Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH3NH3PbI3

The organic-inorganic hybrid perovskite CH3NH3PbI3 has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of CH3NH3PbI3 solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH3NH3PbI3 is thermodynamically unstable with respect to the phase separation into CH3NH3I ＋ PbI2, i.e., the disproportionation is exothermic, independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombie phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH3NH3PbI3 is unchanged. When I is replaced by Br or CI, Pb by Sn, or the organic cation CH3NH3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH3NH3PbI3 and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.

Electronic Structure and Stability of Lead-free Hybrid Halide Perovskites:A Density Functional Theory Study

The most commonly used and studied hybrid halide perovskite is ABX_3, where A usually stands for CH_3NH_3, B for Pb, and X for I. A lead-free perovskite with high stability and ideal electronic band structure would be of essence, especially considering the toxicity of lead. In this work, we have considered 11 metal elements for the B site and three halide elements(Cl, Br, and I) including various combinations among the three halides for the X site. A total number of 99 hybrid perovskites are studied to understand how the crystal structure, band gap and stability can be tuned by the chemistry modification, i.e., the replacement of toxic element, Pb in the original MAPbX_3, with non-toxic metal elements. We find that the favorable substitutes for Pb in MAPbI_3 are Ge and Sn.

NH_(+)^(4)/K^(+)-substitution-induced C–F–K coordination bonds for designing the highest-temperature hybrid halide double perovskite ferroelastic

Ferroelastic materials with switchable spontaneous strain possess widely potential applications in the field of energy and information conversion.Recently,organic-inorganic hybrid halide double perovskites (OIHHDPs) have become a charming new platform for developing various functional materials,such as ferroelectrics,fluorescence and X–ray detection.Nevertheless,OIHHDP ferroelastic materials,especially high-temperature ones,are rare.Herein,we initially synthesized an OIHHDP ferroelastic,(2,2-difluoroethanamine)_(2)[(NH_(4))InCl_6](1),which possesses a ferroelastic phase transition at 407 K.Moreover,thanks to the flexible B-site for OIHHDPs,we replaced the NH_(4)^(+) ions within[(NH_(4))InCl_6]_n^(2n–)formworks with K^(+)ions,which endows with coordination bonds between 2,2-difluoroethanamine organic cations and[KInCl_6]_n^(2n–)formworks.Due to the existence of coordination bonds,the phase transition temperature of (2,2-difluoroethanamine)_(2)[KInCl_6](2) can reach 458 K.As far as we know,this value is the highest reported in OIHHDP ferroelastics.This work offers inspiration for the design of high-temperature OIHHDP phase transition materials including ferroelectrics and ferroelastics.