Combined impurity gettering and defect passivation in polycrystalline silicon solar cells

Abstract

Polycrystalline silicon wafers have been subjected to annealing (700 °C, 1 h) and to a hydrogen plasma (350 °C, 30 min) during the processing of solar cells. The annealing treatment enhances the bulk minority-carrier recombination lifetime by 19%, presumably by impurity gettering. The plasma treatment improves the lifetime by 26%; hydrogen passivation accounts for at least 2/3 of this improvement. Gettering and passivation are found to be complementary: application of both treatments results in a 43% increase in lifetime compared to standard.

Rapid Thermal Dopants Diffusion and Surface Passivation for Silicon Solar Cells Applications

“…Both effects contribute to increase the effective bulk lifetime in the base region. So far, the aluminum gettering process is explained as follows: The molten AlSi alloy acts as an infinite sink for impurities diffusing out of the silicon and fast diffusers, mainly copper and iron [11,12], are readily gettered. Since the solubility of metals in Si does not exceed 1017 cm-3 , the segregation coefficient of the metal between the liquid and Si is of the order of 104, which provides a tremendously large driving force for the metal to segregate into the liquid [12].…”

Selected Solar Cell Types

“…Firstly, the traps at the grain boundaries are saturated and secondly, a p-p+-BSF structure can form on the boundary, because the diffusion rate of aluminium along the grain boundaries is higher by orders of magnitude than in the crystalline nlaterial [41]. Recently, some authors have shown [42], [43], that a simultaneous gettering of phosphorus and aluminium provide the best results. A comprehensive overview of defects and their gettering is given by Schindler [44].…”

Gettering Processes and the Role of Extended Defects

“…An additional beneficial effect of AlG extending throughout a silicon wafer and occuring in connection with a subsequent H-passivation has been attributed to the injection of vacancies [16,76,77] that are thought to enhance H diffusion [16] and promote dissociation of molecular hydrogen [78]. Such effects have been concluded to contribute to material improvement after AlG [79][80][81][82], but still lack a direct proof.…”