Wide Band Gap Cu₂SrSnS₄ Solar Cells from Oxide Precursors

Recent progress in the efficiency of Cu₂ZnSnS₄ (CZTS) solar cells has been relatively slow due to severe bulk band tailing issues that have proven difficult to resolve. Band tails in CZTS are caused by defect-related potential fluctuations, as diagnosed by the large shift between the CZTS band gap and its photoluminescence (PL) peak. In this work, we demonstrate that the PL-band gap shift can be decreased roughly by a factor of 5 when Zn is replaced by the heavier cation Sr. The resulting Cu₂ZnSnS₄ compound is of considerable interest for photovoltaics due to its sharp band edges and suitable band gap (1.95–1.98 eV) for a top absorber in tandem cells. Trigonal CSTS thin films are synthesized by sulfurization of strongly Cu-poor cosputtered Cu₂ZnSnS₄ precursors. The first functioning CSTS solar cells are demonstrated, even though the very high conduction band of CSTS implies that the typical CdS/ZnO electron contact of CZTS solar cells must be redesigned to avoid large voltage losses.