Material exploration via designing spatial arrangement of octahedral units: a case study of lead halide perovskites

Halide perovskites have attracted tremendous attention as semiconducting materials for various optoelectronic applications. The functional metal-halide octahedral units and their spatial arrangements play a key role in the optoelectronic properties of these materials. At present, most of the efforts for material exploration focus on substituting the constituent elements of functional octahedral units, whereas designing the spatial arrangement of the functional units has received relatively little consideration. In this work, via a global structure search based on density functional theory (DFT), we discovered a metastable three-dimensional honeycomb-like perovskite structure with the functional octahedral units arranged through mixed edge-and corner-sharing. We experimentally confirmed that the honeycomb-like perovskite structure can be stabilized by divalent molecular cations with suitable size and shape, such as 2,2′-bisimidazole (BIM). DFT calculations and experimental characterizations revealed that the honeycomb-like perovskite with the formula of BIMPb2I6, synthesized through a solution process, exhibits high electronic dimensionality, a direct allowed bandgap of 2.1 eV, small effective masses for both electrons and holes, and high optical absorption coefficients, which indicates a significant potential for optoelectronic applications. The employed combination of DFT and experimental study provides an exemplary approach to explore prospective optoelectronic semiconductors via spatially arranging functional units.

Suppression of phase transitions and glass phase signatures in mixed cation halide perovskites

Cation engineering provides a route to control the structure and properties of hybrid halide perovskites, which has resulted in the highest performance solar cells based on mixtures of Cs, methylammonium, and formamidinium. Here, we present a multi-technique experimental and theoretical study of structural phase transitions, structural phases and dipolar dynamics in the mixed methylammonium/dimethylammonium MA1-xDMAxPbBr3 hybrid perovskites (0 ≤ x ≤ 1). Our results demonstrate a significant suppression of the structural phase transitions, enhanced disorder and stabilization of the cubic phase even for a small amount of dimethylammonium cations. As the dimethylammonium concentration approaches the solubility limit in MAPbBr3, we observe the disappearance of the structural phase transitions and indications of a glassy dipolar phase. We also reveal a significant tunability of the dielectric permittivity upon mixing of the molecular cations that arises from frustrated electric dipoles.