LAUSR

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.