Workshop on Basic Research Opportunities in Photovoltaics

Abstract

Crystalline silicon continues to be the dominant semiconductor material used for terrestrial photovoltaics. This paper discusses the scientific issues associated with silicon photovoltaics processing and cell design that may yield cell and module performance improvements, both evolutionary and revolutionary in nature. We first survey critical issues in "thick" crystalline silicon photovoltaics, including novel separations processes for impurity removal, impurity and defect fundamentals, interface passivation, the role of hydrogen, and high-throughput, kinetically-limited materials processing.Second, we outline emerging opportunities for creation of a very different "thin-layer" silicon cell structure, including the scientific issues and engineering challenges associated with thinlayer silicon processing and cell design. INTRODUCTIONToday's basic research advances in materials physics and materials synthesis and processing will provide the foundation for a large-scale industrial photovoltaics technology that appears likely to develop over the next 10-30 years. Over this time frame, the photovoltaics industry is expected to expand to a production level on the order of 10's GW/year worldwide, at which point it will be able to provide an important global source of clean energy. In this future, a prototypical photovoltaic manufacturing facility may be anticipated to produce on the order of 1 GW/year -and by simple considerations one can project for example that such a plant will need to achieve a throughput on the order of 10 m 2 of modules per minute. 6 Crystalline silicon is very likely to maintain a quite significant role in photovoltaics technology over this time frame. Indeed, between 1992 and 1998, crystalline silicon has expanded its market share from 73% to 86% of the market relative to other photovoltaics technologies. Shipments of crystalline Si photovoltaics amounted to 132 MW per annum by 1998, and currently the Si photovoltaics industry is growing faster than its large cousin the microelectronics industry. Because of this large and continuing investment in silicon photovoltaics, it will be critical to address fundamental materials physics and materials synthesis issues related to crystalline silicon photovoltaics, since these basic research investments may enable further efficiency improvements and cost-reductions to occur. Several significant scientific, technical and economic advantages accrue to crystalline Si:• Its device physics and materials physics issues are better understood than competing device materials. However, very important basic materials physics issues remain outstanding for crystalline silicon photovoltaics; critical issues related to minority carrier lifetime enhancement in multicrystalline or polycrystalline silicon are unlikely to be addressed by the microelectronics industry in the future.• It is a serendipitous materials system: it has an extremely useful native oxide in SiO 2 ; as an elemental material, it lacks stoichiometry problems; dopants such as Al and P can als...