Fitting of Implicit, Nonlinear Functions

Dear Matlab Experts,

I am taking current-voltage measurements of a photo-voltaic cell (j,V) and would now like to fit the resistance parameters Rs, Rp. Unfortunately the equation is implicit and all the fit functions I have used so far require explicit functions (I usually just use the cftool to fit my measurements).

j=j_0[exp(e(V-j*Rs)/(knT)-1]+(V-j*Rs)/Rp-j_L

As I am not very used to fitting in general every help is very welcome!

best wishes, Chris