Optoelectronic Properties of Strontium and Barium Copper Sulfides Prepared by Combinatorial Sputtering

Optically transparent materials with p-type electrical conductivity can facilitate the development of transparent electronics and improve the efficiency of photovoltaic solar cells. Sulfide materials represent an interesting alternative to oxides for these applications due to better hole transport properties. Here, transparent and conductive Ba-Cu-S thin films are prepared by combinatorial cosputtering and characterized for their composition, structure, and optoelectronic properties. The conductivity and transparency of these films are found to be strongly dependent on their chemical composition and the substrate temperature during growth. The conductivity of BaCu₂S₂ and BaCu₄S₃ can reach 53 S/cm (at 250 °C) and 74 S/cm (at 200 °C), respectively, which is higher than their solution processed/bulk counterparts. The 90% reflectance corrected transmittance is achieved in the wavelength range 600-1000 nm for BaCu₂S₂ and 650-1000 nm for BaCu₄S₃ (at 250 °C). These electrical and optical properties are comparable with other recently presented transparent p-type conductors, while the 200-350 °C processing temperature is low enough to be used in semiconductor devices with limited thermal budgets. Attempts have been made to synthesize the related Sr-Cu-S materials, following the theoretical suggestion of their potential as transparent p-type conductors, but these attempts resulted only in phase-separated SrS and Cu_xS phases. Alloying BaCu₂S₂ with Sr on the Ba site on the other hand increases the conductivity to >100 S/cm while only slightly compromising the transparency of the material. To explain the difference between the Ba and the Sr containing copper sulfides, the lower bounds on the SrCu₂S₂ and SrCu₄S₃ formation enthalpies are estimated. While the doping of the Ba-Cu-S materials presented here is too large for application in transparent electronics, it is promising for potential use as p-type contact layers in thin film solar cells.