Structural and Electronic Impact of an Asymmetric Organic Ligand in Diammonium Lead Iodide Perovskites

Reduced dimensionality forms of perovskites with alternating layers of organic ligands are a promising class of materials for achieving stable perovskite solar cells. Most work until now has focused on phases utilizing two ammonium terminated ligands per formula unit. However, phases utilizing a single diammonium ligand per formula unit are advantageous in that they can potentially have a thinner insulating organic layer between Pb-halide layers, yet the structural effects on their optoelectronic properties are not yet well understood. In this study two organic ligands, butane 1,4-diammonium (BDA) and N,N-dimethylpropane diammonium (DMPD), are investigated as spacers in n = 1, 2D perovskites. Using ultraviolet and inverse photoelectron spectroscopies, BDAPbI₄ is shown to have a larger transport gap by 350 meV and a larger exciton binding energy by 140 meV than DMPDPbI₄. Through density functional theory calculations, the cause of this difference is traced to the out-of-plane tilting of the Pb-halide octahedra provoked by the asymmetric ligand in DMPDPbI₄. Parallel channels of nearly straight PbIPb bonds are formed in one direction, leading to enhanced electronic coupling and higher band dispersion in that direction. In BDAPbI₄, no such channels exist, resulting in greater electronic confinement and a larger bandgap and exciton binding energy.