Photoelectron Spectroscopy Measurements of Valence Band Discontinuities for a-Si:H/c-Si Heterojunction Solar Cells
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1153-A10-03
Photoelectron Spectroscopy Measurements of Valence Band Discontinuities for a-Si:H/c-Si Heterojunction Solar Cells Tetsuya Kaneko1 and Michio Kondo1, 2 1 Innovative and Engineered Materials, Tokyo Institute of Technology, Nagatsuta-cho, Midoriku, Yokohama, 226-8502, Japan 2 RCPV, AIST, Umezono, Tsukuba, 305-8568, Japan ABSTRACT The valence band discontinuity (offset) between a-Si:H-based intrinsic thin layers and cSi 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. INTRODUCTION Heterojunction solar cells consisting of thin a-Si:H layers on c-Si substrate such as “HIT” solar cells [1] have an advantage of higher potential efficiency over conventional c-Si solar cells as already proven by high open circuit voltage and its low temperature coefficient under operation. Although it has been suggested that the device characteristics are affected by the band alignment of a-Si:H and c-Si at the heterointerface, no consensus has been established for the value of the band-offset in spite of a variety of measurements using different methods [2-7]. For instance, Cuniot and Marfaing [3] measured the band discontinuities by internal photoemission; they reported the valence band discontinuities lower than 0.15 eV for a-Si:H/c-Si(p) heterojunctions. In contrast, Mimura and Hatanaka [7] estimated the valence band discontinuities (∆EV) of a-Si:H/ c-Si(n+) heterojunctions using internal photoemission, they obtained ∆EV=0.71 eV. The purpose of the present study is to evaluate the valence band offset at heterojunctions between thin a-Si:H-based layers and c-Si substrates as employed in actual “HIT” devices using photoelectron spectroscopy. EXPERIMENTAL DETAILS Ultraviolet photoelectron spectroscopy (UPS) in combination with x-ray photoelectron spectroscopy (XPS) was used to determine the valence band offset. The UPS was performed
with a He-I resonance line (21.22eV). Fermi level position was determined by measuring a clean Ag surface. The XPS was carried out with a non monochromatic Mg Kα radiation (400W 1253.6 eV). The base pressure of analysis chamber was 5×10-8 Pa. The valence band maximum (VBM) was determined by li
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