Compensation engineering for uniform n-type silicon ingots

This paper addresses a major issue related to the use of upgraded-metallurgical grade silicon for n-type solar cells. We show that n-type silicon ingots, grown from silicon feedstock containing both boron and phosphorus, display a vertical net doping variation which is incompatible with high-yield production of high-efficiency solar cells. As a solution, we propose to use compensation engineering, by means of gallium co-doping, and demonstrate its potential to control the net doping along the ingot height. The resulting material exhibits high minority carrier diffusion length gratefully to compensation but degrades upon illumination due to the activation of the boron-oxygen defect. This latter degradation remains an important though not unsurmountable challenge for making high-efficiency n-type solar cells with upgraded-metallurgical grade silicon.