lasers, [5] single-mode lasers, [6] continuouswave lasers, [7] polariton lasers, [8] and laser arrays. [9] Halide perovskites have also been widely studied in photocatalytic organic reaction, [10] photocatalytic CO 2 reduction, [11] and photocatalytic hydrogen evolution. [12] Due to their broad technological importance, halide perovskites have become the focus of current research.Recently, ferroelectricity has been detected in halide perovskites and quickly attracted widespread interest. [13] Ferroelectricity is a characteristic of spontaneous polarization in certain materials, which can be reversed by applying an external electric field. The discovery of ferroelectricity can be traced back to 1920 [14] (Figure 1a), when Valasek measured the polarization of Rochelle Salt as a function of the applied electric field. Perovskite ferroelectric first appeared on the scene in the early 1940s [15] (Figure 1b). Up to now, perovskite oxides (e.g., BaTiO 3 , [16] PbZr x Ti 1−x O 3 , [17] Bi 4−x La x Ti 3 O 12 , [18] LiNbO 3 , [19] and LiTaO 3 [20] ) as main ferroelectric materials have been widely applied to supercapacitors, [21] memories, [22] sensors, [23] and actuators, [24] which play key roles in modern technologies benefiting human lives. Nevertheless, the fatal weakness of brittleness for most perovskite oxides limits their application in flexible devices. [25] Therefore, perovskite oxide ferroelectrics are losing competitiveness in future technologies pursuing device miniaturization and flexibility. The emergence of halide perovskite ferroelectrics that feature the natural advantages of structural softness and lightweight has thus opened a new chapter in ferroelectric research.Since ferroelectricity was recognized in halide perovskites, the research activities have mainly focused on designing novel halide perovskite ferroelectrics. [35] In the past few years, the collective efforts from interdisciplinary communities have made available a collection of halide perovskite ferroelectrics with distinct compositions and structures (Figure 1c-i), such as 0D (NMP) 3 Sb 2 Cl 9 (NMP = N-methylpyrrolidinium) , [31] 1D (3-pyrrolinium)CdCl 3 , [28] 2D (BEA) 2 PbCl 4 (BEA = benzylammonium), [29] and 3D organometal (AP)RbBr 3 (AP = 3-ammoniopyrrolidinium). [30] Preliminary experiments have revealed the great promise of these materials for applications in ferroelectric photovoltaics, [36] self-powered photodetection, [37] and X-ray detection. [38] On a separate note, the development of halide perovskite ferroelectrics also raises important issues in the mechanistic investigation of halide perovskite in optoelectronics. For example, the possible existence of ferroelectricity was proposed to explain the superior optoelectronic Halide perovskites have gained tremendous attention in the past decade owing to their excellent properties in optoelectronics. Recently, a fascinating property, ferroelectricity, has been discovered in halide perovskites and quickly attracted widespread interest. Compared with traditional perovskite oxide fe...
