Annealing Effects in Low- and High-Stress Silicon Ribbon
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ANNEALING EFFECTS IN LOW- AND HIGH-STRESS SILICON RIBBON* K. A. DUMAS, A. BRIGLIO, JR., AND L. J. CHENG Jet Propulsion Laboratory, California Institute of Technology 4800 Oak Grove Drive; Pasadena, California 91109 ABSTRACT X-ray topography and minority carrier lifetime measurements were used to study the structural and electrical properties in silicon dendritic web ribbon. The effects of annealing on the material quality of high- and lowstress ribbon were investigated. INTRODUCTION A leading candidate for producing low-cost silicon sheet material for the manufacture of solar cells is the dendritic web ribbon process. This technology produces material comparable to Czochralski (Cz)-grown silicon and has demonstrated the capability of being processed into a high-efficiency photovoltaic device (> 15% AM1)[1]. However, technical barriers still exist. Factors limiting the device performance of this material include structural defects created during the growth cycle, for example, those induced by stress/strain effects. Knowledge of how this material reacts to hightemperature processing is important in order to best optimize the solar cell efficiency. Silicon dendritic web ribbon is grown in the form of long, thin, singlecrystal ribbons[2]. Ribbon growth is'initiated by immersing a dendrite seed into a melt, adjusting the temperature so that a button grows laterally to a suitable length, and then raising the button by pulling on the seed (Fig. 1). A dendrite grows downward into the melt from each end of the button, and the
ribbon bridges the gap between these two dendrites. Solidification occurs at a small distance above the melt level, and the single-crystal material that forms is an extension of the seed structure. This seed has a twin plane structure, which is propagated into the ribbon. As the ribbon is pulled from the melt, the ribbon temperature drops rapidly along its length. The temperature profile in the cooling ribbon is important because it strongly affects stresses in the thin ribbon material. Ideally, to ease ribbon handling and reduce loss due to breakage during solar cell and module fabrication processes, the ribbon material should be reasonably flat and have little or no residual stress. However, depending on the conditions under which a ribbon is grown, it can range from flat to highly deformed (buckled, twisted, or rippled) and its stress levels can range from very low values to levels that can cause the material to split or even shatter during handling and processing. Residual stress results when thermal stresses exceed the material's yield point during growth. When this critical yield stress is exceeded, plastic deformation occurs as a result of motion or generation of dislocations, causing a strained lattice that is present when the ribbon is cooled to ambient temperature. Structural defects (e.g., dislocations) can deteriorate solar cell performance. Factors such as the shapes of the temperature profiles, both lateral (across the width) and longitudinal (along the length), of the cooling ribbon, an
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