MICROSTRUCTURE AND MECHANICAL PROPERTIES OF ALUMINIUM BACK CONTACT LAYERS

The overall demand to reduce solar energy costs gives a continuous drive to reduce the thick ness of silicon wafers. Handling and bowing problems associated with thinner wafers become more and more important, as these can lead to cells cracking and thus to high y ield losses. In this paper the microstructure and m echanical properties of the aluminium on the rear side of a s olar cell are discussed. It is shown that the alum inium back contact has a complex composite-like microstructure, consis ting of five main components: 1) the back surface f ield layer; 2) a eutectic layer; 3) spherical (3 - 5 µm) hypere utectic Al-Si particles surrounded by a thin alumin um oxide layer (200 nm); 4) a bismuth-silicate glass matrix; and 5 ) pores (14 vol.%). The Young's modulus of the Al-S i particles is estimated by nanoindentation and the overall Young's modulus is estimated on the basis of bowing measu rements. These results are used as input parameters for an i mproved thermomechanical multiscale model of a silicon solar cell.