Spacer-controlled morphology and mixed ionic-electronic transport in 2D Ruddlesden-Popper perovskites

Two-dimensional Ruddlesden-Popper (RP) halide perovskites are promising alternatives to 3D analogues because their layered structures can improve environmental stability and partially suppress ion migration. At the same time, the insertion of bulky organic spacers enhances structural anisotropy and excitonic confinement, making charge transport strongly dependent on film morphology and crystallographic orientation. Here, we investigate the layered iodide perovskites PEA₂PbI₄ and MeOPEA₂PbI₄, employing phenethylammonium (PEA) and 4-methoxyphenethylammonium (MeOPEA) as closely related organic spacers. The methoxy substitution provides a small perturbation of intermolecular interactions and crystallization kinetics while preserving the n = 1 RP framework. X-ray diffraction confirms phase-pure layered structures for both compounds, while optical measurements reveal pronounced excitonic confinement with optical gaps of 2.27 and 2.33 eV for PEA₂PbI₄ and MeOPEA₂PbI₄, respectively. Despite these similarities, hot-cast films exhibit markedly different microstructures, with MeOPEA₂PbI₄ having a compacted microcrystalline arrangement, whereas PEA₂PbI₄ displays a porous, heterogeneous morphology. Electrical measurements reveal pronounced hysteresis, and slow polarization dynamics characteristic of mixed ionic-electronic conduction. Under illumination, photoconductivity emerges predominantly in the out-of-plane geometry, highlighting the role of grain-boundary connectivity and orientation-dependent transport pathways. Transient measurements further show that light-induced ionic redistribution remains active in these layered films. Overall, the results demonstrate that subtle spacer modification controls optoelectronic behavior primarily through its influence on crystallization and mesoscale connectivity.