Silicon surface and bulk defect passivation by low temperature PECVD oxides and nitrides

The effectiveness of PECVD passivation of surface and bulk defects in Si, as well as phosphorous diffused emitters, is investigated and quantified with respect to the performance improvement of Si solar cells. Significant hydrogen incorporation coupled with high positive charge density in the PECVD SiN layer is found to play an important role in bulk and surface passivation. It is shown that photo-assisted anneal in a forming gas ambient after PECVD depositions significantly improves the passivation of emitter and bulk defects. PECVD passivation of phosphorous doped emitters and boron doped bare Si surfaces is found to be a strong function of doping concentration. Surface recombination velocity of less than 200 cm/s for 0.2 Ohmcm and less than 1 cm/s for high resistivity substrates (~500 Ohmcm) were achieved. PECVD passivation improved bulk lifetime in the range of 30% to 70% in multicrystalline Si solar cell materials. However, the degree of the passivation was found to be highly material specific. Depending upon the passivation scheme, emitter saturation current density (J_oe) can be reduced by a factor of 3 to 9. Finally, the stability of PECVD oxide/nitride passivation under prolonged UV exposure is established