Patric Geiger

Comparing improved state-of-the-art to former EFG Si-ribbons with respect to solar cell processing and hydrogen passivation

Recently, the ASE Company succeeded in enhancing the material quality of their octagon edge-defined film-fed growth (EFG) silicon ribbons resulting in a higher average cell efficiency in the manufacturing line of ASE Americas. In order to quantify and characterize these improvements in material quality, lifetime measurements have been performed on state-of-the-art and, for comparison reasons, also on older EFG material. Moreover, these wafers were subjected to a laboratory solar cell process. Consequently, it has been possible to perform laser beam induced current (LBIC) mappings and to compare the resulting internal quantum efficiency (IQE) mappings to the lifetime measurements performed before cell processing. As a result we found typical, different distributions of lifetime and IQE for the two materials. Further on, the solar cells have been hydrogen passivated using a microwave-induced hydrogen plasma and characterized again so that the impact of this passivation technique on the materials in different states of development could have been investigated. In this way, an appropriate hydrogen defect passivation turned out to reduce the differences between the different kinds of EFG ribbons.

Hydrogen passivation of ribbon silicon-electronic properties and solar cell results

Hydrogen bulk passivation of crystal defects plays a major role during solar cell processing of multicrystalline ribbon silicon material. In this study, the authors concentrate on the diffusion and effusion kinetics of hydrogen in different ribbon silicon materials (RGS (Bayer AG), EFG (ASE), String Ribbon (Evergreen Solar)). Electronic properties like majority charge carrier mobility and concentration were investigated by temperature dependent Hall measurements. Deuterium profiles determined with the SIMS method reveal that the diffusion velocity is strongly linked with the interstitial oxygen concentration present in the different materials. Taking into account these results, a confirmed record high efficiency of 12.5% on Bayer RGS silicon (4 cm/sup 2/) was achieved with an optimized hydrogen passivation step.