Manufacture of mullite substrates from andalusite for the development of thin film solar cells

The purpose of this work is the manufacture of dense thin mullite substrates by tape casting of the natural silicate mineral andalusite. The targeted application for such substrates is the manufacture of thin film solar cells. Indeed, in addition to a good resistance at high temperature (up to 1200 °C), this application requires a good correspondence between the thermal expansion coefficient of the substrate and silicon film in order to lower the stresses in the film and in the substrate after cooling. The work was performed in three successive stages. First, the study of the transformation during sintering of andalusite+alumina mixtures. Second, the optimisation of the slurries and of the parameters for tape casting. Finally, green tapes prepared from various powder compositions were characterised before and after sintering. The addition of alumina to andalusite reduces the final amount of vitreous phase. This limits the risk of impurity diffusion in the silicon film. However, the addition of alumina also slows down the sintering process leading to more porosity in the substrates. A good compromise is obtained with an initial mixture of 80 wt.% andalusite with 20 wt.% alumina. In that case, the thermal expansion behaviour is very close to pure mullite and the mechanical properties of the substrates can be considered as sufficient for the targeted application.