  We investigate the electronic structure and defect properties of Sn- and Ge-
doped ZnTe by first-principles calculations within the DFT+$GW$ formalism. We
find that $(\text{Sn}_\text{Zn})$ and $(\text{Ge}_\text{Zn})$ introduce
isolated energy levels deep in the band gap of ZnTe, derived from Sn-5s and
Ge-4s states, respectively. Moreover, the incorporation of Sn and Ge on the Zn
site is favored in p-type ZnTe, in both Zn-rich and Te-rich environments. The
optical absorption spectra obtained by solving the Bethe-Salpeter equation
reveals that sub-bandgap absorptance is greatly enhanced due to the formation
of the intermediate band. Our results suggest that Sn- and Ge-doped ZnTe would
be a suitable material for the development of intermediate-band solar cells,
which have the potential to achieve efficiencies beyond the single-junction
limit.
