Excess charge carrier injection densities in PERC solar cells at open-circuit voltage and maximum power point

Abstract

Nowadays passivated emitter and rear contact (PERC) solar cells are mainstream cell technology. An accurate knowledge of the excess charge carrier injection density during illumination will help to understand the kinetic behavior of charge carrier sensitive defects such as boron-oxygen related defect, LeTID defect or FeB pair dissociation and thus supports reliability improvements of PERC cells. However, the excess charge carrier injection density is not easily accessible in experiments. The aim of our research is to investigate the distribution of the injection density in PERC cells in the range of 22% to 24% efficiency. Thus, we perform a numerically simulated Design of Experiment varying the base resistivity and the location of recombination to derive the excess charge carrier injection density. However, no relevant combined influences of the location of recombination on the injection density are found. The base averaged excess charge carrier injection density increases with higher PERC efficiencies as well as with higher specific resistivity of the base. For a sufficient description of the injection density at open-circuit condition, the law of mass action under applied voltage considering high-level injection can be used to calculate the injection density. At maximum power point voltage, an effective voltage at the pn-junction is derived considering the voltage drop due to the lumped series resistance. This effective voltage is used to calculate an injection density with the law of mass action considering high level injection.