The search for ever higher efficiency solar cells is at present focussed on multijunction devices. In this field, there is enormous research on tandems consisting of a high bandgap top sub-cell coupled to a Si bottom sub-cell . An emerging top cell candidate is the organic solar cell, given recent impressive breakthroughs in efficiency and stability , thanks in part to non-fullerene ac-ceptors. In this context, we report on approaches to modelling organic solar cells for silicon / organic tandems. The device studied is the three-terminal selective band-offset tandem cell . This innovative design shown in figure 1a is based on the Si interdigitated back contact solar cell , which features a number of fabrication and operating advantages over four terminal and two terminal tandems . The higher gap organic top sub-cell consists of donor and acceptor organic phases in an absorber blend, contacted to hole and electron transport layers, which is in devel-opment within the French ANR project ORGANIST. The modelling of the organic sub-cell is the main focus of this presentation, and is investigated with increasing levels model complexity. The modelling of the complete tandem is first described with a simple first approximation which only considers idealised classic drift-diffusion phenomena of inorganic semiconductors, with optical and band structure data from current best estimates of suitable non-fullerene high bandgap organic solar cell materials . It is shown that this approach is sufficient to quantitatively predict tandem efficiencies with suitable approximations for optical and transport parameters. Figure 1b shows the resulting quantum efficiency of a preliminary 26% tandem without device optimisation, and with a non-textured Si IBC. Organic modelling is then developed from stand-ard open access models , by moving from the widespread effective medium approach treating the absorber blend as a homogeneous material, to a bulk heterojunction model where the accep-tor and donor organic phases are simulated separately. We conclude with lessons learned on the comparative benefits of the modelling approaches for the design and development of high effi-ciency organic solar cells.