Theoretical Insights into Spacer Molecule Design to Tune Stability, Dielectric, and Exciton Properties in 2D Perovskites

Abstract

Two-dimensional organic-inorganic perovskites have garnered extensive interest due to their unique structure and optoelectronic performance. However, their loose structures complicate mechanism illustration and tend to cause uncertainty and diversity of experimental and calculated results. This can generally be rooted in the dynamically swinging spacer molecules through two mechanisms: one is intrinsic geometric steric effect, and the other is related to electronic effect via orbital overlapping and electronic screening. Here, we intentionally design three types of spacer molecules, phenyl methyl ammonium (PMA), thiophene methyl ammonium (THMA), and furan methyl ammonium (FUMA) which adopt different aromatic units. We examine the influence of different aromatic spacers on the structural properties of the inorganic layer of the perovskite based on first-principles calculation and find that a marginal change in the aromatic ending group in the spacer ligand would trigger significant changes in octahedral in inorganic layer. We predict that the use of THMA and FUMA can improve the stability and increase the size of crystal domains due to enhanced binding between the organic and inorganic layers. Compared to prototype phenyl-based perovskite (PMA)2PbI4, thiophene-based perovskite (THMA)2PbI4 has states closer to the band edge, thus boosting carrier transport across inorganic and organic layers. Compared with perovskite using PMA as a spacer cation, the THMA-based perovskite has a higher dielectric constant and a smaller exciton binding energy, suggesting THMA more suitable as an organic spacer and a good passivation agent in 3D perovskites. The difference in screening ability of the molecules induces varying interlayer excitonic binding energy. Our work provides theoretical ground for the engineering of spacer molecules toward high-efficiency light conversion of mixed perovskites.