That's why most silicon-based solar panels have impurities built in - to allow electrons to move around and fill in gaps, creating an electric field.

Elements have different atomic numbers - the number of protons or electrons increases up the table as electrons fill the shells.

The electrons fill the holes.

Suppose the electrons just fill exactly an integral number N of Landau levels.

In this picture electrons fill the all the states except for a few holes near the point.

Although electrons are contributed from a specific atom, the electron fills an MO without regard to its source atom.

Chemists assume that the electrons will fill the lowest energy level MOs first.

I understand there are some irregularities in F orbitals, and I'm not sure exactly in what order the electrons fill the energy levels.

The electrons go in and fill all the states up to some energy, until there are no more electrons left.

Thus, electrons fill orbitals in the order specified by the energy sequence given above.