Exotic Structural and Optoelectronic Properties of Layered Halide Double Perovskite Polymorphs

Discovering new types of layered perovskites has great importance for designing novel optoelectronic devices. In this article, combining first-principle calculations with global structure searching, it is found that Rb₄SnSb₂Br₁₂, a typical halide double perovskite, can unexpectedly possess fertile low formation-energy polymorphs holding van de Walls (vdW) layered structures. Consequently, these polymorphs can be effectively classified into 12 types according to their local octahedral motifs, exhibiting a wide range of bandgap covering the visible spectrum. Interestingly, the structure-dependent bandgap in these polymorphs can be well understood by developing a simple machine learning model. Moreover, as a layered system, the optoelectronic properties of Rb₄SnSb₂Br₁₂ can be effectively tuned by the layer thickness, and both type-I and type-II band alignment can be achieved in single-compound Rb₄SnSb₂Br₁₂ heterojunctions. Finally, it is suggested that the Sn-moderate condition can be considered to grow intrinsic p-type Rb₄SnSb₂Br₁₂ with lower defect density. Those findings not only provide a promising material system for designing the vdW tandem solar cell, but also offer a new opportunity to achieve exotic optoelectronic applications in a single-phase layered perovskite compound.