The  exceptional  properties  of  2D  hybrid  organic-inorganic perovskites (HOIPs) are strongly correlated with atom-level structural details. Stacking disorder (SD) often arises in 2D HOIPs due to quasi-random stacking of inorganic and organic layers, i.e., with no long-range  correlations  of  structural  configurations.  SD  manifests  as diffuse  X-ray  scattering  and  substantially  complicates  an  accurate crystal structure description.

Using combined crystallographic and computational modelling, we  show  that  local  ordering  preferences  exist  among  possible molecular  configurations  and  indeed  affect  the  electronic  structure characteristics  of  one  such  disordered  oligothiophene-based  lead iodide  HOIP.  Only  the  most  stable  herringbone  organic  molecular packing yields a computed type-Ib quantum well (QW) alignment that agrees with observed optical behaviour. Though often ignored in prior work, our study demonstrates that an accurate understanding of the structure (including SD) is crucial for reliable a priori prediction and post facto understanding of HOIP properties.