PV panel -a current source or voltage source?
As usual, the question is about building a model, and how well it conforms to reality.
If you connect a solar panel to a high impedance load (hence expecting a very low current in the panel), modeling the solar panel as a imperfect voltage source (ie. with a series resistor) is certainly the most pertinent.
Else, you need to understand that the physics of a solar panel implies that the current that flows through it is directly proportional to the number of photons impacting the cells. In that case, if you have a (very) low impedance load, the solar panel would be better approximated with a current source.
You can find a more mathy explanation here.