Development steps from an IMM 3J solar cell towards an IMM5J solar cell with BOL design. For comparison, the simulated near-term efficiency potential and the already achieved efficiencies for the IMM3J and IMM4J are shown. Keep in mind that the IMM3J is optimized for EOL conditions, whereas the 4J and 5J structures for BOL. The sketch of the layer structure shows schematically that increasing the number of junctions is not increasing the total epitaxy thickness heavily. Note that for an IMM4J cell with top subcell bandgap up to 2.1 eV, all other absorber material bandgap energies stay the same.