Photovoltaic Technology
Photovoltaics is the field of technology and research
related to the devices which directly convert sunlight into
electricity. The solar cell is the elementary building block of the
photovoltaic technology. Solar cells are made of semiconductor
materials, such as silicon. One of the properties of semiconductors
that makes them most useful is that their conductivity may easily be
modified by introducing impurities into their crystal lattice.
For instance, in the fabrication of a photovoltaic solar cell,
silicon, which has four valence electrons, is treated to increase
its conductivity. On one side of the cell, the impurities, which are
phosphorus atoms with five valence electrons (n-donor), donate
weakly bound valence electrons to the silicon material, creating
excess negative charge carriers. On the other side, atoms of boron
with three valence electrons (p-donor) create a greater affinity
than silicon to attract electrons. Because the p-type silicon is in
intimate contact with the n-type silicon a p-n junction is
established and a diffusion of electrons occurs from the region of
high electron concentration (the n-type side) into the region of low
electron concentration (p-type side). When the electrons diffuse
across the p-n junction, they recombine with holes on the p-type
side. However, the diffusion of carriers does not occur
indefinitely, because the imbalance of charge immediately on either
sides of the junction originates an electric field. This electric
field forms a diode that promotes current to flow in only one
direction. Ohmic metal-semiconductor contacts are made to both the
n-type and p-type sides of the solar cell, and the electrodes are
ready to be connected to an external load.
When photons of light fall on the cell, they transfer their energy
to the charge carriers. The electric field across the junction
separates photo-generated positive charge carriers (holes) from
their negative counterpart
(electrons). In this way an electrical current is extracted once the
circuit is closed on an external load.
Source:
Solar Cell
There are several types of solar cells. However, more than 90 % of the solar
cells currently made worldwide consist of wafer-based silicon cells. They
are either cut from a single crystal rod or from a block composed of many
crystals and are correspondingly called mono-crystalline or
multi-crystalline silicon solar cells. Wafer-based silicon solar cells are
approximately 200 μm thick. Another important family of solar cells is based
on thin-films, which are approximately 1-2 μm thick and therefore require
significantly less active, semiconducting material. Thin-film solar cells
can be manufactured at lower cost in large production quantities; hence
their market share will likely increase in the future. However, they
indicate lower efficiencies than wafer-based silicon solar cells, which
means that more exposure surface and material for the installation is
required for a similar performance.
A number of solar cells electrically connected to each other and mounted in
a single support structure or frame is called a ‘photovoltaic module’.
Modules are designed to supply electricity at a certain voltage, such as a
common 12 volt system. The current produced is directly dependent on the
intensity of light reaching the module.
There are two main types of photovoltaic system. Gridconnected systems
(on-grid systems) are connected to the grid and inject the electricity into
the grid. For this reason, the direct current produced by the solar modules
is converted into a grid-compatible alternating current. However, solar
power plants can also be operated without the grid and are then called
autonomous systems (off-grid systems).
More than 90 % of photovoltaic systems worldwide are currently implemented
as grid-connected systems. The power conditioning unit also monitors the
functioning of the system and the grid and switches off the system in case
of faults.
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