How a solar cell works.

This is a sequence of problems for you to think about and work on.

1. Consider an unbiased pn junction. Suppose a single photon arrives in the depletion region, e.g., around x=0,  and suppose it excites one electron from the valence band (top) to the conduction band (bottom). What happens after that? That is, what happens to the electron? What is the primary influence on that electron? Where does that additional electron go?

Think about this wherever you go today. Discuss with people at the beach or downtown or in the forest. Then post your thoughts (as a comment) later tonight.

Hint: You can assume that the recombination time is pretty long compared to other quantum electron time scales and that the electron has time to go somewhere.

More to follow:

2. Suppose there are many photons incident on the depletion region of a pn junction and that in each second \(4 \times 10^{18}\) of those photons each excite an electron-hole pair. (So that there are \(4 \times 10^{18}\) electrons/sec appearing in the conduction band.) What happens? What if there is a wire connecting the normal p region to the normal n region? What if there isn't a wire? What if there is a 100 ohm resistor along the wire?

3. What if there is a 100 Ohm resistor in the wire? Would a current go through that resistor? Would that create a voltage across the resistor?  V=IR  Would that mean that there is also a voltage across the junction? Would that be positive or negative on the p side?

(Remember the principle of superposition for E&M. There could be a current due to the photons and another current due to the voltage and maybe you can add them and find a self-consistent solution.)