  The Cu$_2$BaSnS$_4$ (CBTS) and Cu$_2$SrSnS$_4$ (CSTS) semiconductors have
been recently proposed as potential wide band gap photovoltaic absorbers.
Although several measurements indicate that they are less affected by band
tailing than their parent compound Cu$_2$ZnSnS$_4$, their photovoltaic
efficiencies are still low. To identify possible issues, we characterize CBTS
and CSTS in parallel by a variety of spectroscopic methods complemented by
first-principles calculations. Two main problems are identified in both
materials. The first is the existence of deep defect transitions in
low-temperature photoluminescence, pointing to a high density of bulk
recombination centers. The second is a low electron affinity, which emphasizes
the need for an alternative heterojunction partner and electron contact. We
also find a tendency for downward band bending at the surface of both
materials. In CBTS, this effect is sufficiently large to cause carrier type
inversion, which may enhance carrier separation and mitigate interface
recombination. Optical absorption at room temperature is exciton-enhanced in
both CBTS and CSTS. Deconvolution of excitonic effects yields band gaps that
are about 100 meV higher than previous estimates based on Tauc plots. Although
the two investigated materials are remarkably similar in an idealized,
defect-free picture, the present work points to CBTS as a more promising
absorber than CSTS for tandem photovoltaics.
