This is a major simplification of a university course I took a decade ago but here we go. SIlicon doesn’t conduct electricity. Silicon can be doped with Phosphorus which means a P atom takes the place of a Si atom in it’s crystal structure. But P has 1 more electron than Si, and this electron becomes free flowing in the crystalline structure, which is now conductive. This is called a N-type (n for negative) semiconductor. You can do the same with Boron except this time we are 1 electron short creating a free flowing “hole” this is called a P-type (P=positive) semiconductor.These days I think they use different elements but the principal is the same.

When you put an Ntype and Ptype semiconductor together the atoms and holes migrate to each other, forming ions and balancing each other out, this creates a zone called the depleted zone and can no longer conduct electricity and has an electric field that repels more electrons (and preventing the entire thing becoming depleted). The depletion zone can’t expand because the electric field prevent any more electrons reaching the Ptype semiconductor.

Now when a photon hits the depletion zone it adds enough energy to the ion to kick the electron out again into the Ntype semiconductor, it wants to get back to a hole in the p-type semiconductor, but it can’t because of that electric field. However if there is a wire connecting the two it will take the long way around to get to the PType semiconductor again, generating a current.