Boron-doped hydrogenated silicon carbide alloys containing silicon nanocrystallites for highly efficient nanocrystalline silicon thin-film solar cells

Boron-doped hydrogenated silicon carbide alloys containing silicon nanocrystallites (p-nc-SiC:H) were prepared using a plasma-enhanced chemical vapor deposition system with a mixture of CH₄, SiH₄, B₂H₆ and H₂ gases. The influence of hydrogen dilution on the material properties of the p-nc-SiC:H films was investigated, and their roles as window layers in hydrogenated nanocrystalline silicon (nc-Si:H) solar cells were examined. By increasing the R_H (H₂/SiH₄) ratio from 90 to 220, the Si - C bond density in the p-nc-SiC:H films increased from 5.20 × 10¹⁹ to 7.07 × 10¹⁹/cm³, resulting in a significant increase of the bandgap from 2.09 to 2.23 eV in comparison with the bandgap of 1.95 eV for p-nc-Si:H films. For the films deposited at a high R_H ratio, the Si nanocrystallites with a size of 3-15 nm were formed in the amorphous SiC:H matrix. The Si nanocrystallites played an important role in the enhancement of vertical charge transport in the p-nc-SiC:H films, which was verified by conductive atomic force microscopy measurements. When the p-nc-SiC:H films deposited at R_H = 220 were applied in the nc-Si:H solar cells, a high conversion efficiency of 8.26% (V_oc = 0.53 V, J_sc = 23.98 mA/cm² and FF = 0.65) was obtained compared to 6.36% (V_oc = 0.44 V, J_sc = 21.90 mA/cm² and FF = 0.66) of the solar cells with reference p-nc-Si:H films. Further enhancement in the cell performance was achieved using p-nc-SiC:H bilayers consisting of highly doped upper layers and low-level doped bottom layers, which led to the increased conversion efficiency of 9.03%.