Molecular Engineering of 2D Hybrid Perovskites: Tailoring Alkyl Chain Length for Tunable Ferroelectricity and Spin Properties

Abstract

While organic–inorganic hybrid perovskites are widely recognized for exceptional photovoltaic performance, their inherent spin–orbit coupling and polarizability also render them compelling candidates for ferroelectrics and spintronics. In this work, we establish a structure–property relationship across the homologous series (CxH2x+1NH3)2CsPb2Br7 (x = 1–5) by integrating density functional theory calculations with symmetry analysis, revealing how progressive alkyl chain growth drives correlated evolution of crystal symmetry, electronic structure, ferroelectric polarization, and spin properties. The chemically distinct sublattices in hybrid perovskites naturally form dual structural-functional zones quantitatively distinguished by the packing factor (PF), in which the inorganic framework with its consistently higher PF acts as a transport zone, while the lower PF organic sublattice serves as an active zone whose conformational degrees of freedom modulate the ferroelectric response. With increasing chain length, bulk hybrid perovskites undergo a progressive transition from quasi-two-dimensional (2D) to intrinsically confined 2D electronic behavior accompanied by symmetry restoration, enabling a robust momentum-independent persistent spin texture protected by C2v symmetry. Our results demonstrate that the organic spacer acts as an active functional component rather than a passive barrier, offering a precise chemical handle for molecular engineering of 2D hybrid perovskites with tunable electronic properties.