Lead-free double perovskites have attracted noteworthy attention due to their compositional flexibility and electronic diversity. In this study, we hydrothermally grow a new class of Cs2Ag x Na1–x FeCl6 (0 ≤ x ≤ 1) perovskite single crystals with high thermal stability. The substitution of B-site cation allows to regulate the crystallographic and band structure, which gives rise to enlarged band absorbance close to the near-infrared region (∼800 nm) via composition engineering. Ultrafast transient absorption spectroscopy (TAS) certifies that the decay time of excited-state absorption is 5.02 and 2450 ps in the case of Cs2NaFeCl6 and Cs2AgFeCl6, respectively. The corresponding charge carrier diffusion length accordingly enhances from 3.7 to 311 nm by means of increasing Ag dopant concentration. Structurally, the primitive cell shrinks due to the partial replacement of [NaCl6]5− octahedra by [AgCl6]5− octahedra. It is proved theoretically as well as experimentally that the introduction of Ag species can effectively enhance the electron mobility (from 1.06 to 15.3 cm2 V–1 s–1) by ∼15 times through realizing stronger orbital coupling of the conductive ions, which enables such a novel double perovskite to be a potential candidate for the optoelectronic and photovoltaic applications.

Multifunctional lead-free double perovskites demonstrate remarkable potential towards applications in various fields. Herein, an environmentally-friendly, low-cost, high-throughput Cs2NaFeCl6 single crystal with exceedingly high thermal stability is designed and grown. It obtains a cubic lattice system in the temperature range of 80–500 K, accompanied by a completely reversible chromatic variation ranging from yellow to black. Importantly, the intriguing thermochromism is proved to own extremely high reproducibility (over 1000 cycles) without a hysteretic effect, originating from its structural flexibility that including (i) the noteworthy distortion/deformation of [NaCl6]5− and [FeCl6]3− octahedra; (ii) order–disorder arrangement transition of [NaCl6]5− and [FeCl6]3− octahedra as the function of temperature. This study paves the way towards a new class of smart windows and camouflage coatings with an unprecedented colour range based on a Cs2NaFeCl6 perovskite.

The general formula of perovskites can be written as A n BX 2+n , where A is monovalent cation like MA + (CH 3 NH 3 + ), [7] FA + ( + HC(NH 2 ) 2 ), [8] GA + ( + C(NH 2 ) 3 ), [9] and so forth, B is a divalent metal, usually Pb 2+ or Sn 2+ , and X is a halogen anion or hybrid halogen anions. When n = 1, the dimension of the perovskite is 3D, as we insert a larger organic group in A site, the structure dimension of perovskite is thereby transformed to 2D, 1D, or even 0D, which enables to enhance the structural stability against the humidity, and also realize the quantum self-trapped effect toward high photoluminescence quantum yield (PLQY). [10,11] For the displaying application, not only the high PLQY is required, but color-tunable emission is also a necessary characteristic, [12,13] which is generally realized through ion doping and/or substitution, such as Cs 4−x A x Sn(Br 1−y I y ) 6 [14] (A = Rb, K; x ≤ 1, y ≤ 1), Cs 2 AgIn x Fe 1−x Cl 6[15](0 ≤ x ≤ 1), PEA 2 (Rb x Cs 1−x ) n−1 Pb n (Br 1−y Cl y ) 3n+1 , [16] etc. Additionally, dimension regulation is also an effective way to control emission wavelength, e.g.

A series of novel perovskite single crystals are innovatively grown. Aiming to enhance the luminescence performance, octahedral distortion coregulation via dual strategies for the as-prepared perovskite single crystals is performed. The distortion of the octahedral structure strengthens the electron− phonon coupling and electron localization, resulting in a more stable self-trapped state, which thereby increases the potential for radiative recombination, accompanied by the self-trapped exciton emission. Accordingly, the luminescence spectra of the as-prepared MA 4 In 0.975 Sb 0.025 Br 7 single crystal can cover the 450− 800 nm range, and the photoluminescence quantum yield is up to 81.25%.

Terpyridine-derived perovskite single crystals displaying tunable low-dimensional structures and outstanding optoelectronic performances suitable for device applications have been developed.