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Perovskite solar cells (PSCs) comprise a solid perovskite absorber sandwiched between several layers of different charge-selective materials, ensuring unidirectional current flow and high voltage output of the devices 1 , 2 . A ‘buffer material’ between the electron-selective layer and the metal electrode in p-type/intrinsic/n-type (p-i-n) PSCs (also known as inverted PSCs) enables electrons to flow from the electron-selective layer to the electrode 3 – 5 . Furthermore, it acts as a barrier inhibiting the inter-diffusion of harmful species into or degradation products out of the perovskite absorber 6 – 8 . Thus far, evaporable organic molecules 9 , 10 and atomic-layer-deposited metal oxides 11 , 12 have been successful, but each has specific imperfections. Here we report a chemically stable and multifunctional buffer material, ytterbium oxide (YbO x ), for p-i-n PSCs by scalable thermal evaporation deposition. We used this YbO x buffer in the p-i-n PSCs with a narrow-bandgap perovskite absorber, yielding a certified power conversion efficiency of more than 25%. We also demonstrate the broad applicability of YbO x in enabling highly efficient PSCs from various types of perovskite absorber layer, delivering state-of-the-art efficiencies of 20.1% for the wide-bandgap perovskite absorber and 22.1% for the mid-bandgap perovskite absorber, respectively. Moreover, when subjected to ISOS-L-3 accelerated ageing, encapsulated devices with YbO x exhibit markedly enhanced device stability. Ytterbium oxide buffer layer for use in perovskite solar cells yields a certified power conversion efficiency of more than 25%, which enhances stability across a wide variety of perovskite compositions.

Fick’s law applicable to isothermal solid-solid n -component diffusion couples has been recently derived by the author directly from the continuity equation. The derivation is briefly reviewed in this paper and general expressions applicable to the determination of interdiffusion coefficients, D ~ ij n ( i , j = 1 , 2 , … n - 1 ) , are developed for isothermal, diffusion couples with constant molar density. Explicit expressions for ternary and quaternary interdiffusion coefficients, D ~ ij 3 ( i , j = 1 , 2 ) and D ~ ij 4 ( i , j = 1 , 2 , 3 ) , are also presented. These expressions developed for the calculation of both main and cross interdiffusion coefficients at a section x include various partial derivatives of [ ( J ~ i ) · ( x - x o ) ] ( i = 1 , 2 , … n - 1 ) with respect to individual concentrations C j , where J ~ i is the interdiffusion flux of component i based on a laboratory-fixed frame and x o is the Matano plane for the couple. In this paper the analysis is applied to the concentration profiles theoretically calculated for an isothermal, binary diffusion couple characterized by a constant interdiffusion coefficient D ~ to illustrate the validity of the analysis. A parabolic representation of the derivative, d [ ( J ~ i ) · ( x - x o ) ] / d C i ( i = 1 , 2 ) that is involved in the expression for D ~ , is also developed as a function of x , and such representation has been shown to be useful for the calculation of D ~ for the binary diffusion couple. The parabolic representation of the terms employed for the determination of the binary D ~ is presented for the first time in this study.

Since the launch of lithium-ion batteries, elements (such as silicon, tin, or aluminum) that can be alloyed with lithium have been expected as anode materials, owing to larger capacity. However, their successful application has not been accomplished because of drastic structural degradation caused by cyclic large volume change during battery reactions. To prolong lifetime of alloy anodes, we must circumvent the huge volume strain accompanied by insertion/extraction of lithium. Here we report that by using aluminum-foil anodes, the volume expansion during lithiation can be confined to the normal direction to the foil and, consequently, the electrode cyclability can be markedly enhanced. Such a unidirectional volume-strain circumvention requires an appropriate hardness of the matrix and a certain tolerance to off-stoichiometry of the resulting intermetallic compound, which drive interdiffusion of matrix component and lithium along the normal-plane direction. This metallurgical concept would invoke a paradigm shift to future alloy-anode battery technologies. Alloy anode materials in lithium batteries usually suffer from fatal structural degradation due to the large volume change during cycling. Here the authors report a design in which Al foil serves as both anode and current collector to circumvent the strain.

Epitaxial heterostructures based on oxide perovskites and III–V, II–VI and transition metal dichalcogenide semiconductors form the foundation of modern electronics and optoelectronics 1 – 7 . Halide perovskites—an emerging family of tunable semiconductors with desirable properties—are attractive for applications such as solution-processed solar cells, light-emitting diodes, detectors and lasers 8 – 15 . Their inherently soft crystal lattice allows greater tolerance to lattice mismatch, making them promising for heterostructure formation and semiconductor integration 16 , 17 . Atomically sharp epitaxial interfaces are necessary to improve performance and for device miniaturization. However, epitaxial growth of atomically sharp heterostructures of halide perovskites has not yet been achieved, owing to their high intrinsic ion mobility, which leads to interdiffusion and large junction widths 18 – 21 , and owing to their poor chemical stability, which leads to decomposition of prior layers during the fabrication of subsequent layers. Therefore, understanding the origins of this instability and identifying effective approaches to suppress ion diffusion are of great importance 22 – 26 . Here we report an effective strategy to substantially inhibit in-plane ion diffusion in two-dimensional halide perovskites by incorporating rigid π-conjugated organic ligands. We demonstrate highly stable and tunable lateral epitaxial heterostructures, multiheterostructures and superlattices. Near-atomically sharp interfaces and epitaxial growth are revealed by low-dose aberration-corrected high-resolution transmission electron microscopy. Molecular dynamics simulations confirm the reduced heterostructure disorder and larger vacancy formation energies of the two-dimensional perovskites in the presence of conjugated ligands. These findings provide insights into the immobilization and stabilization of halide perovskite semiconductors and demonstrate a materials platform for complex and molecularly thin superlattices, devices and integrated circuits. An epitaxial growth strategy that improves the stability of two-dimensional halide perovskites by inhibiting ion diffusion in their heterostructures using rigid π-conjugated ligands is demonstrated, and shows near-atomically sharp interfaces.

Nanoparticle strengthening provides a crucial basis for developing high-performance structural materials with potentially superb mechanical properties for structural applications. However, the general wisdom often fails to work well due to the poor thermal stability of nanoparticles, and the rapid coarsening of these particles will lead to the accelerated failures of these materials especially at elevated temperatures. Here, we demonstrate a strategy to achieve ultra-stable nanoparticles at 800~1000 °C in a Ni 59.9- x Co x Fe 13 Cr 15 Al 6 Ti 6 B 0.1 (at.%) chemically complex alloy, resulting from the controllable sluggish lattice diffusion (SLD) effect. Our diffusion kinetic simulations reveal that the Co element leads to a significant reduction in the interdiffusion coefficients of all the main elements, especially for the Al element, with a maximum of up to 5 orders of magnitude. Utilizing first-principles calculations, we further unveil the incompressibility of Al induced by the increased concentration of Co plays a critical role in controlling the SLD effect. These findings are useful for providing advances in the design of novel structural alloys with extraordinary property-microstructure stability combinations for structural applications. Nanoparticle strengthening provides a crucial basis for developing high-performance materials, which often fails to work due to poor thermal stability. Here, the authors achieve thermally stable nanoparticles at 800~1000 °C in chemically complex alloys via controllable sluggish lattice diffusion.

The elemental interdiffusion between the bond-coat in the thermal barrier coatings (TBCs) system and the superalloy substrate has emerged as a critical factor affecting the service life of the TBCs-coated turbine blades in aero-engines. To address this issue, a NiCrAlY bond-coat with low Al content and high Y content was designed and sprayed on the superalloy using high velocity oxygen fuel method. The performance of the as-deposited coating was assessed through isothermal oxidation test at 1000 °C for durations ranging from 10 to 1000 h. The results revealed the precipitation of c-Y 2 O 3 particles at the grain boundaries of the as-deposited coating. These particles appeared to impede the diffusion of coating elements, resulting in improved oxidation resistance. The oxidation mechanism of the as-deposited coating on the flat region was divided into two stages: the preferential oxidation of Al and the formation of spinel; the inward growth of alumina along with outward growth of spinel. In the concave area, the oxidation mechanism was characterized by a rapid thickening of the spinel phase and NiO, attributed to the thinner coating and special Ni diffusion method. Semi-molten particles formed an overlaying structure after oxidation as the elements diffused in surrounding areas, with only the Ni element remaining inside it. The designed low Al content NiCrAlY coating exhibits excellent oxidation resistance at 1000 °C.

In the present study, the diffusion couple of solid CoCrFeMnNi HEA and liquid pure Al was prepared. The microstructure evolution and relevant interdiffusion behavior of CoCrFeMnNi HEA/Al solid–liquid diffusion couple processed by different parameters were characterized and investigated. Results demonstrated that the interfacial compounds in the order of Al(Co, Cr, Fe, Mn, Ni), Al13(Co, Cr, Fe, Mn, Ni)4 and Al4(Co, Cr, Fe, Mn, Ni) were determined in the interdiffusion area along the direction from CoCrFeMnNi HEA to Al, and the precipitated Al4(Cr, Mn) and Al9(Co, Fe, Ni) phases were formed in the center of Al couple. In addition, the diffusion mechanism and activation energy of growth for each diffusion layer were revealed and determined. More importantly, the growth mechanism of each diffusion layer was also investigated and uncovered in detail. Meanwhile, the activation energy of each intermetallic layer was obtained by the Arrhenius equation and the linear regression method. It is anticipated that this present study would provide a fundamental understanding and theoretical basis for the high-entropy alloy CoCrFeMnNi HEA, potentially applied as the cast mold material for cast aluminum alloy.

Vanadium–titanium magnetite (VTM) is an iron ore abundantly available in China. The dominant utilization route is blast furnace smelting; however, Ti in the ore deteriorates sinter strength, making it urgent to clarify Fe-Ti-Ca interactions during sintering. In this work, single-phase FeTiO3 and Fe2TiO4 were synthesized and each paired with CaO to fabricate diffusion couples. The couples were heated at 1200 °C for 30, 60, 90, and 120 min to investigate their interdiffusion behaviors and microstructure recombination mechanisms. The results show that, at 1200 °C, solid-state diffusion—not interfacial reaction—controls mass transfer in both FeTiO3-CaO and Fe2TiO4-CaO systems. Distinct Fe-rich and Ti-rich sublayers appear within the reaction zone, and banded CaTiO3 forms adjacent to the FeTiO3/Fe2TiO4 matrices. The interdiffusion coefficients were determined to be 4.08 × 10−10 cm2·s−1 and 7.81 × 10−10 cm2·s−1, and the growth of the reaction layer follows a parabolic law, which can be expressed as x2 = 2 × 1.562 × 10−9 t and x2 = 2 × 0.8159 × 10−9 t, respectively. The coefficients of determination exceed 0.90, indicating reliable regression fits.

Sulfide kesterite Cu 2 ZnSnS 4 provides an attractive low-cost, environmentally benign and stable photovoltaic material, yet the record power conversion efficiency for such solar cells has been stagnant at around 9% for years. Severe non-radiative recombination within the heterojunction region is a major cause limiting voltage output and overall performance. Here we report a certified 11% efficiency Cu 2 ZnSnS 4 solar cell with a high 730 mV open-circuit voltage using heat treatment to reduce heterojunction recombination. This heat treatment facilitates elemental inter-diffusion, directly inducing Cd atoms to occupy Zn or Cu lattice sites, and promotes Na accumulation accompanied by local Cu deficiency within the heterojunction region. Consequently, new phases are formed near the hetero-interface and more favourable conduction band alignment is obtained, contributing to reduced non-radiative recombination. Using this approach, we also demonstrate a certified centimetre-scale (1.11 cm 2 ) 10% efficiency Cu 2 ZnSnS 4 photovoltaic device; the first kesterite cell (including selenium-containing) of standard centimetre-size to exceed 10%. The emerging kesterite Cu 2 ZnSnS 4 solar cell offers a potential low-cost, non-toxic, materially abundant platform for next-generation photovoltaics, yet its efficiency has been mired below 10%. Yan et al. now use post-heat treatment of the heterojunction to show device efficiencies that surpass 10%.

Facile ionic transport in lead halide perovskites plays a critical role in device performance. Understanding the microscopic origins of high ionic conductivities has been complicated by indirect measurements and sample microstructural heterogeneities. Here, we report the direct visualization of halide anion interdiffusion in CsPbCl₃–CsPbBr₃ single crystalline perovskite nanowire heterojunctions using wide-field and confocal photoluminescence measurements. The combination of nanoscale imaging techniques with these single crystalline materials allows us to measure intrinsic anionic lattice diffusivities, free from complications of microscale inhomogeneity. Halide diffusivities were found to be between 10−13 and ∼10−12 cm²/second at about 100 °C, which are several orders of magnitudes lower than those reported in polycrystalline thin films. Spatially resolved photoluminescence lifetimes and surface potential measurements provide evidence of the central role of halide vacancies in facilitating ionic diffusion. Vacancy formation free energies computed from molecular simulation are small due to the easily deformable perovskite lattice, accounting for the high equilibrium vacancy concentration. Furthermore, molecular simulations suggest that ionic motion is facilitated by low-frequency lattice modes, resulting in low activation barriers for vacancy-mediated transport. This work elucidates the intrinsic solid-state ion diffusion mechanisms in this class of semisoft materials and offers guidelines for engineering materials with long-term stability in functional devices.