We provide a semi-empirical model based on in situ degradation measurements to predict the durability of hybrid perovskite materials under simulated thermal operation conditions. In the model, the degradation path of MAPbI3 layers is proved to follow an Arrhenius-type law. The predictive role is played by the activation energy combined with its pre-exponential factor. Our comparative study under moisture conditions with respect to vacuum and nitrogen treatments has assessed the occurrence of an intrinsic dynamic exchange of protons between the organic cations and the inorganic cage with a direct impact on the lattice stability, for which the presence of water molecules is not mandatory. This mutual interaction produces defects inside the material and volatile species, such as HI, CH3NH2 or MAI, with an associated experimental activation energy of 1.54 eV measured under vacuum conditions in dark. This value is comparable to that calculated by the density functional theory for defect generation in MAPbI3. In air, the action of water molecules reduces the activation energy for proton exchanges in dark to 0.96 eV. As an alternative solution to increase the material stability, we demonstrate that the substitution of methylammonium (MA(+)) with the formamidinium (FA(+)) cations inside the inorganic cage gives greater robustness to the overall lattice and extends the material durability due to a different interaction between the organic molecules and the inorganic cage. This definitely supports the use of FAPbI3 in applications, provided its structure can be stabilized in the dark phase at room temperature.

Single crystals represent a benchmark for understanding the bulk properties of halide perovskites. We have indeed studied the dielectric function of lead bromide perovskite single crystals (MAPbBr 3 , CsPbBr 3 and for the first time FAPbBr 3 ) by spectroscopic ellipsometry in the range of 1–5 eV while varying the temperature from 183 to 440 K. An extremely low absorption coefficient in the sub-band gap region was found, indicating the high optical quality of all three crystals. We extracted the band gap values through critical point analysis showing that Tauc-based values are systematically underestimated. The two structural phase transitions, i.e., orthorhombic–tetragonal and tetragonal–cubic, show distinct optical behaviors, with the former having a discontinuous character. The cross-correlation of optical data with DFT calculations evidences the role of octahedral tilting in tailoring the value of the band gap at a given temperature, whereas differences in the thermal expansion affect the slope of the band gap trend as a function of temperature.

The role of chloride in the MAPbI3-xClx perovskite is still limitedly understood, albeit subjected of much debate. Here, we present a combined angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and first-principles DFT modeling to investigate the MAPbI3-xClx/TiO2 interface. AR-XPS analyses carried out on ad hoc designed bilayers of MAPbI3-xClx perovskite deposited onto a flat TiO2 substrate reveal that the chloride is preferentially located in close proximity to the perovskite/TiO2 interface. DFT calculations indicate the preferential location of chloride at the TiO2 interface compared to the bulk perovskite due to an increased chloride-TiO2 surface affinity. Furthermore, our calculations clearly demonstrate an interfacial chloride-induced band bending, creating a directional "electron funnel" that may improve the charge collection efficiency of the device and possibly affecting also recombination pathways. Our findings represent a step forward to the rationalization of the peculiar properties of mixed halide perovskite, allowing one to further address material and device design issues.

We study the mechanisms of CH3NH3PbI3 degradation and its transformation to PbI2 by means of X-ray diffraction and the density functional theory. The experimental analysis shows that the material can degrade in both air and vacuum conditions, with humidity and temperature-annealing strongly accelerating such process. Based on ab initio calculations, we argue that even in the absence of humidity, a decomposition of the perovskite structure can take place through the statistical formation of molecular defects with a non-ionic character, whose volatility at surfaces should break the thermodynamic defect equilibria. We finally discuss the strategies that can limit such phenomenon and subsequently prolong the lifetime of the material.

Studying defect formation and evolution in MethylAmmonium lead Iodide (MAPbI 3 ) perovskite layers has a bottleneck in the softness of the matter and in its consequent sensitivity to external solicitations. Here we report that, in a polycrystalline MAPbI 3 layer, Pb-related defects aggregate into nanoclusters preferentially at the triple grain boundaries as unveiled by Transmission Electron Microscopy (TEM) analyses at low total electron dose. Pb-clusters are killer against MAPbI 3 integrity since they progressively feed up the hosting matrix. This progression is limited by the concomitant but slower transformation of the MAPbI 3 core to fragmented and interconnected nano-grains of 6H-PbI 2 that are structurally linked to the mother grain as in strain-relaxed heteroepitaxial coupling. The phenomenon occurs more frequently under TEM degradation whilst air degradation is more prone to leave uncorrelated [001]-oriented 2H-PbI 2 grains as statistically found by X-Ray Diffraction. This path is kinetically costlier but thermodynamically favoured and is easily activated by catalytic species.