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Solar Cell Bias

  1. Aug 29, 2011 #1
    My question is basically how can a solar cell operate in forward bias?

    A normal diode in forward bias (with a battery connected) drives electrons from the N-side towards the pn junction where they are continuously recombining with holes (reducing the depletion zone and allowing for the flow of current).

    A solar cell seems less intuitive though. When in forward bias the electrons travel the opposite way. They go out the external cicuit which connects to the p-side, where the recombine with holes.

    It seems strange to me that both of these are essentially in "forward bias" however the electrons are esentially travelling the opposite way, similar to that of a diode in reverse bias.
  2. jcsd
  3. Aug 29, 2011 #2
    The junction is forward-biased in normal photovoltaic operation, and the resulting diode current is diverted from the load so it should stay small, meaning that a silicon PN cell must be operated around 0.45V at room temperature, even less at heat.

    Though, photons must have >1.12eV to produce a pair, and Sunlight has even higher energies. This explains why Solar cells are inefficient. The quantum photon-to-pair conversion alone would be extremely efficient.

    Improvements include:
    - Gap energies better matched to Sunlight, like InSb or GaAs materials (spacecraft prices)
    - Stacks of materials that harvest each one slice of light energy
    - Concentrated light produce more concentrated current, allowing a higher voltage. Better: it saves semiconductor area.
    - Cool the cells. Especially if Sunlight is concentrated.
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