Hydrogen passivation of newly developed EMC-multi-crystalline silicon

Continuous casting with an electromagnetic cold crucible is a promising way of producing multi-crystalline (mc) silicon on an industrial basis for solar cell production. Casting equipment, capable of producing ingots of 130×130 mm2 cross-section and 600 mm length has been developed and installed. Thorough characterisation of the produced material revealed a relatively high defect density (grain boundaries, dislocations) and small grain sizes of 1–4 mm in diameter. Although the effective minority carrier diffusion length is high on as-cut wafers (>150 μm) it decreases during solar cell processing to values of 50 μm. This can be attributed to standard high temperature processing steps that lead to redistribution and agglomeration of residual impurities (e.g. metals, carbon) at extended crystallographic defects which then act as strong recombination centres. In order to passivate these recombination centres, a PECVD SiNx-layer is deposited which acts as a source of hydrogen and also as an anti-reflective coating. During the firing of the screen-printed metal contacts through the SiNx-layer, atomic hydrogen is released from this layer and diffuses into the bulk of the wafers where it saturates dangling bonds and passivates impurities at crystal defects. After this treatment the minority carrier diffusion length can be restored to values of around 100 μm on finished solar cells.