KCu₃S₂: A Layered Inorganic Chalcogenide Semiconductor as a Promising Light Absorber

A potential replacement that alleviates the shortcomings of the dominant light absorber materials used in solar photovoltaics has been synthesized, and its microstructural, electronic structure, and optical properties have been investigated. KCu₃S₂ crystals were synthesized by the carbonate method. Transmission electron microscopy (TEM) established [010] as the growth direction of the needle-like monoclinic crystals. X-ray photoelectron spectroscopy (XPS) analysis confirmed that sulfur exists in the S^2- state, albeit in two different coordination environments, equally split between 4-fold and 6-fold coordination. The material exhibits an indirect band gap of 1.55 eV, well within the ideal visible range. KCu₃S₂ is a p-type semiconductor, showing a high carrier concentration of the order of ∼10²⁰ cm^-3, with a low hole mobility of 4.41 cm²/(V s). The low mobility is attributed to the flat nature of the valence bands, which leads to a large effective mass of holes, along with weak interlayer interactions that impede carrier transport, as confirmed by our ab initio electronic band structure simulations. Furthermore, our ab initio calculation confirms the directly allowed electronic transitions and a remarkably high optical absorption coefficient of ∼10⁵ cm^-1 in the visible region. A theoretical spectroscopic limited maximum efficiency (SLME) of 26.7% is estimated for a film thickness of 7-9 μm, making KCu₃S₂ a highly promising candidate as a light absorber in single-junction and tandem photovoltaic devices.