TECHNICAL PAPER # 69
UNDERSTANDING SOLAR CELLS
Dennis Elwell & Richard Komp
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703276-1800 * Fax:
Understanding Solar Cells
1990, Volunteers in Technical Assistance
UNDERSTANDING SOLAR CELLS
Volunteers Dennis Elwell and Richard Komp
Solar cells, also called photovoltaic (PV) cells, are a
source of small amounts of electricity.
They are rugged, dependable
devices for converting sunlight directly into electrical
energy. They have no
moving parts and a long working life.
maintenance costs are lower and reliability is much higher
for other power sources.
They can be used on any scale, from
powering a digital watch to running a multi-megawatt
for a public utility.
Because they are usually arranged in modular
panels, it is possible to start with a small system and
expand it as necessary without making the early panels
But because only small amounts of energy are converted by
cell, large-scale electrical requirements require large and
costly arrays of PV cells.
Thus, the main applications of PV
cells have been to supply relatively low demands.
may be considering long-term economics should also consider
selecting PV power helps to achieve a pollution-free
About 1 kilowatt (kW) of radiant energy falls on a square
(sq m) of the earth's tropics at midday.
If a solar panel has an
efficiency of 10%, then each square meter of cell array will
generate a peak of 100 W of electrical power.
A typical 10-W
panel, capable of keeping an automotive battery charged,
31 cm by 35 cm including the frame.
The idea of capturing solar energy in this way is not
copper oxide solar cell was discovered by Antoine Becquerel
1839 and the amorphous-selenium cell came into use for photographic
light meters in the 1890s.
In the 1930s, selenium cells
were used for power on a small scale in remote locations in
Serious development of photovoltaic technology
began, however, when silicon cells were developed and used
U.S. space program.
The first silicon solar cells were used in
the U.S. satellite Vanguard I in 1958.
Their cost was US$600 for
each watt of generating capacity.
It has now (1989) dropped to
less than $6/W for larger systems.
Solar cells are devices that absorb and convert radiant
from the sun directly into electrical energy.
They are made of
materials called semiconductors, which are crystalline
with an electrical conductivity between those of metals and
A thin wafer or sheet of the semiconductor is treated
with chemicals to produce a negative charge (free electrons)
one side and a positive charge (free protons) on the other.
(Virtually all commercial solar cells are made so that the
or top surface is negative.)
The point at which the positive and
negative sides meet is an electronic barrier known as a p-n
The cells convert sunlight into electricity in three major
1. The semiconductor
material absorbs the sunlight.
2. Free positive and
negative charges are generated and separated
different regions of the cell. The
creates a voltage
in the cell.
3. The separated
charges are transferred as electric current
electrical terminals to the intended application.
The processes work this way:
The energy of the incoming sunlight
causes electrons to cross the barrier and remain trapped on
front, or negative, side.
When contacts are made to the front and
back sides of the solar cell, a current flows through wires
devices connecting these contacts.
The current is proportional to
the intensity of the sunlight that falls on the cell.
or positive, electrical contact can be a continuous layer of
metal, but the front contact is made in the form of thin
to allow as much sunlight as possible to reach the back
The cell is usually covered by an anti-reflection coating
protective cover to allow cleaning.
A more detailed explanation
of how photovoltaic cells work is given in references 8 and
The structure of a solar cell is shown in Figure 1.
Until recently most solar cells were made from single
Crystals, usually 10 cm in diameter, are pulled
from ultra-pure molten silicon, then sliced and
process is both costly and wasteful of this expensive,
material. The p-n
junction is made by diffusing phosphorus (which
produces n-type material) into the front surface of a wafer
has been "doped" with boron to make it
p-type. Newer techniques
use technical-grade silicon cast into blocks, sawed into
and fabricated into cells using the same processes as used
single crystal material.
This process is far less expensive and
uses considerably less energy to produce the finished cell;
half of the today's large modules are made in this
approach, still in the pilot plant stage, involves pulling
a silicon thin ribbon that does not need cutting into
Many other new ideas are being explored with the general aim
producing an efficient, long-lived solar cell at lower cost.
Photovoltaic cells are also manufactured from thin films of
amorphous silicon, a glassy material with no regular crystal
this material has proved eminently suitable for
small, low-power uses, like solar pocket calculators,
silicon cells cannot yet be used for power generation panels
because they become less efficient after a period of
addition, their long-term stability is doubtful.
Solar cells should have a useful life of at least 10 years.
Solar cells have also been produced using combinations of
compounds to form the p-n junction.
These are called
heterojunction solar cells.
Copper sulfide/cadmium sulfide cells
are inexpensive but their output also tends to degrade too
alternative materials as copper indium selenide
offer the promise that a so-called thin-film heterojunction
cell can be developed.
Very efficient but very expensive solar
cells can be made from gallium arsenide.
They may be marketed as
the active components of devices that focus the solar
to reduce the size and number of cells needed.
The output characteristics of a typical photovoltaic cell
plotted in Figure 2.
The highest voltage that can be produced by
a cell is called the open-circuit voltage; this is about
volts (V) for silicon.
As more current is drawn from the cell by
the load, the voltage falls.
The maximum current that can be
drawn from a solar cell, the short-circuit current, is about
amperes per square meter in strong sun.
For maximum power, a
silicon cell should be operated at about 0.45 V (in full
90% of the short-circuit current.
As the intensity of solar
radiation falls, the open-circuit voltage falls slowly, but
current falls roughly in proportion to the intensity.
daily cycle, the maximum power output is attained when the
at its highest and, of course, falls to zero between dusk
dawn. Solar output
is reduced on cloudy days, but diffuse sunlight
can still produce a useful fraction of full output.
a solar cell or module can be shorted or left open
circuited indefinitely without being damaged.
The efficiency of a solar cell is defined as the ratio of
electrical power output to the solar power input.
efficiency of a PV module is about 10%.
This means that when 750
W of sunlight is falling on a square meter of solar array
sunlight intensity in most nondesert areas), the solar array
would produce 75 W/sq m
Solar-cell efficiency tends to fall as
the cell temperature rises.
This effect can be serious in hot
climates where the cell may operate at 50 [degrees] C or
Mounting the cell on an energy-absorbing support (heat sink)
tend to keep the temperature down.
Commercial solar arrays or modules are about 35 by 150 cm
are made with laminated tempered glass fronts and extruded
sides. They can
stand temperatures of up to 70 [degrees] C but the
plastic laminating material between the cells and the glass
will yellow with time if exposed to higher
higher temperature use, silicon embedding compounds can be
Since photovoltaic cells give their highest output when
directly at the sun, electrical performance can be optimized
putting them on a moving mount that is always pointed toward
scanning systems are relatively expensive and the
motor and sensor systems are more likely to fail than is the
Moreover, the scanning motors consume electricity.
One available scanner uses as sensors bulbs filled with
Freon, a gas now considered environmentally hazardous.
present conditions, we recommend a simple, static
provide advice on the best angle for mounting a solar
array in a chosen location but a good year-round guideline
point the array directly toward the equator, tilting it at
angle equal to your latitude.
For example, if you are located at
10 [degrees] south latitude, lift the south edge of the
panel until the
panel is tilted 10 [degrees] from horizontal.
Hybrid systems, which provide hot water in addition to
have also been investigated.
Although they work well
for remote homesteads in northern climates they do not seem
economically sound in tropical countries where the need for
water is less urgent.
Exceptions are remote clinics, hospitals,
or other operations that need a reliable supply of hot
Even low temperature steam can be made by a properly
SunWatt Corporation and Alpha Solarco have developed
packaged hybrid modules.
Solar cells are usually sold in panels that vary in size but
of standard voltage.
Connecting individual cells in series adds
the voltages of the individual cells,, while connecting
parallel adds their current-carrying capacity.
Sixteen volts is
a popular choice for a solar panel, because that output
is needed to charge a 12-V storage battery.
Storing and Converting the Energy
In some applications, such as the use of photovoltaic cells
pumping water for irrigation, the change in output of the
through day and night is acceptable since the power is required
only for a few hours in each 24-h period.
For many applications,
however, the solar-cell array should be used together with a
battery storage system that can provide continuous
peak sunlight hours, the batteries are charged by the solar
cells, which produce more power than is required by the
During the night, the batteries discharge to operate
other loads. Use of
a diode is necessary to prevent the batteries
from passing reverse current into the solar cells at night,
voltage-regulating circuit is normally provided on larger
to keep the batteries from being overcharged by the PV
Some voltage regulators will also disconnect the load to
damage if the battery charge gets too low.
Lead-acid batteries specially developed for
applications are generally used, but any deep-cycle
battery may serve if necessary.
Automobile batteries are not
highly satisfactory for this application because daily
discharge cycles greatly shorten their useful life.
purposes, especially in remote locations, the more expensive
nickel-cadmium batteries are preferred since they require
A solar-cell array with battery provides direct current
which has many uses.
A photovoltaic system for d.c. only is shown
in Figure 3. For a
simple arrangement of a few lights and a radio
or TV set, this is the preferred system.
Incandescent lights for
12 V d.c. are available, and are almost twice as efficient
their 220-V or 110-V counterparts.
Small 12-V TV's are very
efficient also, and a small, portable radio draws very
fluorescent lights, refrigerators, etc., designed
to operate on d.c. can be very much more expensive than
counterparts that operate from the 220-V or 110-V
(a.c.) mains in normal industrial and household use.
may therefore be desirable to include an inverter that
the d.c. supply to the 50 Hz or 60 Hz a.c. needed by these
Some loss of power results from the use of the inverter
(at least 10%), but this may be justified if it leads to big
savings in the cost of the appliances.
Alternatively, the inverter
can be used for only the a.c. appliances, while the rest
of the load is operated directly from d.c.
Photovoltaic arrays can now be bought for about $6 to $10
peak watt. This
price has fallen slowly but steadily over the
last few years, and is expected to continue to fall.
battery storage (and regulator, if needed) adds 50% or more
this cost. The total
cost is too high to compete with the local
utility rates in most places, but is far cheaper than the
and operating cost of a petrol or diesel generator.
guideline, if a power line longer than one km must otherwise
built, PV or PV plus wind-generating systems is a cheaper
get small to moderate amounts of electricity.
It is believed that photovoltaics will start to be used
when the price falls to about $2 per peak watt in 1989
At this level, and assuming that whole system costs fall at
similar rate, solar electricity will be competitive with
fossil-fuel generating systems and will be used on a large
scale both by utility corporations and by individuals who
rooftop arrays. Even
now, solar cells are probably cheaper than
diesel generators for most rural applications.
And if prices
fall as predicted, solar cells could be the most economical
electricity source for all applications in remote locations
tropical countries, especially if combined with wind
(W.J. Bifano 1982).
MEETING ENERGY NEEDS WITH SOLAR CELLS
In the next decade, applications of solar cells in
countries will probably be mainly in rural villages.
do not have a power line fed by a central grid system;
the cost of extending a power grid to serve all villages
prohibitive in large countries.
However, pilot solar schemes are
now in progress in most developing countries (W.A. Brainard
1982). See Table 1
for typical village power requirements for a
number of activities that can be powered by solar cells.
Solar powered water pumps are increasingly used for
and community water supplies.
The outstanding advantage of a
pumped system is the ease with which the water supply can be
free of contamination.
From the standpoint of community health,
a pump can be the most important investment a village makes.
As an example, Arco Solar Inc., described a portable photovoltaic
water supply for the village of Boera, Papua New Guinea
Solar Inc. 1982).
The village has a population of about 1,000,
and the system installed produces 440 peak watts, without
storage. This system
delivers about 5,500 liters an hour (L/h) in
full sunlight and about 3,300 L/h under overcast conditions.
Storage is provided by four tanks each of 5,500 L capacity
are normally filled by midday.
The pump is then switched off by a
float valve. The
villagers pay about $0.01 per bucket of water.
portion of the funds is used by the community to maintain
I: TYPICAL VILLAGE POWER REQUIREMENTS
people, 100 homes. Sunlight equivalent
noonday sun. Source:
Water pumping (50 L/person-day)
Lighting - indoor (2 lights/home)
Lighting - outdoor (5 lights/village)
Television (20 sets/village)
Grain Grinder (1 kg grain/person-day)
Communications (1 two-way radio set/village)
Total kW Peak Required
Water for Drinking and Irrigation
Irrigation for agriculture is probably the greatest consumer
energy in rural areas of developing countries.
Animal power and
diesel-fueled pumps are the main competing
quantity of water required for irrigation may range from
13,000 cubic meters per hectare (cu m/ha) over the growing
or 40 to 110 cu m/ha each day.
The required pumping capacity
is therefore about 4 to 10 L/second for each hectare, a
farm being 1 to 3 ha (W.A. Brainard 1982).
As in the case of drinking-water supply, the amount of power
required depends on the depth from which the water must be
pumped. Usually this
is less than 10 m, so the requirement is
for a few hundred W/ha.
If irrigation is to be economical, the
cost of obtaining the water must be less than the value of
increase in crop production.
Wright estimated that irrigation is
not worthwhile unless the water costs less than about
(W.A. Brainard 1982).
He suggested that photovoltaic systems
were two to four times more expensive than their economic
for irrigation. The
break-even point in favorable cases (water
depth less than 5 m) probably already has been reached and
number of photovoltaic-powered irrigation systems is likely
expand in the near future.
Irrigation is important not only for food crops but also in
early stages of reforestation.
Solar power may contribute to the
reversal of deforestation, which has been drastic in such
as India. Another
indirect economic benefit of irrigation
is that it may halt, or even reverse, the population shift
the rural villages to the cities by improving the quality of
village life. And,
according to a recent review, irrigation must
increase by 250% over the next 25 years in order to support
growing world population (J.L. Crutcher 1982).
Thus, the increased
food requirements of world population growth leads to a
prediction of increased use of solar cells.
Photovoltaic-powered desalination units to produce fresh
from sea water have been installed in Saudi Arabia and Qatar
(J.L. Crutcher 1982).
They use reverse osmosis, in which the
dissolved salt is driven through a membrane.
Each liter of drinking
water requires 8 to 20 Wh of electricity, which compares
favorably with 2.4 kWh for a solar still and 200 kWh for a flash
The unit at Jeddah has been in operation since
January 1981 and supplies 2,000 L per day from an 8 kW
array and d.c.-powered pumps.
The system does not use a voltage
regulator; this raises efficiency but leads to fluctuating
rates and pressures.
The Jeddah unit produces water with a
salinity of less than 200 parts per million (= 200
mg/L). In the
Qatar unit, the salinity is below 500 mg/L:
this relaxation in
standards permits 6,000 L/day to be achieved from an 11.2 kW
Desalination is, in general, economically viable
only in relatively affluent communities that have a severe
SunWatt Corporation has demonstrated a small PV/hybrid
based on evaporation and condensation cycles, that produces
fresh water and electricity at the same time.
production of such a machine on a commercial scale requires
PV-powered refrigerators for medical supplies, have become a
regular component of pilot village schemes.
operate on d.c. are available, and it is also possible to
refrigerator with its own independent photovoltaic
reliability of solar-cell systems is vitally important when
storing vaccines and other medical supplies that would
rapidly if not kept cool.
A typical refrigerator requires
about 300 peak watts and consumes about 1 kWh/day.
with 20 refrigerator systems in different countries has
that the units now available require very little maintenance
except of the power supply itself (G.F. Hein 1982).
The performance of a solar-powered grain mill at Tangaye in
Burkina Faso has been well documented.
The mill began operation
in March 1979. The
1.8 kW solar array was used to mill grain for
600 families, relieving the village women of a daily one-to
two-hour task. The
early modules were not very reliable, but by
1982 the original system worked well 98% of the time (D.
1981). No problems
of maintenance or operation were reported.
system was increased in size in May 1981 to 3.6 kW, and an
hammer mill was installed.
By 1982, the mill was grinding
1.2 tons of flour per week and the cooperative that runs the
demonstrated a small operating profit.
Lighting and Communications
Incandescent or the more efficient fluorescent lighting can
greatly improve communal village life by providing increased
opportunities for meetings and social events in the
Battery storage is essential if lighting is included in a
The price of the lights and the greater efficiency of d.c.
be compared with cheaper ballasts for a.c. fluorescent
before deciding whether to buy an inverter; the inverter may
the component with the greatest cost and lowest reliability.
Because they require comparatively little power, television
sets can be operated by solar cells.
The value of TV in rural
education is well documented in many locations, starting in
with Cote d'Ivoire and India.
An emergency radio set is a useful addition to a village and
been included in the development plans of some
Mexican government has installed a solar-powered, rural
station, and solar-powered telephones have also been used in
Saudi Arabia. Solar
power was preferred for a microwave communications
link in Papua New Guinea.
and data-processing microcomputers can also be operated by
cells. VITA has
installed solar-powered packet radio systems
where the computers communicate with each other via radio,
remote areas of Sudan and the Philippines.
This paper was prepared,
in part, in a remote U.S. location on a solar-powered word
processor operating through a 2-kW inverter.
illustrate the variety of ways that solar cells can be used
communications in remote locations.
As in other applications, the
reliability of solar cells is their main advantage.
Can PV arrays assist the development of small
recent review specifically covered small, rural
Mexico and the Philippines, employing fewer than 50 people
producing simple consumer products.
Most industries were found
to require too large an investment in photovoltaics to be
viable at present.
However, viable possibilities do
exist in some industries that use small power tools.
Among small industries, an interesting possibility is the
manufacture of photovoltaic modules themselves.
labor-intensive plants can make modules from purchased
They can even make the cells, from industrial grade silicon,
using recently developed fabrication techniques.
A VITA Volunteer
recently helped set up the first factory in Africa to
panels. Using purchased
cells, the Moroccan plant turns out 100
panels per week. In
plants like this, the economics of using a
few extra workers to replace a large capital investment in
equipment are very favorable.
A detailed analysis of a
500-kW PV plant now being planned for India showed how 11
production workers can displace about $800,000 of capital
Small solar-cell modules to charge batteries for portable
lights, radios, and other small electric appliances can be
in even simpler shops; it can be done on a village level.
Three relatively small-scale plant models at different
production are proposed below.
Cost equivalents are for illustration
and should not be used for planning.
o A small shop producing 5-W to 10-W solar battery chargers.
plastic for cases, etc., are purchased.
2,000 chargers per year, 8 per working day.
1 to 2 persons.
$25,000 startup, $32,000 per year material
o Labor-intensive factory making 40-W, laminated PV modules.
glass, and other supplies are purchased.
1/2 Megawatt (MW) in modules per year
modules, 50 per
18 production workers.
$250,000 startup, $2,000,000 per year
o Plant making solar cells from industrial grade silicon.
grade silicon, the plant casts polysilicon
shapes, cuts them
into square wafers, dopes them, adds metal
1 MW per year (1,000,000 wafers, 4000 per
20 workers (6 highly skilled).
$2,500,000 startup, $3,000,000 per year
SOME CRITICAL COMPARISONS
At present, photovoltaics cannot compete with centrally
electricity except when power lines must be installed over
distances. They are
therefore most likely to be applied in rural
locations, especially in villages.
Their flexibility in use, in
large or small arrays, is a major advantage since a system
carefully tailored to the specific application and expanded
needed. In comparing
the cost-effectiveness of solar and diesel
systems, particular or local economic factors may be
even when maintenance costs and reliability are taken into
Failure problems with the earliest modules appear to have
been solved; thus, wind power is the only serious competitor
PV devices as a renewable source of electricity.
that should also be seriously considered is solar thermal
Hot water or gas can be used to drive a Stirling engine, for
example in irrigation, and some engineers argue that this is
currently the most effective method.
Refrigerators and air conditioners
can also be driven by warm water, but need small
powered pumps. Here
as elsewhere, one must choose from many
alternatives the one that offers the best combination of
The choice of solar cells or wind generators for electricity
depends on the location.
However, it is likely that a combination
of these will become the major source of electricity in
areas that are not supplied with a central grid that
for example, hydroelectric or geothermal energy.
The cost of
solar cells is still high and there are few applications in
a strong economic benefit can be demonstrated to justify
However, there is no doubt that solar arrays can
greatly improve the quality of rural village life.
decade should see a great expansion in solar-cell
prices fall to the predicted $1 to $2 per peak watt.
Ideally, developing countries can follow the lead of India,
Morocco, and Mexico by starting to develop their own
for solar-cell production.
Thus, a country can begin now to
develop technological capabilities in a field where future
1. Arco Solar
Inc. Applications Bulletin A-18-82A
Woodland Hills, California:
Arco Solar Inc., 1982.
2. Bifano, W.J.,
"Economic Viability of Photovoltaic Power for
Institute of Electrical
Engineers, Proceedings of the 16th Photovoltaics
Conference (San Diego, California), vol.
3. Brainard, W.A.,
"The Worldwide Market for Photovoltaics in the
Sector." Institute of Electrical
and Electronics Engineers,
the 16th Photovoltaics Specialists Conference
(San Diego, California), vol. 3, pp.
4. Chiles, James
R., "Tomorrow's Energy Today."
York, New York,
vol. 92, pp. 58-72, 1990.
5. Crutcher, J.L.;
Cummings, A.B.; Norbedo, A.J., "Photovoltaic-Powered
Desalination Systems: Experience in Two
of Electrical and Electronics
Proceedings of the 16th Photovoltaics Specialists
Diego, California), vol. 3, pp. 1400-1404,
6. Day, J. F.,
"An American View of Photovoltaics in Developing
Countries." Proceedings of
the Third European Conference on
7. Elwell, D.,
"Solar Electricity Generation in Developing
Countries." Mazingira, vol.
5, no. 3, pp. 30-41. (1981)
8. Hankins, Mark,
Renewable Energy in Kenya, Nairobi, Kenya:
9. Hein, G.F.,
"Design, Installation, and Operating Experiences
Photovoltaic Medical Refrigerator Systems on Four
Institute of Electrical and Electronics
the 16th Photovoltaics Specialists
Diego, California), vol. 3, pp. 1394-1399,
10. Komp, Richard
J., Practical Photovoltaics:
Solar Cells, 2nd ed. Ann Arbor,
Michigan: AATEC Publications,
11. Maycock, Paul
D.; Stirewalt, Edward, Photovoltaics:
in One Step. Andover,
Publishing Co., 1981.
12. Wright, D.E.,
"The Use of Photovoltaic Pumps for Small-Scale
the Developing World: a Progress Report
Project." Proceedings of the Third
Solar Energy, pp. 117-123, 1981.
The main U.S. suppliers of photovoltaic modules and related
equipment are listed below:
Alpha Solarco, 11534 Gondola Drive, Cincinnati, Ohio 45241
Arco Solar Inc., P.O. Box 4400, Woodland Hills, California
Photocomm Inc., 7861 East Gray Road, Scottsdale, Arizona
Solarex Corp., 1335 Piccard Drive, Rockville, Maryland 20850
SunWatt Corporation, RFD Box 751, Addison, Maine 04606