  We propose type-II GaAsSb/GaAsN superlattices (SLs) lattice-matched to GaAs
as a novel material for the 1 eV sub-cells present in highly efficient
GaAs/Ge-based multi-junction solar cells. We demonstrate that, among other
benefits, the spatial separation of Sb and N allows a better control over
composition and lattice matching, avoiding the growth problems related to the
concomitant presence of both elements in GaAsSbN layers. This approach not only
reduces clustering and improves crystal quality and interface abruptness, but
also allows for additional control of the effective bandgap in the 1.0-1.15 eV
spectral region through the SL period thickness. The optimized SL structure
exhibits a type-II band alignment and strong electronic coupling at 0 V. Both
effects cooperate to increase the minority carrier collection and leads to a
net strong enhancement of the external quantum efficiency (EQE) under
photovoltaic conditions with respect to bulk layers of equivalent thickness.
