New Interpretation of the Effect of Hydrogen Dilution of Silane on Glow-Discharged Hydrogenated Amorphous Silicon for Stable Solar Cells

Abstract

The effect of hydrogen dilution on glow-discharged hydrogenated amorphous silicon (a-Si:H) is investigated at substrate temperatures of 100–200° C. The dependence of the properties of a-Si:H on the hydrogen dilution ratio γ (γ=[ H 2 gas flow rate]/[ SiH 4 gas flow rate]) can be explained in terms of two different effects: i.e., decrease of the film deposition rate at a low γ and implantation of hydrogen atoms into a-Si:H during and after film deposition at a high γ. The latter effect, which is similar to that of hydrogen plasma post-treatment, increases the hydrogen content ( C H ) and optical gap ( E opt ) of a-Si:H with no significant deterioration in photoconductivity or SiH 2 /SiH ratio estimated from infrared absorption. It is found that the electric conductivity and defect density of a-Si:H, both in the annealed state and light-soaked state, have a better correlation with the hydrogen content with SiH 2 bond configurations ( C SiH 2 ) than with C H or E opt . A conversion efficiency of 8.8% is achieved after light soaking (1.25 sun, AM-1.5, 48° C, open load, 310 h) for a single-junction a-Si:H solar cell using an a-Si:H i-layer with reduced C SiH 2 .