The Use of Injection-Level Dependent Lifetime Measurements for Determining Solar Cell Parameters

Abstract

The open-circuit voltage of a silicon solar cell is well known to be directly related to the effective recombination lifetime of the substrate as measured under 1-sun illumination. However, an important feature of the recombination lifetime is that it is dependent on the concentration of excess carriers present, often very strongly. This can result in a significantly lower recombination lifetime at maximum-power as compared to open-circuit conditions, and consequently cause a reduction in the cell fill factor. In high efficiency multicrystalline silicon cells, such a varying lifetime is often the result of impurities and defects residing in the bulk of the material. Injection-level dependent lifetime measurements on such wafers are shown to provide a good explanation of low fill factors observed in finished cells. The impact of these bulk recombination centres on fill factors in commercially produced multicrystalline silicon cells is modelled and discussed.