In this work package, epitaxial structures are produced and optimized for the different sub-cells by means of metal organic vapour phase epitaxy. The current world record solar cell with 46% efficiency is assumed as the starting point. Improvements can for example. result from an optimization of the transition between emitter and window layer of the upper cell. The direction of growth also has an influence on the quality of the layers, and it has been found that it is unfavourable to expose the GaInP top cell to high temperatures > 500 °C for a longer time. This knowledge is transferred into optimized cell structures and at the same time new materials are introduced and evaluated.