Characteristics curve of solar cell

The I-V characteristic of a solar cell shows the relation between the output current and voltage under constant sunlight and temperature. The equivalent circuit is a current source (photocurrent Iph) in parallel with a diode, with small series and shunt resistances.

I = Iph − I₀ [exp(qV / nkT) − 1]

 I (A)
 Isc |-----------.
     |            '.
     |   MPP -> *   '.
     |          |     '.
     |          |       |
     +----------+-------+---- V
              Vmpp     Voc

Important points

• Short-circuit current (Isc): current at V = 0; it is directly proportional to the irradiance.
• Open-circuit voltage (Voc): voltage at I = 0 (about 0.6 V for silicon); it rises only slowly (logarithmically) with irradiance and drops with temperature.
• Maximum power point (MPP): the point (Vmpp, Impp) where the product V×I is largest. Pmax = Vmpp × Impp.
• Fill factor (FF): FF = Pmax / (Voc × Isc), usually 0.7–0.85. It shows the squareness of the curve.
• Efficiency: η = Pmax / (G × A), where G is irradiance and A is the cell area.

Effect of sunlight: higher irradiance raises the current (curve moves upward). Effect of temperature: higher temperature reduces Voc (about −2 mV/°C per cell), and hence reduces power (about −0.4 to −0.5% per °C).