Solar Cells

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Solar Cells

• Typically 2 inches in diameter and 1/16 of an
  inch thick
• Produces 0.5 volts, so they are grouped together
  to produce higher voltages. These groups can then
  be connected to produce even more output.
• In 1883 the first solar cell was built by Charles
  Fritts. He coated the semiconductor selenium with
  an extremely thin layer of gold to form the
  junctions. The device was only around 1
  efficient.

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Generations of Solar cells

• First generation
• large-area, high quality and single junction
  devices.
• involve high energy and labor inputs which
  prevent any significant progress in reducing
  production costs.
• They are approaching the theoretical limiting
  efficiency of 33
• achieve cost parity with fossil fuel energy
  generation after a payback period of 5-7 years.
• Cost is not likely to get lower than 1/W.

3
Generations of Solar cells

• Second generation-Thin Film Cells
• made by depositing one or more thin layers (thin
  film) of photovoltaic material on a substrate.
• thickness range of such a layer varies from a few
  nanometers to tens of micrometers.
• Involve different methods of deposition
• Chemical Vapor deposition the wafer (substrate)
  is exposed to one or more volatile precursors,
  which react and/or decompose on the substrate
  surface to produce the desired deposit.
  Frequently, volatile by-products are also
  produced, which are removed by gas flow through
  the reaction chamber.

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Thin Film deposition techniques

• Electroplating
• electrical current is used to reduce cations
  (positively charged ions) of a desired material
  from a solution and coat a conductive object with
  a thin layer of the material.
• Ultrasonic nozzle
• spray nozzle that utilizes a high (20 kHz to 50
  kHz) frequency vibration to produce a narrow drop
  size distribution and low velocity spray over the
  wafer
• These cells are low cost, but also low efficiency

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The Third Generation

• Also called advanced thin-film photovoltaic cell
• range of novel alternatives to "first generation
  and "second generation cells.
• more advanced version of the thin-film cell.

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Third generation alternatives

• non-semiconductor technologies (including polymer
  cells and biomimetics)
• quantum dot technologies
• also known as nanocrystals, are a special class
  semiconductors. which are crystals composed of
  specific periodic table groups. Size is small,
  ranging from 2-10 nanometers (10-50 atoms) in
  diameter.
• tandem/multi-junction cells
• multijunction device is a stack of individual
  single-junction cells
• hot-carrier cells
• Reduce energy losses from the absorption of
  photons in the lattice
• upconversion and downconversion technologies
• Put a substance in front of the cell that
  converts low energy photons to higher energy ones
  or higher energy photons to lower energy ones
  that the solar cells can convert to electricity.
• solar thermal technologies, such as
  thermophotonics(TPX)
• A TPX system consists of a light-emitting diode
  (LED) (though other types of emitters are
  conceivable), a photovoltaic (PV) cell, an
  optical coupling between the two, and an
  electronic control circuit. The LED is heated to
  a temperature higher than the PV temperature by
  an external heat source. If power is applied to
  the LED, , an increased number of electron-hole
  pairs (EHPs) are created.These EHPs can then
  recombine radiatively so that the LED emits light
  at a rate higher than the thermal radiation rate
  ("superthermal" emission). This light is then
  delivered to the cooler PV cell over the optical
  coupling and converted to electricity.

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Efficiency and cost factors

• In 2002 average cost per peak watt was
  2.90-4.00. Coal fired plant is 1.00/watt.
• Efficiency is not great.
• Recall, 77 of the incident sunlight can be used
  by the cell.
• 43 goes into heating the crystal.
• Remaining efficiency is temperature dependent
• Average efficiency of a silicon solar cell is
  14-17
• The second and third generation technologies
  discussed are designed to increase these
  efficiency numbers and reduce manufacturing costs

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Novel approaches

• UA astronomer Roger Angel
• Uses cheap mirrors to focus sunlight on 3rd
  generation solar cells (triple junction cells)
  which handle concentrated light
• 1.00 per watt achievable-competitive with coal
  plants
• Potential 1 solar farm 100 miles on a side could
  provide electricity to the whole nation
• Does not have to be all in one place

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Solar Cooling

• Consider a refrigeration system with no moving
  parts.
• Heat the coolant (say ammonia gas dissolved in
  water) and force it via a generator into an
  evaporator chamber where it expands into a gas
  and cools. Move it to a condenser and cool it
  back to a liquid and repeat the process.
• These systems actually have existed for a number
  of years, refrigerators in the 1950s were sold
  with this technology (gas powered and there
  was/is a danger of CO emissions).
• Energy to heat the coolant and drive it through
  the system comes from burning fuel or a solar
  cell to provide electricity to do the heating.
• Need what is called a concentrating collector
  (lens or other system to concentrate more light
  on the solar cell).
• Ideally, you could do this with a flat plate
  collector system, though you do not obtain as
  much cooling.
• Devices are not widely used, due to the
  intermittency of sunlight

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