Polycrystalline Silicon‐Film™ Thin‐film Solar Cells: Advanced Products

Abstract

Thin-film polycrystalline silicon has th e potential to achieve the cost reduction and performance improvem ent necessary for large-scale electricity markets. Reduced cost is achieved by capitalizing on the benefits of thin films grown on low-cost , large-area substrates. Improved efficiency is realized, in spite of reduced material quality , by incorporating enhanced optical absorption and back-surface passivation. The cornerstone of A stral'ower's thin-film solar cell technology is the Silicon-Film' P" process: a method for th e manufacture of solar cell-quality , polycrystalline films of silicon on a variety of low-cost, supporting substrates . Three thin-film solar cell design s, based on this technology, are currently under development: This pap e" presents the k ey design fe atures of these three products and briefiy reviews the current status of the development of the k ey technologies that comprise th e advanced thin-film solar cell products. INTROD UCTIONT hin-film polycrystalline silicon solar cells offer significant potential for the reduction of cost and improvement of conversion efficiency of photovoltaic power. This potential stems from several features. First, well-designed thin-film silicon solar cells minimize the mass of feedstock silicon required per watt of power output and improve conversion efficiency. Second, low-cost supporting substrates enable very large-area solar cells (> 1000 crrr') and monolithic, series-connected submodules (with an insulating substrate) to be manufactured. Finally, an established silicon technology already exists. This includes a well-developed material and solar cell technology base in both fabrication and modeling, demonstrated long-term electrical stability and low environmental impact both for manufacture and long-term deployment.The key to the acceptance of photovoltaics for large-scale electricity markets is the achievement of module costs of the order of $1.00 per watt and conversion efficiencies over 15% (in order to manage the balance-of-system costs). At 15% efficiency, a 50-llm thick film of silicon will yield close to 1.3 kW per kilogram of silicon. Therefore, even with electronic-grade silicon feedstock (at $45 kg -1) and including substrate materials (at $5 kg ""), the cost of materials for a 'thin-film wafer' can be less than $0.10 per watt. The challenge, however, is in developing a low-cost, high-yield and high-throughput manufacturing technology for the production of thin-film polycrystalline silicon material.Thin-film silicon solar cells can borrow heavily from the large body of solar cell fabrication and module manufacturing technologies available for Czochralski (Cz) and multicrystalline solar cells. Presently, the cost of solar cell fabrication and module manufacture represents roughly 50% of the cost