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.

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.

Methylammonium lead iodide (CHNHPbI) is an extensively used perovskite material with a remarkable potential for solar energy conversion. Despite its high photovoltaic efficiency, the material suffers from fast degradation when aging in atmospheric conditions and/or under sunlight. Here we review the principal degradation mechanisms of CHNHPbI, focusing on the thermodynamic, environmental and polymorphic parameters that impact the stability of the material. A critical analysis of the available data indicates that degradation under ambient conditions is a defect-generation process that is highly localized on surfaces and interfaces, while it is further enhanced above the tetragonal-cubic transition at ∼54 °C. Within this context, we discuss the conservative role of N and propose strategies for the emergence of industrially viable hybrid photovoltaics.

We investigate the degradation path of MAPbI3 (MA=methylammonium) films over flat TiO2 substrates at room temperature by means of X-ray diffraction, spectroscopic ellipsometry, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The degradation dynamics is found to be similar in air and under vacuum conditions, which leads to the conclusion that the occurrence of intrinsic thermodynamic mechanisms is not necessarily linked to humidity. The process has an early stage, which drives the starting tetragonal lattice in the direction of a cubic atomic arrangement. This early stage is followed by a phase change towards PbI2 . We describe how this degradation product is structurally coupled with the original MAPbI3 lattice through the orientation of its constituent PbI6 octahedra. Our results suggest a slight octahedral rearrangement after volatilization of HI+CH3 NH2 or MAI, with a relatively low energy cost. Our experiments also clarify why reducing the interfaces and internal defects in the perovskite lattice enhances the stability of the material.

One of the main challenges to exploit molybdenum disulfide (MoS) potentialities for the next-generation complementary metal oxide semiconductor (CMOS) technology is the realization of p-type or ambipolar field-effect transistors (FETs). Hole transport in MoS FETs is typically hampered by the high Schottky barrier height (SBH) for holes at source/drain contacts, due to the Fermi level pinning close to the conduction band. In this work, we show that the SBH of multilayer MoS surface can be tailored at nanoscale using soft O plasma treatments. The morphological, chemical, and electrical modifications of MoS surface under different plasma conditions were investigated by several microscopic and spectroscopic characterization techniques, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), conductive AFM (CAFM), aberration-corrected scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS). Nanoscale current-voltage mapping by CAFM showed that the SBH maps can be conveniently tuned starting from a narrow SBH distribution (from 0.2 to 0.3 eV) in the case of pristine MoS to a broader distribution (from 0.2 to 0.8 eV) after 600 s O plasma treatment, which allows both electron and hole injection. This lateral inhomogeneity in the electrical properties was associated with variations of the incorporated oxygen concentration in the MoS multilayer surface, as shown by STEM/EELS analyses and confirmed by ab initio density functional theory (DFT) calculations. Back-gated multilayer MoS FETs, fabricated by self-aligned deposition of source/drain contacts in the O plasma functionalized areas, exhibit ambipolar current transport with on/off current ratio I/I ≈ 10 and field-effect mobilities of 11.5 and 7.2 cm V s for electrons and holes, respectively. The electrical behavior of these novel ambipolar devices is discussed in terms of the peculiar current injection mechanisms in the O plasma functionalized MoS surface.

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