Ellipsometry study of Dielectric Function and Thermal Degradation of 2D L2PbI4 (L=PEA, BA) Perovskite Thin Films

Two-dimensional (2D) Ruddlesden-Popper (RP) metal halide perovskites have been widely utilized to enhance the stability of perovskite solar cells (PSCs), owing to their superior stability over their 3D counterparts. However, to date, there remain limited studies focused on the thermal stability of 2D perovskite thin films. In this study, the thermal degradation of (PEA)₂PbI₄ and (BA)₂PbI₄ thin films is systematically characterized using X-ray diffraction, scanning electron microscopy, photoluminescence, and spectroscopic ellipsometry. The optical band gaps of (PEA)₂PbI₄ and (BA)₂PbI₄, obtained from their respective dielectric functions, are 2.42 ± 0.002 eV and 2.43 ± 0.002 eV, respectively. Through an investigation of the dynamic evolution of the dielectric function, we found that (PEA)₂PbI₄ exhibits superior stability compared to (BA)₂PbI₄. The enhanced stability of (PEA)₂PbI₄ can be attributed to its lower density of defect states and the presence of a rigid aromatic ring spacer cation within the crystal structure. This aromatic ring spacer cation contributes to the structural stabilization via strong CH···π interactions with the alkyl chains of adjacent cations. A comprehensive understanding of the dielectric function and thermal degradation of these two 2D perovskite thin films is essential for the future design and optimization of 2D/3D PSCs.