Acetamidinium-Substituted Methylammonium Lead Iodide Perovskite Solar Cells with Higher Open-Circuit Voltage and Improved Intrinsic Stability

Unprecedented Richness of Temperature- and Pressure-Induced Polymorphism in 1D Lead Iodide Perovskite

Inherent features of metal halide perovskites are their softness, complex lattice dynamics, and phase transitions spectacularly tuning their structures and properties. While the structural transformations are well described and classified in 3D perovskites, their 1D analogs are much less understood. Herein, both temperature- and pressure-dependent structural evolutions of a 1D AcaPbI3 perovskitoid incorporating acetamidinium (Aca) cation are examined. The study reveals the existence of nine phases of δ-AcaPbI3, which present the most diverse polymorphic collection among known perovskite materials. Interestingly, temperature- and pressure-triggered phase transitions in the 1D perovskotoid exhibit fundamentally different natures: the thermal transformations are mainly associated with the collective translations of rigid polyanionic units and ordering/disordering dynamics of Aca cations, while the compression primarily affects inorganic polymer chains. Moreover, in the 1-D chains featuring the face-sharing connection mode of the PbI6 octahedra the Pb···Pb distances are significantly shortened compared to the corner-sharing 3D perovskite frameworks, hence operating in the van der Waals territory. Strikingly, a good correlation is found between the Pb···Pb distances and the pressure evolution of the bandgap values in the δ-AcaPbI3, indicating that in 1D perovskitoid structures, the contacts between Pb2+ ions are one of the critical parameters determining their properties.

Phase Transitions and Dynamics in Mixed Three- and Low-Dimensional Lead Halide Perovskites

Lead halide perovskites are extensively investigated as efficient solution-processable materials for photovoltaic applications. The greatest stability and performance of these compounds are achieved by mixing different ions at all three sites of the APbX3 structure. Despite the extensive use of mixed lead halide perovskites in photovoltaic devices, a detailed and systematic understanding of the mixing-induced effects on the structural and dynamic aspects of these materials is still lacking. The goal of this review is to summarize the current state of knowledge on mixing effects on the structural phase transitions, crystal symmetry, cation and lattice dynamics, and phase diagrams of three- and low-dimensional lead halide perovskites. This review analyzes different mixing recipes and ingredients providing a comprehensive picture of mixing effects and their relation to the attractive properties of these materials.

Composition-Property Relations for GAx FAy MA1- x - y PbI3 Perovskites

Halide perovskites are materials for diverse optoelectronic applications owing to a combination of factors, including their compositional flexibility. A major source of this diversity of compositions comes from the use of mixed organic cations in the A-site of such compounds to form solid solutions. Many organic cations are possible for this purpose. Although significant progress is made over years of intensive research, the determination of systematic relationships between the compositions and properties of halide perovskites is not exploited accordingly. Using the MAPbI3 prototype, a wide range of compositions substituted by formamidinium (FA+ ) and guanidinium (GA+ ) cations are studied. From a detailed collection of experimental data and results reported in the literature, heat maps correlating the composition of GAx FAy MA1- x - y PbI3 solid solutions with phase transition temperatures, dielectric permittivity, and activation energies are constructed. Considering the characteristics of organic cations, namely their sizes, dipole moments, and the number of N─H bonds, it is possible to interpret the heat maps as consequences of these characteristics. This work brings a systematization of how obtaining specific properties of halide perovskites might be possible by customizing the characteristics of the A-site organic cations.

In Situ and Operando Characterizations of Metal Halide Perovskite and Solar Cells: Insights from Lab-Sized Devices to Upscaling Processes

The performance and stability of metal halide perovskite solar cells strongly depend on precursor materials and deposition methods adopted during the perovskite layer preparation. There are often a number of different formation pathways available when preparing perovskite films. Since the precise pathway and intermediary mechanisms affect the resulting properties of the cells, in situ studies have been conducted to unravel the mechanisms involved in the formation and evolution of perovskite phases. These studies contributed to the development of procedures to improve the structural, morphological, and optoelectronic properties of the films and to move beyond spin-coating, with the use of scalable techniques. To explore the performance and degradation of devices, operando studies have been conducted on solar cells subjected to normal operating conditions, or stressed with humidity, high temperatures, and light radiation. This review presents an update of studies conducted in situ using a wide range of structural, imaging, and spectroscopic techniques, involving the formation/degradation of halide perovskites. Operando studies are also addressed, emphasizing the latest degradation results for perovskite solar cells. These works demonstrate the importance of in situ and operando studies to achieve the level of stability required for scale-up and consequent commercial deployment of these cells.