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1 Introduction
The sun is the basic for all life process on earth. Apart from nuclear energy and the earth’s heat,
all usable energy forms are either a direct or indirect utilization of solar energy. The conventional
forms of energy are stored solar energy in the form of fossil fuels (oil, gas etc.) of in the form of
biomass (e.g. wood). Fossil fuels are produced over a period of many millions years from solar
energy. These reserves are limited and cannot last indefinitely. Mankind has to find new forms of
energy as soon as possible.
Solar energy is available abundantly all over India. It is therefore natural to think of using energy
from the sun, which radiates 20,000 times the world’s energy requirement on the surface of the
earth every year. In the past few years, efforts to use solar energy directly have been identified.
One of the most spectacular ways of using solar energy is in the form of electricity, using
photovoltaic (PV) modules. Rapid progress has been made in this in the last twenty years and
many photovoltaic systems are now in market.
The photovoltaic process converts solar radiation into electrical energy using a solar cell (photolight, voltaic-electricity). The photovoltaic effect was discovered as early as 1839 by Alexander
Becquerel. Becquerel was experimenting in the field of electrochemistry with two metal plates
dipped in a dilute acid. He found that the equipment produced more energy when exposed to
sunlight. It was 50 years later that Charles Firth proved the photovoltaic effect using the
selenium cell. Relatively low efficiency about 1 to 2%, together with high cost at the time
prevented any wide application of the technology.
In the year 1955, Bell Laboratories of the USA developed a silicon solar cell with an efficiency
of 6%. After this development, silicon solar cells were used in spaceships. The first satellite
(named Vanguard) with solar cells was launched in the year 1958. The solar generator was made
of 108 solar cells and delivered energy beyond the operating lifetime of the satellite. In spite of
the high costs, photovoltaics for space application had no competition. Further development for
solar cell and the corresponding price reduction in terms of watt peak is shown in Figure 1.1.
This reduction in the cost of PV modules also allowed the terrestrial use of photovoltaics.
Nowadays, a wide range of photovoltaic system is available, from milliwatts (mW) in calculators
or watches to megawatts (MW) in large central power stations. An overview of the PV power
required for various applications along with cost, as against conventional energy sources, is
given in Figure 1.2. The large spectrum of possible power typical of photovoltaic system is not
possible for any other energy source. In practice, one can achieve the desired power range by a
modular constructions of PV modules connected in series and parallel with each other. The often
held impression that photovoltics are not yet economical, holds true only if one compares the
energy costs of PV generators with that of conventional power houses. There already exist
several economically attractive applications of photovoltics (as shown in Fig 1.2). In general, the
lower the desired power for a system, the more attractive is the photovoltaic option the
continuous decreasing trend in cost of PV, allows more and more applications of PV. Thus PV
makes an increasing contribution towards total energy supply.
Solar photovoltaic are cells that produce electricity directly from sunlight. They are usually made
of silicon-material that is found in send. The cells are wafer thin circles are rectangles, about 6 to
8 cm across. Solar cells operate according to what is called the photovoltaic effect, ‘bullets’ of
sunlight-photons-striking the surface of semiconductor material such as silicon, liberate electrons
from the material’s atoms. Certain chemicals added to the material’s composition help establish a
path for the freed electrons. Through the photovoltaic effect, atypical 10 cm silicon solar cell
produces about one watt of direct current electricity (Fig1.3).
In the most common process for the production of PV cells, very pure silicon is reduced to its
molten form. Through a painstaking and time-consuming process, the silicon is reformed in to a
solid single crystal cylinder called an ingot. Extremely thin slices cut from the ingot are
chemically treated to form solar photovoltaic cells-sometimes referred to as solar batteries. Wires
attached to the negative and positive surface of all the cells complete the electrical circuit. Direct
current electricity flows through the circuit when the cell is exposed to light. For efficiency and
practicality, multiple cells are wired together in series /parallel and placed in a glass-covered
housing called a module. The modules themselves can then be wired together into arrays. SPV
arrays can produce as much direct current electricity as desired through the addition of more
modules.
Solar photovoltaic (SPV) modules and arrays produce direct current (DC) electricity. Since most
applications and equipment are designed to be powered by alternating current (AC), SPVproduced electricity must be converted. This is accomplished using an inverter. Inverters are
solid state devices that convert DC to AC current, compatible with that sent over utility grids. As
a result, PV installations may be interconnected with a utility grid, sending power onto the grid
whenever there is an excess, and drawing electricity from the utility when sunlight is not
available. Most inverters have a fail-safe relay that disconnects the PV system from the utility
grid whenever the grid fails, ensuring the safety of utility repair personnel.
Solar photovoltaic modules are currently expensive and are not yet competitive with readily
available utility power. However, PV costs are decreasing. When the first photovoltaic system
was used by NASA to power orbiting space satellites, the costs were as high as US$ 1000 per
peak watt (peak watt is the amount of electricity produced by a PV cell when bright sunlight is
available). One can now buy solar cells at a rate of about Rs 180 per peak watt. This cost is also
likely to come down.
The introduction of PV has to be seen, not in terms of price per kWh, but from many other
important considerations, especially in the small power ranges. In fact, there are a number of
battery-operated systems where kWh price is several hundred rupees. In many other areas too,
the price of delivered power is relatively high; for example diesel generators (Rs 3-10/kWh),
auto batteries (Rs 1-6/kWh) etc. as seen from fig 1.2. For small applications, where the cable
costs become much higher, PV is an obvious solution. Also in remote areas where grid and
distribution costs become very high, an integrated system depending upon PV, diesel, wind and
water would be much more economical. There is no optional cost in solar PV systems and there
are many systems like solar home systems, emergency lighting and so on where initial costs are
affordable and which make much better sense then going in for diesel generators etc.
Material and manufacturing costs are the two major factors that influence the price of
photovoltaic cells. Even though silicon is the second most abound material on earth, the silicon
used for PV cells must be very pure. Refining high grade silicon to remove most of its impurities
is an expensive process. PV cells and systems are now being fabricated indigenously and the cost
therefore are likely to full further.
More efficient cells are also help to lower the cost somewhat. The limit of efficiency for silicon
PV cells is estimated to be about 25%. Currently most PV cells operate at about 10% efficiency.
When the cell and systems can be made to operate at higher efficiency levels, the cost of a
system will be lower because fewer cells will be needed to generate the desired amount of
electricity.
Presently, photovoltaic research is focused on two areas- manufacturing and applications. In the
area of manufacturing, both methods and materials are being explored. Scientists are
investigating the use of polycrystalline and amorphous silicon in PV cells. Semiconductor
materials other then silicon are also receiving attention. Manufacturing methods being researched
include new ways to purify silicon to ‘solar grade’, better methods of slicing cell wafers from
silicon ingots, and more efficient production of cell material by casting it into blocks, drawing it
into ribbons or sheets, or depositing a thin film of the material on an inert base. In the area of
SPV systems, integrations of components such as SPV module, battery, charge controller and
inverter etc. is important.
Many remote uses of photovoltaics are now cost effective and practical. Photovoltaics are
generating power for both on- and off-shore traffic control systems, crop irrigation systems,
bridge corrosion inhibitors and radio relay stations. They are also providing electricity to remote
cabins, villages medical centers and other isolated sites where the cost of photovoltaics is less
then the expense of extending cables from utility power grids or producing diesel generated
electricity. When system cost are reduced, several more options will be feasible. Residence may
have their south facing roofs converted with photovoltaic modules, either as an integral part of
the roof structure or mounted on supports designed for the purpose (Fig. 1.4).
Such residential PV systems will probably be connected to the utility grid as well as home (Fig.
1.5). In the way excess power would be sent on to the grid for credit during sunny periods, and
power could be drawn from the utility at night and on cloudy days. In another option, clusters of
homes may jointly own or shear a common photovoltaic array located at a central site. Such
centralized installation also could be owned and operated by a utility company. Because
maintenance needs are generally low for photovoltaic systems, on-site crews and auxiliary
equipment could be kept to a minimum, cutting utility operating costs.
This manual is meant to provide basic information about solar photovoltaic cells, modules and
systems and illustrate the applications.
2 The sun as the Energy Source
2.1 THE SUN
The sun is a sphere consisting of hot gases. Its diameter is 1.39 million km (109 times that of
earth). On an average, the sun is 150 million km away from earth. The sun ray requires 9 min. to
cover this distance.
At the core of the sun, four hydrogen nuclei fuse together to form one helium nucleus .In this
process of fusion, part of the mass is converted into energy. The process is described by Einstein
well-known mass-energy relationship, E=MC2, where c is the speed of light.
The energy created by the process of fusion results in very high temperature (of the order of 15
million degree Celsius) at the core of the sun. The external surface has temperature around 6000
deg. C. A large part of solar energy originates from this external layer. Of the total energy
radiated by the sun, the earth receives a 0.5 billionth part, equivalent to about 1.3×1017 w which
is 20,000 times the world energy requirement.
2.2 COMPOSITION OF SOLAR RADIATION
The radiation from the sun consists of several wavelengths. In simple terms, this radiation lies in
the visible and invisible region. In the visible region, the wavelength corresponds to the various
colors in the rainbow. The regions of invisible radiations are the infrared and ultraviolet. Here
also the wavelength determines the type of radiation. The intensity of radiation in various
wavelength regions is given in the figure 2.1
The spectrum of solar radiation varies depending on whether it is measured in space (AM 0) or
on the earth (AM 1.5). For solar cells this spectrum is very important, because a solar cells reacts
only to definite region of the spectrum, just like a human eye does. A solar cell can use only a
part of solar spectrum. The radiation outside the goes waste and cannot be converted into
electrical energy. The proportions of various wavelengths in the solar spectrum outside earth
region are given in table 2.1.
Table 2.1: intensity of solar in three wavelength regions
Wavelength
µm
Power
(w/m2)
Percentage Power
Ultraviolet
(UV)-Region
(0-0.38)
Visible
Region
(0.38-0.78)
infrared(IR)
Region
(0.78-)
95
640.0
618.0
7
47.3
45.7
Summing together, one gets the power of 1353 w/m2 .this power of solar radiation is called the
solar constant.
This power is reduced by various factors, like air mass, pollution etc. till it reaches the earth. On
the surface of the earth, one receives at most 1000 w/m2. This radiation is called as global
radiation.
2.3 SOLAR RADIATION ON THE EARTH
The solar radiation on the earth is measured in two ways:
(a) The number of sunshine hours (per day, month and year).
(b) The global radiation, G.
A solar sunshine hour corresponds to the condition when the solar radiation is more than 200
W/m. The total period of sunshine fluctuates from place to place. For India, the total no. of
sunshine hours is approximately 4000 in a year. Till a few year ago it was the practice in
meteorological offices to register only the solar sunshine hours. To design a solar energy
system, however, information only about the sunshine hours is insufficient. This error is
recognize when the solar and global radiation values began to be measured throughout the
world. Nowadays, data is available about the global radiation and ambient temperature in the
form of tables or graphs to help the designer in sizing a solar system.
The solar radiation incident on a horizontal surface is comprised of direct radiation and
diffuse radiation. The diffuse radiation is, in turn, comprised of sky radiation and the
reflected radiation shown in figure 2.2.
Solar cells use direct as well as diffuse radiation to produce electricity. The power produced
by solar cells is proportional to the intensity of global radiation. System which works with
the concentrated form of solar radiation, use only the direct radiation. For photovoltaics,
however, one uses such systems rarely. Daily global radiation on a horizontal surface at
different places in India is given in figure 2.3
.
For optimal use in the northern hemisphere, a solar system is oriented southwards at an
inclination from the horizontal. The appropriate inclination angle, α, is dependent upon the
latitude and on the time of the year. In spring and in the beginning of fall, the sun shines
exactly overhead the equator. The biggest deviation from this position occurs at the
beginning of the summer and winter. It varies between ±23.45. To achieve optimal energy
yield throughout the year, the position of the solar cell would have to be adjusted frequently.
However intermediate position are usually sufficient, as given in table 2.2.
Time of year
Optimal inclination
angle of south
oriented solar
module
Summer
Spring and autumn
Winter
For fixed inclination(depending
upon storage and use
Latitude-15
Latitude
Lalitude+15•
25•-45•
2.4 SOLAR RADIATION AND INCLINED SURFACES
Measurement of solar radiation and the available data are usually given for a horizontal
surface. For inclined surface and for places at latitudes above 10˚N, the solar radiation falling
on inclined surface is usually higher. The optimal inclination of the solar cell corresponds to
the condition when the direct radiation is incident normal to the surface.
Another factor which has to be considered while installing the solar modules is the skyline.
In any case any obstacle which does not subtend an angle of more than 10 deg. With the
horizontal can to be assumed to have no influence on the radiation incident on the solar
panel. The incident solar radiation is also dependent on whether. The important astronomical
factor are elliptical path of the earth around the sun and the daily rotation of the earth. These
determine the sun’s altitude.
In order to use the sun optimally throughout the sun shine hours, the solar cell system has to
be moved along two axes. Usually, however, this is troublesome and the inclination of the
modules will have to be optimized for a definite period in a year. In such cases one does not
bother about the daily movement of the sun.
To convert horizontal radiation into radiation incident on an inclined SPV module, one
multiples the available solar radiation on a horizontal surface by a correction factor given in
table 2.3 for various climatic zones in India (Figure 2.5)
3 The Solar Cell
3.1 Working Principle
A solar cell is essentially a diode in which light energy is converted into electrical energy.
During irradiation by the sun, a typical solar cell yields between 0.5 and 1.2 Volts across the
metallic contacts. The power delivered by a solar cell is, in general, proportional to the incident
light energy. The ratio between the deliverable electrical power and the incident optical power is
known as the efficiency of a solar cell. Depending upon the material and type of cell, the
efficiency is between 5 and 20%. Technically, one can achieve efficiencies up to 30%, i.e. the
same order as in other energy conversion process. Just as in a battery, the electrical energy in a
PV system is produced without any moving parts and without any noise. In a PV cell the process
of electrical conversion is non-chemical, in contrast to the process in a battery.
3.1.1 Materials
Semiconductor materials can be used to produce solar cells. There are several selection criteria
for the choice of material: basic physical characteristics like energy band gap, doping strength,
cell structure, lifetime, environmental friendliness, material purity and costs of manufacturing.
Electrical power in a cell is produced due to the photoelectric effect, i.e. as a result of photon
(light) absorption, an electron-hole pair is produced. This phenomenon of electron-hole
generation limits the choice of semiconducting material for electricity generation. Table 3.1
gives the important materials presently used for solar cell production, with respective efficiencies
and the status of their development. The dominant material is silicon, though some other
materials are also being used for mass production of solar cells.
Crystalline silicon is the basic material for semiconducting technology. It shows
excellent stability. So far there has been no noticeable degradation in solar cells manufactured
from monocrystalline silicon. Recently there has been a growing interest in amorphous solar
cells made from a thin film of one-thousandth of a millimeter thickness. One third of the world’s
market has already been captured by amorphous solar cells because they work well in small
applications (watches, calculators etc.). For power generation, amorphous cells still don not play
an important role due to long term instability and degradation.
3.1.2 Construction of a solar cell
The important characteristics of a solar cell are its structure for keeping the charge carriers
(electrons and holes) separate, in general a p-n junction, and the contacts for the current flow. A
schematic diagram of a crystalline solar cell is shown in figure 3.1. Usually a layer of 100- 150
µm is sufficient to ensure complete absorption of relevant photons. In practice however, a
greater thickness is usually used to overcome stability and rigidity problems. As already
mentioned, amorphous Si-cells are also manufactured using thin film technologies. The film is
usually only 1 to 3 µm thick. This helps in saving costly semiconducting material but one
additionally need a base, usually a glass plate. Thin amorphous Si (a-Si) films are created on
glass using methods like thin-film evaporation, chemical deposition, and so on.
Table 3.1: Solar cell materials and respective efficiencies
Laboratory
Comercial
Area
(cm2)
Efficiency
(%)
Area
(cm2)
Efficienc
y (%)
Status
Monocrystalline
4
22.2
4
15
Large Scale
Monocrystalline with concentrators
0.15
27.526
26
17.2
Large Scale
Polycrastalline
100
15.8
100
13.5
Large Scale
EFG-Band
50
14.7
50
11.5
Small Production
Dendrite web
4
17.0
8
15.5
Small Production
Amorphous Si
1
11.5
10
7.4
Large Scale
Monocrystalline with concentrators
1
22.2
4
17.0
Small production
Monocrystalline
0.2
25.9
Polycrystalline thin film
8
8.8
CdS/Cu2S
10
9.2
CdS/CdTe
1.0
10.9
CdZnS/CuinSe2
8
10.5
Material
Silicon
GaAs (Gallium Arsanide)
Small Scale
Cd-Cu (Cadmium Copper)
Small Scale
Small Scale
100 Kw/a
Usual monocrystalline Si cells are manufactured by creating a p-n junction for separation of
charge. A solar cell is, therefore, essentially a well-known semiconducting diode, which can be
manufactured by known technical process. For special purposes, such as concentration, complex
=
structures are necessary. For thin-film amorphous solar cells, one uses a cell construction with
intrinsic semiconductors (undoped) as a middle layer (p/i/n or n/i/p cells). In this middle layer,
the charge carriers are usually produced and this allows higher efficiency due to smaller
recombination rate of the charge carriers.
Along with the selection of materials, other factors that play an important role in the
working of a solar cell are the anti-reflection coating and the metallic contacts. Contacts should
ideally be transparent to light, while at the back of the solar cell the contacts can be opaque. In
a-Si cells one uses conducting oxides solar cell a grid structure is used. If the grid design is
good, the shadding is not more than 5%.
Semiconducting materials, in general, have a high refractive index. This implies,
unfortunately, a higher reflection coefficient for the incident light. In the case of Si, for example,
30% of the incident solar light is reflected and therefore does not contribute to electricity
production. It is, therefore, usual to use antireflection coatings on the solar cell. A simple anitreflective (AR) coating reduces the reflection by 12% and with poly-AR coatings; one over the
other, reflection can be further reduced. A two-layer system of magnesium fluoride and titanium
oxide, for example reduces the reflection losses to only 3%.
Figure 3.2 shows schematically the three main regions of a solar cell – a strongly n-doped
diffused emitter, the space charge region and the p-doped basis. Alight quantum of sufficient
energy falls on the upper surface of the solar cell, passes through the transparent emitter and
space charge region, and gets absorbed in the p-region. The absorption leads to the creation of
an electron-hole pair, the electron in the conduction band and hole in the valance band. Since the
electrons in the p-region are
in a minority, one denotes the electrons as minority charge carriers and holes as the majority
charge carriers. These electrons diffuse in the p-region till they come across the boundary of the
space charge region where the governing electric field accelerates the electrons and brings them
to the side of the emitter. This phenomenon, therefore, leads to charge separation, the medium of
separation being provided by the electric field. The assumption for this separation is that the
diffusion length of the electron is long enough to reach the boundary of the space charge region.
For a shorter diffusion length, a recombination would take place and the energy of the photon
goes waste.
Absorption of a photon in the n-region, on the other hand, again leads to the creation of
an electron-hole pair, but now the holes are in minority. If the diffusion length of the hole is
long enough to enable it to reach the boundary of the space charge region, if gets accelerated by
the electric field and reaches the p-base. It is, therefore, clear that due to the presence of the
space charge region in the p-n junction the electric and positive charges, created by the
absorption of solar radiation, are immediately separated.
A result of electron migration to the n-region and migration of holes into the p-region
there is an excess of electrons in the n-region and deficiency in the p-region. If now the p-region
and n-region are connected together through a conductor and the load, the generated voltage by
charge separation gives rise to current and power.
Depending upon the material solar cells archives an open-circuit voltage Voc of 0.5 to
1.2V. Voc varies only slightly with the irradiation intensity. Up to a certain limit, the current in
the solar cell is linearly proportional to the intensity. The usual parameter to characterize a solar
cell is the short-circuit current Isc.
The variation of Voc and Isc with irradiated solar intensity is shown in Figure 3.3.
The maximum power Pmax from a solar cell is proportional to the product Voc and Isc. The
proportionality constant is known as the fill factor, FF. The maximum efficiency of a solar cell
is given by
where Pin is the incident power. FF usually has values between 0.8 and 0.85 and it is adversely
affected by the series resistance Rs of the cell. The characteristics parameters of a solar cell are
measured under special conditions, namely an irradiation intensity of I kW/m2, normal incidence,
solar spectrum corresponding to AM 1.5 and a cell temperature of 25˚C.
In practice, several losses occur during power conversion:
a) Power decreases by about 0.5%/˚C for Si solar cells with increasing temperatures;
b) Oblique incidence leads to higher reflection loss; and
c) Low radiation intensity yields lower cell efficiency.
Mathematically, current in a solar cell is given by
where I is current in a solar cell (A), e is electronic charge (1.6 x 10-9 Coulombs), k is Boltzmann
Constant (1.38 x 10-23 J/K), T is temperature (K), I0 is dark current (A) and Isc is short-circuit
current (A).
3.2 Characteristic Curve of a Solar Cell
When the solar cell is not irradiated it is like a large surface of a semiconductor diode. The I-V
curve of this device exactly follows the I-V curve of a diode. When a solar cell is irradiated,
then a constant current flows through the circuit in a direction opposite to that of diode current.
The I-V curve is drawn on the downward side (Figure 3.4).
Normally, the I-V curve of a solar cell is used independent of the diode curve. Therefore,
the I-V curve is drawn in the 4th quadrant. For I-V characteristics of a solar cell, three important
points are determined,
a) Open-circuit voltage is measured across the cell when there is no load in the ciruit. It is
more or less independent of the incident intensity of solar radiation.
b) The short-circuit current Isc is proportional to the intensity of solar radiation (Figure 3.3).
c) The point corresponding to maximum power (Maximum Power Pint, MPP) of the solar
cell is found by calculating the product of current and voltage for every point. For the
case of open-circuit voltage and short-circuit current, the solar cell does not produce any
power. Only at a particular point is the power maximum. This point is called MPP
(Figure 3.5). In practice it is desirable to connect such loads to the PV modules so that
they work always around the MPP. The energy transfer from a solar cell to the user is
maximum when the user’s resistance is equivalent to the internal resistance of cell; i.e.
RL = R S
with the help of a DC-DC converter called the Maximum Power Tractor (MPPT), it is possible to
achieve the load matching.
3.3 Measurement of Characteristic Curve
The characteristics curve of a solar cell can be measured with the help of a simple circuit as
shown in Figure 3.6. By pressing the key S, and using the variable resistance, R, one can
measure the current and voltage to get a few points for the I-V curve. When S is open, one
measures the open circuit voltage Voc and by pressing S and putting the load to zero one
measures the ISC. One should use a voltmeter of very high resistance and ammeter of very low
resistance; otherwise the measurements will be incorrect.
Measurements of solar radiation are always difficult. The equipment that measures the
solar radiation over the entire spectral region is very costly and needs some expertise for it’s
operation. For simple measurements in commercial use, one can use a calibrated solar cell, the
open-circuit voltage of which corresponds to a particular incident solar radiation. The measured
voltage can then be converted either into a absolute value, W/m2, or in a relative value (%).
3.4 Solar Irradiation and the Power of a Solar cell
As we have already seen, for a normal solar cell, the open-circuit voltage changes very little, but
the short-circuit current varies almost linearly with the solar intensity. Therefore, the power of
the solar cell also changes with solar radiation. The change in the power is almost linear with the
solar radiation (Figure 3.7). At a constant temperature, the efficiency of a solar cell varies in a
logarithmic fashion with the solar intensity. With increasing temperature, the efficiency of a
solar cell goes down.
3.5 Effect of Temprature
Under continued irradiation, the temperature of a solar cell increases and the power decreases
with increasing solar cell temperature. The open-circuit voltage decreases by a value of
approximately 3mV/K for each degree centigrade rise in temperature. A solar cell with a V oc of
0.6V at 25˚C reaches a value of 0.45V at 75˚C. This is a considerable reduction, which can be
about 25% in practice.
The short-circuit current increases at increasing temperatures at a rate of about 0.1%/K.
A solar cell with short circuit current of 2.0 A at 25˚C reaches a value of 2.1 A at 75˚C. This
means an increase of about 5% (Figure 3.8). The reduction in voltage is much greater than the
corresponding increase in current. This affects the power, which decreases at a rate of about
0.44% per degree rise in
3.6 Effect of Area
The size of the solar cell determines its power as well. The relationship is linear. A solar cell of
100 cm2 has a short-circuit current (e.g. 2A) which is double that of a solar cell of area 50 cm 2
(e.g. 1A). The open circuit voltage is not dependent upon the area of solar cell. The largest solar
cells have an area of 100 cm2 and they can have a short-circuit current of about 3A at a cell
temperature of 25˚C. this corresponds to a peak power (Wp) of 1.5 Wp.
3.7 Effect of spectral Sensitivity
For an expert, an important effect is the spectral sensitivity of a solar cell. It can be determined
for the measured values of the short-circuit current as a function of the wave length (λ) of the
radiation, where it is constant. Spectral sensitivity is represented by the symbol S(λ). For an Sisolar cell, this spectral sensitivity is shown in Figure 3.9. Solar cells made from other materials
have entirely different spectral sensitivities. This allows interesting combinations of solar cells
fabricated from different materials. Solar cells in pocket calculators, which need to work in
artificial light, have entirely different spectral sensitivity to solar cells which are irradiated by the
sun. The data on a solar cell is therefore meaningful only if the light source with its spectral
distribution is defined. Expert measurements of spectral distribution need expensive measuring
equipment.
3.8 Ageing Effects
Like most other devices, ageing of a solar cell also affects its power. For commercially available
mono or poly-crystalline silicon solar cells, the problem of ageing is minor. Solar cells which
are properly encapsulated have a very long life, and power from them does not reduce in any
significant manner. The effect of ageing is more severe in amorphous Si-solar cells.
3.9 Equivalent Circuit of a Solar Cell
A solar cell can be studied in detail by drawing its equivalent circuit. The solar cell is in fact a
combination of a constant current source and a diode. A load can be connected at the terminals
(Figure 3.10). the current which flows through the load is
IL = Isc - ID
In practice, this simplified circuit is not sufficient because there are two more important
parameters that need consideration:
Rp: Parallel to the constant current source, there is a loss resistance Rp. theis resistance
should be as large as possible to limit the power losses. Rp is essentially influenced by crystal
disorders, doping defects and other (Figure 3.11).
Rs: The series resistance is caused due to a number of individual resistance (resistance of
the semiconducting material, the metal semiconductor contacts and the electrical contacts).
Measurements show that for commercial, monocrystalline solar cells of area between 20 and 80
cm2, series resistance could be between 80 mΩ and 400 mΩ and the parallel resistance R p
between 400 Ω and 600 Ω, depending upon the cell type.
3.10 Fill Factor of a solar Cell
The influence of the series and parallel resistance on the characteristic curve and power of a solar
cell is described with the help of a fill factor, FF. The fill factor of a solar cell is the product of
current Im and voltage Vm at a point of maximum power divided by the product of short circuit
current Isc and open circuit voltage Voc, i.e.
FF = (Im. Vm)/(Isc.Voc)
The maximum fill factor is obtained for the smallest series resistance, Rs and the biggest parallel
reasistance, Rp (Fig. 3.12).
3.11 The efficiency
The power of a solar cell and, therefore, its efficiency is limited by a number of factors.
Reflection losses
A part of solar radiation intercepted by a solar cell is reflected from top surface and it lost.
Untreated normal silicon reflects 36% or more of the solar radiation. By treating the top surface
of a solar cell (texturization) and applying special anti-reflective coatings, one can reduce the
reflection losses to about 4%.
Unabsorbed solar radiation
A part of the solar radiation has very low energy photons that are not capable of raising electrons
in the conduction band (power loss 24%). This part of solar radiation is absorbed in the solar cell
and the cell temperature increases, resulting in further losses.
Excessively strong radiation
When the radiation is much more energetic than what is needed to raise electrons to the
conduction band, the balance gets lost in the form of heat (30% power loss).
The above two factors are responsible for maximum losses form a solar cell. The efficiency is,
however, reduced due to other factors also. These are:
Collection Efficiency
Not all charge carriers are able to reach the p/n layer. Many electrons combine with holes before
that and do not contribute to the conduction. The ratio of charge carriers that are able to reach p/n
layer, to the total number of produced charge carriers is known as collection efficiency. It can be
influenced during the manufacturing process itself.
Series and parallel losses
Series resistance (Rs) and parallel resistance (Rp) also limit the efficiency. These can also be
controlled during manufacturing.
Self-shadowing
The conduction stripes on the top surface of a solar cell reduce the area available for interception
of solar radiation.
Temperature losses
Due to increased temperature the efficiency of a solar cell reduces. With increasing temperature,
the balance electrons move with more velocity. At a temperature of abut 300˚C, the energy band
gap losses its function.
All these above factors result in an efficiency of 15 to 25%. The proportion of different types of
losses in a solar cell are represented in Figure 3.13.