Solar Cells
• Early development of solar tech. starts
in the 1960s
• Conversion of sunlight to electricity –
by photovoltaic effect
• In 1974 only 6 private homes in North
America were entirely heated by solar
power (increasing rapidly since 1979
energy crisis)
• Photovoltaic production growth ~ 40%
per year since 2000
Nellis Solar Power Plant, Nevada
• The second larger solar power plant
in North America
• Located in Nellis Air Force Base, Nevada,
northeast of Las Vegas
• Generate 25 M Kw-h, supply 25% of the
power used in the base, more than
12,000 military and civilians at Nellis
• Contain 70,000 solar panels,
Theory
• Photovoltaics (PV): a method of generating electrical powere by converting solar radiation
into direct current electricity using semiconductor exhibiting photovoltaic effect
• Photovoltaic effect: creation of a voltage (or corresponding electric current) in a material
upon exposure of light. Generated electrons are transferred from valence to conduction bands,
resulting in the buildup of a voltage between two electrodes
PN Junction (Diode)
• When N-type and P-type dopants are introduced side-by-side in
a semiconductor, a PN junction or a diode is formed.
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Depletion Region
• As free electrons and holes diffuse across the junction, a
region of fixed ions is left behind. This region is known as
the “depletion region.”
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Current Flow Across Junction:
Drift
• The fixed ions in depletion region create an electric field that results
in a drift current.
• At equilibrium, the drift current flowing in one direction cancels out
the diffusion current flowing in the opposite direction, creating a net
current of zero.
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Diode in Reverse Bias
• When the N-type region of a diode is connected to a
higher potential than the P-type region, the diode is under
reverse bias, which results in wider depletion region and
larger built-in electric field across the junction.
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Diode in Forward Bias
• When the N-type region of a diode is at a lower potential
than the P-type region, the diode is in forward bias.
• The depletion width is shortened and the built-in electric
field decreased.
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Diode in Forward Bias
• Minority charge profile should not be constant along the
x-axis; otherwise, there is no concentration gradient and
no diffusion current.
• Recombination of the minority carriers with the majority
carriers accounts for the dropping of minority carriers as
they go deep into the P or N region.
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I-V (current-voltage) characteristic of a p-n junction
VD
I D  I S (exp  1)
VT
• The current and voltage relationship of a PN junction is
exponential in forward bias region, and relatively constant
in reverse bias region.
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Light-emitting diode (LED)
• A semiconductor light source
• Introduced as a practical electronic component in 1962
• Earlier LED with red light, modern LED available
across visible, UV and infrared wavelengths
• The first blue LED were made in 1971
by Jacques Pankove (GaN LED) at RCA Lab.
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Applications of LED
•
•
•
Advantages over incandescent light: lower energy consumption, longer
life time, improved robustness, small size, faster switching
Compared to fluorescent lamp for room lighting: relatively expensive,
require more precise current and heat management
Used for used as indicator lamps, aviation lighting, traffic signals,
lighting, automotive lighting
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Applications of LED
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Operations of LED
•
Electroluminescence effect
electron-hole recombination
release photons,
the color is determined
by the energy gap
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Operations of LED
•
Electroluminescence effect
electron-hole recombination
release photons,
the color is determined
by the energy gap
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