimes have been found [15]. In general, the group five compounds under
consideration seem to be difficult to prepare as pure single phase.
The group VI compounds seem to possess only limited ranges of homogeneity;
they can usually be prepared stoichiometrically and as single phase compounds.
The difficulty however lies in the preparation of large single crystals. The
preparation of group VII and VIII compounds is also difficult. In certain cases like
TiSe2, TiTe2, VSe2 and VTe2 no compound higher than dichalcogenide be easily
synthesized from elements while in other cases trichalcogenides can be
obtained easily, with use of excess of chalcogen source, where to synthesize
dichalcogenides special care has to be taken.For the preparation of single
crystals, the discovery of the chemical vapour transport technique has been of
great importance. It enabled researchers to grow single crystal of substances
otherwise known as difficult materials. Scheffer has explained it in more detail
[16]. However the preparation of single crystals of materials is comparatively
tidious and difficult than recent techniques of synthesis of materials in the form
of thin films. There is tremendous development in synthesis of these materials
which is causes the afore mentioned exponential growth in interest about this
family of materials and it opened the door to careful examination of the
extremely fascinating properties of materials.
29
The TMDC’s can be classified according to the group in the periodic table of the
transition metal. An introductory description of the crystal structures of these
compounds is inevitable here.
1. B.1.1. Crystal Structures of TMDC’s:
The structures of TMDC’s fall into two classes i) Layered and ii) Non-layered.
The non-layered members are found in groups VII and VIII, they are MnCh2,
FeCh2, RuCh2, OsCh2, CoCh2, IrS2, IrSe2, NiS2 and NiSe2. The ditellurieds of
cobalt and Rhodium can also adopt a CdI2-type of structure, the other
compounds occur in one or more of the following types. i) The pyrites, ii) The
Marcasite, iii) The IrSe2 and PdS2 type.
In the layered compounds under consideration the types of structures are i) CdI2
type a 6:3 co-ordination exists in which the metal atom is surrounded
octahedarally by the chalcogen ii) another 6:3 co-ordination in which the metal
atom is surrounded in a trigonal prismatic fashion is found in the MoS2 type of
structure iii) the mixtures of these two, contain alternate prismatic and
octahedaral co-ordinate layers. In the layered compounds the internal bonding in
the slabs is strong. The inter layer forces between slabs are week and this leads
to the marked cleavage perpendicular to the hexagonal / trigonal symmetry axes
making this materials very anisotropic both mechanically and electrically and
enabling one to introduce metal atoms or molecules of organic complexes into
the Vander Waals gap between the slabs.In several compounds chains of metal
atoms with short metal-metal distances are found e.g.NbTe2, TaTe2, WTa2, MoTe2, TeS2 and TeSe2 etc. This is due to distortion of the layers; the metal
atoms are actually displaced from the center of the co-ordination units. As a
result the chalcogenide sheets buckle somewhat to accommodate this
displacement.
1. B.1.2. Polymorphism and Polytypism in TMDC’s:
Since TMDC’s are layered compounds, where so many variations in staking
order of the sandwiches exists, therefore number of compounds show
30
polytypism. Polymorphism is the ability of the same chemical compound to exist
in the more than one crystalline form and polytypism is a special kind of one
dimensional polymorphism. Polytypism is mostly observed in the group V and
VI.The various modifications in structure are assembled in three groups i.e.
i)Those having the prismatic surrounding of the metal by the chalcogen atoms,
ii) those having an octrahedral surrounding,
iii)those having alternating layers with prismatic and octrahedral surrounding.
In all cases the thickness of the repeat unit in the C-direction is indicated by
the number of slabs 1, 2, 3,…,6, and if necessary higher. The symmetry is
denoted by letter T for trigonal, H for Hexagonal or R for Rhombohedral, if
suppose in the 2H modification (see Fig.1.6 to 1.10) these 2 are not sufficient to
identify the polytype uniquely, So it is common in literature to add a 3rd lower
case letter a,b,c,….etc. to a newly discovered staking sequence.
 Layers with a prismatic surrounding of the metal atom.:
A one slab structure is not known and not likely to exist, due to the very unstable
lattice that will be formed. For a two layered structure there are three staking
sequence possible the structure of figure 1.6 is some times called the TaS2
structure and generally indicated as 2H(a) it has space group P63/mmc while
figure 1.7 represents the structure of Molybdenite, MoS2 and is generally
indicated with 2H(b) it has space group P63/mmc.
 Layers with an octahedral surrounding of the metal atom. :
Here a one slab structure is known to give stable lattice, several compounds are
known to crystallize in this so called 1T modification presented in figure 1.8, it
has space group
ml. Only one 2H modification is possible which has space
group P63/mmc, shown in figure 1.9 and only one 3R modification is possible
which has space group R3m and shown in figure 1.10 the common examples of
all these three modifications are TaS2, TaSe2.
31
 Structures with layers having a prismatic surrounding of the metal atom in
the layers.
Figure 1.6
Figure 1.7
 Structures with layers having octahedral surrounding of the metal atom in
the layers.
Figure 1.8
Figure 1.9
Figure 1.10
32
1. B.1.3. Bond Character in TMDC’s:A number of researchers focused on the point that there are differences in
TMDC compounds which are due to the differences in bonding of the TMDC.
That imply there are substantial differences in their ionicity, such differences in
bonding forbid the use of rigid band-scheme for these compounds. White et al.
and Lucovsky et al.[17] assumed that chalcogenides of group IV and VI
members are ionic due to the electronegativity differences in their bonding
atoms. Investigating the consequence of this assumption they arrived at
conclusion that TiS2, ZrTe and HfTe2 are metals due to an overlap of the
chalcogenide s- and p- states, with d states of the metal atoms. A similar
metallic band scheme is assumed for TiSe2 and TiTe2. ZrS2, HfS2 and HfSe2 as
well as ZrSe2 are expected to be ionic which is in accordance with binding
energy calculations of Murray, Bromley and Yoffe [18].
For MoS2, MoSe2, WoS2 and WSe2
found predominantly covalent bonding
which is also in agreement with calculations of Bromley et al [19]. Huisman et al
[20] have suggested that in d0, d1 and d2 members, d-covalency provides a
stabilizing factor for trigonal prismatic coordination observed in group VI and V
members against the more symmetric, electrostatically favored octahedral
coordination observed in the IV and some V group compounds and in one VI
group compound. Gamble (1974)[21]showed that the differences in the crystal
parameters of compounds is due to the natural and remarkably smooth function
of the elemental electronegativity differences of the bonding atoms. Also the
shorter intra layer and inter layer Ch-Ch distances in the group VI member do
not mean the Ch-Ch bonds but are the result of the smaller size of the almost
neutral chalcogens.
33
1. B.2. Literature Survey of Mo, W-Transition Metal Dichalcogenide Thin
Films:
The VI-Group Transition Metal Dichalcogenides are also refered as VIB-VIA
compounds which includes chalcogenides of Molybdenum (Mo) and Tungsten
(W) i.e. MoS2, MoSe2, WS2, WSe2 and WTe2. The sulphides of Mo and W are
gray black colored. Both are having lubricating and semiconducting properties.
MoS2 is naturally occurring mineral called ‘Molybdenite’ which is primary source
of Molybdenum, similarly WS2 is also naturally occurring mineral called
‘Tungstenite’. Both the compounds are having 2H and 3R polytypes. The
tellurides of Mo and W have different structures. MoTe2 possess layered MoS2
type of structure [22], while WTe2 is having different and more closely related to
CdI2 type of structure. In this present work author has reported the study of
MoSe2, WSe2 and solid solution phase of the both i.e. Mo(x)W (1-x)Se2.
1. 1. B.2.1. MoSe2:
Fig.1.11. Crystal Structure of MoSe2
MoSe2 is not known to occur naturally. But characteristically it is like MoS2. In
the system Mo-Se three phases are known- MoSe2, MoSe3 and Mo3Se4 [23].
Two polytypes of MoSe2 are known that are 2H and 3R. The synthesis of MoSe2
is carried out at higher temperature and pressure. The Synthesis from the
elements at temperatures 340oC seem to be the most convenient procedure,
which always leads to the synthesis of 2H form [23,24] to obtain the compound
34
in a well crystallized form, however higher temperatures in the range of 900oC to
1100oC have to be applied for periods between one day to two weeks [23].
Brixner has reported this period reduced to 10 to 15 hours at 600oC to 700oC
[25].
Brixner, Wildervanck, Al-Hilli and Evans have described crystal growth
study of MoSe2 [26]. For MoSe2 the characteristic temperature in the bromine
assisted transport process were TH= 910oC TL= 730oC for a period of three
days with 2 mg bromine cm-3 tube volume. According to Al-Hilli’s report there
growth experiments with bromine resulted in a mixture of 2H and 3R crystals.
However crystals grown without bromine were of the 2H type only and showed
well defined hexagonal growth spirals.
1. B.2.2. WSe2:
Fig.1.12. Crystal Structure of WSe2
At crystal level tungsten diselenide is reported as semiconducting compound
with relatively low electrical conductivity increasing with temperature and a high
seebeck coefficient. The system W-Se was extensively studied by Hicks [27]
where no evidence was found for the existence of higher or lower stoichiometric
compound than WSe2. The structure of WSe2 reported in the literaute is like
WS2 with the metal trigonal prismatically surrounded by selenium [28]. The only
known modification of WSe2 is 2H, the rhombohedral diselenide is not known.
35
Several reports regarding the synthesis of WSe2 exist in literature,
Silverman’s high pressure-high temperature experiments cover very wide range
of pressure and temperature profile upto 70 Kbar and 2400oC repectively. It
gave only hexagonal form. Wildervanck’s, Al-Hilli et al. and Kershaw[29] ,
Brixner [30]et al. Revolinsky et al. [31] and Hick’s have reported the synthesis of
polycrystalline substance by direct reaction of the elements in stoichiometric
ratio. DTA experiments suggests 480oC temperature of formation for WSe,
however most of the described procedures utilize higher temperature. In all
cases the yield consisted of 2H- WSe2.
Wildervanck’s growth experiments on WSe2 with both bromine and iodine
were used and reactions were usually between 2.5 to 3 weeks with bromine
transport took place from TH=800oC - 850oC  TL= 750oC and with iodine the
gradient was TH=1085oC  TL = 890oC. The produced samples were described
as thin small crystals having the hexagonal 2H-MoS2 type lattice. In its synthesis
with and without use of halogen as carrier transport the produced material
consisted of thin buckled crystals with no obvious surface features. Crystals
grown without the aid of Br2 were plates which will well define the hexagonal
features.
1. B.2.3. MoxW(1-x)Se2
Brixner and Tuefer have studied the solid solutions of VI group members
dichalcogenides in single phase form [32]. Hicks and Revolinsky also studied
the semiconducting and electrical properties and substituted WSe2, MoSe2 and
MoTe2 compounds. Brixner synthesized solid solution by heating the constituent
elements in an evacuated sealed quartz ampoules analogs to 10 to 15 mL at
600 to 700 oC. Then the reaction mixture given at second heat treatment for
another 10 to 15 hours a homogeneous polycrystalline graye black substance is
obtained. At such higher temperature though the complete solid solubility was
observed between WSe2 and MoSe2, the small difference in their cell
parameters results in a linear variations of the crystallographic parameters of the
ternary compounds with composition.
All above reports about the transition
36
metal dichalcogenide available in literature are attempts of researchers to
synthesise and study these materials at bulk substance level, which required
very high temperature and pressure as well as long time period to complete the
processes in hours, days and weeks in certain cases. The field of thin films
changes all this scenario and desired compounds started to be synthesized in
fine thin film from with much accuracy. R. Bichchsel and F. Levy have studied
electrical and optical properties of MoSe2 thin films prepared by Rf magnetron
sputtering[33].
Goldberg
et
al.
have
reported
the
optical
absorption
measurement on MoSe2. The energy gap determined by extra-polating different
plots of r Vs E to zero absorption Eg = 1.06  0.02eV [34]. The morphological
and compositional properties of MoSe2 films prepared by Rf magnetron
sputtering have also been reported by R. Bichchsel and F. Levy [35]. S.H. ElMahalawy and B.L. Evans have studied the temperature dependence of the
electrical conductivity and Hall coefficient in 2H – MoS2, MoSe2, WSe2 and
MoTe2 [36]. The semiconducting character of molybdenum diselenide has been
mentioned by Brixner [25] as well as Brixner and Teufer [32] who have found an
n-type and a high Seebeck effect. Hicks [27] has mentioned  = -900V deg.-1
and  = 20cm at 1000C, and
= 15cm2 V-1Sec-1 at room temperature.
Tungsten diselenide is also semiconducting with a p-type an optical absorption
threshold at 780A0 (Eg = 1.6 eV)[37]. The resistivity and Seebeck effect in
MOSe2-WSe2 solid solutions have been investifgated by Revolinsky and
Beernsten [31]. The activation energy remains around 0.1eV. All compositions
(including MoSe2) have been found to be of p-type. Marked anisotropy has been
observed, as it is to be expected in the C7 (MoS2) structure. Photoelectron
spectra using He(I) and He(II) excitation confirmed the 2H crystal lattice of
MoSe2 and WSe2 that reported by F.R. Shephard and P.M.Williams [38].
Recently, MX2 (where M = Nb, Ta, Mo, W, Sn, and Ti, X = S, Se) films have
been solution processed using a technique based on forming ultra thin platelets
through Li intercalation and exfoliation in water followed by self assembly of the
platelets to a dipped hydrophilic substrate [39,40]. A well grown molybdenum
and tungsten dichalcogenide crystals (MX2, X=S, Se, Te) in contact with
37
electrolyte (wet solar cells) have yielded solar energy conversion efficiency
exceeding 15% [41]. PEC cells using tungsten and molybdenum dichalcogenide
in junction with the I-/I2 couple in aqueous solution have been developed with
good stability experimentally confirmed [42,43]. The MoSe2 and WSe2 in
particular have favorable band gap around 1.5eV an advantage for MoSe2
based cells can still be realized from the smaller value of the direct band gap for
MoSe2 (1.4eV) compared to that for MoS2 (1.7eV) [44]. The efficient
photoelectrochemical solar cells were constructed with nMoSe2 and WSe2
photoanodes [45] reported by G. Kline and K. Kam etc., in this increasing
efficiency above 10% was reported by B.A. Perkinson and A. Heller and others
[46,47]. For improvement of energy conversion efficiency, by specific chemical
treatment of n-MoSe2 and n-WSe2 was reported by D. Canfield and B.A.
Perkinson [48]. A.J. Grant and others have shown WSe2 is a layer type
semiconductor with band gap of 1.35eV [49]. J.C. Bernede and S. Benhida have
studied primary and secondary crystallization processes of WSe2 thin films [50].
Transistion metal dichalcogenide MX2 (M=Mo,W and X=S and Se)are
semiconductors that can act as photovoltaic materials, the promising results
have been obtained in the realization of photoelectrochemical [51-53] or solid
devices solar cells [54,55]. D.Palit, S.K. Srivastav and others have studied the
layer disorder and microstructural –parameters of molybdenum-tungsten mixed
sulphoselenide Mo0.5W0.5 Sx Se2-x (0 x  2) by X-ray linear profiler analysis [56].
The electronic structure of some layered TMDC’s were studied [57-62]. P.
Pramanik, S. Bhattacharya have synthesized MoSe2 by chemical bath
deposition method (CBD) and reported band gaps for MoS Eg=1.17eV as well
as for MoSe2 Eg =1.1eV [63]. The MoSe2 films were 0.5 -0.6 m in thickness
with n-type conductivity, =104cm. The optical absorption in single crystals of
InxMoSe2 (0 x  1) was reported by A.M. Vora and P.K. Gard [64]. The
performance of synthetical n-MoSe2 in electrochemical solar cells was reported
by J. Gobreacht and others [65]. S.Menezes and F.Z. Disalvo have studied
photoeletrochemical compatibility of n-WSe2 and n-MoSe2 with various redox
systems [66].
Wrighton and co-workers showed that n-MoSe2 and n-MoS2
38
electrodes have good stability in concentrated electrolytes containing LiCl, even
in presence of chlorine evolution [67]. M. K. Agarwal et al. studied the etching
and dissolution kinetics of MoSe2 single crystal [68]. M.K.Agarwal and Babu
Joseph have carried out electron microscopic observations of dislocations in
MoSe2
single crystal [69]. The single crystals of MoSe2
were also studied
extensively for various applications [70]. J.C. Bernede and others have
synthesized MoSe2
thin films depositing them by DC diode sputtering
technique [71].The electrical studies of (Mo W)Se2 carried out by M.K. Agarwal
and P.D. Patel and Ajalkar et al. revealed that the material is semiconducting
[72]. S. Chandra and Sahu S.N. have reported the electro deposition technique
for MoSe2 thin films. They found that the deposition of Se is faster than Mo.
[73]. B.L. Evans and R.A. Hazelwood
have measured dielectric constant of a
MoSe2 thin films [74]. J.H. Zhan and Z.D. Zhang have synthesized MoSe2 by
solvothermal route from conversion of MoO3 [75]. J. Pouzet and J.C. Bernede
have studied properties of MoSe2 thin films obtained by solid state reaction s
between thin films and by d.c. diode sputtering[76]. Measurement of Hall
Coefficient and resistivity carried out on a series of semiconducting layer
structures and shown that in these materials the charge carriers have rather low
but strongly temperature dependent mobilities reported by R. Fivaj and E.
Mooser [77].The melt grown samples of TMDC show improved photoactivity
reported by W.K.Hoffmann and H.J.Lewerenz [78,79]. H.J.Lewerenz and others
have reported that there are disadvantages of the technique of preparation of
TMDC like melt growing technique since irregular growth was observed [80]. In
this technique stoichiometry variation was also reported by S.Menezes and coworkers [81]. MoSe2 and WSe2 have shown considerable photoactivity [82,83].
In electrochemical system solar to electrical conversion efficiency was reported
to be increased by more than 10 % [84,85]. Haruo Naruke and Noboru
Wakalsuki and others have reported synthesis of MoSe2 at 1273k for 24 hours.
The X–ray study of the compound to be non-stoichiometric in the approximate
range of 1.85  Se/Mo  2.0 [86]. N. Guettri and I.Ouerfelli have reported
photoconductive nature of WSe2 thin films by post annealing treatment of
39
W/Se/W……... W/Se/W thin layers sequentially deposited on to a thin Ni layer
[87]. M.K.Agarwal and L.T.Talele have studied transport properties of
Molybdenum sulphoselenide single crystals [88].
1. B.3. Aim and Objectives of the Present Research Work:
With an enormous rise in world population during the past few decades
the energy requirements have been also increased at even larger rate. Our
increasing appetite (need for consumption) for energy is one of the causes for
our energy crisis. There is ever increasing problem of environmental pollution
and global warming due to use of conventional and non-renewable sources of
energy. In addition to this according to some projections we may run out of fossil
fuels by 2025. This fact makes considerable increase in interest about the
searching and exploiting non-conventional and renewable sources of energy.
Transition Metal Dichalcogenides (TMDC) is the group of compounds
having semiconducting properties with band gap suitable for its application as
photoanode. The material MoSe2 and WSe2 were extensively studied for their
semiconductor properties. However the major part of the study was carried out
on single crystal and bulk material. Therefore its study in the form of thin film as
well as study of their combination in the form of mixed transition metal
dichalcogenide thin film has been planned. This ternary mixed TMDC is
expected to have much suitable band gap and improved stability against
decomposition as a photoanode which will increase the efficiency of the solar
cell and lasts for longer as a photoanode in solar cell.
In the recent years, there has been
highly increasing interest in
developing the thin film solar cells as one of the alternative energy source since
it is clean, non-polluted way of energy generation which need comparatively little
maintenance and abundantly available solar energy. Photovoltaic energy
conversion through semiconductor / liquid junction route is developing fastly and
becoming one of the popular alternatives to present energy crisis. Binary and
mixed composite thin films of chalcogenides of groups II-IV, IV-VI, V-VI, VI-VI, IIV-VI families have a great importance. A large number of techniques have been
40
developed and used to deposit these materials in pure single crystal and mixed /
alloyed thin films. Out of these electrodeposition is one of the attractive method
for the preparation of elemental, binary and ternary compound thin films. It has
some advantages over the other methods viz, it is easy, less expensive. It is an
isothermal process mainly controlled by electrical parameters, which are easily
adjusted to control film thickness, morphology, composition etc. In the recent
years, electrodeposited thin film semiconductors are becoming popular in the
field of solar cell, optoelectronic devices, solar selective coatings etc.
The
present
study
deals
with
chemosynthesis
of
MoSe2,
electrosynthesis of WSe2 and synthesis of MoxW (1-x)Se2 thin films by hybrid
electro-chemosynthesis from aqueous bath. The preparative parameters such
as deposition potential, concentration of the precursors, bath temperature, pH of
the bath, deposition time and speed of substrate rotation, etc. were optimized.
As deposited and post treated binary TMDC thin films of W and Mo and hybrid
thereof will be characterized using following characterization techniques to
check suitability of these films as a photoelectrode in energy conversion devices
through PEC cell.
The characterization of thin films will be carried out using different techniques
such as X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy
Dispersive X-ray analysis (EDS), optical absorption, Electrical Conductivity,
Thermoelectric Power (TEP) and PEC studies. The available data will be then
interpreted and the results will be reported in thesis.
41
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