Microstructure and mechanical properties of aluminum back contact layers

The overall demand to reduce solar energy costs gives a continuous drive to reduce the thickness 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 yield losses. In this paper the microstructure and mechanical properties of the aluminum on the rear side of a solar cell are discussed. It is shown that the aluminum back contact has a complex composite-like microstructure, consisting of five main components: (1) the back surface field layer; (2) a eutectic layer; (3) spherical (3-5 μm) hypereutectic Al-Si particles surrounded by a thin aluminum oxide layer (200 nm); (4) a bismuth-silicate glass matrix; and (5) pores (14 vol%). The Young’s modulus of the Al-Si particles is estimated by nanoindentation and the overall Young’s modulus is estimated on the basis of bowing measurements. These results are used as input parameters for the improved thermomechanical multiscale model of a solar cell.