Lets consider a solar cell as an ideal pnjunction, for simplicities sake even without the current contributions from the space charge region, but with a built in series resistance R_{ser} and a shunt resistance R_{shunt}  
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 pnjunction of a solar cell might have weak points (locally, e.g. at the edge) which shortcircuit 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 I_{phot} (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. I_{junct} = I_{diode}(U) – I_{phot}. I_{phot} thus simply moves the total characteristics of the diode downwards on the current scale.  
Take the following schematic curve of IUcharacteristics 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 currentvoltage relationship.  
Discuss qualitatively the influence of the two resistors with particular respect to:  
The opencircuit voltage U_{OC}  
the shortcircuit current I_{SC}  
The fill factor FF (the degree of "rectangularism" of the characteristics).  
The efficiency h which is proportional to U_{OC}, I_{SC}, and FF, i.e.  


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© H. Föll (Semiconductor  Script)