= Measuring a solar cell's current-voltage characteristic
Keep the illumination and cell <temperature> approximately constant. Connect an <ammeter> and an adjustable <resistor> as a <series circuit> across the <solar cell>; connect a high-resistance <voltmeter> across its terminals. Vary the <electrical resistance> and record pairs of <electric potential difference> $V$ and delivered <electric current> $I$. This measures the illuminated <current-voltage characteristic>.
A load close to a <short circuit> estimates $I_{\rm sc}$ at $V\simeq0$. With the load disconnected, an <open circuit> measurement using a high-resistance <voltmeter> estimates $V_{\rm oc}$ at $I\simeq0$. Neither endpoint delivers useful <power>: $P=VI$ vanishes there. The intermediate readings locate the maximum of $VI$ and determine the <solar-cell fill factor>. Holding illumination fixed matters because the photogenerated <electric current> changes with <light intensity>; holding <temperature> fixed matters because the <diode saturation current> and junction response change with it.
This experiment distinguishes producing an <electric potential difference> from delivering <energy> to a load. To determine conversion efficiency as well, measure the incident <power> on the illuminated area, rather than equating it to a lamp's electrical rating.
\Image[/past-exam-of-the-quantum-condensed-matter-course-of-the-university-of-cambridge/2012-solar-cell-output.png]
{title=The maximum-power point of an illuminated solar cell lies between short circuit and open circuit}
{description=An illustrative ideal <diode> model, with $eV_{\rm oc}/(k_BT)=20$, shows the <current-voltage characteristic> and delivered <power>. These are calculated curves, not measured data.}
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