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A3.06 - Amorphous Silicon Passivation of Nano-Textured Silicon for Application in Solar Cells Exhibiting an Implied Open Circuit Voltage Above 700 mV and Reflectivity Below 5% 
Date/Time:
April 22, 2014   3:30pm - 3:45pm
 
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Nano-textured “black” silicon (b-Si) exhibits a reflectivity of only a few percent in the visible range as well as excellent light scattering properties [1]. It is therefore especially well-suited to enhance the short circuit current (jsc) of silicon solar cell concepts like silicon hetero-junction (SHJ) solar cells on thin wafers [2] or polycrystalline (poly-Si) thin film solar cells [3]. However the nano-textured surface is prone to high defect densities and thus a major challenge is the development of suitable passivation layers. Up to now solar cell concepts comprising b-Si rely on surface passivation with Al2O3 [4], or thermal silicon oxides [5]. Such dielectric passivation layers must be opened locally for contact formation. An alternative but up to now unexplored route is the passivation with hydrogenated amorphous silicon. This approach is advantageous since passivation and contacting of the nanostructured surface can be achieved with a single layer and furthermore the amorphous-crystalline SHJs excellent passivation enables the highest open circuit voltages of any silicon wafer-based technology [2].In this communication we present nano-textured amorphous-crystalline silicon hetero-junctions which exhibit excellent passivation quality and low reflection values. Minority carrier lifetimes above 1.3 ms, and implied open circuit voltage above 700 mV on nano-textured silicon surfaces with reflectivity below 5 % are reached using plasma-enhanced chemical vapor deposition of approximately 5 to 6 nm thick intrinsic amorphous silicon layers for the passivation of b-Si. Nano-textured silicon has been implemented in wafer-based and thin film amorphous-crystalline SHJ solar cells. For both cell types the reflectivity was strongly reduced and an effective reflectivity below 5% was obtained. For the thin film-based solar cell an increase in jsc by 30% has been obtained. For wafer-based solar cell jsc was increased as compared to a planar reference, however due to inferior blue response jsc is lower than for a conventional textured reference. Numerical simulations indicate that this loss is due to a photo inactive region inside the nanotexture. Furthermore these early prototype cells experience a drop of the implied voltage during cell processing, which leads to a final open circuit voltage of 614 mV. Options to preserve the excellent initial passivation of the intrinsic amorphous silicon layers during the whole process and to improve the poor blue response of these solar cells will be discussed. Utilizing these improvements may open a viable route towards high-efficiency b-Si solar cells with thin absorbers.[1] S. Koynov et al., Appl. Phys. Lett. 88 (2006) 203107[2] M. Taguchi et al., IEEE J. of Photovoltaics (2013) in press[3] D. Amkreutz et al., Prog. Photovolt: Res. Appl 19 (2011) 937[4] P. Repo et al., Energy Procedia 38 (2013) 866-871[5] J. Oh et al., Nature Nanotechn. 7 (2012) 743-748
 


 
 
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