I'm reading about PV behaviour and am confused on whether a PV panel/cell would be considered to be a voltage source or current source or both or neither (from the characteristic IV curve). The IV curve looks like a combination of both constant current and constant voltage. It seems that from (a) panel is unloaded to (b) panel is loaded to max power--the operating point travels from Voc to MPP(If this is the case; I'm confused as to the reason behind why a PV cell is modelled as a current source). Do we use only this region under load conditions? Under what conditions does the operating point traverse from MPP to Isc point? Why is a solar cell modelled as a current 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.
Most PV inverters have an MPPT (max. power point tracker) in them. It's purpose is to keep the panel operating at the MPP for what should be obvious reasons.
You do NOT want to load the panels so that the voltage drops (i.e. to the left of the MPP) and you are moving toward Isc. This, of course, operates away from the MPP and also causes excess current to flow which may, over time, damage the panel.
You do, often, operate to the right of the MPP since there are times when the inverter must "throttle down" the output of the panel or even switch them off. Here the current drops and the voltage approaches Voc. That rightmost point is where you are operating an unconnected panel.
The reason a PV panel is modelled at a current source is that is how they behave.