Electromotive force
Solar cell photo emf

Solving the illuminated diode's above simplified current–voltage relationship for output voltage yields:

which is plotted against I / I 0 {\displaystyle I/I_{0}} in the figure.

The solar cell's photo emf E p h o t o {\displaystyle {\mathcal {E}}_{\mathrm {photo} }} has the same value as the open-circuit voltage V o c {\displaystyle V_{oc}}, which is determined by zeroing the output current I {\displaystyle I}:

It has a logarithmic dependence on the light-induced current I L {\displaystyle I_{L}} and is where the junction's forward bias voltage is just enough that the forward current completely balances the light-induced current. For silicon junctions, it is typically not much more than 0.5 volts. While for high-quality silicon panels it can exceed 0.7 volts in direct sunlight.

When driving a resistive load, the output voltage can be determined using Ohm's law and will lie between the short-circuit value of zero volts and the open-circuit voltage V o c {\displaystyle V_{oc}}. When that resistance is small enough such that I ≈ I L {\displaystyle I\approx I_{L}} (the near-vertical part of the two illustrated curves), the solar cell acts more like a current generator rather than a voltage generator, since the current drawn is nearly fixed over a range of output voltages. This contrasts with batteries, which act more like voltage generators.
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