Fabrication and performance of p + layer by SiO 2 nanospheres assisted liquid boron diffusion
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Fabrication and performance of p+ layer by SiO2 nanospheres assisted liquid boron diffusion Junkui Zhu1 · Honglie Shen1 · Dongli Hu2 · Hao Gu1 · Kai Gao1 Received: 4 May 2020 / Accepted: 10 July 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Boric acid mixed with S iO2 nanospheres was used as a boron source in the spin-on method to improve p + layer doping quality in the surface of p-type single crystal silicon (c-Si) wafers with pyramid structure texture. The effects of boric acid concentration, diffusion temperature and diffusion time on diffusion uniformity, surface hole concentration, junction depth and minority carrier lifetime were investigated. It was found that the addition of SiO2 nanospheres in boric acid could result in a high degree of diffusion uniformity more than 96%. When the boric acid concentration was 5 g/100 mL and diffusion was at 940 °C for 40 min, the obtained p + layer in silicon wafer showed a best overall performance with a surface hole concentration of 8 × 1018 cm−3, a sheet resistance (RS) of 70.2 Ω/□, a junction depth of 0.28 μm and minority carrier lifetime of 3.39 μs.
1 Introduction In the preparation of solar cell with a back surface field (BSF), a p/p+ structure is normally adopted. The p/p+ structure will generate a built-in electric field from the p region to the p+ region, which accelerates the photo-generated carriers and increases the effective diffusion length of the carriers to reduce the surface recombination losses and dark current. At present, industrialized solar cells are mainly based on p-type silicon wafers [1]. For the manufacturing process of p-type silicon solar cells, the p+ region of BSF is usually fabricated by boron doping and evaporated aluminum sintering [2]. Compared with aluminum, the solubility of boron atoms in silicon has an advantage of several orders higher in magnitude. Boron doping can reduce the resistivity and activation energy of silicon surface significantly to improve photovoltaic performance. With the increase of doping concentration, the carrier concentration increases first and then * Honglie Shen [email protected] 1
College of Materials Science & Technology, Jiangsu Provincial Key Laboratory of Materials and Technology for Energy Conversion, Nanjing University of Aeronautics & Astronautics, 29 Jiangjun Avenue, Nanjing 211106, People’s Republic of China
Key Laboratory of Silicon Based Electronic Materials of Jiangsu Province, Jiangsu GCL Silicon Material Technology Development Co., Ltd., No. 88 Yangshan Road, Xuzhou 221000, People’s Republic of China
2
becomes saturated, while the conductivity and activation energy gradually approach optimized values [3]. According to a report [4], compared with 11.2% efficiency of the traditional Al-BSF solar cells, the heavily boron-doped p + layer was capable to improve the efficiency of BSF ones to 12.9%. Therefore, the usage of simpler methods to improve the quality of boron BSF (B-BSF) stimulated the attention of many researchers in recent year
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