IMPACT OF GAS AMBIENT ON THE QUALITY OF SCREEN-PRINTED PHOSPHORUS EMITTERS Ali Hamdi, Abdullah Uzum, Saori Nagashima, Shota Suzuki, Hidenori Suzuki, Shuhei Yoshiba, Marwan Dhamrin and Koichi Kamisako Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan Hiroaki Sato, Katsuhiko Katsuma and Kuniyasu Kato The Nippon Synthetic Chemical Industry Co., Ltd. (NIPPON GOHSEI), 1-1-88, Oyodonaka, Kita-ku, Osaka 531-0076, ABSTRACT: Impact of gas ambient on the quality of screen-printed phosphorus emitters was investigated. 2.1 cm monocrystalline silicon (CZ-Si) and 1.2 cm multicrystalline (mc-Si) p-type wafers with a surface area of 55 cm 2 were used. Phosphorus diffusion paste was screen-printed on both sides of the wafers and diffused in a conventional furnace at peak temperatures of 875 o C and 900 o C for durations of 30 min in Ar, N 2 , O 2 and N 2 +O 2 ambient. Sheet resistances between 40 to 120 /sq were achieved by changing either the temperature profile or ambient. Carrier lifetime was improved up to 8 folds than the initial values after phosphorus diffusion reaching to 88 s and 86 s in CZ- and mc-Si p-type wafers, respectively. Keywords: Screen-printing, Phosphorus Diffusion, Lifetime, Monocrystalline Silicon, Multicrystalline Silicon. 1 INTRODUCTION Quality improvement of solar cells and reduction of manufacturing costs are the main demands for photovoltaic industry in the recent years. The formation of p-n junction is one of the most important processes for the fabrication of higher quality solar cells. Gettering of metallic impurities by phosphorus diffusion is a well known technique for improving the carrier lifetime of silicon wafers, especially in relatively contaminated material such as multicrystalline or ribbon silicon. There are various methods to form the p-n junctions in industry. The current technologies use batch diffusion furnaces which require additional procedures for wafers arrangement, cleaning and maintenance and have disadvantages such as long warm up and cool down cycle times, additional drive-in steps or pre-deposition during diffusion. It is also so hard to avoid the diffusion on both sides which requires further edge isolation process such as plasma, laser and back etchers which increases manufacturing costs. In addition, the current diffusion process cannot utilize the emitter selectivity regions and result in a whole surface with the same emitter properties. Screen-printing diffusion sources can be easily printed uniformly on the pre-designed surfaces allowing better dopant controlling and at the same time, it can be utilized for the formation of selective emitter solar cells and considered as an attractive mass production process for photovoltaic industry to replace current batch furnaces with belt furnaces. In this work, phosphorus diffusion was formed on p- type CZ-Si and mc-Si wafers using screen-printable phosphorus paste. Emitter sheet resistance was estimated to examine the impact of gase ambient on the quality of the phosphorus emitter. Carrier lifetimes were measured to evaluate the bulk quality improvement after diffussion process. 2 EXPRIMENTAL DETAILS Textured 1.2 cm multicrystalline and 2.1 cm monocrystalline silicon p-type wafers with a surface area of 55 cm 2 were used. mc-Si p-type wafers were textured in acid solution containing HF and HNO 3 in order to obtain light trapping effect and facilitate the printing process. CZ-Si p-type wafers were textured in alkaline solution. The experimental procedure is shown in Figure 1. Phosphorus diffusion paste was screen- printed on both sides of the wafers by the conventional screen-printing technology and dried at 150 o C for 3 min [1]. After drying, thermal diffusion was applied in a conventional quartz tube furnace at 875 o C and 900 o C for 30 minutes in Ar, N 2 , O 2 and N 2 +O 2 ambient. The thermal diffusion conditions are summarized in Table 1. After the diffusion, 5% HF was used to remove the phosphorus silica glass (PSG) from the wafer surfaces and the sheet resistance was measured using the four- points-probe method. Figure 1: The experimental procedure used in this work. Damage Removal, HF:HNO 3 Chemical Passivation Using 3% Iodine/Ethanol Lifetime Measurement by QSSPC Wafer Cleaning (Ethanol and Chemical Etching) Screen-Printing of Phosphorous Paste Diffusion Process (Different Gas Ambient and Temperature) PSG Removal Sheet Resistance Measurement n + Layers Removal Chemical Passivation Using 3% Iodine/Ethanol Lifetime Measurement by QSSPC 26th European Photovoltaic Solar Energy Conference and Exhibition 1289