Solar Cells

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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.
CH2 Basic Physics of Semiconductors
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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.”
CH2 Basic Physics of Semiconductors
11
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.
CH2 Basic Physics of Semiconductors
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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.
CH2 Basic Physics of Semiconductors
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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.
CH2 Basic Physics of Semiconductors
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CH2 Basic Physics of Semiconductors
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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
CH2 Basic Physics of Semiconductors
21
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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