The transient supersaturation in a system undergoing Ostwald ripening is related to the cluster formation energy E fc as a function of cluster size n. We use this relation to study the energetics of self-interstitial clusters in Si. Measurements of transient enhanced diffusion of B in Si-implanted Si are used to determine S͑t͒, and inverse modeling is used to derive E fc ͑n͒. For clusters with n . 15, E fc ഠ 0.8 eV, close to the fault energy of ͕113͖ defects. For clusters with n , 10, E fc is typically 0.5 eV higher, but stabler clusters exist at n ഠ 4 (E fc ഠ 1.0 eV) and n ഠ 8 (E fc ഠ 0.6 eV). [S0031-9007(99)09311-4]

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.

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.

We study the optical response of CH3NH3PbI3 layers to light solicitation under different environmental gas and temperature conditions. The measurements were performed in nonreactive (Ar or N2) and reactive (O2 or humid air) gas in the range 40–80 °C crossing the tetragonal–cubic transition (∼50 °C). With respect to truly inert Ar, the use of N2 not only assures the reversibility of the optical constants during thermal cycles but also improves the optical response of the material. While in N2 and Ar atmospheres the optical parameters of the material can be recovered at the end of the cycle, in contrast, the presence of humidity in the air causes the absorption coefficient to monotonically and inexorably decrease in the whole visible range, especially after the lattice has moved to cubic. The use of N2 thus represents an effective strategy to improve the absorption under thermal operation conditions.