Martin Käs

Hydrogenation in Crystalline Silicon Materials for Photovoltaic Application

In this contribution an overview of hydrogenation issues for (multi-)crystalline silicon material is given. Crystalline silicon material for photovoltaic application contains more defects than material used for other semiconductor device fabrication. Therefore passivation of bulk defects has to be performed to reach higher efficiencies and exploit the cost reduction potential of these materials. Especially minority charge carrier lifetimes of ribbon silicon can be drastically improved by hydrogenation in combination with a gettering step. Apart from bulk passivation atomic hydrogen plays an important role in surface passivation via dielectric layers. Performance of single dielectric layers or stack systems can be increased after a hydrogenation step. It is believed that hydrogen can passivate defects at the silicon/dielectric interface allowing for lower surface recombination velocities. In industrial application hydrogenation is performed via deposition of a hydrogen-rich PECVD SiNx layer followed by a belt furnace annealing step. Surface passivation for characterization of charge carrier bulk lifetime is often performed with the same technique, omitting the annealing step to avoid in-diffusion of hydrogen. It is shown that for some crystalline silicon materials even the PECVD SiNx deposition alone (without annealing step) can cause significant bulk defect passivation, which in this case causes an unwanted change of bulk lifetime.

Investigations on the recombination activity of grain boundaries in MC silicon

This paper focuses on the influence of the effective intra-grain minority charge carrier diffusion length and surface recombination velocity at grain boundaries on solar cell parameters. Both can be extracted in principle from Light- and Electron Beam Induced Current measurements (LBIC and EBIC). Multicrystalline floatzone (mc FZ) silicon with different grain sizes was processed to solar cells with and without hydrogenation step, followed by LBIC and EBIC characterization. A theoretical model is used which can be applied to measured LBIC or EBIC profiles in order to obtain values for the effective intra-grain diffusion length and the recombination velocity at grain boundaries. Efficiencies reached on the processed solar cells (up to 16.0%) are the highest reported so far for material with such a small grain size, and the positive effect of hydrogenation can clearly be seen. The obtained results are very useful for other cost effective small grained mc silicon materials.