Two-dimensional (2D) organic-inorganic hybrid perovskites (2DHPs) are stoichiometric compounds that crystallize into structures of alternating inorganic monolayers and organic mono- or bilayers. Electronically, 2DHPs are akin to quantum well superlattices. However, they do not exhibit the interfacial roughness common to molecular beam epitaxially-grown materials and are more like monolayer transition-metal dichalcogenides. We synthesize and characterize derivatives of the 2DHP phenethylammonium lead iodide ((PEA)2PbI4) in which the para H on the cation is replaced with F, Cl, CH3, or Br. These substitutions increase the length of the cation but leave the cross-sectional area unchanged, resulting in structurally similar PbI42- frameworks with increasing spacing between inorganic layers. Longer cations result in broader, blue-shifted excitonic absorption spectra with reduced or eliminated structure, indicating greater energetic disorder. Photoluminescence spectra are largely invariant and insensitive to cation length, suggesting polaron formation stabilizes a structural and electronic minimum. Temperature-dependent linewidth analysis reveals excitons couple to a vibration on the organic framework that is weakly sensitive to these cation substitutions, and Raman spectra and electronic structure calculations support the presence of such a cationic mode. Despite carriers being confined to the inorganic framework, the length of the organic cation alters the optical and electronic properties of 2DHPs. Our findings suggest that stiffening or constraining the organic lattice by reducing the volume available to the cation enhances excitonic structure, increases exciton-phonon coupling, and reduces energetic disorder in 2DHPs, which are important design rules when considering 2DHPs for optical and electronic applications.

Longer cations increase energetic disorder in excitonic 2D hybrid perovskites

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.