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.