New Screen-printed Metal Paste Options for PV Manufacturing
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New Screen-printed Metal Paste Options for PV Manufacturing N. Bakhshizadeh and S. Sivoththaman Centre for Advanced Photovoltaic Devices and Systems (CAPDS), Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, Ontario N2L3G1, Canada. While the global market for photovoltaic (PV) modules continues to grow 30% - 40% annually, manufacturers are looking at advanced technologies and cell concepts in order to improve the cost performance and reliability. One of the challenges faced by the silicon (Si) PV manufacturers is the lack of efficient metal screen-printing technologies. While screen-printing is a long-established technology, the traditional silver-based pastes have technical limits in terms of paste composition, drying and thermal-firing conditions, line-width, aspect ratio, contact resistance, etc. Such limits keep the industrial cell structure from being close to an ideal PV cell architecture. In an effort to develop alternate printable pastes electrically, electrically conductive adhesives are considered in this study to provide fine-line contacts and low thermal budget so that performance-driven solar cell designs can be reliably implemented in manufacturing. Mechanical properties of solar cell printed by conductive adhesives are experimentally investigated in this work and confirmed with the simulation results. INTRODUCTION Solar cell metallization in screen printing technology is considered to be a major efficiency limiting and a cost determining step. Traditionally screen printing has involved silver and aluminum/silver pastes for solar cell contacts. In the contact firing process that follows printing and drying, the cell rapidly undergoes a temperature profile that peaks typically at about 800°C. This process results in melting the glass frits which are usually composed of lead silicates and appear to play an important role in contact formation [1]. The presence of lead is also a concern. Therefore, developing lead-free and environmentally friendly materials as alternatives to currently available metallic pastes and conventional Sn/Pb solders has been a topic of interest in recent years [2-4]. The first alternative developed for screen printing metallization is lead-free metallic pastes containing metal oxides by Xu. et al. [2] who investigated lead-free frit formulations. An interesting alternative would be electrically conductive adhesives (ECAs) which have been studied extensively for electronic packaging applications since 1956 [5]. Great efforts have also been made on ECAs to replace the Sn/Pb solders [6,7]. However, no research was ever conducted on the possibility of using ECAs in screen printing technology. A number of studies have been conducted on ECA development, discussing their benefits and drawbacks and investigating their main technological applications [4,7-9]. The most important advantages and disadvantages of ECAs over conventional metallic pastes are shown in table I. The use of low process temperature of ECAs leads to another important advantage of redu
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