New interpretation of the effect of hydrogen dilution of silane on glow-discharged hydrogenated amorphous silicon for stable solar cells

The effect of hydrogen dilution on glow-discharged hydrogenated amorphous silicon (a-Si:H) is investigated at substrate temperatures of 100-200 degrees C. The dependence of the properties of a-Si:H on the hydrogen dilution ratio gamma (gamma=[H-2 gas flow rate]/[SiH4 gas flow rate]) can be explained in terms of two different effects: i.e., decrease of the film deposition rate at a low gamma and implantation of hydrogen atoms into a-Si:H during and after film deposition at a high gamma. 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 SiH2/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 SiH2 bond configurations (C-SiH2) 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 degrees C, open load, 310 h) for a single-junction a-Si:H solar cell using an a-Si:H i-layer with reduced C-SiH2.