Minority-carrier lifetime enhancement in edge-defined film-fed grown Si through rapid thermal processing-assisted reduction of hydrogen-defect dissociation

Abstract

This paper demonstrates that a very short, 1-s, simultaneous firing of screen-printed Al at the back and SiNx antireflection (AR) coating at the front can significantly enhance the minority-carrier lifetime in edge-defined film-fed grown (EFG) ribbon Si via SiNx-induced hydrogen passivation of defects. It was found that 1-s firing in a rapid thermal processing system at an optimum temperature improved the average minority-carrier lifetime from 3to>80μs, resulting in ∼16% efficient 4-cm2 screen-printed EFG Si cells. It is proposed that rapid thermal firing enhances the retention of hydrogen at defect sites by minimizing the hydrogen-defect dissociation. A combination of simulations and experiments reveals that the dissociation of hydrogen is extremely rapid at conventional firing temperatures of ∼700°C. An activation energy of 2.4–2.6eV was determined for the hydrogen-defect dissociation in EFG Si. This activation energy, in conjunction with the room-temperature photoluminescence data, suggests that the impurity-decorated dislocations are the dominant hydrogenation and dehydrogenation sites in the EFG Si. Based on the above understanding, a manufacturable process, involving rapid co-firing of SiNx AR coating, screen-printed Al-doped back surface field (Al-BSF), and screen-printed Ag front grid, was developed in a conventional belt furnace to minimize the degree of dehydrogenation while producing good Al-BSF and ohmic contacts. This process produced 4-cm2 screen-printed EFG Si cells with an efficiency of 15.9%.