Phosphorus deactivation mechanisms by hydrogenation in the n+ emitter region and its effect on defects passivation in n+pp+ poly-silicon solar cells

Abstract

Doping level of the n+ emitter region is an essential parameter that controls the performance of the n+ pp+ poly-silicon solar cells. Also, most poly-silicon n+ pp+ solar cell manufacturers apply hydrogenation from the phosphorus emitter n+ side to improve photovoltaic efficiency. Although hydrogen can passivate defects as well as it changes initial phosphorus doping level through phosphorus-hydrogen complex formation. Consequently, phosphorus deactivation can have a harmful effect on photovoltaic efficiency. In this context, the primary purpose of this work is to investigate the phosphorus deactivation in n+ emitter region and its effect on defects passivation of hydrogenated n+ pp+ poly-silicon solar cells. To do this, hydrogenation is performed by microwave plasma discharge involving an electron cyclotron resonance system. Besides, hydrogen passivates defects in poly-silicon, at the same time it deactivates phosphorus. For this reason, we have chosen to separate these simultaneous effects. So, we performed phosphorus deactivation on Schottky diodes-based mono-silicon, while defect passivation was operated in n+ pp+ poly-silicon solar cells. Our results reveal that hydrogen effectively deactivates phosphorus dopant. This effect is deeper in Schottky diodes with low initial phosphorus doping level where hydrogen diffuses easily in the bulk. This behavior is clearly revealed in open circuit-voltage values (Voc) measured on n+ pp+ samples. In fact, solar cells with low phosphorus concentration in n+ region revealed 319 mV compared to 230 mV for high doping level. Also, all n+ pp+ poly-silicon solar cells show a saturation of Voc at high microwave plasma power. Reasons for such case were explained and discussed in detail.