Goal

1
Solar Car Solar Cells
2
Solar Cells

• Photovoltaics
• Photo Light Voltaic Electricity
• Silicon is a semiconductor which absorbs certain
  frequencies of light (the Energy of the absorbed
  light is transferred to the semiconductor)
• If the Energy is of a certain level (1.1 - 1.4eV
  for most cells), electrons are knocked loose and
  allowed to flow freely (current!)

3
Flowing Electrons

• Pure Silicon is a poor conductor because all its
  valence electrons are tied up in bonds, making
  them harder to knock free
• Doping silicon with small quantities of
  impurities (such as Phosphorous which has an
  extra electron that is unbonded in silicon)
  allows the extra electrons to break free with
  much less energy than the electrons of pure
  silicon

4
Flowing Electrons

• Free electrons are called free carriers
• Each free electron leaves a corresponding hole
  or positive charge
• Even so, the material is still electrically
  neutral, because each atoms electrons are
  balanced by a proton in the nucleus
• Silicon that is doped with a material that
  supplies extra electrons is called n-type
• Silicon that is doped with a material that
  supplies extra holes is called p-type

5
Using Electrons and Holes to our Advantage

• By putting n-type and p-type silicon together, an
  electric field is formed
• Free electrons from the n-side rush to fill free
  holes on the p-side, until equilibrium is reached
  and a barrier is formed

n-type silicon

Barrier Region
- - - - - - - - - - - - - - -
p-type silicon
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n-type silicon

Barrier Region
- - - - - - - - - - - - - - -
p-type silicon

• This creates a gradient where electrons can only
  flow in one direction
• An electron can cross from the p-side to the
  n-side, but not the other way around
• When free carriers (electrons) get close to the
  barrier, they tend to slide across, causing an
  electrical imbalance (charge)

7
Using the Charge

• When light strikes a Photovoltaic cell, atoms are
  bombarded with photons, and give up electrons,
  which eventually cross the barrier
• More light means more electrons crossing, thus
  more charge
• By adding an external current path, the electrons
  will flow through the path, back to their
  original side (the p-side) to join with the holes
  left behind when the electrons crossed the
  barrier, and we can use the current to do work
  along the way

8
Solar Cell in Use
Light Photons knock electrons loose. The
electrons wandering near the barrier cross to
the n-side and become trapped. Likewise, holes
cross to the p-side and become trapped.
External Current Path allows the trapped
electrons a way to flow back to the p-side to
join with the holes.
n-type silicon
Hole

Load
Barrier Region
- - - - - - - - - - - - - - -
p-type silicon
Electron
The current flow can be used to do work along
the way
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Problems with Silicon

• In order for solar cells to work, light must be
  absorbed
• Silicon is highly reflective
• In order for the electrons to flow through the
  current path, a good conductor plain must be
  provided on both the top and the bottom of the
  cell
• If you cover the top with an opaque conductive
  plain, no light can be absorbed

10
Solutions

• Solar cells must be coated with an
  anti-reflective surface so that light can be
  absorbed
• Solar cells are covered with a metallic contact
  grid to cover the least surface area of the cell
  but still provide good conductance

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Layers of a Solar Cell
Anti-reflective Coating
Ground Conductor Grid
n-layer
p-layer
Power Conductor