Commercial High-Efficiency Silicon Solar Cells.pdfVIP

7 Commercial High-Efficiency Silicon Solar Cells R. Hezel 7.1 Introduction The present rapidly expanding photovoltaic market is dominated by mono- and mul- ticrystalline silicon solar cells based upon the standard, decades-old screen-printing approach [1, 2]. Despite significant progress in both performance and cost reduction, there are limitations in efficiency that arise from using the screen-printing process to apply the front contact [1, 3]. Currently, there are three improved processing sequences for high-efficiency de- vices that are commercially available [1, 4]. These include the Point-Contact solar cell of Sun Power, the HIT cell of Sanyo Electric and the laser-grooved buried con- tact (LGBC) cell of BP Solar. Using completely different approaches in design and process technology, and deviating significantly from conventional silicon solar cells, these nonconventional cells have the potential to reach cell efficiencies beyond 20%. Details about the fabrication process are not available from the manufacturers. Basic features and properties of the high-efficiency structures are presented in the follow- ing. 7.2 The Point-Contact Solar Cell The high-efficiency cell of Sun Power Corporation (founded 1988) is a simplified version of the highly sophisticated point-contact solar cell designed for concentrator applications. These cells were developed by Prof. Swanson together with NASA in 1973 and have been fabricated with conventional IC processing technology. They were successfully applied for niche applications, such as solar cars and solar air- planes [5]. Originally several photolithography steps were applied to define the rear fea- tures (i.e., diffusions, contact openings and fingers), so that one-sun efficiencies approaching 23% could be achieved. In order to reduce the fabrication costs and to make processing suitable for mass production, a novel screen-printing technol- ogy for masking purposes was developed [4, 6]. It is evident that these low-cost metho