Photocell Sensors

1
Photocell Sensors

• Chris Rogers
• Mechatronics ECE 5320

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Outline

• Photocells Introduced
• Materials Used
• First, Second and Third Generation Photocells
• Limitations
• Areas of Research and Future opportunities
• Web links

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Information References

• Wikipedia -http//en.wikipedia.org/wiki/Solar_cel
  ls
• HowStuffWorks -http//science.howstuffworks.com/so
  lar-cell1.htm
• http//www.pv.unsw.edu.au/Research/3gp.aspblah

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What are Photocells?

• Photo cells convert photon energy, light, into
  electricity.
• All photo cells make use of the photovoltaic
  effect, which is a two step process.

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How the Photovoltaic effect drives power cells

• The photovoltaic effect is realized in two steps
• 1 Excitation of charge carriers on the surface
  of the cell
• 2 Separation of charge carriers to conductive
  material which can carry current

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Cell Materials

• Similar to transistors and diodes, photocell
  materials are often made up of n-type and p-type
  silicon materials.
• N-type materials are semiconductors with an
  excess of electrons, or negative type.
• P-type materials are semiconductors with a recess
  of electrons, and have carrier holes. They get
  their name from the positive charge, and are
  hence positive type.

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Let there be Light

• When light hits the sandwiched N-type and P-type,
  each photon of light releases an extra electron,
  concurrently creating an extra carrier where the
  electron was.
• The extra electrons and carriers force current to
  flow when an external load is connected.

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Finishing the Cell

• Silicon is an extremely reflective material, and
  silicon alone isnt suited to trap light on its
  surface.
• Anti-reflective coatings are employed to reduce
  photon loss through reflection.
• Image reference
• fdashttp//science.howstuffworks.com/solar-cell7.h
  tm

• A Glass cover
• B Anti-reflective coating
• C Electrical contact
• D N-type material
• E P-type Material
• F Electrical contact

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Efficiency and Materials

• Silicon is by far the most widely used among
  photocell materials. Crystalline silicon is put
  into three categories
• Mono crystalline wafers
• Poly crystalline wafers
• Ribbon Silicon

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First Generation Photocells

• First generation photocells are photocells in the
  simplest sense they are nothing but PN junctions
  that convert sunlight to current.
• They arent very efficient and dont produce very
  much current.

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Mono Crystalline Wafers

• Mono crystalline wafers use the Czochralski
  process to grow silicon crystals by seeding
  molten silcon.
• The molten silicon is placed in a vat, and a free
  moving arm with a seeding silcon crystal is
  placed in the liquid. As the arm is raised, it
  spins, using centrifugal forces to move rapidly
  cooling and crystallizing silicon to the outer
  edges.

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Poly Crystalline Wafers

• Poly crystalline wafers are made from carefully
  cooled and solidified molten silicon ingots.
• The ingots can be cut in square dimensions, and
  are valuable for large panel arrays.

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Rubber silicon

• Rubber silicon is a hybrid of mono and poly
  crystalline processes. The molten silicon is
  drawn out, forming thin flat ribbons.
• These processes are sometimes grown horizontally
  with substrate material and seeding crystals.

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Second Generation Photocells

• In an attempt to improve the current output of
  first generation photovoltaic cells, the second
  generation photocells combine several layers of
  PN junctions and output current.
• Each layer is tweaked to handle a different
  wavelength from the suns spectrum.

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Third Generation Photocells

• Third generation photocells attempt to limit the
  bandwidth of incoming light in order send more
  usable light to the photocell.
• Other areas of research seek to tune the bandgap,
  which tunes the emission wavelength of incident
  light. This furthers the efficiency of the wafer.

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Efficiency and Materials
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Limitations

• Photo cells convert, on average, 15 of the suns
  light into electrical energy.
• Many wavelengths of light emanating from the sun
  do not have the energy to break electrons free.
• More area is needed to create more current,
  however, the larger the arrays of photocell
  material, the farther current must travel. These
  long contact lines into the photocell material
  can be high in resistance.

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Current Research Topics

• Silicons biggest stumbling block is that its
  not always pure.
• Many methods have been worked on to purify
  silicon and create more mobility for charge
  carriers, increasing overall electrical current
  potential.
• Another major hindrance is cost. Solar panels
  have a much higher USD/Watt than other fossil
  fuels. Money talks.

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Show me the money!

• In attempts to cheapen photo cells for power
  generative purposes, thin film solar cells are a
  promising area of research. They use less than
  1 of raw material compared to wafer based solar
  cells.
• Thin film solar cells are amorphous silicon,
  where nano crystalline and proto crystalline
  silicon can be obtained.
• Nano and proto crystalline structures have higher
  bandgaps, and higher bandgaps mean more energy
  absorption from the sun, which means more energy
  production.

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Other Web References

• Denim Solar Panels
• http//www.newscientist.com/article.ns?iddn3380
• Scientists Create New Solar Cell
• http//www.sciam.com/article.cfm?chanIDsa003arti
  cleID0004C094-02CC-1CD0-B4A8809EC588EEDF
• Spray On Power Cells
• http//news.nationalgeographic.com/news/2005/01/01
  14_050114_solarplastic.html
• Photovoltaics for buildings
• http//www.nrel.gov/buildings/pv/factsheets.html
• Full Solar Spectrum Photovoltaic Materials
  Identified
• http//www.lbl.gov/msd/PIs/Walukiewicz/02/02_8_Ful
  l_Solar_Spectrum.html
• Solar Panel Newsgroup
• alt.solar.photovoltaic