Lecture 14 - Lecture 14 Lecture 14 Direct vs. indirect gap...

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Lecture 14 1 © Jeffrey Bokor, 2000, all rights reserved Lecture 14 Direct vs. indirect gap Some semiconductors are good absorbers, and absorb all above-bandgap light in a layer of a few microns thick. These are called direct-bandgap semiconductors. In others, called indirect-gap semi- conductors, which include crystalline silicon, the absorption process is weaker. In this case, a phonon (a quantum of the lattice vibration) is necessary to conserve momentum in the light absorption pro- cess. In silicon, a layer several hundred microns thick is required. Solar cell structure bus bar fingers finger anti-reflection coating p-type base n-type emitter backside contact
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Lecture 14 2 © Jeffrey Bokor, 2000, all rights reserved The top contact structure typically consists of widely spaced thin metal strips to allow the light to pass through, with a larger bus bar connecting them all to extract the current. An anti-reflection coating on top of the cell can be used to minimize reflection loss from the top surface. The light generation current in the diode is in the reverse direction, so we can write to total current as the difference between the two: The I-V characteristic now looks like this: Maximum power point No power is generated under open or short circuit.
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Lecture 14 - Lecture 14 Lecture 14 Direct vs. indirect gap...

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