# Exercise 3.4.1

| 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
|  | 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
|  | 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.
| 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.
|  | Take the following schematic curve of I-U-characteristics as a reference and for the definition of the following terms
|  | The fill factor is the relation between the area of the yellow rectangle to the pinkish area under the characteristics.
| Derive the complete current-voltage relationship.
| Discuss qualitatively the influence of the two resistors with particular respect to:
|  | The open-circuit voltage UOC
|  | the short-circuit current ISC
|  | The fill factor FF (the degree of "rectangularism" of the characteristics).
|  | The efficiency h which is proportional to UOC, ISC , and FF, i.e.
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.

With frame

3.4.1 Junction Diodes

Solar Cell Primer

Solution to 3.4.1

© H. Föll (Semiconductors - Script)