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a question re solar cells

Discussion in 'Electronic Design' started by RichD, Apr 30, 2013.

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  1. RichD

    RichD Guest

    I recently attendesd a seminar on photovoltaic research. The
    speaker showed a graph, how the power efficiency drops as
    the recombination time of the minority carriers decreases.
    Can anyone expound on this?

    And, the quantum efficiency drops, as well. What does
    that mean?
  2. mike

    mike Guest

    The electrons have to get to the wire. That takes time.
    The any electron that recombines never gets out the wire.
  3. Martin Brown

    Martin Brown Guest

    If the electron recombines before it reaches the external connection
    that the energy that it had is just thermalised as heat.
    I was a bit surprised to see how much they suffer when hot. eg
  4. benj

    benj Guest

    So what is "quantum efficiency"? It's simply the ratio of photons in to
    electrons out the wires. So if a photon comes in creates an electron-hole
    pair but they recombine before being collected for the output. Then that
    photon has no output. Hence QE is lower. Simple.
    What do you mean. Only some of the technologies (especially the
    "compromise" ones) experience severe drop-off. The problem is
    efficiencies so low as to limit utility. They won't be saving the planet
  5. RichD

    RichD Guest

    How is that different than power conversion efficiency?
  6. RichD

    RichD Guest

    So, it's possible to see high quantum efficency,
    but low power output efficiency?

    That would be an argument for thin junctions, yes/no?
  7. benj

    benj Guest

    No. IF Quantum efficiency is the ratio of photons in to electrons out,
    then any inefficiency will reduce that ratio.
    It's not that simple. Thin junctions reduce recombination but you lose
    efficiency if the light goes clear through the thin junction without
    being absorbed. So there are lots of compromises here. It's why this sort
    of thing has a lot of "art" to the designs.
  8. Jasen Betts

    Jasen Betts Guest

    I can't see power efficiency being synonymous with quantum efficiency
    solars photons go in with an mean energy of around 2eV and you get
    electron current out (of a silicon photocell) at less than a third of that.
  9. Guest

    You can get higher quantum efficiency by throwing a tarp over it,
    reducing the temperature. Makes 'em last longer too.
  10. Guest

    I designed a really nifty solar space-heater a few years ago, only to
    realize the winter I designed it was running overcast/cloudy 8.5 days
    out of 10. (I measured "cloudy" intensity at 3-to-5% of clear, on
    typical days, so that's a lot of cold days.)

    Fast-forwarding to today, there's a guy locally with a lot of solar
    panels selling for $1/W. This inspires an idea--air blown up under
    the panels could cool the panels, harvest the waste heat for heating,
    and I could be without heat or electricity both, all winter long.

  11. Photoelectric efficiency is the power of the light falling on the cell
    divided by the energy output of the cell. It is seldom more than 20%.

    There are several reasons for this - first, not all quanta are absorbed,
    some are reflected.

    Second, some of the quanta which are absorbed do not create
    electron/hole pairs, and some electrons and holes recombine before they
    reach the cell's electrodes.

    The proportion of quanta absorbed which produce electrons which reach
    the electrodes is often called the quantum efficiency, though
    technically the term is to mean the proportion of the quanta which fall
    on the cell which produce electrons which reach the electrodes. It is
    measured in electrons per photon.

    Third, light falls on the cell in quanta with energies somewhere between
    1.6eV (for red light) and 2.8 eV (for blue light). When a quantum is
    absorbed, some will have high initial energy and some lower initial
    energy, but the electrons they push out will all have the energy of the
    bandgap when they leave the electrodes, and some energy is always lost here.

    Fourth, the cell has some electrical resistance.

    fifth ...

    -- Peter Fairbrother
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