Thiocyanate Containing Two-Dimensional Cesium Lead Iodide Perovskite, Cs₂PbI₂(SCN)₂: Characterization, Photovoltaic Application, and Degradation Mechanism

We explored thiocyanate (SCN)-based two-dimensional (2D) organometal lead halide perovskite families toward photovoltaic applications. Using an SCN axial ligand and various cation species, we examined AA′PbI₂(SCN)₂-type 2D perovskite by replacing the cation species (AA′) between methylammonium (MA), formamidinium (FA), and cesium. Among various cation compositions, only all-inorganic cesium-based SCN perovskite, Cs₂PbI₂(SCN)₂, film showed high thermal stability compared to known 2D perovskites. Perovskite solar cells (PSCs) using the Cs₂PbI₂(SCN)₂ absorber yielded approximately 2% conversion efficiency on the mesoscopic device. Relatively low efficiency is attributed, in addition to optical properties (large band gap (2.05 eV) and exciton absorption), to the orientation of perovskite layer parallel to the layered structure, preventing carrier extraction from the light-absorber perovskite. In device stability, the Cs-based 2D perovskite was stable against oxygen (oxidation), whereas it was found to be unstable against humidity. X-ray diffraction and X-ray photoelectron spectroscopy measurements showed that, unlike long alkylammonium-based 2D perovskite families such as BA₂PbI₄ (BA = butylammonium), the Cs-based 2D perovskite can undergo hydrolysis due to the hydrophilic Cs cations.