Abstract
Metal halide perovskites feature crystalline-like electronic band structures and liquid-like physical properties. The crystal–liquid duality enables optoelectronic devices with unprecedented performance and a unique opportunity to chemically manipulate the structure with low energy input. In this work, we leverage the low formation energy of metal halide perovskites to demonstrate multicolor reversible chromism. We synthesized layered Ruddlesden-Popper FAn+1PbnX3n+1 (FA = formamidinium, X = I, Br; n = number of layers = 1, 2, 3 … ∞) and reversibly tune the dimensionality (n) by modulating the strength and number of H-bonds in the system. H-bonding was controlled by exposure to solvent vapor (solvatochromism) or temperature change (thermochromism), which shuttles FAX salt pairs between the FAn+1PbnX3n+1 domains and adjacent FAX “reservoir” domains. Unlike traditional chromic materials that only offer a single-color transition, FAn+1PbnX3n+1 films reversibly switch between multiple colors including yellow, orange, red, brown, and white/colorless. Each colored phase exhibits distinct optoelectronic properties characteristic of 2D superlattice materials with tunable quantum well thickness.
Highlights
Metal halide perovskites feature crystalline-like electronic band structures and liquid-like physical properties
Reversible chromism is enabled by a second “reservoir” phase in the film composed of excess FAX salt, which allows FAX to reversibly shuttle between the reservoir and Metal halide perovskite (MHP) layers
Wide-angle X-ray scattering (WAXS) data shows that FAn+1PbnI3n+1 powder exhibits Bragg diffraction peaks that correspond to a mixture of 2D FA2PbI4 (n = 1) with staggered octahedral layers and 3D α-FAPbI3 (3C/3R) that corresponds to n ≥ 2 (Supplementary Fig. 1)
Summary
Metal halide perovskites feature crystalline-like electronic band structures and liquid-like physical properties. MHPs are inherently thermochromic materials exhibiting significant optical changes induced by crystal phase transitions between the α-phase, a black high-symmetry perovskite phase composed of corner-sharing [MX6]4− octahedra, and the δ-phase, a yellow-to-colorless non-perovskite hexagonal or orthorhombic phase composed of face- or edge-sharing [MX6]4− octahedra, respectively[26,27,28] Utilizing this phase transformation, the mixedhalide perovskite CsPbI0.5Br2.5 was employed in switchable photovoltiac window that achieved a 4.69% PCE with a phase transition temperature of 105 °C22. We synthesize composite films composed of layered FA-based MHPs of the general formula FAn+1PbnX3n+1 (X = I, Br) and their mixed-halide compositions We leverage their liquid-like physical properties to demonstrate a third mechanism of dynamic chromism in MHPs through reversible layer formation and coalescence to form compounds that span 2D FA2PbX4 (n = 1) to 3D α-FAPbX3 (n = ∞) and 1D δ-FAPbI3. The colored phases are 2D superlattice materials with tunable quantum well thickness
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