Analysis of multi-crystalline silicon solar cells at low illumination levels using a modified two-diode model

Current–voltage characteristics of multi-crystalline silicon solar cells measured under several low illumination levels are analyzed. The fitting analysis is conducted using a modified two-diode equivalent circuit accounting for an additional ohmic series resistance in the vicinity of grain boundaries and allowing for variable diode ideality factors. Apart from the shunt resistance—a key factor at very low illumination levels—the model indicates two other current transport mechanisms at low illumination levels, both related to recombination, but each of them can be allocated to a different region in the cell: (1) recombination in the crystalline region and (2) recombination at grain boundaries. The role played by each mechanism is studied depending on illumination level.