Stresses and their relation to defects in multicrystalline solar silicon

We report research on the characterization of stresses in relation to defects in multicrystalline silicon (mc-Si) solar cell material. Photoelasticity and micro-Raman spectroscopy (μRS) techniques are combined to understand this correlation at different length scales. While photoelasticity can probe small residual thermal stresses distributed over the entire volume of the sample as well as local stresses caused by defects, μRS can measure very localized intrinsic stresses of defects existing at the sample surface. Defects such as dislocations and inclusions are usually accompanied by stresses. In combination with thermal stresses, it results in a global residual stress state in the material. Thermal stresses are induced by the mismatch of different coefficients of thermal expansion between inclusions and silicon as well as due to local inhomogeneous temperature gradients during crystallization. Thermal stresses that relax by forming efficiency limiting defects can be reduced below the yield stress values by optimizing the crystallization conditions using photoelasticity.