That would say that the short-circuit (V=0) panel current is 261.88A. I would like to see your panel...
My apologies for the misunderstanding. The equation is actually:
I=0.5407v^5+4.8027v^4+14.649v^3+16.603v^2-8.9411v+261.88
where I is the output current supplied by the PV cell and V is the voltage across it.