Solar Cell Formula - I/V from Load Resistance

Background: I have the following simplified formula in a VisualBasic program that gives me current output (Icell) from variable input Volts (cellVoc) for a simple PV cell simulation.
By varying cellVoc I can plot IV curve, scaled output & calculated load resistance agrees with my physical 255w PV panel in test circuit & LTspice. (I am not intending to accurately model a PV cell, my sim program will interact with a PIC micro code simulator )

'Inputs
Io1 = 0.0000000000114   ' PV Diode current
PVt = 0.02568           ' PV T
CellVoc = 0.616         ' **(variable 0 to CellVoc)
CellIsc = 8.78          'Illumination current
'Formula
Icell = Cell_Isc - (Io1 * (Exp(cellVoc / PVt))) Question: What would be the formula so that I can input Load resistance as the variable. I am thinking I need [Vcell = ... Rload ...] as input into above formula.

I have tried but I am stuck on how to implement the exponent for the non linear relation between Load resistance & cell volt where Ipv drops off . Thanks