Broad Orange Emission from Piperazine-Based Layered Halide Perovskites

The excellent optoelectronic properties of layered halide perovskites make them ideal for devices such as solar cells, light-emitting diodes, and photodetectors. In this study, we report five two-dimensional (2D) halide perovskites: (C₆H₁₄N₂)PbBr₄, (C₈H₁₈N₂)PbBr₄·H₂O, (C₆H₁₄N₂)PbI₄, 1-(C₈H₁₈N₂)PbI₄, and 2-(C₈H₁₈N₂)PbI₄, (C₆H₁₄N₂ = N-Ethylpiperazine, C₈H₁₈N₂ = 1-Butylpiperazine). The optical band gaps of these compounds range from 2.63 to 3.35 eV. (C₆H₁₄N₂)PbBr₄ exhibits broad orange emission with a peak at 624 nm, a full width at half-maximum (fwhm) of 246.8 nm, and a Stokes shift of 237 nm. In contrast, (C₈H₁₈N₂)PbBr₄·H₂O shows a peak at 640 nm, fwhm of 262.8 nm, and a larger Stokes shift of 286 nm, attributed to exciton self-trapping induced by strong electron-phonon coupling and structural distortions. This significant Stokes shift is a notable feature of 2D materials. Our findings expand the database of layered halide perovskites, providing valuable insights for the development of next-generation optoelectronic technologies.