Solar Cells and Solar Technology

• Solar energy is an obvious source of energy
  • it provides most of the heat for our planet
  • it provides energy source for photosynthesis
    • and ultimately created all the fossil fuels
• It comes automatically, free
• Can we harvest it in other ways?
• Photovoltaic Panels of Solar Cells
  • start with a slab of silicon
  • like carbon, 4 valence electrons
    • 4 single bonds to neighbors
    • all electrons are tied up
    • not an electrical conductor
  • electrons bound by about 1 eV
    • eV = electron volt = energy unit
  • light (green, 500 nm)
    • photon has energy of 3 eV
    • enough to knock an electron loose
      • and give it another 2 eV of energy
    • enough energy to pump around the electrons
  • shows up as 2 volts
  • in practice, only 10-20% of the electrons show up
    • the others recombine, go back into the bond
    • so every 5 photons produces one electron
    • device is 10-20% efficient

• the bigger the panel, the light captured
  • the more electrons
  • current is proportional to area
  • energy is proportional to current and voltage
• can wire cells in parallel to form a panel
  • (+ to +) and (- to -)
  • get same voltage
  • get double the current and power
• can wire cells in series
  • (+ to -)
  • get same current
  • get double voltage and doubled power

Simple to Use

• Set a panel in sunlight
• connect the two wires to ....
  • lamps, motors, telephones, calculator
  • electrolysis (make H2 for example)
  • batteries (save power)
• Disdavantages?
  • obviously, useless at night
  • reduced power on overcast days
  • often teamed with storage batteries

Economics are marginal

• panels are relatively expensive to make
• panels have good lifetime, but not indefinite
• for large power generator
  • need lots of area
  • intricate frames to hold panels
  • ideally, motors to track the sun
• as a rule, little or no maintenance
  • perhaps wipe off the panel
  • replace water proofing materials
• text: 28 cents per kilowatt hour
  • 5 cents for KWH from coal
• why would anyone do this?

1. In some areas, far from power lines

• cheaper than bringing in power lines
• for large units, generators= more economical
• for small power needs, generators = expensive , troublesome
• cheaper than driving out, replacing batteries

• telephones along isolated highways
• remote weather stations
• garden lighting (low level lights to mark path)

• NASA and satellites and spacecraft
  • perhaps out to Jupiter
  • marginal further out (light too faint)
• Internet connections where no electrical power (seems strange set of priorities)

2. Tax Credits

• manufactures and taxpayers
• significant rewards for solar installations
• about enough to offset actual costs
• why?
  • to encourage solar use
  • to encourage solar development
  • to test panels in realistic situations

• special case: calculators
  • first consumer product with solar cells
  • Japanese panel manufacturers
  • Japanese calculator manufacturers
    • teamed up to make solar powered calculators at a net loss over battery units
    • to create demand to create the industry

The Promise

• first, the promise has been around for over 20 years now; always the same promise
  • expensive now, but soon cheap
  • materials are intrinsically cheap
  • technology will improve, lower costs (it has)
  • quantity manufacture lowers costs
  • possibility of great technological progress

• in operation
  • no fossil fuel consumption
  • no CO₂, no NOx, SOx
  • no radioactivity
  • no thermal pollution
  • (some manufacturing pollution inevitable)

Need to discuss purity, doping of semiconductors

• metals are conductors:
  • have electrons, free to move
• silicon (pure) is an insulator
  • in light, it becomes a photoconductor
  • light frees some electrons
• really need very pure material
  • (<1 ppm impurities)

• Chemically, 99.9% purity is good for chemical processes, but we need 99.9999 %
• Start with a cylinder of clean silicon
  • add a ring heater
  • melt a tiny zone across the bottom of the rod
    • either have a container
    • or use surface tension to hold the liquid
  • slowly raise the heater
    • silicon freezes out at as the bottom cools
    • very pure silicon
    • impurities stay in the liquid
  • pass zone though material
    • impurities end up in top 5-10% of the rod
  • repeat with additional passes

Doping a Semiconductor

• let's add 1 ppm of P (5 valance electrons)
• still same as silicon
  • 4 bonds [per atom
  • all electrons tied up
  • P just takes a Si place
• there's now one more electrons
  • not used in the bonds
  • free to move
• now electrically conducting like a metal
  • but 1/1,000,000 as good
• said to be N-doped
  • negative charge carriers= electrons

P-doping

• lets add 1% Al instead
• still looks just like Si solid
• Al atom has four bonds
  • but is one electron short
• Electron from neighboring bond can fill hole
  • of course that bond is now short an electron
  • the "hole" moves
• since the hole is one electron below neutral
  • the hole is though of as a positive entity
• such semiconductors are P-doped

Combining N-doped and P-doped materials

• makes electronic devices
• diodes, transistors
• solar panels

• The best solar panels are made from nearly perfect Si crystals
  • expensive, especially in larger sizes
  • sliced into thin layers (costly process)
  • traditional design, unlikely to be the future
• Can make glass like material
  • comes out as a strip
  • poly-crystalline
  • much cheaper, but less efficient
  • preferred area for development
• Can make non-crystalline material (amorphous)
  • much easier and cheaper
  • must less efficient panels
  • would be best, but is less promising now

Cost?

• find 1-2 sq inch panels in $10 calculators
• garden lights $20
• 120 Watt panel (24 volts)
  • $609 (72 cells, polycrystaline material)
  • 28" x 57" (28 lbs)
• (These are consumer prices in a small market)

Toledo Company-- First Solar

• factory to manufacture
• deposits from vapor
• expect products January 2002
• First Solar Web site