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Chap 4 : Photovoltaic (Solar Cells)
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Introduction
The history of photovoltaics goes back to the year 1839,
when Becquere discovered the photovoltaic effect, but no
technology was available in the 19th century to exploit this
discovery. The semiconductor age only began about 100
years later. After Shockley had developed a model for the
pn junction, Bell Laboratories produced the first solar cell
in 1954; the efficiency of this, in converting light into
electricity, was about 5%.
Photovoltaics offers the highest versatility among
renewable energy technologies.
Theoretically, PV systems could cover the whole
electricity demand of most countries in the world.
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Introduction
Worldwide, the installed photovoltaics capacity and the
share of electricity generated by PV are still low, despite
impressive market growth. The political environment and
magnitude of market introduction programmes will
determine the future of this technology.
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The photo effect
Light, with its photon energy, can provide the energy to lift
an electron to a higher orbit. The photon energy is given
by :
E =
h ⋅ c
The energy sufficient to lift the electron to orbit E∞ is also
called the ionization energy (external photoelectric effect).
photovoltaic cells mainly convert to electricity photons of
visible, ultraviolet and infrared light, therefore, the internal
photo effect determines the effect of light in a solar cell.
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The photo effect
The highest fully occupied band is called the valence band
The next highest band, which can be partially occupied or totally
empty, is called the conduction band
The space between VB and CB is called the forbidden band
The energy gap between the band is called the band gap
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