Silicon nitride cover layer prepared by silane-free plasma chemical vapor deposition for high quality surface passivation of silicon solar cells

Abstract

Passivated emitter and rear contact (PERC) silicon solar cell is one of the most promising high-efficiency solar cells, which uses aluminum oxide (Al2O3) and hydrogenated silicon nitride (SiNx:H) stack as the rear passivation layer. The SiNx:H not only serves as a protective layer but can provide hydrogen atoms that further passivate the dangling bonds on silicon wafer surface. In this study, the SiNx:H cover layers are deposited using high density plasma chemical vapor deposition with a silane-free gas mixture of tetramethylsilane (TMS) and ammonia (NH3). Effects of the TMS gas flow rate on properties of the SiNx:H cover layers and effective minority carrier lifetime of silicon wafers are investigated. The experimental results show that the film density is positively related to the SiN bond proportion. The secondary ion mass spectrometry measurement shows that the dense SiNx:H can make hydrogen atoms move toward wafer surface instead of out-diffusion to the environment. The best SiNx:H film is deposited at a TMS flow rate of 35 sccm. The minority carrier lifetime can be improved from 70 μs for as-cleaned wafer to 373 μs for wafer/Al2O3 and to 1006 μs for wafer/Al2O3/SiNx:H. The surface recombination rate of 23 cm/s is obtained. Therefore, this study demonstrates the dense, silane-free SiNx:H films with high passivation quality for PERC solar cells.