Performance degradation in CZ(B) cells and improved stability high efficiency PERT and PERL silicon cells on a variety of SEH MCZ(B), FZ(B) and CZ(Ga) substrates

The cells made on CZ(B) (boron doped Czochralski) silicon substrates often exhibit degraded minority carrier lifetimes corresponding to poorer cell pe rformance. The first part of this paper reports our cell results on a varie ty of CZ(B) substrates. Most of these CZ(B) cells showed a severely reduced cell performance after being stored for a period of time. The cells on hig her resistivity CZ(B) substrates from Crysteco Inc., showed no degradation due to its low boron level of 10 Omega -cm resistivity. The second part of this paper describes work avoiding this CZ(B) degradatio n problem by using different substrate growth methods or different dopant, such as MCZ(B) (boron doped magnetically confined Czochralski grown), CZ(Ga ) (gallium doped Czochralski), and FZ(B) (boron doped float zone) substrate s. All these substrates were supplied by Shin-Etsu Handotai Co, Japan, (SEH ). High efficiency PERL (passivated emitter, rear locally-diffused) cell an d Pert (passivated emitter, rear totally-diffused) cell structures were use d for these cells. The best energy conversion efficiency of 24.5% has been demonstrated by a PERT cell on a MCZ(B) substrate. This is the highest ener gy conversion efficiency ever reported by a silicon cell made on a non-FZ s ubstrate. All these cells have shown stable performances after a period of storage. The PERT cell structure has also considerably reduced the cell ser ies resistance from higher resistivity substrates. A total rear boron diffu sion in this PERT structure also appears to improve the surface passivation quality of MCZ(B) and some FZ(B) substrates. Hence, higher open-circuit vo ltages were observed for these PERT cells. It is also believed that these M CZ(B) and other SEH materials have considerably improved material qualities under SEH's current effort to develop high quality substrates particularly for photovoltaic applications. Copyright (C) 2000 John Wiley & Sons, Ltd.