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.

Abstract

A cost-effective and high-throughput material named perovskite has proven to be capable of converting 15.9% of the solar energy to electricity, compared to an efficiency of 3.8% that was obtained only four years ago. It has already outperformed most of the thin-film solar cell technologies that researchers have been studying for decades. Currently, the architecture of perovskite solar cells has been simplified from the traditional dye-sensitized solar cells to planar-heterojunction solar cells. Recently, the performance of perovskite in solar cells has attracted intensive attention and studies. Foreseeably, many transformative steps will be put forward over the coming few years. In this review, we summarize the recent exciting development in perovskite solar cells, and discuss the fundamental mechanisms of perovskite materials in solar cells and their structural evolution. In addition, future directions and prospects are proposed toward high-efficiency perovskite solar cells for practical applications.