Synergy of phosphorus gettering and hydrogenation in multicrystalline silicon wafers and cells

Abstract

In mc-silicon the minority carrier diffusion length (L) is improved by external gettering or hydrogenation techniques. LBIC maps show that L increases except in "bad regions" which contain large densities of defects and impurities. It is proposed to use two steps: first gettering (1) then hydrogenation (2). Step (1): P diffusion at 850/spl deg/C, for 20 min, from a POC13 source, at low pressure (200 mbars), followed by a slow cool down to 700/spl deg/C; step (2): post-gettering deposition of a SiN-H antireflection coating by PECVD at 350/spl deg/C, followed by metallisation and firing at 700/spl deg/C. After step(1) L achieves 200 to 220 /spl mu/m, out of "bad regions". After a subsequent step(2) the recombination strength of extended defects is neatly reduced, especially in the bad regions, giving rise to an increase of the conversion efficiency by 1.5 to 3 percent absolute.