Investigation of thin aluminium films on the rear of monocrystalline silicon solar cells for back surface field formation

The trend to thinner crystalline silicon wafers in the production of silicon solar cells instigates a re-evaluation of the back surface field (BSF) formation. Aluminium layers, whose thickness is typically over 10 μm, are commonly used for BSF formation. Such thick films, however, cause severe warping on thin solar cells (<250 μm). This study investigates the use of thin Al films of between 0.1 and 2.0 μm and focuses on the aluminium/rear surface structure after sintering in a mixed nitrogen and oxygen gas atmosphere at temperatures between 820°C and 995°C. It is shown that the rear surface morphology has a strong influence on the reaction of Al with Si. Secondary electron and focused ion beam microscopy reveal that thin Al layers form Al islands on smooth surfaces but the Al agglomerates at the apex of pyramids when the surface is textured. In both cases the Al dissolves the Si rear surface inhomogeneously. The structural results are correlated with electrical measurements of laser grooved buried contact solar cells. The correlation shows increasing short circuit current with increasing Al film thickness. The results are used to explain deviations of the standard theory of the Al:BSF formation when using thin Al films.