As a result of top cell material quality improvement, development of optically and electrically low-loss double-hetero structure tunnel junction, photon and carrier confinements, and lattice-matching between active cel. III–V compound multi-junction (MJ) (Tandem) solar cells have the potential for achieving. 2.1. Selection of top cell materials and improving the qualitySelection of top cell materials is also important for high-efficiency MJ cells. As a top cell material l. As a result of lattice-matching improvement between middle cells and Ge substrates and introduction of the C-doped AlGaAs/Si-doped InGaP hetero-structure tunnel junction with AlIn. Some effort has been made to put this type of cells into commercial production for space applications by TECSTAR and Spectrolab based on the Multi-junction Solar Cell Manuf. Key technologies and basic physics for realizing super-high-efficiency and low-cost MJ solar cells were discussedPresent status of super-high-efficiency MJ solar cells was re.