Semiconductor Junction I - Basic Principles

Most solar cells work based on p-n junctions.  P-n junctions are created when a p-type material is joined (possibly glued) to a n-type material.  The outcome electronic band shape is shown in the diagram below.  On the left is the p-type material, where the Fermi level is closer to the valence band, the majority charge carriers are holes, and the fixed doping atoms in the lattice are negatively charged.  On the right is the n-type material, where the Fermi level is closer to the conduction band, the majority charge carriers are electrons, and the fixed doping atoms in the lattice are positively charged.

The slope in the middle of the 2 materials is indicative of the electric field from the n-type material to the p-type material.  This is because the Fermi level must be constant across the entire p-n junction, but the energy differences between conduction and valence bands and the Fermi level will vary in the space charge region.  The electric field arises from the space charge region (depletion zone) in the middle when the 2 materials are joined together to form a p-n junction.  The majority charge carriers in the zone firstly diffuse across the boundary between the 2 materials due to the large density gradient.  Next, they recombine with the majority charge carriers across the boundary (the p-n interface), thereby forming a space charge region with negligible charge carriers.  Hence, the fixed doping atoms in the lattice of the 2 materials are left to form the electric field (see diagram below).  This electric field, which causes drift, will act on minority charge carriers, eg. moving electrons from the p region to the n region.

This is the situation when the p-n junction is at thermal equilibrium in the dark.  It is important to note that diffusion can be increased by increasing the density of the majority charge carriers, or by reducing the width of the depletion zone.  The drift can be increased by increasing the density of the minority charge carriers, or by increasing the electric field E over the depletion zone.  The electric field E is described as follows:

E = q V_bi

where q is the charge of a hole or electron, and V_bi is the built-in voltage at the depletion zone.

The p-n junction's equilibrium can be affected by applying a bias voltage over the p-n junction, or by shining a light source to increase the minority charge carrier densities, thereby creating a solar cell.

Reference:
2.4.1 Semiconductor Junction I - Basic Principles, Delft University of Technology, https://www.youtube.com/watch?v=xN7gUmt-BnQ