Requirements for achieving extremely low surface recombination velocity and negligible optical loss in Cat-CVD SiNx/a-Si stacked passivation

Extremely low surface recombination velocity (SRV) of crystalline silicon (c-Si), as low as 1.3 cm/s or even lower, is achieved with silicon nitride (SiNx) and amorphous silicon (a-Si) stacked passivation layers, all prepared by Cat-CVD (Hot Wire CVD). In Cat-CVD, the source gases are decomposed by catalytic cracking reactions with heated metal wires such as tungsten (W) wires, and thus, high quality films can be deposited on c-Si without any surface damages due to plasma. Avoiding plasma damage appears a reason for obtaining low SRV, however, the other major reasons for achieving such low SRV are studied along with the influence of optical absorption in a-Si of SiNx/a-Si stacked layers. It is discovered that the penetration of nitrogen (N) atoms into c-Si, which occurs during SiNx deposition, degrades the quality of c-Si itself, and the simple direct deposition of the conventional SiNx will limit the improvement of performance of c-Si solar cells, although SiNx is well-used material in c-Si solar cells. The insertion of a-Si layer with a thickness of 10 nm or more can suppress such N penetration and helps to obtain low SRV. Although the insertion of thicker a-Si layer may cause the optical loss, it is also found that all photo-carriers generated in a-Si of SiNx/a-Si stacked layers are not recombined inside a-Si layer and transported into c-Si when the thickness of a-Si layer is 10 nm or less. Thus, a-Si does not induce optical loss at all, when the thickness of it is 10 nm. It is concluded that extremely low SRV is achieved without any optical loss by using SiNx/10 nm-thick a-Si stacked passivation layer.