Altermatt Lecture:   The PV Principle


2.7:  Currents through the diode (in the dark)

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?

Current injection in the dark

Is this a likely path of current through the solar cell in the dark?
  Are you sure? Do you remember that the p-type contact is selective to holes, so hardly any electrons from the conduction band can pass through it?
  The p-type contact is selective to holes, because there are only few electrons in the conduction band near it. This makes the current-path shown above very unlikely.

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 pn, 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.

Dark IV curve

Figure: Dark current–voltage curve.


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