Using alternative to function handle (@) for Matlab embedded in Simulink

hi! I am a very unexperienced matlab / simulink user with a small problem. I have an implicit function of which I would like to calculate the two unknowns. In matlab I used the ode45 function to solve the function, this worked perfectly but when trying to use ode45 in combination with function handle (@) in the Matlab-block from simulink, it allways say that the @-function handle feature is not supported.

How can I define/declare this function alternatively?? The equation I would like to solve is the one-diode model of a photovoltaic cell.

thank you very much cheers!