The detailed balance approach has been used to analyze the optimum use of band gaps in a multi-junction device of up to 6 sub-cells. Results for the AM1.5G spectrum suggest that as the number of sub-cells increases the importance of the bottom sub-cell band gap becomes less critical, assuming the optimum band gap combination for that value can be obtained. Given this greater freedom in choice, the potential for the use of silicon as an active substrate is investigated along with a cell thinning 'current sharing' approach to improve current mismatch in the device. Results show a more robust design space of multi-junctions with active silicon substrates when the current sharing approach is used, with performances close to the optimum for a completely free choice of band gaps. The use of the AM1.5D spectrum for a concentration ratio of 100, shows similar results for the substrate and a slight increase in band gap sensitivity for the upper band gaps in the stack. Inclusion of optical coupling between the sub-cells lowers limiting efficiency, with luminescent coupling mitigating the band gap sensitivity. The results and approach outlined are useful for determining how best to deploy new photovoltaic materials in multi junction solar cells. (C) 2016 Elsevier Ltd. All rights reserved.