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Solar Cells

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  • Solar Cells
  • Physics


Connexions module: m11343 1 Solar Cells ∗ Bill Wilson This work is produced by The Connexions Project and licensed under the Creative Commons Attribution License † Abstract Solar Cells Now let us look at the opposite process of light generation for a moment. Consider the following situation. P-N diode under illlumination Figure 1 We have just a plain old normal p-n junction, only now, instead of applying an external voltage, we imagine that the junction is being illuminated with light whose photon energy is greater than the band-gap. In this situation, instead of recombination, we will get photo-generation of electron hole pairs. The photons simply excite electrons from the full states in the valance band, and "kick" them up into the conduction band, leaving a hole behind. As you can see from Figure 1 (P-N diode under illlumination), this creates excess electrons in the conduction band in the p-side of the diode, and excess holes in the valance band of the n-side. These carriers can diffuse over to the junction, where they will be swept across by the built-in electric field in the depletion region. If we were to connect the two sides of the diode together with a wire, a current would flow through that wire as a result of the electrons and holes which move across the junction. Which way would the current flow? A quick look at Figure 1 (P-N diode under illlumination) shows that holes (positive charge carriers) are generated on the n-side and they float up to the p-side as they go across the junction. Hence positive current must be coming out of the anode, or p-side of the junction. Likewise, electrons generated on the p-side fall down the junction potential, and come out the n-side, but since they have negative charge, this flow represents current going into the cathode. We have constructed a photovoltaic diode, or solar cell (Figure 2 (Schematic representation of a photovoltaic cell))! Here is ∗ Version 1.1: Jun 20, 2003 12:00 am -0500 †

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