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

Experimental and first-principles spectroscopy of Cu$_2$SrSnS$_4$ and Cu$_2$BaSnS$_4$ photoabsorbers