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ReviewJ. Jpn. Soc. Colour Mater., 96〔4〕，147–151（2023）

Optoelectronic Properties of Expanded Halide Perovskite Analogs Enrichedby Molecular Diversity
Daiki Umeyama＊,†＊Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan†Corresponding Author, E-mail: umeyama.daiki@nims.go.jp(Received January 18, 2023; Accepted February 17, 2023 )Abstract
Hybrid materials offer the potential of coupling the electronic properties of covalent organic molecules and ionic inorganic solids. In thefamily of hybrid halide perovskites, the diversity of organic molecules has not been observed in three-dimensional ( 3D) structures due tothe small size of the cuboctahedral cage, whereas two-dimensional ( 2D) derivatives can hold aliphatic and aromatic molecules of a wide
range of sizes between the inorganic sheets. Expanded 3D perovskite analogs are emerging materials that mitigate the limitation on themolecular variety in the 3D perovskite, where a single metal-halide octahedron in the perovskite is replaced by a dimer of edge-sharing
octahedra. Unlike 3D perovskites, the expanded analogs can hold large molecular dications while maintaining a dispersed inorganic band
structure. Flexible choices about organic cations allow us to tune the energy levels of molecular orbitals relative to the inorganic valence andconduction bands. The interplay between active molecular orbitals and inorganic bands affords intriguing optoelectronic properties.
Key-words: Hybrid halide perovskites, Lattice expansion, Bandgap tuning

1．Introduction
The diversity of organic molecules provides organicinorganic hybrid materials with rich structural andelectronic variety, which is one of the most attractive
features of this class of materials. 1 - 4 ) In the family of
hybrid halide perovskites, two-dimensional ( 2D) structures
can accommodate organic molecules of a wide range ofsizes between the inorganic sheets. 5,6 ) The well-definedstructure results in the quantum-well-like electronic structureof 2 D hybrid perovskites, enabling the manipulation of
the relative energy levels of the organic and inorganic
components. 7-9 ) On the other hand, due to the small size ofthe cuboctahedral cage for organic cations, the moleculardiversity in three-dimensional ( 3D) hybrid perovskites isseverely limited. Indeed, the 3D hybrid perovskite A[PbI 3 ]is reported to accommodate only methylammonium (MA)and formamidinium (FA) cations as stoichiometric A-site

cations, 10,11 ) which limits the electronic diversity in 3D
hybrid perovskites. In this regard, a general method toaccess the electronic diversity seen in 2D perovskites while
preserving the 3D inorganic network and dispersed band
structure has been desired.
Recently, we and others reported a method to expand the3D lattice of perovskite, where the basic building unit of theinorganic lattice is replaced by the edge-sharing dimer of BX6octahedra (B = Pb 2+, Sn 2+; X = I－, Br－, Cl－). 12-18 ) The newbuilding unit, B 2 X 10 , shares corners to form 3D inorganic
structures in an analogous way perovskites’ BX6 does (Fig. 1).
Because of the similarity in the connectivity of the basic
building unit, these expanded structures are called “perovskiteanalogs” or “perovskitoids.” The expanded perovskite〔氏名〕 うめやま だいき〔現職〕（国研）物質・材料研究機構機能性材料研究
拠点 研究員〔趣味〕 将棋，育苗〔経歴〕 2015 年 3 月 京 都 大 学 大 学 院 工 学 研 究 科 合成・生物化学専攻博士後期課程修了，博士（工学）。2015 年 4 月スタンフォード大学化学科，博士研究員。2018 年 1 月物質・材料研究機構国際ナノアーキテクトニクス研究拠点，独立研究者。2020 年 4 月より現職。

Fig. 1 Conceptual transformation of the inorganic sublatticefrom (A) a 3D perovskite to (B) its expanded analog,
where the lead-halide octahedra in (A) are replaced withedge-sharing dimers of octahedra in (B). Reprinted fromRef. ( 14 ) with permission from Wiley-VCH.

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