Is There an Optimal Spacer Cation for Two-Dimensional Lead Iodide Perovskites?

Two-dimensional lead iodide perovskites have attracted significant attention for their potential applications in optoelectronic and photonic devices due to their tunable excitonic properties. The choice of organic spacer cations significantly influences the light emission and exciton transport properties of these materials, which are vital for their device performance. In this Perspective, we discuss the impact of spacer cations on lattice dynamics and exciton-phonon coupling, focusing on three representative 2D lead iodide perovskites that exhibit distinct types of structural distortions. Minimizing structural distortions, such as dynamic out-of-plane octahedral tilting and lone pair distortion, appears to be essential for achieving narrow photoluminescence (PL) emission peaks, high PL quantum yields, and rapid exciton diffusion by suppressing exciton-phonon coupling, as demonstrated in 2D perovskites based on phenylethylammonium cation or its derivatives. We propose that designing spacer cations with enhanced intermolecular interactions and denser packing, combined with the close packing of inorganic ions to minimize the motions of both organic and inorganic lattices, would be the ideal scenario for yielding the most favorable optoelectronic properties in these materials.