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Highly Efficient Silicon Solar Cell Based on Asymmetric Nanowire Structure

Highly Efficient Silicon Solar Cell Based on Asymmetric Nanowire Structure
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A novel asymmetric silicon nanowire (SiNW) to improve an efficiency of the solar cells has been investigated using experimental results and numerical simulations. The solar energy has been highlighted for future renewable clean energy resources due to its free, safety, and inexhaustible properties. However, the solar energy requires high production costs and shows the limitation of conversion efficiency from light to electrical energy. To reduce the production costs and improve the efficiency, the various types of the solar cells have been proposed. Among them, the SiNW solar cells have been received great attentions due to their unique one-dimensional characteristics. The inherent characteristics of the SiNWs, such as the direct carrier collection paths and high surface to volume ratio improve, improve the photo-excited carrier transport and light absorption, which results in an increase of the conversion efficiency. Recently, to the use of low-quality silicon while maintaining the efficiency, the radial p-n junction SiNW design was proposed. Despite the radial p-n junction SiNW solar cells have been improved the efficiency and reduced the production costs, their efficiency are still low, and many challenges, such as investigating modification of the SiNW structure, optimization of the structural parameters, anti-reflection coating (ARC) layer, and reduction of the front contact resistance and back contact recombination, have to be resolved to reach the theoretical limitation of efficiency. Therefore, this study investigates the analytic researches to resolve those challenges and presents a new SiNW structure to boost the efficiency. First, the asymmetric radial SiNW structure is proposed using three-dimensional numerical simulations to improve the efficiency of the SiNW solar cells. This nanostructure is designed by shrinking the bottom core diameter from the vertical SiNW with holding the top core diameter, which results in a total reflection of the incident light at the outer wall of the shell due to the difference in refractive indices between silicon and ARC layer. The reflection enhances the incident light trapping, which results in a 10 times higher optical carrier generation rate and larger light absorption as compared with the vertical symmetric radial SiNW structure. As a result, the efficiency increases by over 10% when the bottom core diameter decreases from the symmetric SiNW to asymmetric SiNW due to improved optical carrier generation and light absorption. Furthermore, the efficiency of the radial SiNW solar cell is enhanced by increasing shell doping concentration, applying ARC and back surface field layers, and optimizing geometrical parameters, i.e. core diameter, shell thickness, and height. Second, the fabrication process of the symmetric and asymmetric vertical radial SiNW solar cells have been developed using the top-down method with photo-lithography and selective dry etching process. Firstly, before the SiNW patterning, the backside of wafer was ion implanted to reduce the recombination at interface between the back metal contact and silicon substrate. A single step deep-reactive-ion-etching (SDRIE) was used to make the asymmetric SiNWs. By tuning the composition of the gas mixture of the SDRIE process, a slant bottom angle between the substrate and sidewall of the SiNW was controlled, which make it possible to pattern the asymmetric SiNWs with various bottom diameters. The poly-Si layers were deposited on the patterned SiNW to form a p-n junction, which is applicable to the production of the low-quality silicon solar cells. Also, by forming the front and back metal patterns, the series and contact resistance of the vertical SiNW solar cells could be reduced. Finally, the optical and electrical characteristics of a highly efficient radial SiNW solar cells based on the asymmetric SiNW have been presented. Without anti-reflection coating, a maximum short circuit current density of 27.5 mA/cm2 and an efficiency of 7.53% have been demonstrated. The improvement in efficiency arises by changing the SiNW structure from symmetry to asymmetry
and this efficiency is higher than that of a planar solar cell due to increased short circuit current density. With total reflection at the sidewall, there is an increase of the light trapping path and charge carrier generation in the radial junction of the asymmetric SiNW, which causes high external quantum efficiency and short circuit current density. The proposed asymmetric structure has great potential to significantly improve the efficiency of the SiNW solar cells.
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