The most successful high efficiency design, and one of the oldest, is the multi-junction solar cell. There are a range of multijunction solar cell terminal configurations, the specificities of which are reviewed, concluding with noting the increased attention being given to three terminal designs. This introduces a new device design which is the Three Terminal Selective Band Offset Barrier tandem solar cell. The physical operation of this new device is examined, and embodiments in prototype materials relying on materials properties from the literature. We present projected performance evaluated by two dimensional numerical modelling. Identifying shortcomings on two fronts of materials and optical properties of the multilayer stack, we describe theoretical progress in optimising these properties via ab initio materials modelling and combined ray and wave optics. This theoretical context introduces the experimental results obtained in the first year of the project. This consists of successful integration of the selective band offset barrier on a suitably modified IBC structure using prototype materials previously reported. This paper thereby presents detailed analysis of the three terminal selective band offset barrier tandem solar cell, and announces the first experimentally fabricated three terminal selective band offset barrier solar cells, together with preliminary conclusions of experimental characterisation which is underway. .