Characterization of Solar Cell Substrates Using Diode Array Technique

In current manufacturing regimes, silicon solar cells are made from low-cost silicon wafers, which are produced in one of three ways: low-grade, Czochralski-type single-crystal wafers; multicrystalline silicon wafers grown by casting; or shaped ribbons produced by directional solidification. These materials contain high concentrations of impurities and defects, which lead to spatial variations inthe material properties. A detailed characterization of the substrates is needed to understand the nature of defects and impurities and their influence on solar cell performance. Although many techniques can be applied to map material properties, the influence of the various materials on cell performance is difficult to determine. We have developed a technique to fabricate arrays ofedge-passivated diodes (small-area solar cells), which can be probed using an automatic prober to generate dark and illuminated I-V characteristics of each device. These devices can be readily used for other measurements, such as measurement of minority-carrier diffusion length, light or electron beam induced current, C-V analyses, and DLTS. The data from diode arrays are used to study influenceof defects and impurities on cell performance and to predict the highest large-area cell performance.