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

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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,

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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

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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.

CH2 Basic Physics of Semiconductors
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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.

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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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

• 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
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CH2 Basic Physics of Semiconductors
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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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