21-05-2013, 12:36 PM
A Review of Photovoltaic Cells
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Abstract
Photovoltaic cells provide an additional method of
acquiring energy, converting sunlight directly into electricity
through the use of semiconductors. Effective photovoltaic
implementation is reviewed, focusing on semiconductor
properties and overall photovoltaic system configuration.
INTRODUCTION
Energy policies have pushed for different technologies to
decrease pollutant emissions and reduce global climate
change. Photovoltaic technology (PV), which utilizes sunlight
to generate energy, is an attractive alternate energy source
because it is renewable, harmless, and domestically secure.
Because PV technology’s basic component is the PV cell
which produces less than three watts on average, cells must be
bundled in series/parallel configurations known as PV
modules or solar cells to achieve high powered tasks.
PV arrays produce power only when illuminated, and it is
therefore standard to employ a large energy storage
mechanism, most commonly a series of rechargeable batteries.
To prevent harmful battery overcharge and overdischarge
conditions and to drive AC loads, a charge controller and an
AC to DC converter must be implemented [1]. The primary
objective is to optimize PV cells and energy storage and to
increase overall system efficiency. In order to discuss
optimization, one must have a basic understanding of how PV
cells and storage mechanisms function.
FUNCTIONALITY
PV functionality relies upon the absorption of light within a
bulk or semiconductor material, most commonly a silicon pn
diode, providing a medium in which incident photons can be
converted to energy, usually in the form of heat. When
absorbed, a photon transfers energy to an electron in the
absorbing material and if the magnitude of incident photon
energy is greater than the electron’s work function, the photon
may raise an electron’s energy state or even liberate an
electron. Once liberated, the electrons are then free to move
around the semiconductor material influenced by present
phenomena of diffusion, temperature, and electric field [1, 2,
3].
PV CELL DESIGN
Because a PV cell is a simple pn diode, the well known
voltage transfer characteristic equations will be incorporated
into the design process. As such, these characteristic
equations provide a means of determining ideal PV cell
performance limits. The VTC graph in figure 3 illustrates that
the cell has both a limiting voltage and current so open circuit
and short circuit operating conditions will not be detrimental
to its function. Under zero applied voltage, the short circuit
current simply becomes the photon induced current while the
open circuit voltage can be found by setting the cell current to
0, as shown by equations 3a and 3b respectfully [1,8].
FUTURE ADVANCEMENTS
Although 86% of PV cells are designed with this first
generation semiconductor approach, second and third
generation cells consist of thin film deposits and electron
confined nanoparticle materials. Thin film technologies
reduce the required mass of light absorbing material, resulting
in reduced processing costs but also reduced energy
conversion efficiency. Because these thin films are nearly
mass-less, they can be stacked to form multiple layer film cells
which yield an average of 30% efficiency while standard
semiconductor efficiency is limited to 14% [9]. Utilizing the
same thin-film light absorbing materials, nanocrystalline solar
cells increase efficiency as they are covered with an extremely
thin coating of mesoporous metal oxide whose high surface
area helps to increase internal reflections and ultimately light
absorption probability and efficiency. This increase of
internal reflection helps to boost nanocrystalline PV cell
efficiency to over 40% [10,11].
APPLICATIONS
With function and design in mind, one must inquire about
PV applications. PV cells are ideal energy candidates in areas
where electric-grid extensions are not offered, and where a
clean, environmentally friendly power source is desired.
Common examples of PV devices include roof-top
residential/commercial systems, remote water pumping
stations, telecommunications equipment, and traffic lights [6].
In the most popular application of a solar powered house,
PV cells absorb photons, send DC current through an inverter
which transforms the signal to 120 or 240-volt to utilize AC
appliances. The AC power enters the utility panel in the house
and is then distributed to appliances throughout the house.
Electricity that is not used will be recycled and reused in other
facilities [6,7].
CONCLUSION
PV cells are a proven environmentally benign power source
whose attractive characteristics will continue to further
photovoltaic research. Because current PV systems are still
highly inefficient and uncommon, they are not yet cost
competitive with fossil fuel-based generators and are only
regularly used where there is no nearby power source.
Photovoltaic advancements in the fields of thin film and
nanocrystalline materials will continue to flourish and soon
increase PV efficiency to over 50%. As efficiency increases,
PV technology will attract a greater number of people,
resulting in reduced cost. Because the sun delivers ten
thousand times more energy than people currently consume,
photovoltaic improvements will one day replace
environmentally unfriendly power plants with a proven and
clean energy source [13].