Semiconductor Junction II - The Solar Cell

For a p-n junction at thermal equilibrium in the dark, its diffusion current equates the drift current (diagram above).  When we apply a forward bias to the device, which means a more positive voltage to the p-region as compared to the n-region, this forward bias voltage will act in the opposite direction to the V_bi and decrease the depletion zone voltage.  Hence, the width of the depletion zone is also reduced.  This means that the diffusion current dominates the drift current, leading to a net current over the depletion zone and in the electrical circuit.  Electrons will move from the n-region to the p-region across the depletion zone, and across the circuit back to the n-region.  This results in the splitting of the Fermi level in the depletion zone, which are now called the quasi Fermi levels for electrons and holes (see diagrams below).

Energy gap between both quasi Fermi levels = qV
where V represents the forward bias voltage at the p-n junction.

When we apply a reverse bias to the device, which means a more negative voltage to the p-region as compared to the n-region, this reverse bias voltage will be in the same direction as the V_bi and increase the depletion zone voltage and width.  This means that the electron drift current density dominates the electron diffusion current density, leading to a net current in the circuit, where electrons move from the p-region to the n-region across the depletion zone, and then across the circuit back to the p-region.  However, this current is very small because the drift current is formed by minority charge carriers.

A p-n junction device with this behaviour is known as a diode, offering high conductance in forward bias and low conductance in reverse bias.

When light is shone on the device and the photons absorbed in both p and n regions, electron and hole pairs will be generated, which significantly affects only the density of the minority charge carriers.  This means that the drift current is increased, where there's more holes in the n-region and electrons in the p-region (see diagram above).  Hence, by illuminating a p-n junction device (a solar cell), current is generated.

If we connect the terminal contacts of the p-region to the n region under illumination, there will be a short circuit current I_sc resulting from this short circuiting of the device.  This current will follow the direction of the current of the device under reverse bias, but is of a much larger magnitude.  Electrons will drift from the p-region to the n-region across the depletion zone, and then through the n-region terminal contact and the circuit to the p-region terminal contact to recombine with holes.  Holes will drift from the n-region to the p-region across the depletion zone, and then diffuse to the p-region terminal contact (the back contact) to recombine with the incoming electrons from the rest of the circuit.

If the terminal contacts are unconnected (open circuit), the illumination will cause electrons to drift to the n-region and holes to drift to the p-region, resulting in a photogenerated electric field that's opposite to V_bi (in the same direction as a forward bias voltage), and consequently reducing the field and net drift current again.  This build up of electrons in the n-region and holes in the p-region will continue until both drift currents are in equilibrium (in addition to the diffusion currents).  Hence, the electric field, and correspondingly the open circuit voltage V_oc, will grow to a maximum, quite quickly.  V_oc results from the photogenerated electric field that's opposite to V_bi.

Energy gap between both quasi Fermi levels (where the electron quasi-Fermi energy is higher) = qV_oc

Reference: 2.4.2 Semiconductor Junction II - The Solar Cell, Delft University of Technology, https://www.youtube.com/watch?v=qSmTojJF-ls