To understand how a solar cell operates under illumination, it is very useful to look first at how the
current flows through the diode in the dark.
When one applies a bias to a diode in the dark, a current flows through it. By applying the bias,
electrons are injected at the n-type contact into silicon. How do they flow through the diode?
These are the likely current paths:
This implies that the electrons, injected at the n-type contact, are able to exit through the p-type contact only if they
jump down from the conduction to the valence band somewhere in the cell. In other words, only those electrons can pass through the
cell that recombine somewhere in the cell. This behavior is shown in the bottom figure.
How does the current depend on the applied bias V? On the previous page, you learned that the minority carrier density increases
exponentially with applied bias, and because the recombination rate is proportional to p⋅n, the recombination rate increases
exponentially with applied bias. Since all the current that passes through the cell is recombination, the current through the cell
increases exponentially with applied bias in the dark. See the figure below.
Figure: Dark current–voltage curve.