Photoelectron Spectroscopy Measurements of Valence Band Discontinuities for a-Si:H/c-Si Heterojunction Solar Cells

Abstract

The valence band discontinuity (offset) between a-Si:H-based intrinsic thin layers and c-Si substrates was estimated using ultraviolet photoelectron spectroscopy (UPS) in combination with x-ray photoelectron spectroscopy (XPS). A core level shift measured by XPS was utilized to correct the shifts of UPS spectra after UV light illumination. Thin films of a-Si:H, a-SiO:H and a-SiC:H were prepared by plasma-enhanced chemical vapor deposition (PECVD) using SiH₄, CO₂ and CH₄ gases. The valence band offset of 0.11 eV was obtained from the a-Si:H/c-Si heterojunction, whereas 0.27 eV was obtained from the a-SiO:H/c-Si heterojunction. Moreover, the valence band offset between the c-Si and the a-SiC:H deposited with [CH₄]=10 SCCM and [CH₄]=20 SCCM were determined to be 0.25 eV and 0.36 eV, respectively. The c-Si-based heterojunction solar cells with estimated i layer in this study were fabricated, reduction of FF with increasing the valence band offset was observed. It is likely that increasing of the valence band offset contributes to the reduction of FF.