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p-type Window Layers for pin Solar Cells Entirely Fabricated by Hot-Wire CVD Urban Weber, Markus Koob, Chandrachur Mukherjee, D. Chandrashekhar,1 Rajiv O. Dusane,1 and Bernd Schroeder Dept. of Physics and Research Center of Materials Science, University of Kaiserslautern, Kaiserslautern, Germany. 1 Dept. of Met. Engg. & Materials Science, Indian Institute of Technology, Bombay, India. ABSTRACT We investigate a-SiC:H p-layer deposition for a-Si:H-based solar cells by Hot-Wire CVD using alternatively methane, ethane, and acetylene. Carbon incorporation in the film results from gas-phase reactions and not from direct dissociation at the hot filament for all hydrocarbon gases. Ethane can be dissociated more easily than methane allowing less extreme deposition conditions. With all types of materials the requirements of high dark conductivity and high band gap for the use as window layers in solar cells can be fulfilled. Highest conductivity is observed with ethane indicating a better network structure, which is supported by the IR signatures. A larger band gap (>2 eV) can be obtained at a similar conductivity with the use of acetylene. We compare these results with the utilization of µc-Si:H p-layers. All types of p-layers are incorporated into pin solar cells. Methane- and ethane-based a-SiC:H-p-layers yield similar Voc and FF (~850 mV and 72%). Acetylene-p-layer-based solar cells yield higher current and higher Voc (890 mV) but lower fill factor (~67%). Microcrystalline p-layers improve Voc and FF up to 900 mV and 72%, respectively, however higher absorption leads to lower short circuit current and prevents an increase of initial efficiency beyond 8%. Using ethane for p-layer deposition, a significant improvement of the stability of all-Hot-Wire CVD pin solar cells is achieved.

INTRODUCTION Recently, pin and nip solar cells were fabricated entirely by Hot-Wire (HW) or thermocatalytic CVD with initial efficiencies up to 8.8% [1,2]. In pin-type cells, the front 10 to 20 nm region (TCO/p, p-layer, and p/i interface) has a dominant influence on overall device performance. The p-layer especially acts as the “substrate” for i-layer deposition and also influences the field structure near the p/i interface. A general requirement on p-type window layers is to obtain simultaneously high conductivity and low absorption, e.g. a large optical band gap. At the same time deposition conditions should be close to that of device quality intrinsic film such as to yield a dense and smooth material. Normally, a-SiC:H p-layers were grown with methane as precursor gas and it requires extreme deposition conditions. This results in a hydrogen-rich and porous material, which leads to a higher degradation in solar cells. In this paper we compare the use of methane (CH4), ethane (C2H6) and acetylene (C2H2) as carbon precursor gases as well as p-µc-Si:H and discuss the resulting cell performance including stability. EXPERIMENTAL DETAILS Details of the deposition system have been published earlier [1]. Optical properties were determined by reflect