  Many advanced solar cell concepts propose surpassing the Shockley Queisser
(SQ) limit by introducing multiple quasi-Fermi level separations that are
arranged in series and/or in parallel. Exceeding the SQ limit with any parallel
arrangement involves intermediate states that deliver additional charge
carriers at, ideally, the same electro-chemical potential as the other elements
in the parallel network. This can be thought of as voltage matching individual
parallel components and in intermediate band materials is intricately linked to
solar concentration and \'etendue mismatch between absorption and emission.
Generally, to achieve voltage matching under sub-optimal conditions, an
additional degree of freedom in the absorption thresholds of the material
through a carrier relaxation or ratchet step is required. We explain why the
ideal ratchet step decreases with solar concentration and how it depends on
radiative efficiency and emission \'etendue of the individual transitions. For
solar cell concepts that use Auger type carrier-carrier interactions or
molecular triplet states for energetic up- or down-conversion, ideal bandgap
combinations and achievable efficiencies also depend on interaction rates. We
show that Auger assisted solar cells suffer more strongly from finite
interaction rates than carrier multiplication devices.
