The simplest model of a photovoltaic cell is a model consisting of two elements connected in parallel:
1. A source of current proportional to the power of solar radiation on the surface of the element. This source determines the short circuit current.
2. A diode shunting this source. The voltage drop across this diode determines the open circuit voltage (approximately 0.7V for silicon).
This model perfectly describes the characteristics of a photocell in both the forward and reverse directions. If desired, it can be complicated with series and parallel resistors.
Article Model of photovoltaic cell with consideration of load variability