The thermal instability of organic–inorganic halide perovskite solar cells (PSCs) is one of the most important factors restraining their commercialization. It has recently been reported that modifications at the interface between the electron transport layer (ETL) and the perovskite layer could effectively promote the long-term thermal stability of high-efficiency PSCs. However, few studies have disclosed the effect of the microstructure of ETLs, such as SnO₂ or TiO₂, on the thermal degradation pathway of the perovskite layer at nano-, or even atomic-scale, resolution. In this work, a variety of characterization techniques were used to detect the diffusion of the oxygen element from SnO₂/TiO₂ layers into organic–inorganic hybrid perovskite (OIHP) layers, which was accompanied by the thermal decomposition of MAPbI₃ and FA_0.9Cs_0.1PbI₃ films. Moreover, a preferential thermal decomposition of the perovskite layer at the SnO₂–perovskite interface was confirmed to be an adverse factor inducing the performance degradation of PSCs. First-principles calculations further elucidate that dangling bonds at the SnO₂ or TiO₂ ETL–perovskite interface down-regulate the oxygen vacancy formation energy. Furthermore, a low segregation energy for oxygen diffusion (0.19–0.55 eV and 0.26 eV at SnO₂ and TiO₂–perovskite interfaces, respectively) accelerates the transfer of the oxygen element from SnO₂ or TiO₂ into OIHP layers and promotes the transfer of protons in the perovskite layer. Furthermore, the fabrication of planar OIHP solar cells based on an O₂-plasma treated ETL SnO₂ layer effectively improved their photoelectric performance and thermal stability, further confirming the important role of interfacial oxygen in modulating the stability of PSC devices.

Ceria is one of the most important functional rare-earth oxides with wide industrial applications. Its amazing oxygen storage/release capacity is attributed to cerium’s flexible valence conversion between 4+ and 3+. However, there still exists some debate on whether the valence conversion is due to the Ce-4f electron localization-delocalization transition or the character of Ce–O covalent bonds. In this work, a mixed valence model was established and the formation energies of oxygen vacancies and electronic charges were obtained by density functional theory calculations. Our results show that the formation energy of oxygen vacancy is affected by the valence state of its neighboring Ce atom and two oxygen vacancies around a Ce⁴⁺ in CeO₂ have a similar effect to a Ce³⁺. The electronic charge difference between Ce³⁺ and Ce⁴⁺ is only about 0.4e. Therefore, we argue that the valence conversion should be understood according to the adjustment of the ratio of covalent bond to ionic bond. We propose that the formation energy of oxygen vacancy be used as a descriptor to determine the valence state of Ce in cerium oxides.

The structural and electronic properties of interfaces play an important role in the stability and functionality of solar cell devices. Experiments indicate that the SnO₂/perovskite interfaces always show superior electron transport efficiency and high structural stability even though there exists a larger lattice mismatch. Aiming at solving the puzzles, we have performed density-functional theory calculations to investigate the electronic characteristics of the SnO₂/perovskite interfaces with various stresses and defects. The results prove that the PbI₂/SnO₂ interfaces have better structural stability and superior characteristics for the electron transport. The tensile stress could move the conduction band minimum (CBM) of CH₃NH₃PbI₃ upward, while the compressive stress could move the CBM of SnO₂ downward. By taking into account the stress effect, the CBM offset is 0.07 eV at the PbI₂/SnO₂ interface and 0.28 eV at the MAI/SnO₂ interface. Moreover, our calculations classify V_I and I_i at the PbI₂/SnO₂ interface and Sn-I, I_i and Sn_i at the MAI/SnO₂ interface as harmful defects. The I_i defects are the most easily formed harmful defects and should be avoided at both interfaces. The calculated results are in agreement with the available experimental observations. The present work provides a theoretical basis for improving the stability and photovoltaic performance of the perovskite solar cells.

Compounds of rare earth zirconates with pyrochlore structure are candidates for the application of thermal barrier coatings of next generation. In order to modify the mechanic properties and maintain the low thermal conductivity, other trivalent rare-earth element substitution is commonly used. Presently, investigation on the evaluation of the property of thermal expansion is attracting more attention. In this paper, a feature parameter of thermal expansion coefficient at high temperature (α_∞) was proposed by combining Grüneisen’s equation and the Debye heat capacity model. Using α_∞ model, the thermal expansion property of different compounds can be easily figured out by first principles. Firstly, α_∞ of ZrO₂, HfO₂, were calculated, and results are in good agreement with the experimental data from the literature. Moreover, α_∞ of La₂Zr₂O₇, Pr₂Zr₂O₇, Gd₂Zr₂O₇, and Dy₂Zr₂O₇ were calculated, and results demonstrated that the model of α_∞ is a useful tool to predict the thermal expansion coefficient at high temperature. Finally, Gd₂Zr₂O₇ with 4 different Yb dopant concentrations (Gd_1-xYb_x)₂Zr₂O₇ (x = 0, 0.125, 0.3125, 0.5) were calculated. Comparing with the experimental data from the literature, the calculation results showed the same tendency with the increasing of Yb concentration.

To investigate the comprehensive effects of the Al and Zr element contents on the microstructure evolution of the AlNbTiVZr series light-weight refractory high entropy alloys (HEAs), five samples were studied. Samples with different compositions were designated Al_1.5NbTiVZr, Al_1.5NbTiVZr_0.5, AlNbTiVZr, AlNbTiVZr_0.5, and Al_0.5NbTiVZr_0.5. The results demonstrated that the actual density of the studied HEA samples ranged from 5.291 to 5.826 g·cm⁻³. The microstructure of these HEAs contains a solid solution phase with a BCC structure and a Laves phase. The Laves phase was further identified as the ZrAlV intermetallic compound by TEM observations. The microstructure of the AlNbTiVZr series HEAs was affected by both the Al and Zr element contents, whereas the Zr element showed a more dominant effect due to Zr atoms occupying the core position of the ZrAlV Laves phase (C14 structure). Therefore, the as-cast Al_0.5NbTiVZr_0.5 sample exhibits the best room temperature compression property with a compression strength (σ_p) of 1783 MPa and an engineering strain of 28.8% due to having the lowest ZrAlV intermetallic compound area fraction (0.7%), as characterized by the EBSD technique.