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 sol ar cell production. Casting equipment, capable of producing ingots of 130 x 130 mm(2) 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 le ngth is high on as-cut wafers (>150 mu m) it decreases during solar cell pr ocessing to values of 50 mu m. This can be attributed to standard high temp erature processing steps that lead to redistribution and agglomeration of r esidual impurities (e.g. metals, carbon) at extended crystallographic defec ts which then act as strong recombination centres. In order to passivate th ese recombination centres, a PECVD SiNx-layer is deposited which acts as a source of hydrogen and also as an anti-reflective coating. During the firin g of the screen-printed metal contacts through the SiNx-layer, atomic hydro gen is released from this layer and diffuses into the bulk of the wafers wh ere it saturates dangling bonds and passivates impurities at crystal defect s. After this treatment the minority carrier diffusion length can be restor ed to values of around 100 mu m on finished solar cells. (C) 1999 Elsevier Science S.A. All rights reserved.