Electronic surface passivation of crystalline silicon solar cells by a-SiC:H

Hydrogenated amorphous silicon carbide (a-SiC:H) provides excellent electronic surface passivation for crystalline silicon solar cells. The hydrogen and carbon content of the passivation layers control the surface passivation depending on hydrogen bonding and annealing temperature. The carbon content c_C of the amorphous layers varies depending on the methan-to-silane gas flow ratio during deposition. The electronic passivation quality exhibits best thermal stability for an optimum c_C = 2.3 at.%. Annealing this sample under forming gas atmosphere up to T_FG = 550°C enables excellent effective minority carrier lifetimes τ_eff = 1.2 ms. Hydrogen effusion measurements relate this result to an increase in H-content with rising c_C and to a simultaneous shift of the effusion peaks to higher temperatures. A higher carbon content reduces the diffusion of atomic hydrogen out of the amorphous layers. The Si-H bonding configurations in the amorphous layers, analyzed from infrared absorption spectroscopy, reveal that a-SiC:H layers with lower carbon content have a higher density. Increasing c_C induces voids and microvoids in the amorphous structure, favoring the diffusion of molecular hydrogen out of the a-SiC:H layers. We show the implementation of the thermally most stable a-SiC:H as back side of an industrial silicon solar cell. Evaporated and tempered Al point contacts through the amorphous layers enable the current transport through a-SiC:H. Compared to a full-area back side metallization, the lower recombination velocity of the a-SiC:H back side enhances the open circuit voltage, demonstrating the benefit of a-SiC:H passivation for industrial crystalline silicon solar cells.