I don't know what PV model you're using, but if you're interested in the Rs current then put Rs in place of n.
Here's an example with a LED and a bv source.
Are you saying that at a given irradiance,
the Voltage V from the panel = 0.5407*I**5 + 4.2807*I**4 + 14.649*I**3 + 16.603*I**2 + 8.4911*I + 261.88, where I is the Current output of the panel?
The attached .asc file is how to model that:
I1 is the independent variable of the simulation. The red trace shows the MPPT.
That would say that the short-circuit (V=0) panel current is 261.88A. I would like to see your panel
The way I interpreted the equation is that the open-circuit (I=0) output voltage of the panel is 261V and per my plot, the short-circuit (v=0) output current of the panel is ~4.2A, which is more like real panels I have worked with.