Chapter 1
General Introduction
1.1 Solar Cell
The world population grows fast and will reach 10 billion in 2050. In order to
provide the growing population with high living standards, further economic
development is essential. This requires more energy than what we use today. What the
global community is much concerned for is to look for environmental friendly
renewable energy sources. By renewable energy we understand that it is obtained
from the continuing flows of energy occurring in the natural environment such as
solar energy, hydropower and energy from biomass.
It is believed that the renewable energy sources may fulfil the great energy
demand which is increasing day by day as industries grow and remains challenging
the survival of humanity. Similarly, global warming threat due to excess of carbon
dioxide is also disconcerting human life on earth. To overcome the above situation
research is going on to find an alternate energy source. Solar energy is one of the best
choices naturally available among such non-conventional energy sources to nurture
today’s great need for electrical energy. The fundamental mechanism involved in
exploiting solar energy includes solar thermal system, converting solar energy into
heat, solar fuel system that converts this energy to chemical energy, and solar
electrical system, focussing on generating electrical energy.
Today, the application based solar energy materials in fabricating solar cells is
fast emerging research area of science and technology. Physicists, chemists, material
2
scientists, engineers and personnel of R&D industries continuously attempt to develop
novel materials for greatly efficient and cost effective solar cells.
The process involved in converting solar energy to electrical energy needs an
electrical device, called solar cell, also known as photovoltaic cell (PVC) that helps
change energy of light directly into electricity by photovoltaic effect. This is one of
the most advanced technologies for clean power production from abundant solar
energy. This method of converting solar energy to electricity is pollution free and is
the best solution for the global energy problem. Hence, the hypothesis has been set to
seek the pragmatic feasibility of using oxide thin films, particularly copper oxides that
may help develop economically competitive, much-higher-efficiency and/or muchlower-cost solar cells. There are considerable developments in solar photovoltaic
applications.
The world’s largest oil company Shell has published a vision on future energy
consumption and potential energy sources (Van der veer, J et al., 1997). In future, the
energy supply will become more diversified and hence more robust. It is interesting to
notice that Shell expects the photovoltaic (PV) solar energy to become a major energy
source within fifty years. The renewable solar energy source is based on the
continuing flows of energy that is considered inexhaustible from the point of view of
human civilisation. Solar radiation represents such an infinite source of energy for the
Earth. The Sun delivers 1.2 × 1014 kW energy on the Earth, which is about 10,000
times more than the present energy consumption. The energy that the Earth receives
from the Sun in just one hour is equal to the total amount of energy consumed by
humans in one year. As solar power is naturally available in abundance the
3
researchers have been prominently engaged in converting this energy into electrical
energy.
Therefore, solar energy is considered to be one of the most sustainable energy
resources for future energy supplies. The conversion of light energy into electricity
was invented by Edmund Becquerel in 1839.
1.1.1 Photovoltaic solar energy
The direct conversion of solar radiation into electricity is often described as a
photovoltaic (PV) energy conversion because it is based on the photovoltaic effect. In
general, the photovoltaic effect means the generation of a potential difference at the
junction of two different materials in response to visible or other radiation. The whole
field of solar energy conversion into electricity is therefore denoted as the
“photovoltaics”. Photovoltaics literally means “light-electricity”, because “photo” is a
stem from the Greek word “phos” meaning light and "Volt” is an abbreviation of
Alessandro Volta’s (1745-1827) name who was a pioneer in the study of electricity.
Developing the PV solar energy which is clean and environment friendly is
considered at present noble mission. One of the important design criteria in the
development of an effective solar cell is to maximize its efficiency in converting
sunlight to electricity.
A large-scale use of PV solar energy can lead to a substantial decrease in the
emission of gases such as CO2 and SOx and NOx that pollute the atmosphere during
the burning of the fossil fuels. At present, the total energy production is estimated to
be 1.6 × 1010 kW compared to 1.0 × 106 kWp that can be delivered by all solar cells
installed worldwide. PV starts to make a substantial contribution to the energy
4
production and consequently to the decrease in the gas emissions depending on the
growth rate of the PV solar energy production. When the annual growth of PV solar
energy production is 15% then in year 2050 solar cells will produce 2.0 × 108 kWp.
The annual growth of 25% will result in the solar electricity power production of 7.5
× 109 kWp in 2040 and the annual growth of 40% will lead to power production of
2.4 × 1010 kWp in 2030. This demonstrates that there must be a steady growth in solar
cells production so that PV solar energy becomes a significant energy source after a
period of 30 years.
The solar cells and solar panels are already on the market. An advantage of the
PV solar energy is that the solar panels are modular and can be combined and
connected together in such a way that they deliver exactly the required power. We
refer to this feature as “custom made” energy. The reliability and very small
operations and maintenance costs, as well as modularity and expandability, are
enormous advantages of PV solar energy in many rural applications. There are two
billion people mostly in rural parts of the world that have no access to electricity and
the solar electricity may help them develop their life as it is most cost effective
solution. Bringing solar electricity to these people represents an enormous market.
Some companies and people have realised that solar electricity can make money now
and this fact is probably the real driving force to a widespread development and
deployment of the PV solar energy.
The PV solar systems are already an important part of our lives. The simplest
PV solar systems power many of the small calculators and wrist watches that we use
every day. More complicated systems provide electricity for pumping water,
5
powering communications equipment, and even lighting our homes and running our
household appliances.
To make the energy of solar radiation converted into electricity, materials that
behave as semiconductors are used. At present, silicon-based solar cells occupy the
majority of the solar energy market due to their well-developed fabrication techniques
and relatively high energy conversion efficiencies. It accounts for about 90% of total
installed solar cell in the world. However, their high fabrication cost and limited
feedstock of silicon prompt us to search for new alternative absorber materials which
are low cost, abundant, and environmental friendly for long-term sustainability. In this
respect, novel solar cells with various inorganic and organic materials have been
investigated extensively.
Thin film solar cell described as part of solar cell technology in which layers
of materials are deposited one after another by physical or chemical deposition
method. Thin film may encompass a considerable thickness range, varying from a few
nano meters to tens of micro meters.
Thin film solar cells offer a wide variety of choice in terms of the device
design and fabrication. A variety of substrates can be used for deposition of different
layers using different techniques (sputtering, CVD, PVD, spray pyrolysis, etc.).
Cheaper solar cells can be produced only if cheaper materials and lower cost
technology are utilized. It is where thin film technology offers promising potential as
an alternative to silicon photovoltaic technology. The following features of thin film
processes have shown how they are interested in solar cell technologies:
6
1.
A variety of physical, chemical, electrochemical, plasma based and hybrid
techniques are available for depositing thin-films of the same material.
2.
Microstructure of the films of most materials can be varied from one extreme of
amorphous/nanocrystalline to highly oriented and/or epitaxial growth, depending
on the technique, deposition parameters and substrate.
3.
A wide choice of shapes, sizes, areas and substrates are available.
4.
Because of relaxed solubility conditions and a relaxed phase diagram, doping and
alloying with compatible as also, in many cases, incompatible materials can be
obtained.
5.
Surface and grain boundaries can be passivated with suitable materials.
6.
Different types of electronic junctions, single and tandem junctions, are feasible.
7.
Graded band gap, graded composition, graded lattice constants, etc., can be
obtained to meet the requirements for a solar cell.
8.
In case of multi component materials, composition, and hence band gap and other
optoelectronic properties, can be graded in desired manner.
9.
Surfaces and interfaces can be modified to provide an interlayer diffusion barrier
and surface electric field.
10. Surfaces can be modified to achieve desired optical flectance / transmission
characteristics, haze and optical trapping effects.
11. Integration of unit processes for manufacturing solar cells and integration of
individual solar cells can be easily accomplished.
7
12. Besides conservation of energy and materials, thin-film processes are in general
eco-friendly and are thus ‘Green’ processes.
1.1.2 Principle operation of solar cell
Practically, all conventional photovoltaic devices are based on the p-n
junction. A solar cell is basically a semiconductor diode. A photovoltaic cell consists
of a light absorbing material which is connected to an external circuit in an
asymmetric manner. Sunlight is a spectrum of photons distributed over a range of
energy. The charge carriers are generated from the material by the absorption of
photons of light. The important parameter in PVC is band gap energy Egap of the
semiconductor. Photons whose energy is greater than the band gap energy (the
threshold energy) can excite electrons from the valence to conduction band. All
photons with h >Egap will each contribute to the photo generated electron-hole pairs
with the excess energy lost because of thermalization. In the second step of the energy
conversion process, the photogenerated electron-hole pairs are separated with
electrons drifting to one of the electrodes and the holes drifting to the other electrode
because of the internal electric field caused by p-n junction structure. This light
driven charge separation establishes a photo voltage at open circuit, and generates a
photocurrent at short circuit. When a load is connected to the external circuit, the cell
produces both current and voltage and can do electrical work (Rai, B.P., 1988).
Thus, PV cells can produce electricity without operating at high temperature
and without mobile parts. These are the salient characteristics of photovoltaics that
explain safe, simple, and reliable operation.
8
1.1.3 Theory of I-V Characterization
PV cells can be modelled as a current source in parallel with a diode. When
there is no light present to generate any current, the PV cell behaves like a diode. As
the intensity of incident light increases, current is generated by the PV cell.
Fig. 1.1 Current generation in a PV model
In an ideal cell, the total current I is equal to the current I! generated by the
photoelectric effect minus the diode current ID, according to the equation:
!"
#
$
%
=
#
$
% &"'
()*+,
$ -.
where I0 is the saturation current of the diode, q is the elementary charge 1.6/ -0"19C,
k is a constant of value 1.38x10-23J/K, T is the cell temperature in Kelvin, and V is
the measured cell voltage that is either produced (power quadrant) or applied (voltage
bias).
Short Circuit Current (ISC)
The short circuit current ISC corresponds to the short circuit condition when
the impedance is low and is calculated when the voltage equals zero. I (at V=0) = ISC.
ISC occurs at the beginning of the forward-bias sweep and is the maximum current
9
value in the power quadrant. For an ideal cell, this maximum current value is the total
current produced in the solar cell by photon excitation.
ISC = IMAX = I! for forward-bias power quadrant
Open Circuit Voltage (VOC)
The open circuit voltage (VOC) occurs when there is no current passing
through the cell. V (at I=0) = VOC. VOC is also the maximum voltage difference
across the cell for a forward-bias sweep in the power quadrant.
VOC= VMAX for forward-bias power quadrant
Maximum Power (PMAX), Current at PMAX (IMP), Voltage at PMAX (VMP)
The power produced by the cell in Watts can be easily calculated along the I-V
sweep by the equation P=IV. At the ISC and VOC points, the power will be zero and
the maximum value for power will occur between the two. The voltage and current at
this maximum power point are denoted as VMP and IMP respectively (Fig.1.2).
Fig 1.2 -Maximum Power for an I-V Sweep
10
Fill Factor (FF)
The Fill Factor (FF) is essentially a measure of quality of the solar cell. It is
calculated by comparing the maximum power to the theoretical power (PT) that would
be output at both the open circuit voltage and short circuit current together. FF can
also be interpreted graphically as the ratio of the rectangular areas depicted in
Fig. 1.3.
FF =
1234
15
=
621 721
689 7:9
Fig. 1.3 Fill Factor from the I-V Sweep
A larger fill factor is desirable, and corresponds to an I-V sweep that is more squarelike. Typical fill factors range from 0.5 to 0.82. The fill factor is also often
represented as a percentage (Wurfel, P., 2005).
Efficiency ( )
Efficiency is the ratio of the electrical power output Pout, compared to the solar
power input, Pin, into the PV cell. Pout can be taken to be PMAX since the solar cell can
be operated up to its maximum power output to get the maximum efficiency.
;<=>
Efficiency =
;?@
; Maximum Efficiency
max =
;ABC
;?@
11
Pin is taken as the product of the irradiance of the incident light, measured in W/m2 or
in suns (1000 W/m2), with the surface area of the solar cell/m2 (Green, M.A., 1982).
1.2 Scope of the work
Photovoltaic technology continues to probe into the field of better preparing such
photovoltaic cells. The two basic requirements for materials to be used for solar cell
windows are high optical absorption in the visible range and low electrical resistivity.
Synthesis of inorganic nano structures with reliable low cost and well defined
morphology have drawn attention for the structural, optical and electrical
characterisation and their applications in various fields. Also, the research work on
thin film coating has been carried out throughout the world. The researchers are
searching for new coating material that has electro optical properties like transparent
conducting, smart window etc. Among different metal oxide materials Cu based
materials are of great interest because of their applications. Oxide thin films have
been used for Solar cell application by the researchers.
Copper oxide has fascinating properties for alternative photovoltaic devices and
photo electrodes in high efficiency photo electrochemical cells. It is a semiconductor
and also the best absorbing material for photovoltaic devices. A good absorbing
material should have i) a low band gap ii) a good absorption coefficient iii) minority
carrier diffusion length. The diffusion length depends on the structure of a material
(Verka Georgieva et al., 2011). To use Cuprous or Cupric oxide in Photo Electro
Chemical Cell it should be prepared as a thin film type electrode. A thin layer of
Cuprous and Cupric oxide can absorb sun light radiation. For both cuprous and
cupric oxide the band gap value is less than 2.5eV. They have high absorption
coefficient and photosensitive properties which are well suitable for solar cells.
12
Further, these Copper oxides have low toxicity and good environment acceptability
and their constituents are cheap and plentiful in nature. These properties make Cu-O
compound the best absorber layers in photovoltaic devices. Hence, the Copper oxide
thin film is advantageous for the photovoltaic applications.
Theoretical calculations have predicted electrical power conversion efficiency
approximately 20%. So far the highest efficiency of approximately 2% for copper
oxide solar cell has been reported by the researchers like Mittiga (Alberto Mittiga,
2006) using high temperature annealing method and an expensive vacuum
evaporation technique. Different kinds of Cu2O based solar cells have been fabricated
with metal/p-Cu2O Schottky junctions, p-n hetero and p-n homo junctions. But copper
oxide thin film for solar cell application can be prepared by simple electrodeposition
method. If cuprous oxide is prepared at high temperature, it may have high leakage
current due to the shorting path created during formation of the material and so it will
produce low conversion efficiency. Therefore the objective of the researcher is to
prepare cuprous oxide at low temperature. For the future of a copper oxide based thin
film solar cell system its physical properties have to be investigated in deeper detail.
So it is highly considerable to optimise the physical properties for developing the best
copper oxide thin films.
Metal oxide semiconductors have wide applications in technological areas
such as electronics, opto-electronics, bio-chemical sensors, coating systems and
catalysis. This technology requires novel materials. One such material is copper oxide
coated over indium tin oxide. In all technological devices, an in-depth understanding
of the material’s structures, morphology and properties are necessary. The optical,
structural and electrical properties of absorbing materials in solar cells are the key
13
parameters which determine the performance of the solar cell. Hence it is necessary to
study these properties for high efficient devices.
The objectives of the thesis are as follows:
1) To study the mechanism to grow Cuprous and Cupric oxide.
2) To grow a nano thin film of Cuprous and Cupric oxide on ITO coated glass
plate in a low temperature by electrodeposition technique.
3) To study the effect of pH and the temperature of the electrolyte of Cuprous
and Cupric oxide thin films and to optimise the value of pH and temperature
of the electrolyte.
4) To study the effect of potential applied on the working electrode for the
deposition of oxide thin films and to find an optimised value of potential.
5) To study the structural, optical and electrical properties and to analyse the
surface morphology of the films prepared using various characterisation
techniques.
6) To develop a heterojunction layer with Cuprous and Cupric oxide.
7) To find the photoelectric response of the prepared films.
Thus the research work is a comprehensive attempt to prepare a thin layer of
copper oxide on a transparent conducting substrate which has a wide scope for
preparing solar cell with better efficiency.
14
References
Alberto Mittiga, Enrico Salza, Francesca Sarto, Mario Tucci, and Rajaraman
Vasanthi, 2006. Heterojunction solar cell with 2% efÞciency based on a Cu2O
substrate. Applied Physics Letters, 88: 163502.
Green, M.A., 1982. Solar Cells; Operating Principles, Technology and System
Applications. Prentice-Hall.
Kunhee Han, 2009. Electrodeposited Cuprous Oxide Solar Cells. Ph.D Thesis, The
University of Texas at Arlington.
Rai, B.P., 1988. Cu2O Solar cells: a review. Solar Cells, 25: 265.
Van der Veer, J., and Dawson, J., 1997. Shell International Renewables. Transcript of
a Press conference in London.<http:/www.shell.com>
Verka Georgieva, Atanas Tanusevski and Marina Georgieva, 2011. Low Cost Solar
Cells Based on Cuprous Oxide. Solar Cells – Thin-Film Technologies.
Wurfel, P., 2005. Physics of Solar Cells: From Principles to New Concepts. WileyWCH, Weinheim.