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Lets consider a solar cell as an ideal pn-junction, for simplicities sake even without
the current contributions from the space charge region, but with a built in series resistance
Rser and a shunt resistance
Rshunt |
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We have the following equivalent circuit diagram (also defining what is meant by a shunt resistance). See
also the "Solar Cell Primer" in a basic module |
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The shunt resistance takes into account that the huge area of the pn-junction of a solar cell might have weak
points (locally, e.g. at the edge) which short-circuit the junction somewhat. These defects are summarily described by a
shunt resistor. |
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The constant current source mimics the current generated in the junction by light. it simply defines a current value
Iphot (not to be mixed up with the terminal current I) that is given by the light
and added (with a negative sign) to the junction current, i.e. Ijunct = Idiode(U)
– Iphot
. Iphot thus simply moves the total characteristics of the diode downwards on the current scale. |
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Take the following schematic curve of I-U-characteristics as a reference and for the
definition of the following terms |
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The fill factor is the relation between the area of the yellow rectangle to the pinkish area under the characteristics. |
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Derive the complete current-voltage relationship. |
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Discuss qualitatively the influence of the two resistors with particular respect to: |
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The open-circuit voltage UOC |
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the short-circuit current ISC |
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The fill factor FF (the degree of "rectangularism" of the characteristics). |
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The efficiency h which is proportional to UOC,
ISC , and FF, i.e. |
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| h | = |
const · UOC · ISC · FF |
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Link to several exercises to solar cells, far exceeding this one, with the solutions. There, look at chapter 8. |
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© H. Föll (Semiconductors - Script)
With frame
3.4.1 Junction Diodes