Conducting two-phase silicon oxide layers for thin-film silicon solar cells
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1123-P03-09
Conducting two-phase silicon oxide layers for thin-film silicon solar cells Peter Buehlmann1, Julien Bailat2, Andrea Feltrin1, Christophe Ballif1 1 IMT, University of Neuchâtel, Neuchâtel, Switzerland 2 Now at Oerlikon Solar-Lab, Neuchâtel, Switzerland ABSTRACT We present optical properties and microstructure analyses of hydrogenated silicon suboxide layers containing silicon nanocrystals (nc-SiOx:H). This material is especially adapted for the use as intermediate reflecting layer (IRL) in micromorph silicon tandem cells due to its low refractive index and relatively high transverse conductivity. The nc-SiOx:H is deposited by very high frequency plasma enhanced chemical vapor deposition from a SiH4/CO2/H2/PH3 gas mixture. We show the influence of H2/SiH4 and CO2/SiH4 gas ratios on the layer properties as well as on the micromorph cell when the nc-SiOx:H is used as IRL. The lowest refractive index achieved in a working micromoph cell is 1.71 and the highest initial micromoph efficiency with such an IRL is 13.3 %. INTRODUCTION Thin film silicon technologies have a high potential for further cost reduction of terrestrial photovoltaics for energy production. In this technology, multi-junction solar cells are candidate to achieve efficiencies comparable to the well established crystalline silicon solar cells. One of the most promising structures is the micromorph cell where a high band-gap amorphous top cell and a low band-gap microcrystalline bottom cell are stacked upon each other to better exploit the solar spectrum. The individual cells composing a multi-junction cell are electrically connected in series and special care has therefore to be taken to match their currents. Matching of currents can be achieved by adjusting the thickness of the thin absorber layers or by the insertion of an intermediate reflecting layer (IRL). The second solution is especially interesting for the micromorph cell where the top cell has to be kept reasonably thin (< 300 nm) to limit the effects of the light induced Staebler-Wronski degradation of the amorphous material [1]. For high reflectivity, the IRL should have a refractive index (n) as low as possible whereas for the electrical requirements, the IRL must have a transverse conductivity of at least 10-5 S/cm to avoid blocking the device. Very low in-plane conductivity is desired to avoid series interconnection of shunts in the elementary cells. The first results on micromorph cells with IRL were presented in 1996 by D. Fischer [2] who used ZnO as low-n material. Later a different approach was introduced by K. Yamamoto [3] without specifying the material used. In this study we will investigate on the use of n-doped, hydrogenated silicon sub-oxide containing silicon nanocrystals (nc-SiOx:H) as proposed by our group [4] and A. Lambertz [5] in 2007. EXPERIMENTAL DETAILS The n doped nc-SiOx:H films were prepared from a mixture of SiH4, CO2, H2 and PH3 gases in a capacitively coupled very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) system. In this study
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