The underappreciated lone pair in halide perovskites underpins their unusual properties
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Introduction Lead and tin halide perovskites have recently been shown to exhibit impressive performance in a range of optoelectronic applications ranging from photovoltaics to radiation detection to lasing, but are unusual in several respects relative to conventional semiconductors. These materials can be prepared under mild conditions from modest purity precursors, and yet they are highly crystalline and exhibit sharp optical absorption onsets. They can incorporate orientationally rotating molecular cations and are mechanically soft, but exhibit low nonradiative recombination rates on par with high-quality III–V semiconductors.1 Initially, many of the favorable features of these materials were attributed to dynamically disordered molecular cations, but it is increasingly clear that the performance and properties of the all-inorganic variants qualitatively match those of their hybrid counterparts.2–7 The chemistry and bonding of the 6s2 (5s2) lone-pair electrons on Pb (Sn) are essential to understanding several of the unusual properties of these materials. This article pre sents a brief introduction to lone pairs and their stereochemistry, and establishes the connections between the electronic
configuration of the metal cations and the resulting electronicband structures and lattice dynamics of the compounds. The lone pairs are shown to have a decisive impact on electronic properties, directly causing a favorable broad valence band (VB) and light holes. The lattice dynamical impacts are profound, with the lone pairs leading to substantially elevated lattice polarizability.
Lone pairs and their stereochemistry Students of high-school chemistry will be familiar with the molecular structures of simple molecules such as water and ammonia from the valence-shell electron pair repulsion (VSEPR) theory of Sidgwick and Powell,8 later fine-tuned by Gillespie and Nyholm,9 as shown in Figure 1a. By counting electrons and assigning bond orders, one concludes that water has two lone pairs of electrons, leading to the familiar bent geometry, and to a substantial permanent dipole. Similarly, ammonia has one lone pair and adopts a distorted pyramidal geometry. In extended solids of heavier elements, similar principles apply. While competing long-range forces, relativistic contraction (expansion) of 6s (5d) orbitals, and effects of
Douglas H. Fabini, Max Planck Institute for Solid State Research, Germany; [email protected] Ram Seshadri, Materials Department and Department of Chemistry and Biochemistry, University of California, Santa Barbara, USA; [email protected] Mercouri G. Kanatzidis, Northwestern University, USA; [email protected] doi:10.1557/mrs.2020.142 • VOLUME • JUNE © 2020 Materials Research Society Uppsala Universitetsbibliotek, on 16 Jun 2020 at 16:21:58, subject to theMRS BULLETINCore 45 use, 2020 •atmrs.org/bulletin Downloaded from https://www.cambridge.org/core. Cambridge terms of available https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2020.142
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The underappreciated lon
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