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NANO EXPRESS

Open Access

Phosphorus diffusion gettering process of multicrystalline silicon using a sacrificial porous silicon layer Derbali Lotfi* and Ezzaouia Hatem

Abstract The aims of this work are to getter undesirable impurities from low-cost multicrystalline silicon (mc-Si) wafers and then enhance their electronic properties. We used an efficient process which consists of applying phosphorus diffusion into a sacrificial porous silicon (PS) layer in which the gettered impurities have been trapped after the heat treatment. As we have expected, after removing the phosphorus-rich PS layer, the electrical properties of the mc-Si wafers were significantly improved. The PS layers, realized on both sides of the mc-Si substrates, were formed by the stain-etching technique. The phosphorus treatment was achieved using a liquid POCl3-based source on both sides of the mc-Si wafers. The realized phosphorus/PS/Si/PS/phosphorus structures were annealed at a temperature ranging between 700°C and 950°C under a controlled O2 atmosphere, which allows phosphorus to diffuse throughout the PS layers and to getter eventual metal impurities towards the phosphorus-doped PS layer. The effect of this gettering procedure was investigated by means of internal quantum efficiency and the dark current–voltage (I-V) characteristics. The minority carrier lifetime measurements were made using a WTC-120 photoconductance lifetime tester. The serial resistance and the shunt resistance carried out from the dark I-V curves confirm this gettering-related solar cell improvement. It has been shown that the photovoltaic parameters of the gettered silicon solar cells were improved with regard to the ungettered one, which proves the beneficial effect of this gettering process on the conversion efficiency of the multicrystalline silicon solar cells. Keywords: Multicrystalline silicon, Porous silicon, Defect density, Grain boundaries, Gettering, WTC-120 lifetime tester

Background Due to the high price of fossil energy source and huge pollution of environmental issue, in the recent years, the development of renewable energy has regained our attention. The development of the solar energy industry is one of the most popular technologies in renewable energy. The majority of solar cells are made from crystalline silicon; more than half of the crystalline silicon solar cell production is based on multicrystalline silicon (mc-Si). The quality of multicrystalline silicon (mc-Si) wafers may become worse in the future for several reasons; the main reason is that lower-quality feedstock will probably be used for cost reduction and availability reasons [1]. Improving conversion efficiency using a low* Correspondence: [email protected] Photovoltaïc laboratory, Research and Technology Center of Energy, Technopôle de Borj-Cédria. BP 95, Hammam-Lif 2050, Tunisia

cost material development is the main research activity in the photovoltaic field [2,3]. Several experiments demonstrated that the electrical properties of crystalline silicon solar cells can be improved by