Solar Cells

A solar cell is a semiconductor device that converts sunlight energy into electrical
energy directly without going through any intermediate energy conversion steps.
It is a fundamental block of solar photovoltaic (PV) technology. Many solar cells
are connected together to form solar PV modules. Several solar PV modules are
connected together to make PV array in small power applications as well as in big
power plant applications. Therefore, it is important to understand how does a solar
cell work, how to identify a solar cells, what are its parameters, how much power a
solar cell can generate, how the generated power depends on sunlight falling on it,
etc. This chapter focuses on providing the fundamental understanding of solar cells,
its parameters and how the variation in parameter and ambient conditions affect the
performance of solar cells of different technologies.

3.1 The electricity is conventionally generated by using coal energy, hydro energy or
nuclear energy. One of the most common ways of generating electricity is using coal
How Solar Cells are Better energy. In India, about 55% of electricity is generated using coal energy. A typical
than any Conventional coal power plant, shown in Figure 3.1, involves several steps before the energy
Sources of Electricity? of coal gets converted into useful energy form, the electricity. The power plant
process starts from the burning of coal and ends with the generation of electricity
by generators. In the whole process, only fraction of coal energy is eventually used
in running our appliances. Most of the coal energy is wasted in the conversion
process and transmission of electricity from power plant to our homes. Other than
the waste of energy, there is also environmental pollution caused of coal based
power plants. Also, coal as a source of energy is not available in infinite quantity,
which means that sooner or later we will run in the shortage of coal. Considering
these facts, one must look for alternate source of energy.
One of the modern ways of generating electricity is using solar cells or solar
Photovoltaics (PV); a technology that converts sunlight into electricity. Solar cell
and its technology have drawn lots of attention of engineers, researchers, industries
In conventional energy conversion and governments in recent times. Therefore, in this chapter, we will focus on solar
process, several steps are involved in
converting fuel energy (like coal, diesel, cells. So, let us see what is a solar cell? How is current generated by it and what
hydro energy, etc.) into electricity. are its applications?

39
40 Solar Photovoltaic Technology and Systems: A Manual for Technicians, Trainers and Engineers

FIGURE 3.1
A typical coal power plant generating
electricity using coal.

3.2 Solar cell is a semiconductor device which directly converts sunlight into electricity.
Solar cell converts sunlight into electricity by photovoltaic effect. Hence, they are
What is a Solar Cell? also called photovoltaic cell. A typical commercial silicon solar cell is shown in
Figure 3.2.
A solar cell converts sunlight into
electricity directly, without any
intermediate conversion steps.

FIGURE 3.2
Front side and back side of a typical
solar cell.

The solar cell generates current and voltage at its terminals when sunlight falls on
it. The amount of electricity generated by a solar cell depends on the amount of
sunlight incident on it. The electricity generated by solar cell depends upon the
intensity (amount) of light, the area of a cell and the angle at which light falls on it.
The higher is the intensity of sunlight, the more is the electricity generated by solar
cell. If area of a solar cell is increased, the current generated by it increases. The
power generated by the solar cell is optimum when sunlight falling is perpendicular
to the front side of solar cell.
In common, all solar cells, irrespective of the technology and material used have
only two terminals (positive and negative terminals) as output. Typically solar cells
have front contact at the top, emitter-base junction or p-n junction in the middle
and the back contact at the bottom. At the emitter-base junction, the separation
A solar cell is a two terminal power of negative and positive charge takes place. Electricity is supplied to a load by
generating device, one is a positive
(anode) terminal and another is a connecting its terminals to the front and back contacts of a solar cell or solar module
negative terminal (cathode). or solar panel as shown in Figure 3.3.

FIGURE 3.3
Solar cell converting day light into
electricity; front and back terminals
connected to torch bulb, (a) switch
is OFF and (b) switch is ON.
 Chapter 3: Solar Cells 41

3.3 The sunlight falling on the earth is basically the bundles of photons or bundles
of small energy. Each photon in a bundle has a finite amount of energy. In solar
How Solar Cell Generates spectrum, there are many photons of different energy. For generation of electricity,
Electricity? photons must be absorbed by solar cell. The absorption of photon depends upon
the energy of photon and the band-gap energy of semiconductor material of a solar
cell. The photon energy and the band-gap energy of semiconductor is expressed in
terms of Electron-volt (eV). The eV is a unit of energy.
So, the working of a solar cell can be explained as follows:
1. Photons in the sunlight falling on the solar cell’s front face are absorbed by
semiconducting materials.
2. Free electron-hole pairs are generated. Electrons are considered as negative
charge and holes are considered as positive charge. When solar cell is
connected to a load, electron and holes near the junction are separated from
each other. The holes are collected at positive terminal (anode) and electrons
at negative terminal (cathode). Electric potential is built at the terminals due
to the separation of negative and positive charges. Due to the difference
between the electric potentials at the terminals we get voltage across the
terminals.
3. Voltage developed at the terminals of a solar cell is used to drive the current
in the circuit. The current in the circuit will be direct current or DC current.
So, the solar cell with day light falling on it can directly drive DC electrical
appliances. But, the amount of electricity generated is proportional to the amount
of light falling. So, the amount of electricity generated throughout the day is not
constant. The current generated also depends on several other parameters. In the
following section, we will now see why the generated current is not constant?

3.4 A solar cell converts the sunlight into electricity. How nicely a solar cell does
the conversion of sunlight into electricity is determined the parameters of solar
Parameters of Solar Cells cells. There are several parameters of solar cells that determine the effectiveness
of sunlight to electricity conversion. The list of solar cell parameters is following:
 Short circuit current (Isc),
 Open circuit voltage (Voc) and
 Maximum power point
 Current at maximum power point (Im)
 Voltage at maximum power point (Vm)
 Fill factor (FF)
 Efficiency (h),
These parameters can be best understood by Current-Voltage curve (I -V curve) of
a solar cell. The representation of I -V curve is plotted in Figure 3.4. The Y-axis is
normally plotted as current axis and X-axis is plotted as voltage axis.

FIGURE 3.4
Schematic of solar cell I-V curve and
its parameters.
42 Solar Photovoltaic Technology and Systems: A Manual for Technicians, Trainers and Engineers

Using Figure 3.4, the cell parameters are defined here. Normally, the value of the
cell parameters are given by a manufacturer or scientist at standard test conditions
(STC) which is corresponding to 1000 W/m2 of input solar radiation and 25°C cell
operating temperature.
 Short circuit current (Isc): It is the maximum current a solar cell can
produce. The higher the Isc, better is the cell. It is measured in Ampere (A)
or milli-ampere (mA). The value of this maximum current depends on cell
technology, cell area, amount of solar radiation falling on cell, angle of
cell, etc. Many times, people are given current density rather than current.
The current density is obtained by dividing Isc by the area of solar cell (A).
The current density is normally referred by symbol, ‘J’, therefore, the short
circuit current density, Jsc is given by Isc/A.
 Open circuit voltage (Voc): It is the maximum voltage that a solar cell
produce. The higher the Voc, the better is the cell. It is measured in volts
(V) or sometimes milli-volts (mV). The value of this maximum open circuit
voltage mainly depends on cell technology and operating temperature.
 Maximum power point (Pm or Pmax): It is the maximum power that a solar
cell produces under STC. The higher the Pm, the better is the cell. It is
given in terms of watt (W). Since it is maximum power or peak power, it
is sometimes also referred as Wpeak or Wp. A solar cell can operate at many
current and voltage combinations. But a solar cell will produce maximum
power only when operating at certain current and voltage. This maximum
power point is denoted in Figure 3.4 as Pm. Normally, the maximum power
point for a I-V curve of solar cells occurs at the ‘knee’ or ‘bend’ of the
curve. In terms of expression Pm is given as:
Pm or Pmax = Im  Vm
 Current at maximum power point (Im): This is the current which solar cell
will produce when operating at maximum power point. The Im will always
be lower than Isc. It is given in terms of ampere (A) or milli-ampere (mA).
 Voltage at maximum power point (Vm): This is the voltage which solar cell
will produce when operating at maximum power point. The Vm will always
be lower than Voc. It is given in terms of volt (V) or milli-volt (mV).
 Fill factor (FF): As the name suggests, FF is the ratio of the areas covered
by Im-Vm rectangle with the area covered by Isc-Voc rectangle (both shown
by dotted line in Figure 3.4), whose equation is given below. It indicates the
square-ness of I-V curve. The higher the FF, the better is the cell. The FF
of a cell is given in terms of percentage (%). Cell with squarer I-V curve
is a better cell.
I ¥ Vm
FF = m
I sc ¥ Voc

Pm
or FF =
I sc ¥ Voc
Here the expression for Pmax or Pm can alternatively be written in terms of
Isc, Voc and FF as:
Pm = Isc  Voc  FF
 Chapter 3: Solar Cells 43

 Efficiency (h): The efficiency of a solar cell is defined as the maximum

output power (Pm or Pmax) divided by the input power (Pin). The efficiency of
a cell is given in terms of percentage (%), which means that this percentage
of radiation input power is converted into electrical power. Pin for STC is
considered as 1000 W/m2. This input power is power density (power divided
by area), therefore, in order to calculate the efficiency using Pin at STC, we
must multiply by solar cell area. Thus, efficiency can be written as:
A solar cell performance depends on Pm I ¥ Voc ¥ FF
its parameters or the cell parameters h= = sc
and determines the performance Pin Pin ¥ A
of a solar cell under the sunlight,
particularly the amount of power it will Let us now see what the possible values of solar cell parameters and how the values
produce in a given condition. that depend on the various solar cell technologies.

WORKSHEET 3.1: Fill below in Table 3.1, the various solar cell parameters and their units by which
they are presented.

TABLE 3.1 Solar Cell Parameters and their Units

S. No. Name of parameter Unit of parameter

1
2
3
4
5
6
7

EXAMPLE 3.1 The current density of a solar cell having an area of 100 cm2 at Standard Test
Condition (STC) is given as 35 mA/cm2. Find out the output current of the solar cell.
Solution First, we write the formula for current density of a solar cell given by
I sc
Current density (J sc ) = (mA/cm 2 )
A
where,
Jsc = Current density (mA/cm2)
Isc = Output current (mA)
A = Area (cm2)
Given that, Jsc = 35 mA/cm2
So, the expression for solar cell current can be written as:
Output current (Isc) = Jsc  A (mA)
Now, given that area of solar cell is 100 cm2, then
Output current (Isc) = 35 mA/cm2  100 cm2 = 3500 mA or 3.5 A
Similarly, we calculate output current for different values of solar cell area in the
Table 3.9.
EXAMPLE 3.2 A solar cell gives a current of 0.6 A and voltage of 0.5 V at maximum power point.
What is the maximum power point of the solar cell?
44 Solar Photovoltaic Technology and Systems: A Manual for Technicians, Trainers and Engineers

Solution First, we write formula for the maximum power point of a solar cell, given by
Pm or Pmax = Im  Vm
Given that, Im = 0.6 A
Vm = 0.5 V
Therefore, the maximum power point, Pm = 0.6 A  0.5 V = 0.3 W
EXAMPLE 3.3 A solar cell having an area of 100 cm2 gives 3.1 A current at maximum power
point and 0.5 V at maximum power point at STC. The cell gives 3.5 A short circuit
current and 0.6 V open circuit voltage. What is the maximum power point of the
solar cell? Also, find out the efficiency of the cell.
Solution First, we write the formula for the maximum power point of a solar cell, given by
Pm or Pmax = Im  Vm
Given that,
Isc = 3.5 A
Im = 3.1 A
Voc = 0.6 V
Vm = 0.5 V
Maximum power point, Pm = 3.1 A  0.5 V = 1.55 W
Now, we write the formula for efficiency of a solar cell given by
Pmax
h=
Pin ¥ A
where,
h = Efficiency in per cent (%)
Pmax = Output power in watt (W)
Pin = Light input power per unit area in watt/square meter (W/m2)
A = Solar cell area in square meter (m2)
h= ?
We know, Pm = 1.55 W and at STC, Pin = 1000 W/m2
First, we convert the unit of area from square centimetre (cm2) to square metre
(m ) by dividing area in cm2 by 10000.
2

Here, A = 100 cm2 = 100  10–4 m2 = 0.01 m2

Now, putting the number we can calculate the efficiency of the cell.
Pmax 1.55 watt
h= = ¥ 100 = 15.5%
Pin ¥ A 1000 W/m 2 ¥ 0.01 m 2
From an I-V curve of a solar cell, all
solar cell parameters can be derived. Thus, efficiency of the solar cell is 15.5%.
EXAMPLE 3.4 Refer the characteristic curve (Figure 3.5) and find out the Fill Factor for the solar
cell.

FIGURE 3.5
Figure for Example 3.4.
 Chapter 3: Solar Cells 45

Solution Short circuit current (Isc) = 0.45 A

Open circuit voltage (Voc) = 0.7 V
Current at maximum power point (Im) = 0.40 A
Voltage at maximum power point (Vm) = 0.5 V
Now,
Maximum power point, Pm or Pmax = Im  Vm = 0.40  0.5 = 0.2 W
I m ¥ Vm
Fill Factor, FF =
I sc ¥ Voc
Pm 0.2
or FF = = ¥ 100 = 63.49%
I sc ¥ Voc 0.45 ¥ 0.7
Note: In order to represent the FF value in ‘percentage’, multiply by 100.
EXAMPLE 3.5 A solar cell having an area of 25 cm2 gives a current of 0.85 A and voltage
0.55 V at maximum power point. The short circuit current is 0.9 A and open circuit
voltage is 0.65 V. What is the Fill Factor, maximum power point and efficiency of
the solar cell? Consider STC.
Solution Given, Short circuit current (Isc) = 0.9 A
Open circuit voltage (Voc) = 0.65 V
Current at max power point (Im) = 0.85 A
Voltage at maximum power point (Vm) = 0.55 V
Light input power (W/m2) = 1000 W/m2
Area = A = 25 cm2 = 25  10–4 m2 = 0.0025 m2
Now,
Maximum power point, Pm or Pmax = Im  Vm = 0.85  0.55 = 0.4675 W
I m ¥ Vm
Fill Factor, FF =
I sc ¥ Voc
Pm 0.4675
or FF = = ¥ 100 = 79.91%
I sc ¥ Voc 0.9 ¥ 0.65
Pmax 0.4675
Efficiency (h) = = ¥ 100 = 18.7%
Pin ¥ A 1000 ¥ 0.0025
Note: In order to represent the FF and efficiency values in ‘percentage’, multiply
by 100 in both cases.)
EXAMPLE 3.6 A solar cell having Fill Factor (FF) 60% gives 2.5 A current at maximum power
point at STC. The cell gives 3 A short circuit current and 0.5 V open circuit voltage.
What is the voltage at maximum power point of the solar cell?
Solution Given that,
Isc = 3 A
Im = 2.5 A
Voc = 0.5 V
Vm = ?
FF = 60%
First, we write formula for Fill Factor of a solar cell given by
I ¥ Vm
Fill Factor (FF) = m
I sc ¥ Voc