Thin film silicon alloys with enhanced stability made by PECVD and HWCVD for multibandgap solar cells
This paper presents the development of high deposition rate protocrystalline Si:H and /spl mu/c-Si:H as obtained by hot wire chemical vapor deposition (HWCVD, or catalytic CVD) and very high frequency plasma enhanced CVD (VHF PECVD) and protocrystalline SiGe:H as obtained by conventional PECVD. Using the latter two low bandgap materials as absorber layer, it is feasible to obtain stable devices showing virtually no light-induced changes in fill factor. We have obtained stable proto-SiGe:H cells using high H/sub 2/ dilution. A disadvantage, however, is the very low deposition rate for proto-SiGe:H (/spl sim/0.6 /spl Aring//s). By HWCVD, much higher deposition rates have been achieved for silicon-based thin films (both proto-Si:H and pc-Si:H). During the last few years, improved control of parameters is also obtained in HWCVD, which presently makes this method interesting for reducing the production cost of thin film solar cells. Recently, we have demonstrated that protocrystalline Si:H can be obtained from undiluted silane at 250 /spl deg/C in HWCVD at a high deposition rate of 1 nm/s. The light induced degradation in single junction n-i-p cells is less than 10%. Here, we also present for the first time p-i-n cells on commercial SnO/sub 2/:F with proto-Si intrinsic layers deposited at rates up to 3 nm/s, with again remarkable stability. Further, HWCVD microcrystalline materials with a crystalline fraction V/sub f/ of /spl sim/50% possess excellent electronic quality: the dangling bond density extracted from computer simulations, is only 2 /spl times/ 10/sub 15/ cm/sup -3/ and therefore we could obtain fill factors of 0.77 and 0.75 in dual-junction stacked /spl mu/c-Si:H//spl mu/c-Si:H cells, and proto-Si:H//spl mu/c-Si:H 'micromorph' n-i-p type solar cells, respectively. Both VHF PECVD /spl mu/c-Si:H made in the high pressure depletion regime and HWCVD /spl mu/c-Si:H have excellent potential for multibandgap solar cells.
Productivity potential of an inline deposition system for amorphous and microcrystalline silicon solar cells
Abstract Amorphous and microcrystalline silicon single layers and p–i–n solar cells were produced dynamically using an inline deposition system called “line source”. A highly uniform deposition of thin-film silicon layers with layer-thickness variations of less than ±5% was achieved. Amorphous and microcrystalline silicon single junction solar cells were dynamically fabricated with initial efficiencies of 8.3% and 6.3%, respectively. The dynamic deposition rate of these solar cells is 6.75 nm m/min in case of a-Si:H and 3.3 nm m/min for μc-Si:H. In this work it will be shown that an enhancement of the deposition rate up to 15.6 nm m/min during the i-layer deposition of a-Si:H solar cells has only a weak negative influence on the initial efficiencies of the cells. Further on, the effect of substrate velocity on solar cell characteristics of a-Si:H solar cells is investigated. Finally, a productivity estimation of the line source concept is presented.
ITO surface ball formation induced by atomic hydrogen in PECVD and HW-CVD tools
Abstract The formation of granule-like balls on the surface of indium-tin oxide (ITO) films induced by atomic hydrogen (H-) treatment in plasma-enhanced chemical vapor deposition (PECVD) and hot-wire chemical vapor deposition (HW-CVD) tools is reported in this paper. The ball formation on the film surface is responsible for the reduction in optical transmittance and electrical conductivity of ITO films. The X-ray diffraction patterns have revealed that the H-treated ITO surfaces are composed of complicated compounds, including metallic indium, indium tin oxides, and indium hydroxides. Auger electron spectroscopy has shown that the balls are rich in indium atoms and areas outside the balls are rich in oxygen atoms. Finally, we have shown that by capping the ITO surface with a PECVD silicon oxide thin film, the degradation of ITO films can be completely prevented.