Study of Si Nanowires Produced by Metal-Assisted Chemical Etching as a Light-Trapping Material in n-type c-Si Solar Cells.

Abstract

Si nanowires (SiNWs) produced by metal-assisted chemical etching on n-type Si were investigated for their use as a light-trapping material in c-Si solar cells. The nanowires were fabricated before junction formation (on a lightly doped Si substrate) so that their core was bulk and nonporous. The above fabrication process was implemented in solar cell fabrication. The SiNW reflectivity was tested at different steps of solar cell processing and found to be lower than that of conventional random pyramids used in c-Si solar cells. Contact formation on the front side of the cell was investigated by considering metal deposition either directly on the nanowires or on bulk areas in between the nanowire areas. The superiority of this second case was demonstrated. Three different Si nanowire lengths were investigated, namely, 0.5, 1, and 1.5 μm, the case of 1 μm giving better results in terms of solar cell characteristics and external quantum efficiency. The electronic quality of the Si nanowire surface was investigated using the corresponding metal-oxide-semiconductor capacitors with atomic-layer-deposited alumina dielectric. Successful reduction of surface recombination centers at the large Si nanowire surface was achieved by reducing structural defects at their surface through a specific chemical treatment. Finally, using the determined optimized conditions for Si nanowire formation, chemical cleaning, and process implementation in solar cell fabrication, we demonstrated ∼45% increase in solar cell efficiency with 1 μm SiNWs compared to that from a flat reference cell processed under similar conditions. The above study was made on test solar cells without surface passivation.