how to write an equation the it is function of itself (solar cell current equation)

Answer 1

[Torsten]

I guess you want to solve for J ?

Then define

fun_J = @(J) J - (J_PH - J0*(exp(q*(V+J*R_S)/(k_B*T))-1)-(V+J*R_S)/R_SH)

and use fzero or fsolve to get a solution.

Or do you want to make a curvefit ?