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Solar Cells: An Introduction *
What are solar cells?
Unlike solar hot water heaters, which convert sunlight directly into heat, solar cells are devices
that can convert the energy of sunlight directly into electricity. This electricity can then be
used to power electrical equipment such as lights and television, just like electricity from a
power point. Solar cells rely on a quantum mechanical process known as the 'photovoltaic effect'.
They are thus also known as 'photovoltaic' cells, a word that comes from the Greek word
photo, meaning 'derived from light' (hence the word photograph), combined with the
name of the Italian physicist Alessandro Volta, who invented the first battery (hence the word
voltage).
Conventional methods of generating electricity (eg. burning coal and other
fossil fuels) can produce pollutants such as carbon dioxide, the main gas responsible for global
warming. Furthermore, fossil fuels and uranium are non-renewable resources. The only resource
needed to power a solar cell is sunlight. Since sunlight is clean, abundant and virtually
limitless, solar cells are a non-polluting and renewable alternative to more conventional energy
sources. Moreover, since there are no moving parts, solar cells can continue to operate reliably
for many years without maintenance.
How do solar cells work?
Solar cells are made using semiconductors such as silicon. Semiconductors have interesting
electrical properties, making them useful for electrical devices such as microprocessors used in
computers. One of their properties is that they can be treated in different ways to become either
'positive' (p-type) or 'negative (n-type).
A solar cell consists of two of semiconductor, one p-type and the other n-type,
sandwiched together to form a 'pn junction'. This pn junction induces an electric field
across the device. When particles of light ('photons') are absorbed by the semiconductor, they
transfer their energy to some of the semiconductor's electrons, which are then able to move about
through the material. For each such negatively charged electron, a corresponding positive charge
is created called a 'hole'. In an ordinary semiconductor, these electrons and holes recombine
after a short time and their energy is wasted as heat.
In a solar cell, however, the electrons and holes are swept across the pn
junction in opposite directions by the action of the electric field. This separation of charge
induces a voltage across the device. By connecting the device to an external circuit, the
electrons are able to flow - and this flow of electrons is what we call electricity.
Semicondutor diagram **
What are solar cells good for?
In the mid-1950s, the builders of early spacecraft required an energy source that was reliable,
long lasting and required no maintenance. There are no power points in space and as we all know,
batteries go flat pretty quickly. Solar cells, however, are an ideal choice for this application.
They are reliable, maintenance-free and their energy source - sunlight - is abundant and virtually
everlasting. The first practical solar cells were therefore developed for these space
applications.
Unfortunately, these first solar cells were very expensive to produce. In the
early 1970s, the oil crisis spurred development of solar cell technology for applications on
Earth. A terrestrial photovoltaic was soon established and has continued to grow ever since. To
date, however, solar cells have usually been too expensive to compete directly with electricity
derived from other energy sources as coal. Nevertheless, solar cells have been used extensively
in remote applications such as to provide electricity for telecommunications equipment or for
people who live away from the electricity grid. In such applications, solar cell technology is
often cheaper than alternatives such as connecting to the electricity grid or installing a diesel
generator and transporting fuel.
What about the future?
Recent developments suggest that photovoltaic technology may soon be playing a much larger role
in all our lives. Spearheading this push has been the Photovoltaics Special Research Centre at
the University of New South Wales.
Whilst efficiencies have been implemented, the cost of the solar cells
themselves has steadily fallen in recent years and should continue to do so over the coming
decade. Development of system technology and integration of solar cells into building materials
such as roof tiles is also reducing the overall system costs. These developments should see
photovoltaic technology become cost competitive with conventional energy sources in the near
future.
As costs continue to fall over the coming decade, we are likely to see solar
cells become commonplace in everyday applications such as on people's roofs. Those who elect to
place solar cells on their roof will be able to sell electricity to their local utility during
the day when the sun is shining and buy it back at night when it is not. These changes should
have a significant impact on reducing emission of greenhouse gases and other pollutants, and
will takes us one step closer to a cleaner energy future.
* Kennedy et al The speed of light. Sydney : The Faculty Of Engineering,2001
** Cotter et al The speed of light 2 : the 1999 World Solar Challenge.
Sydney : Key Centre for Photovoltaic Engineering, 2001
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