Fill factor limitations and non-ideal diode behaviour of Czochralski silicon solar cells due to light-induced recombination centres

The light-induced activation of the deep-level recombination centre specific to boron-doped, oxygen-contaminated Czochralski (Cz) silicon leads to a shoulder in the current-voltage curve of solar cells fabricated on this material. This effect is demonstrated by means of numerical simulation and measurements on different types of high-efficiency Cz silicon solar cells. The physical reason for the non-ideal diode behaviour, characterised by a local ideality factor greater unity, is the strongly injection-level-dependent bulk lifetime produced by the characteristic recombination centre. The increased ideality factor causes a degradation in fill factor of up to 15% with the magnitude of degradation depending on the doping concentration of the Cz silicon. The optimal base resistivity for high-efficiency solar cells made on boron-doped Cz silicon is shown to be in the vicinity of 3 omega cm. The comparison of current-voltage curves measured before and after degradation clearly shows that the reduction in fill factor is not due to device issues (i.e., series resistance, shunt, recombination in the space charge region), but is an inherent property of boron-doped, oxygen-contaminated Cz silicon as it is widely used in the production of solar cells.