Solution-Processed Earth-Abundant Cu₂BaSn(S,Se)₄ Solar Absorber Using a Low-Toxicity Solvent

Cu2BaSn(S,Se)4 (CBTSSe) has recently gained substantial attention as an alternative absorber material for photovoltaic (PV) and photoelectrochemical (PEC) applications due to the abundance of the constituent elements, a large absorption coefficient, tunable band gap ranging from 1.5 to 2 eV, and reduced tendency for antisite disorder relative to Cu2ZnSn(S,Se)4. In this study, as an alternative to more expensive vacuum-based film-deposition processes, we report a low-toxicity solution-based process for the fabrication of high quality CBTSSe absorber layers with micrometer-scale film thickness and grain size. The facile process involves spin-coating an environmentally benign solution of highly soluble, inexpensive, and commercially available precursors, Ba(NO3)2, Cu(CO2CH3)2, and SnI2, followed by sequential sulfurization/selenization annealing. A high-temperature prebaking step under sulfur vapor is needed for each film layer to avoid forming the difficult-to-remove impurity phase, Ba(SO4), when starting from the soluble Ba(NO3)2 reagent. The solution-based CBTSSe films have been employed in a Pt/TiO2/CdS/CBTSSe photocathode structure (e.g., for water splitting), exhibiting an ∼10 mA/cm2 current density at 0 VRHE, comparable to that of vacuum-deposited CBTSSe PEC devices. Our approach for the fabrication of CBTSSe absorbers represents a first step in achieving low-cost and large-scale solution-processed solar devices based on this material.