Resolving Rotational Stacking Disorder and Electronic Level Alignment in a 2D Oligothiophene-Based Lead Iodide Perovskite

Two-dimensional
(2D) hybrid organic–inorganic perovskites
(HOIPs) represent diverse quantum well
heterostructures composed of alternating inorganic and organic layers.
While 2D HOIPs are nominally periodic in three dimensions for X-ray
scattering, the inorganic layers can orient quasi-randomly, leading
to rotational stacking disorder (RSD). RSD manifests as poorly resolved,
diffuse X-ray scattering along the stacking direction, limiting the
structural description to an apparently disordered subcell. However,
local ordering preferences can still exist between adjacent unit cells
and can considerably impact the properties, particularly the electronic
structure. Here, we elucidate RSD and determine the preferred local
ordering in the 2D [AE2T]­PbI4 HOIP (AE2T: 5,5′-bis­(ethylammonium)-[2,2′-bithiophene]).
We use first-principles calculations to determine energy differences
between a set of systematically generated supercells with different
order patterns. We show that interlayer ordering tendencies are weak,
explaining the observed RSD in terms of differing in-plane rotation
of PbI6 octahedra in neighboring inorganic planes. In contrast,
the ordering preference within a given organic layer is strong, favoring
a herringbone-type arrangement of adjacent AE2T cations. The calculated
electronic level alignments of proximal organic and inorganic frontier
orbitals in the valence band vary significantly with the local arrangement
of AE2T cations; only the most stable AE2T configuration leads to
an interfacial type-Ib band alignment consistent with observed optical
properties. The present study underscores the importance of resolving
local structure arrangements in 2D HOIPs for reliable structure–property
prediction.