INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 3, ISSUE 1, JANUARY 2014 ISSN 2277-8616 1 IJSTR©2013 www.ijstr.org Efficiency Improvement Of Crystalline Silicon Solar Cells By Optimizing The Doping Profile Of Pocl 3 Diffusion Hocine Ghembaza, Abdellatif Zerga, Rachid Saïm Abstract: The emitter formation constitutes a crucial step in the manufacturing of the crystalline silicon solar cells. Several techniques are used in the photovoltaic industry and the most well-known one is based on the POCl3 diffusion in cylindrical quartz tube. Despite the efficiency of this technique to be reproducible, economic and simple, it presents the major inconvenient to have a heavily doped region near the surface which induces a high minority carrier recombination. To limit this effect, an optimisation of diffused phosphorous profiles is required. Our modelling of phosphorus profiles is summarized in the presence of an erfc distribution near to the surface and other Gaussian distribution in the bulk region of the emitter. However, this work is devoted to study the effects of the temperature, diffusion time, surface concentration and doping profile on the crystalline silicon solar cells performances by using the new parameters. The first results of our numerical modelling carried out by the Silvaco Atlas® simulation package show the possibility to improve the efficiency by 2.78%. This result is also confirmed by the IQE calculus which present an obvious enhancement in short wavelength region (380-450nm) about 23%. Index Terms: Crystalline silicon solar cells, Emitter, Phosphorus, POCl3 diffusion. ———————————————————— 1 INTRODUCTION During the last decades, the research in crystalline silicon solar cells is mainly focused on improving the cell efficiency and reducing the cost of watt peak. The emitter is the most relevant element of solar cell. It is formed by POCl 3 diffusion which is actually the most used technique in silicon solar cells processing. Unfortunately, this technique imposes a high concentration of dopants in surface which limits considerably the passivation of surface. Thus, the design of an efficient emitter must take into account a low surface concentration of dopants without affecting the quality of screen printed contacts. In the literature, the classical phosphorus diffusion profile in crystalline silicon is constituted by a plateau in the region over 10 20 cm -3 , a kink in the 10 19 cm -3 region and a tail in the region below 10 18 cm -3 [1]. Actually, in order to improve solar cells efficiencies, several optimization of emitter have been proposed by many researchers and they are based on varying the design of experiments. However, few theoretical models with analytical solutions have been used to study emitter [2-3]. Dunham [4] and Bracht [5] describe the phosphorus diffusion in silicon with the pair-diffusion model where phosphorus diffuses in association with interstitials (I) and vacancies (V). Fermi level dependencies of various charge states are taken into account to capture the kink and tail of the diffusion profile [6]. In this work, the principal motivation is to optimize the emitter according the different diffusion parameters by using a new modelling of POCl 3 diffusion. In our modelling, the analytical considerations of Dunham and Bracht are taken into account and integrated in the numerical code which it was accomplished by Silvaco-TCAD package [7]. However, an experimental phosphorous diffusion profile was used to validate our numerical simulation results. This experimental profile was obtained with phosphorus diffusion in crystalline silicon at 825°C during 30 minutes. The sheet resistance of emitter was about 40Ω/sq. The phosphorous profile was determinate by the Secondary Ion Mass Spectroscopy (SIMS) [8]. 2 MODELING 2.1 Process description The phosphorus diffusion process was carried out in an industrial diffusion tube furnace under a low-pressure atmosphere of about 200mbar. The wafers are vertically placed into a quartz support for loading 156x156 mm² wafers. Every wafer is placed at a distance of 3 mm next to each other. The support is placed into the quartz tube and is heated up to 800°C. A flow of nitrogen is used to carrier the liquid dopant source POCl 3 via a bubble trap. Before POCl 3 injection, the O 2 gas is introduced into the diffusion tube to ensure the thermal oxidation of silicon. At the higher temperatures, phosphorus diffuses into silicon matrix forming the p-n junction with the p-type substrate doped initially at 10 16 cm -3 (0.1Ω.cm resistivity). All the wafers have a thickness of 200μm. Usually, the diffusion process consists of an active dopant deposition step followed by a drive-in step. During the deposition, a Phospho-Silicate Glass PSG is formed on surface of substrate. The PSG glass is a mixture of phosphorus pentoxide (P 2 O 5 ) and silicon dioxide (SiO 2 ) [9]. This glass is etched by fluorydric acid 10% during 5 minutes before SIMS and sheet resistance characterizations. _________________________  Hocine Ghembaza, Materials and Renewable Energies Research Unit (URMER), Tlemcen University, PO Box 119, 13000 Tlemcen, Algeria, PH-+21343215890. E-mail: hocineghembaza@gmail.com  Abdellatif Zerga, Materials and Renewable Energies Research Unit (URMER), Tlemcen University, PO Box 119, 13000 Tlemcen, Algeria, PH-+21343409131. E-mail: a-zerga@yahoo.fr  Rachid Saïm, Laboratoire Energétique et Thermique Appliquée (ETAP), Tlemcen University, PO Box 230, 13000 Tlemcen, Algeria.