Exploration and control of short-chain spacer cations propylammonium and propanediammonium for tuning structure property performance relationships in 2D perovskites

The structural diversity of 2D perovskites, resulting from the integration of various spacer cations, remains insufficiently explored, necessitating a thorough investigation into the mechanisms that govern their structural, optoelectronic, surface moisture resistance, mechanical stability, and photovoltaic performance. This study examines the use of short-chain spacer cations to adjust and control these properties. We analyse the impact of Propylammonium (PA⁺) and Propanediammonium (PDA²⁺) on the structure-property-performance relationships in Ruddlesden-Popper-type (PA₂(MA)Pb₂I₇) and Dion-Jacobson-type (PDA(MA)Pb₂I₇) perovskites, based on first-principles calculations. Our results demonstrate that incorporating PDA²⁺ yields shorter intercalation distances, a higher volumetric density (3.87 g/cm³), and a more compact structure, resulting in monoclinic symmetry and a narrower band gap (1.8 eV). This configuration enhances absorption and charge carrier transport by lowering the excitation binding energy, resulting in a more pronounced Rashba effect due to reduced symmetry. This, in turn, enhances carrier lifetime through indirect transitions. These factors contribute to a higher short-circuit current density (19.64 mA/cm²) and better SLME efficiency (27 %) compared to that of PA₂(MA)Pb₂I₇ (11.16 mA/cm², 19 %), which adopts a less compact orthorhombic structure (3.17 g/cm³) with a wider band gap (2.17 eV). Mechanical analysis reveals that both perovskites are ductile, with the PDA²⁺-based perovskite showing greater rigidity and fracture resistance. In contrast, the PA⁺ cation induces higher ductility, as well as a higher water migration barrier and better resistance to water absorption compared to the PDA²⁺ cation. These results highlight the crucial importance of spacer cation selection and crystalline symmetry in determining the properties of 2D perovskites.