p-type nc-SiOx:H emitter layer for silicon heterojunction solar cells grown by rf-PECVD
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p-type nc-SiOx:H emitter layer for silicon heterojunction solar cells grown by rf-PECVD Henriette A. Gatz1, Yinghuan Kuang1, Marcel A. Verheijen1, Jatin K. Rath2, Wilhelmus M.M. (Erwin) Kessels1, 3, and Ruud E. I. Schropp1, 3 1 Department of Applied Physics, Eindhoven University of Technology, The Netherlands 2 Department of Physics and Astronomy, Utrecht University, The Netherlands 3 Solliance, Eindhoven, The Netherlands ABSTRACT Silicon heterojunction solar cells (SHJ) with thin intrinsic layers are well known for their high efficiencies. A promising way to further enhance their excellent characteristics is to enable more light to enter the crystalline silicon (c-Si) absorber of the cell while maintaining a simple cell configuration. Our approach is to replace the amorphous silicon (a-Si:H) emitter layer with a more transparent nanocrystalline silicon oxide (nc-SiOx:H) layer. In this work, we focus on optimizing the p-type nc-SiOx:H material properties, grown by radio frequency plasma enhanced chemical vapor deposition (rf PECVD), on an amorphous silicon layer. 20 nm thick nanocrystalline layers were successfully grown on a 5 nm a-Si:H layer. The effect of different ratios of trimethylboron to silane gas flow rates on the material properties were investigated, yielding an optimized material with a conductivity in the lateral direction of 7.9×10-4 S/cm combined with a band gap of E04 = 2.33 eV. Despite its larger thickness as compared to a conventional window a-Si:H p-layer, the novel layer stack of a-Si:H(i)/nc-SiOx:H(p) shows significantly enhanced transmission compared to the stack with a conventional a-Si:H(p) emitter. Altogether, the chosen material exhibits promising characteristics for implementation in SHJ solar cells. INTRODUCTION Silicon heterojunction solar cells in a back-contacted configuration hold the record efficiency of 25.6% for crystalline silicon solar cells.1 One of the main features enabling this outstanding efficiency is the increased short circuit current density Jsc, which can be attributed to the fully back contacted design. However, this design requires a much more complicated production procedure compared to conventional SHJ solar cells. Therefore, several groups are working towards the goal of increased Jsc by implementing more transparent window layer in the front side of the conventional bifacially contacted SHJ cell, thus increasing the Jsc of SHJ cells but in addition keeping the more simple cell design. Possible candidates are e.g. amorphous silicon carbide (a-SiC:H)2, molybdenum oxide (MoOx), x 0.0067 the increase of boron incorporated in the material is the main influence on most material properties. This increase is attributed to a higher boron fraction of the process gasses. The increased boron incorporation facilitates a decrease in band gap.11 The activation energy scales with the band gap and therefore also diminishes. The rise in active boron incorporation in the film can also lead to a gain in conductivity.12 A suitable candidate for SHJ solar cell emitter has to exhibit a
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