Building an All-Sulfide TaS2/Cu2-II-Sn-S4/CdS Solar Cell and Putting it on Silicon

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A few transition metal dichalcogenides are high work-function metallic compounds and could therefore be effective hole contacts in a variety of (opto)electronic devices. Nevertheless, when considering - for example - the field of photovoltaics, it is difficult to find reports of conductive contacts that are not elemental metals or transparent conductive oxides. Here, we incorporate the metallic compound TaS2 into the device structure of emerging photoabsorbers (Cu2BaSnS4 and Cu2SrSnS4) and fabricate all-sulfide solar cells. Compared to reference cells built with a standard elemental metallic contact (Mo), the all-sulfide solar cells are more efficient by about 10% relative. We will discuss some of the properties of the TaS2 hole contact, its stability, and the possible reasons for the efficiency improvement. For the growth of Cu2BaSnS4 and Cu2SrSnS4 we propose an oxide precursor route involving thermal conversion of sputtered oxide films (Cu2BaSnO4 and Cu2SrSnO4) in an H2S atmosphere at the same temperature normally used for the more common non-oxide precursors. Interestingly, Cu2BaSnS4 and Cu2SrSnS4 crystallize in a trigonal structure where Cu, Ba(Sr), and Sn have distinct coordination environments. This major structural difference from the well-studied tetrahedrally-coordinated kesterite Cu2ZnSnS4 implies that substitutional defect formation is in general unfavorable in Cu2BaSnS4 and Cu2SrSnS4. In fact, both compounds are found to have sharper absorption and emission edges than kesterite, and their room-temperature photoluminescence peak is well aligned to their band gap.
Finally, there is a long-standing dream of depositing sulfide semiconductors as wide band-gap absorbers on silicon solar cells, in order to realize highly efficient double-junction (tandem) cells. Progress towards this ambitious goal has been hampered by the relatively low efficiency of sulfide absorbers, and by integration issues with the Si bottom cell due to the relatively high temperatures needed to grow and crystallize many sulfides. Here we attempt to address the second issue by incorporating a very thin TiN diffusion barrier between the two sub-cells, which helps preserve the silicon cell during a sulfurization process at 550°C. Efficiencies up to 3.3% for Cu2ZnSnS4/Si tandems have been reached so far when incorporating the TiN barrier layer.
Original languageEnglish
Publication date2019
Publication statusPublished - 2019
Event2019 MRS Fall Meeting - Boston, United States
Duration: 1 Dec 20196 Dec 2019


Conference2019 MRS Fall Meeting
Country/TerritoryUnited States

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