A Study Of Photovoltaic Loss-Mechanisms Due to Defects and Grain Boundaries in Polycrystalline Silicon

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Abstract
Various photovoltaic loss-mechanisms associated with defects and grain boundaries (g.b.'s) in polycrystalline silicon have been experimentally studied. Analysis was carried out on two types of substrates/cells viz. Wacker Silso and laser crystallized RTR ribbons. Solar cells were fabricated on selected regions of the substrates and their characteristics (VOC, ISC, F.F.) related the substrate structure. Mechanisms related to photovoltaic losses are divided into two categories: electronic and physical. Parameters describing electronic loss mechanisms, such as changes in minority carrier diffusion length, dark current and local photo-current losses were measured and their dependence on density and type of defects was determined. A variety of analytical techniques were used for this study. These include I-V characterization of solar cells, I-V characterization of g.b.'s and light intensity dependences of some material parameters. Loss mechanisms associated with physical effects are defect-defect and impurity-defect interactions. It is shown that physical effects such as impurity segregation and defect annihilation can lead to significant loss/gain in photovoltaic characteristics.

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