FACULTY OF ENGINEERING
LAB SHEET
EEN2056 PHYSICAL ELECTRONICS
TRIMESTER 2 (2010-2011)
PHE 2 – Characteristics of a Solar Cell
*Note: On-the-spot evaluation may be carried out during or at the end of the experiment. Students
are advised to read through this lab sheet before doing experiment. Your performance, teamwork
effort, and learning attitude will count towards the marks.
EEN2056 Physical Electronics
Experiment PHE2
Experiment PHE2: Characteristics of a Solar Cell
Introduction
Solar cell directly converts sunlight to electrical energy and is the most promising of all
alternative resources known to date. Solar cells are inexhaustible in power generation with no fuel
except sunlight for nearly 30 years of lifetime guarantee. Solar cells are basically large area pn
junction photo diode. Basing on the junction structure, solar cells are categorized into two types
(a) homogeneous junction Si, Ge or GaAs solar cells (b) heterogeneous junction solar cells are
based on II-VI compounds like CdS/CdTe, CdS/CIGS etc. Homogeneous solar cells may be
either single crystal or poly crystalline material. The wafers are suitably doped to get ‘n’ or ‘p’
type materials. A suitable ohmic contact from the top of the solar cell is formed. For the n-type,
aluminium is one of the suitable materials and for p-type silver is the apt one.
The current-voltage characteristics of a solar cell are to be measured at different light
intensities, the distance between the light source and the solar cell being are to be varied. The
dependence of no-load voltage and short-circuit current on temperature is to be determined. As
additional tasks, the series resistance and ideality factor (of the diode) are to be figured out.
Equipment:
Solar battery, 4 cells, 2.5x5 cm
Thermopile, molltype
Universal measuring amplifier
Rheostat, 330 Ohm, 1.0 A
Lamp socket E27, mains conn.
Filament lamp, 220 V/120 W, w. ref l.
Hot-/Cold air blower, 1000 W
Meter scale, demo, l = 1000 mm
Tripod base
Right angle clamp
Plate holder
Universal clamp
G-clamp
Glass pane, 150x100x4 mm, 2 off
Digital multimeter
Lab thermometer, -10...+100 oC
Connecting cord, l = 500 mm, red
Connecting cord, l = 500 mm, blue
1
1
1
1
1
1
1
1
2
2
1
1
2
1
2
1
3
2
Objectives:
1. To collect the light intensity with the thermopile at various distances from the light source.
2. To measure the short-circuit current and open-circuit (no-load) voltage at various distances
from the light source.
3. To test the dependence of open-circuit (Voc) voltage, and short-circuit current (Isc) on
temperature.
4. To analyse the current-voltage characteristics (I-V) at different light intensities.
5. To compare the current-voltage (I-V) characteristics under different operating conditions:
cooling the equipment with a blower, no cooling, shining the light through a glass plate.
6. To calculate the Series Resistance of the solar cell.
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EEN2056 Physical Electronics
Experiment PHE2
7. To determine the Ideality factor of the Solar cell. (Optional)
Fig. 1: Experimental set-up for determining characteristic curves.
Fig. 2: Circuit for measuring the current-voltage characteristic.
Theory
Pure silicon is deliberately ‘impurified’ (doped) with tri- and pentavalent impurity atoms
to make a p- or n-type semi-conductor. If we put a p-and n-type crystal together we get a junction
(pn-junction, Fig. 3) whose electrical properties determine the performance of the solar cell.
In equilibrium (with no external voltage) the Fermi characteristic energy level E F will be
the same throughout. Because of the difference in the concentrations of electrons and holes in the
p- and n-regions, electrons diffuse into the p-region and holes into the n-region. The immobile
impurity atoms create a space charge-limited current region; the diffusion current and the field
current offset one another in equilibrium. The diffusion potential UD in the pn-junction depends
on the amount of doping and corresponds to the original difference between the Fermi energy
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EEN2056 Physical Electronics
Experiment PHE2
levels of the separate p- and n-regions. The distance between the valence band and the conduction
band in silicon at room temperature is E = 1.1 eV. For silicon, the diffusion potential is UD = 0.5
to 0.7 V.
If light falls on the pn-junction, the photons create electron-hole pairs separated by the
space charge. The electrons are drawn into the n-region and the holes into the p-region. Photons
are absorbed not only in the pn-junction but also in the p-layer above it. The electrons produced
are minority carriers in those areas: their concentration is greatly reduced by recombination and
with it their efficiency. The p-layer must therefore be sufficiently thin for the electrons of
diffusion length LE to enter the n-layer. LE >> t, where t = thickness of p-layer.
Fig. 3: pn-junction in the energy-band diagram-acceptors, + donors, UD is the diffusion
potential, EF is the Fermi characteristic energy level, and e is the elementary charge.
Fig. 4: Construction of a silicon solar cell.
If g is the number of electron-hole pairs produced per unit area and of a voltage U is applied
across the pn-junction, a stream of electrons and holes of density
i = e . (exp eU/kT – 1)…………………………………. (1)
(n0 De t/L2e + p0 Dh/Lh) – e . g
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Experiment PHE2
is produced, where e is the elementary charge, k is Boltzmann’s constant, T is the temperature, L
is the diffusion length of electrons and holes, D is the diffusion constant for electrons and holes,
n0 and p0 are equilibrium concentrations of the minority carriers.
Fig. 5: Light intensity J at distances s normal to the light source.
The short-circuit current density (U = 0)
is = – e . g…………………….. (2)
is proportional to the intensity of the incident light at fixed temperature.
g becomes very slightly greater (less than 0.01 %/K) as the temperature rises.
The voltage U can become as high as the diffusion potential UD but no higher. As the temperature
rises the no-load voltage decreases typically by –2.3 mV/K, since the equilibrium concentrations
n0and p0 increase with the temperature:
n0 = exp (– E/2kT)
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Experiment PHE2
Experimental Procedures:
Task 1:
Measure the light intensity with the thermopile and digital multimeter/amplifier with the
equipment at different distances from the light source e.g. 10 points beginning at 200cm till
50cm. The inlet aperture marks the position of the thermopile. The distance between the lamp and
the thermopile should be at least 50 cm, since the angular aperture of the thermopile is only 20
degree. The light intensity is varied by varying the distance between the light source and the solar
cell. To determine the intensity with the thermopile it is assumed that all the light entering the
aperture (dia. 2.5 cm) reaches the measuring surface. (see Fig.5 for reference). Collect the data
and plot the graph for report.
Notes: The maximum output voltage of the amplifier is 10 V in case if used. The sensitivity is
0.16 mv/mW. By extrapolating the straight line one can determine the intensity at distances s
cm.
Task 2:
The solar cell measures the diffused light as well as the direct light from the lamp. As the
lamp has a slim light cone of approx. 30 degree, the diffused light chiefly arises as a result of
reflection from the bench top, and can be suppressed by covering the bench with a black cloth.
Using the measured values in Fig. 5 one can obtain the relationship between the light intensity
and the short-circuit current and open-circuit voltage measured at various distances away from the
light source. Collect the Ligh Intensity (J)-Short Circuit Current (Isc)-Open circuit voltage (Voc)
data and later prepare graph for report. (see Fig. 6 for reference).
Task 3:
The open circuit voltage (Voc) and the short-circuit current (Isc) of the solar cell depend
on temperature. To demonstrate the temperature effect, blow hot air (no air, medium hot, high hot
air and screening with glass plate: 4 categories) over the solar cell.
A glass plate which absorbs light in the infrared region can be used to reduce a rise in temperature
of the solar battery.
To measure the temperature, put the thermocouple tip along any side of the panel and
insert the socket to the multimeter. Do not touch the cell as its thin p-layer can easily be damaged.
Now, measure Voc and Isc individually and tabulate the data.
Task 4 & 5:
Please refer to figure 2 for circuit setup for I-V measurement. Change the voltage with
the potentiometer (also called voltage divider) and record the current to get the I-V
characteristics. Repeat the I-V characteristics for various light intensities and temperatures from
the readings of the Voltmeter and Ammeter. Make sure to put at High Impedance and
amplification factor of 105. The differences are in the distance and the temperature variation by
external factors (like hot blowers). Ask the supervisor if any confusion happens.
Notes: The short circuit current is proportional to the light intensity (see 2). If the distance
between lamp and solar cell exceeds 50 cm, the temperature rise caused by radiation can be
disregarded in comparison with that caused by the hot air.
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Experiment PHE2
Fig. 6: Short-circuit current Is and open
circuit (no-load) voltage Uo as a function of
the light intensity J.
Fig. 7: Current-voltage
characteristic at different
light intensities J.
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EEN2056 Physical Electronics
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Fig. 8: Current-voltage characteristics of
the solar battery
a) with blower cooling
b) with no blower cooling
c) when screened with a glass plate.
When measuring the effect of temperature on U0 and Is, the temperature distribution over the hot
air area must be taken into account. The measurements can provide only a rough order of
magnitude of this.
Fig. 9: Spectrum of the sun (T approx. 5800 K) and of an incandescent lamp (T approx. 2000 K),
and the spectral sensitivity of the silicon solar cell.
The change in short-circuit current with the temperature cannot be measured.
The maximum power output is at the turning points on the curves (joined by the broken line) at
which the load resistor has the same value as the internal resistance Ri of the solar battery. The
internal resistance decreases with increasing light intensity.
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EEN2056 Physical Electronics
Experiment PHE2
If we compare the maximum power output with the incident power, we obtain an efficiency of
approx. 6% (area of solar battery 50 cm2).
Fig. 8 shows the effect of the various ‘operating modes’ as for reference.
Task 6:
To measure the series resistance, connect the solar cell and the measurement apparatus as
shown in figure 10. This experiment has to be carried out under Dark condition (you have to
cover the solar cell area with dark cloth as provided).
Step 1: Supply the voltage to the cell from the power supply until the reading on the digital
ammeter is equal or close to the Short-circuit current Isc measured in previous
experiment.
Step 2: Record the applied voltage Va. A higher voltage that the open circuit voltage (Voc) is
necessary to obtain the current value equal to Isc. This is because of the additional
voltage drop across the series resistance (Rs). This resistance can therefore be determined
by the difference between those two voltages:
Va - Voc= Rs x Isc
Therefore,
Rs 
Va  Voc
Isc
A
+
Solar
Cell
V
_
_
_
Fig. 10: Series Resistance measurement setup
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EEN2056 Physical Electronics
Experiment PHE2
Task 7: Experiment on Ideality factor of Solar cell (optional)
To determine the ideality factor of a given solar cell illuminated at different intensities.
The wires are connected to the voltmeter and ammeter to measure current when the solar cell is
exposed to the illuminating source. Ideality factor indicates the formation of an ideal diode (as
varies in between 1 - 2). The ideality factor is obtained from the equation,
qV
 nkT

  I sc …………….(1)
I  Io 
e

1




Under load condition, current will flow; the total current is given by,
 Rs I )
 q (VnkT
  V  Rs I 
 ………..(2)
I  I sc  I o  e
 1  

  Rsh 
Where ‘Io’ is the reverse saturation current, ‘n’ is the ideality factor, ‘Isc’ is the photocurrent, Rs
and Rsh are the series and shunt resistance of the solar cell, ‘k’ is the Boltzmann constant
respectively. Hence from the above equation (1), the reverse saturation current ‘Io’ and the
ideality factor ‘n’ can be determined.
1. Construct the circuit shown in Fig 11 for measuring the ideality factor of a solar cell.
2. The solar cell is connected in series with a resistance box (variable resistance or voltage
divider) and an ammeter. A voltmeter is connected across the solar cell.
3. Cover the whole cell with a black cloth to prevent the external light / radiations.
4. Illuminate the light source to the solar cell. Maintain the solar cell and light source in the
same height. Measure the intensity of the light by intensity meter or LUX meter
(mW/cm2 or LUX or Lumens).
5. By varying the load resistance to maximum, record the open circuit voltage V oc for
different intensities ranging from 10 to 100 mW/cm2 insteps of 10 mW/cm2.
6. Short circuit the solar cell (fix the zero resistance in the resistance box) and measure the
short circuit current, Isc at different light intensities ranging from 10 to 100 mW/cm2
insteps of 10 mW/cm2.
7. Tabulate the values of Voc, Isc and intensity and estimate the ideality factor and reverse
saturation current form the graph.
8. Plot the graph between Voc and ln Isc. Estimate the ideality factor ‘n’ from the slope of
q
,
nkT
q
n
,
kT  slope
the curve. slope 

9. Extrapolate the IV curve, where it cuts the y-axis is the reverse saturation current Io.
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EEN2056 Physical Electronics
Experiment PHE2
A
+
Solar
Cell
V
R
Fig 11. Setup for Ideality factor experiment
S.No
1
2
:
9
10
_
_
_
Table on data exploration of Ideality factor
Intensity
Open circuit
Short circuit ln Isc
Voltage Voc
current Isc
(mW/Cm2)
(V)
(mA)
10
20
:
90
100
Cautions: Never put the black cloths over the lamp. Put the thermocouple tip any side of the solar
panel. Double check the connections before powering on. Please do not handle any of the
apparatus roughly and the way that differs from this lab sheet. Any questions should immediately
be asked to the instructor/lecturer.
References:
1. Solar Cells: Operating Principles, Technology, and System Applications (Prentice-Hall
series in solid state physical electronics) by Martin A. Green; Publisher: Prentice Hall
(October, 1981) ISBN: 0138222703.
2. Any other “Semiconductor Devices” related books in the MMU library could be good
references.
NOTE on Report Writing:
Students should submit their computer composed report within 7 days of performing the
experiment to the same laboratory. Report should include clear graphical representation of the
results obtained. Moreover, logical explanations/discussions on the results are to be described for
each experiment. Personal comments regarding the topic/experiment are also welcome based on
the practical experience. Please refer to all the graphs in this sheet for your own one.
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EEN2056 Physical Electronics
Experiment PHE2
Marking Scheme
Lab
(10%)
Assessment Components
Hands-On & Efforts (2%)
On the Spot Evaluation
(2%)
Lab Report
(6%)
Details
The hands-on capability of the students and their efforts during the
lab sessions will be assessed.
The students will be evaluated on the spot based on the lab
experiments and the observations on the solar cell characteristics.
Each student will have to submit his/her lab final report within 7
days of performing the lab experiment. The report should cover the
followings:
1. Introduction, which includes background information on
the working principle of solar cell and their structures.
2. Experimental section, which includes the general summary
of the lab experiment work.
3. Results and Discussions, which include the measured
results, analysis, and evaluations, with neat graphs/images
of the results and recorded data.
4. Conclusion, which includes a conclusion on the
experimental.
5. List of References, which includes all the technical
references cited throughout the entire lab report.
The report must have references taken from online scientific
journals (e.g. www.sciencedirect.com,
http://ieeexplore.ieee.org/xpl/periodicals.jsp,
http://www.aip.org/pubs/) and/or conference proceedings (e.g.
http://ieeexplore.ieee.org/xpl/conferences.jsp).
Format of references: The references to scientific journals and text
books should follow following standard format:
Examples:
[1] William K, Bunte E, Stiebig H, Knipp D, Influence of low
temperature thermal annealing on the performance of
microcrystalline silicon thin-film transistors, Journal of
Applied Physics, 2007, 101, p. 074503.
[2] Hodges DA, Jackson HG, Analysis and design of digital
integrated circuits, New York, McGraw-Hill Book Company,
1983, p. 76.
Reports must be typed and single-spaced, and adopt a 12-point
Times New Roman font for normal texts in the report.
Any student found plagiarizing their reports will have the
assessment marks for this component (6%) forfeited.
The lab report has to be submitted to the Electronics lab staff.
Please make sure you sign the student list for your submission. No
plagiarism is allowed. Though the electrical characteristics of the
measured sample from the same group can be similar, the report
write-up cannot be duplicated for group members. The individual
report has to be submitted within 7 days from the date of your lab
session. Late submission is strictly not allowed.
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