19 05 2014

Another question – ‘I was wondering if you could possibly do an example of an example of calculating Pmax for a solar cell please‘.

This one needs a bit more explanation than a one liner – so here goes.  Starting with the circuit schematic for the solar cell with a load resistor, which can be found in figure 11.3

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Hence, the current through the load resistor is the photo-current (IL) and a reverse direction p-n junction, which is described by Shockleys equation (see the second year notes!).  This gives equation 11.1.

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From this, it is possible to calculate the short circuit current, by setting the voltage term in Shockley’s equation to zero, giving equation 11.3 – but realise that a lot of the terms at the front of the equation are in fact Is – the saturation current!

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You can also balance the two currents (IL and the p-n junction) to get Voc (i.e. the voltage when no current flows) and so this gives equation 11.2

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Having got all this, the last step is to look at the maximum power.  Because you get zero power at Isc (because V=0) and at Voc (because I=0) there is a voltage and current at which the maximum occurs and this is given by Voc x Isc x FF.  Here the fill factor (FF) is an experimentally extracted value (quoted to you if required)

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