Halide double perovskites have proved to be a promising environmentally friendly optoelectronic and photovoltaic material for their inherent thermodynamic stability, high defect tolerance, and appropriate band gaps. However, no ferroelectric material based on halide double perovskites has been discovered until now.  

In a study published in Angew.Chem.Int.Ed., a research group led by Prof. LUO Junhua from Fujian Institute of Research on the Structure of Matter (FJIRSM) of Chinese Academy of Sciences, reported the first halide double perovskite ferroelectric, (n-propylammonium)₂CsAgBiBr₇, which exhibits distinct ferroelectricity with a notable saturation polarization of about 1.5 μC/cm^-2.  

The researchers found a new environmentally friendly two-dimensional (2D) halide double perovskite ferroelectric material, (n-propylammonium)₂CsAgBiBr_7, through the metal-hetero-substitution and organic-cation intercalation of CsPbBr₃. 

They used a fascinating strategy, that is, combining trivalent (Ga³⁺, Al³⁺, Tl³⁺, In³⁺, Bi³⁺, Sb³⁺) and monovalent metals (Rb⁺, K⁺, Na⁺, Li⁺, Ag⁺, Tl⁺) to replace two divalent lead cations (Pb²⁺), thus constructing a series of lead-free halide double perovskites. 

Besides, the researchers found that single-crystal photodetectors of (n-propylammonium)₂CsAgBiBr₇ exhibit extraordinary performance containing high on/off ratios of about 10⁴, fast response rates of 141 μs, and detectivity as high as 5.3×10¹¹ Jones. Such halide double perovskite ferroelectric shows high heat, light and moisture stable compared to lead perovskites. 

This study opens a new way to design high-performance perovskite ferroelectrics, and provides a viable approach in the search for stable and lead-free optoelectronic materials as an alternative to the lead-containing system. 

Schematic illustration of the metal-hetero-substitution and cation intercalation from 3D lead halide perovskites to 2D double halide perovskites. (Image by Prof. LUO’s group) 

Contact: 

Prof. LUO Junhua  

Fujian Institute of Research on the Structure of Matter 

Chinese Academy of Sciences 

E-mail: jhluo@fjirsm.ac.cn