Issues with undefined variables in curve fitting

Hi I'm new to MATLAB and I'm trying to curve fit some data based on the theoretical model to find the various parameters of the electrical component. I keep getting:

>> Formal_Report_Curve_Fitting
Unrecognized function or variable 'Isf'.

Error in Formal_Report_Curve_Fitting (line 13)
p = [Isf,Isr,Bf,Br];

From the following script:

            data = readmatrix("RAW_Zener_Diode_I-V_Data.xlsx") ; 
            vD = data(:,1);
            iD = data(:,2);
                
            plot(vD, iD, "*");
                
            double Isf;
            double Isr;
            double Bf;
            double Br;
                
            zener_response = @(p,vD)(p(1).*(exp(p(3).* vD)-1) - p(2).*(exp(-p(4).*vD) - 1));
            p = [Isf,Isr,Bf,Br];
                
            Isf_guess = 70/1000;
            Isr_guess = 20/1000;
            Bf_guess = ((25/1000)*(1))^-1;
            Br_guess = ((25/1000)*(2))^-1;
                
            p0 = [Isf_guess, Isr_guess, Bf_guess, Br_guess];
            p1 = lsqcurvefit(zener_response, p0, vD, iD);
                
            iDfit = zener_response(p1, vD);
            plot(vD, iD, vD, iDfit);

Answer 1

[Torsten]

data = readmatrix("RAW_Zener_Diode_I-V_Data.xlsx") ; 
vD = data(:,1);
iD = data(:,2);
zener_response = @(p,vD)(p(1).*(exp(p(3).* vD)-1) - p(2).*(exp(-p(4).*vD) - 1));
Isf_guess = 70/1000;
Isr_guess = 20/1000;
Bf_guess = ((25/1000)*(1))^-1;
Br_guess = ((25/1000)*(2))^-1;

p0 = [Isf_guess, Isr_guess, Bf_guess, Br_guess];
p1 = lsqcurvefit(zener_response, p0, vD, iD);

iDfit = zener_response(p1, vD);
plot(vD, iD, vD, iDfit);