Crossover between Tilt Families and Zero Area Thermal Expansion in Hybrid Prussian Blue Analogues

Materials in the family of Prussian blue analogues (C₃H₅N₂)₂K[ M (CN)₆], where C₃H₅N₂ is the imidazolium ion and M =Fe, Co, undergo two phase transitions with temperature; at low temperatures the imidazolium cations have an ordered configuration (C 2/c ), while in the intermediate‐ and high‐temperature phases (both previously reported as ) they are dynamically disordered. We show from high‐resolution powder neutron diffraction data that the high‐temperature phase has zero area thermal expansion in the ab ‐plane. Supported by Landau theory and single‐crystal X‐ray diffraction data, we re‐evaluate the space group symmetry of the intermediate‐temperature phase to . This reveals that the low‐to‐intermediate temperature transition is due to competition between two different tilt patterns of the [ M (CN)₆]³⁻ ions. Controlling the relative stabilities of these tilt patterns offers a potential means to tune the exploitable electric behaviour that arises from motion of the imidazolium guest.     

Guanine-Stabilized Formamidinium Lead Iodide Perovskites

Formamidinium (FA) lead iodide perovskite materials feature promising photovoltaic performances and superior thermal stabilities. However, conversion of the perovskite α-FAPbI₃ phase to the thermodynamically stable yet photovoltaically inactive δ-FAPbI₃ phase compromises the photovoltaic performance. A strategy is presented to address this challenge by using low-dimensional hybrid perovskite materials comprising guaninium (G) organic spacer layers that act as stabilizers of the three-dimensional α-FAPbI₃ phase. The underlying mode of interaction at the atomic level is unraveled by means of solid-state nuclear magnetic resonance spectroscopy, X-ray crystallography, transmission electron microscopy, molecular dynamics simulations, and DFT calculations. Low-dimensional-phase-containing hybrid FAPbI₃ perovskite solar cells are obtained with improved performance and enhanced long-term stability.     

Understanding Hydrogen Bonding Interactions in Crosslinked Methylammonium Lead Iodide Crystals: Towards Reducing Moisture and Light Degradation Pathways

Methylammonium lead halide perovskite‐based solar cells have demonstrated efficiencies as high as 24.2 %, highlighting their potential as inexpensive and solution‐processable alternatives to silicon solar cell technologies. Poor stability towards moisture, ultraviolet irradiation, heat, and a bias voltage of the perovskite layer and its various device interfaces limits the commercial feasibility of this material for outdoor applications. Herein, we investigate the role of hydrogen bonding interactions induced when metal halide perovskite crystals are crosslinked with alkyl or π‐conjugated boronic acid small molecules (‐B(OH)₂). The crosslinked perovskite crystals are investigated under continuous light irradiation and moisture exposure. These studies demonstrate that the origin of the interaction between the alkyl or π‐conjugated crosslinking molecules is due to hydrogen bonding between the ‐B(OH)₂ terminal group of the crosslinker and the I of the [PbI₆]^4? octahedra of the perovskite layer. Also, this interaction influences the stability of the perovskite layer towards moisture and ultraviolet light irradiation. Morphology and structural analyses, as well as IR studies as a function of aging under both dark and light conditions show that π‐conjugated boronic acid molecules are more effective crosslinkers of the perovskite crystals than their alkyl counterparts thus imparting better stability towards light and moisture degradation.     

Hybrid Organic–Inorganic Antiperovskites

Substitution of A-site and/or X-site ions of ABX₃-type perovskites with organic groups can give rise to hybrid perovskites, many of which display intriguing properties beyond their parent compounds. However, this method cannot be extended effectively to hybrid antiperovskites. Now, the design of hybrid antiperovskites under the guidance of the concept of Goldschmidt's tolerance factor is presented. Spherical anions were chosen for the A and B sites and spherical organic cations for the X site, and seven hybrid antiperovskites were obtained, including (F₃(H₂O)_x)(AlF₆)(H₂dabco)₃, ((Co(CN)₆)(H₂O)₅)(MF₆)(H₂dabco)₃ (M=Al³⁺, Cr³⁺, or In³⁺), (Co(CN)₆)(MF₆)(H₂pip)₃ (M=Al³⁺ or Cr³⁺), and (SbI₆)(AlF₆)(H₂dabco)₃. These new structures reveal that all ions at A, B, and X sites of inorganic antiperovskites can be replaced by molecular ions to form hybrid antiperovskites. This work will lead to the synthesis of a large family of hybrid antiperovskites.     

Lead‐ and Iodide‐Deficient (CH3NH3)PbI3 (d‐MAPI): The Bridge between 2D and 3D Hybrid Perovskites

3D and 2D hybrid perovskites, which have been known for more than 20 years, have emerged recently as promising materials for optoelectronic applications, particularly the 3D compound (CH₃NH₃)PbI₃ (MAPI). The discovery of a new family of hybrid perovskites called d ‐MAPI is reported: the association of PbI₂ with both methyl ammonium (MA⁺) and hydroxyethyl ammonium (HEA⁺) cations leads to a series of five compounds with general formulation (MA)_1−2.48x(HEA)_3.48x[Pb_1−xI_3−x]. These materials, which are lead‐ and iodide‐deficient compared to MAPI while retaining 3D architecture, can be considered as a bridge between the 2D and 3D materials. Moreover, they can be prepared as crystallized thin films by spin‐coating. These new 3D materials appear very promising for optoelectronic applications, not only because of their reduced lead content, but also in account of the large flexibility of their chemical composition through potential substitutions of MA⁺, HEA⁺, Pb²⁺ and I⁻ ions.     

A Nickel(II) Nitrite Based Molecular Perovskite Ferroelectric

The X‐site ion in organic–inorganic hybrid ABX₃ perovskites (OHPs) varies from halide ion to bridging linkers like HCOO^?, N₃^?, NO₂^?, and CN^?. However, no nitrite‐based OHP ferroelectrics have been reported so far. Now, based on non‐ferroelectric [(CH₃)₄N][Ni(NO₂)₃], through the combined methodologies of quasi‐spherical shape, hydrogen bonding functionality, and H/F substitution, we have successfully synthesized an OHP ferroelectric, [FMeTP][Ni(NO₂)₃] (FMeTP=N‐fluoromethyl tropine). As an unprecedented nitrite‐based OHP ferroelectric, the well‐designed [FMeTP][Ni(NO₂)₃] undergoes the ferroelectric phase transition at 400 K with an Aizu notation of 6/mmmFm, showing multiaxial ferroelectric characteristics. This work is a great step towards not only enriching the molecular ferroelectric families but also accelerating the potential practical applications.     

Methylammonium‐Mediated Evolution of Mixed‐Organic‐Cation Perovskite Thin Films: A Dynamic Composition‐Tuning Process

Methylammonium‐mediated phase‐evolution behavior of FA_1−xMA_xPbI₃ mixed‐organic‐cation perovskite (MOCP) is studied. It is found that by simply enriching the MOCP precursor solutions with excess methylammonium cations, the MOCPs form via a dynamic composition‐tuning process that is key to obtaining MOCP thin films with superior properties. This simple chemical approach addresses several key challenges, such as control over phase purity, uniformity, grain size, composition, etc., associated with the solution‐growth of MOCP thin films with targeted compositions.