Influence of crucible and coating quality on the properties of multicrystalline silicon for solar cells

The effect of silica crucible and silicon nitride coating quality on the minority carrier lifetime has been investigated by casting standard solar-grade silicon in a pilot-scale furnace with crucibles and coatings of different purities. A crucible made from high-purity silica and a crucible produced from standard-grade silica with a layer of high-purity silica located between the crucible and the silicon nitride coating have been tested. A new concept involving various nitride/oxide concentration layers developed to ensure improved loading and transfer condition has also been tested. The minority carrier lifetime has been used to evaluate the quality of the resulting ingots and has been compared to that of ingots cast in reference crucibles.It has been found that the purity of the crucible and coating has a significant effect on the lifetime. A higher purity crucible and coating will double the lifetime of the ingots over ingots cast with reference-grade crucible and coating. A layer of high-purity silica between the crucible and silicon nitride coating may also have a positive effect as long as the layer is sufficiently thick. A mechanically resistant layer of silica on top of the silicon nitride facing the melt in the crucible will allow the silicon nitride to be applied by the crucible producer and will facilitate crucible charging. The dissolution of the oxygen from this coating into the melt has been investigated and evaluated through lifetime and cells characteristics.