The stellarator is a type of fusion energy device that—if properly designed—could provide clean, safe, and abundant energy to the grid. To generate this energy, a stellarator must keep a hot mixture of charged particles (known as a plasma) sufficiently confined by using a fully shaped magnetic field. If this is achieved, the heat from fusion reactions within the plasma can be harvested as energy. We present a novel method for designing reactor-relevant stellarator magnetic fields, which combine several key physical properties. These include plasma stability, excellent confinement of the fast-moving particles generated by fusion reactions, and reduction of the turbulence that is known to limit the performance of the most advanced stellarator experiment in the world, Wendelstein 7-X.

Entropy transfer in metal halide perovskites, characterized by significant lattice anharmonicity and low stiffness, underlies the remarkable properties observed in their optoelectronic applications, ranging from solar cells to lasers. The conventional view of this transfer involves stochastic processes occurring within a thermal bath of phonons, where the lattice arrangement and energy flow from higher- to lower-frequency modes. Here, we unveil a comprehensive chronological sequence detailing a conceptually distinct coherent transfer of entropy in a prototypical perovskite ${\mathrm{CH}}_3$${\mathrm{NH}}_3$${\mathrm{Pbl}}_3$. The terahertz periodic modulation imposes vibrational coherence into electronic states, leading to the emergence of mixed (vibronic) quantum beat between approximately 3 and 0.3 THz. We highlight a well-structured bidirectional time-frequency transfer of these diverse phonon modes, each developing at different times and transitioning from high to low frequencies from 3 to 0.3 THz, before reversing direction and ascending to around 0.8 THz. First-principles molecular dynamics simulations disentangle a complex web of coherent-phononic coupling pathways and identify the salient roles of the initial modes in shaping entropy evolution at later stages. Capitalizing on coherent entropy transfer and dynamic anharmonicity presents a compelling opportunity to exceed the fundamental thermodynamic (Shockley-Queisser) limit of photoconversion efficiency and to pioneer novel optoelectronic functionalities.

Durable and reliable photovoltaic (PV) modules are critical to enabling an efficient transition to sustainable energy generation. The rate at which new module designs and materials are developed and deployed currently outpaces the rate at which we can identify failure mechanisms and understand degradation rates. Increasing the service life of PV modules, and our ability to predict performance over time, requires more durable materials and designs, better durability testing, more extensive material characterization, robust modeling, and methods to cross-examine historical performance data to extract meaningful results. This is a multidisciplinary challenge that requires expertise from a broad range of fields and, therefore, benefits significantly from a collaborative approach. In this Perspective, we outline the approach taken by the Durable Module Materials Consortium (DuraMAT), present a few case studies where our approach was successful, and provide an outlook on where this approach might be applied as the PV technology landscape continues to rapidly evolve.

Analytical diode models that relate the current to key material parameters are invaluable for understanding the device physics in photovoltaic (PV) devices. However, a diode model that accurately describes the current in thin-film PVs based on low-mobility semiconductors (including organic solar cells) limited by bimolecular recombination has been lacking. Previous models neglect effects of injected charge carriers, which are the predominant source for bimolecular recombination in thin-film PV systems with Ohmic contacts. In this work, we derive a unified diode equation for thin-film PVs based on undoped semiconductors limited by bimolecular recombination. Based on a regional approximation approach, we derive an analytical model for the current accounting for the interplay between charge-carrier extraction, injection, and bimolecular recombination. The analytical model is validated by numerical simulations and tested experimentally on organic solar cells. Our findings provide key insights into the mechanisms driving and limiting charge collection, and ultimately the power-conversion efficiency, in low-mobility PV devices. The presented framework is material agnostic and generally applicable to sandwich-type thin-film PV devices, including photodiodes and indoor light-harvesting cells.

The search for materials with large thermopower is of great practical interest. Dirac and Weyl semimetals have recently proven to exhibit superior thermoelectric properties, particularly when subjected to a quantizing magnetic field. Here, we consider whether a similar enhancement arises in nodal-line semimetals, for which the conduction and valence band meet at a line or ring in momentum space. We compute the Seebeck and Nernst coefficients for arbitrary temperature and magnetic field and we find a wealth of different scaling regimes. Most strikingly, when a sufficiently strong magnetic field is applied along the direction of a straight nodal line or in the plane of a nodal ring, the large degeneracy of states leads to a large linear-in-$B$ thermopower that is temperature independent even at low temperatures. Our results suggest that nodal-line semimetals may offer significant opportunity for efficient low-temperature thermoelectrics.

Recent advances in machine learning (ML) are expediting materials discovery and design. One significant challenge facing ML for materials is the expansive combinatorial space of potential materials formed by diverse constituents and their flexible configurations. This complexity is particularly evident in molecular mixtures, a frequently explored space for materials, such as battery electrolytes. Owing to the complex structures of molecules and the sequence-independent nature of mixtures, conventional ML methods have difficulties in modeling such systems. Here, we present MolSets, a specialized ML model for molecular mixtures, to overcome the difficulties. Representing individual molecules as graphs and their mixture as a set, MolSets leverages a graph neural network and the deep sets architecture to extract information at the molecular level and aggregate it at the mixture level, thus addressing local complexity while retaining global flexibility. We demonstrate the efficacy of MolSets in predicting the conductivity of lithium battery electrolytes and highlight its benefits in the virtual screening of the combinatorial chemical space.

Synthetic power grids enable real-world energy system simulations and are crucial for algorithm testing, resilience assessment, and policy formulation. We propose a novel method for the generation of synthetic transmission power grids using exponential random graph (ERG) models. Our two main contributions are (1) the formulation of an ERG model tailored specifically for capturing the topological nuances of power grids and (2) a general procedure for estimating the parameters of such a model conditioned on working with connected graphs. From a modeling perspective, we identify edge counts per bus type and $k$-triangles as crucial topological characteristics for synthetic power-grid generation. From a technical perspective, we develop a rigorous methodology to estimate the parameters of an ERG constrained to the space of connected graphs. The proposed model is flexible, easy to implement, and successfully captures the desired topological properties of power grids.

The bulk photovoltaic effect (BPVE) in ferroelectric materials has attracted much attention due to its above-band-gap photovoltage and switchable photoresponse. However, the exact correlation between the photocurrent and spontaneous polarization remains unclear. Here, we report an anomalous reversal of the photocurrent, even for the same polarization direction, in van der Waals (vdW) ferroelectric ${\mathrm{Cu}\mathrm{In}\mathrm{P}}_2{\mathrm{S}}_6$ (CIPS). Careful analysis reveals that the relocation of $\mathrm{Cu}$ ions into the vdW gap upon thickness reduction causes the current reversal, even though it does not change the polarization direction. By combining theoretical and experimental studies, we show clearly the decoupling of the BPVE photocurrent and electric polarization. This discovery significantly improves our understanding of the BPVE and demonstrates yet another example of the lattice dimensionality effect in vdW ferroelectrics.

Lithium reservoir-free solid-state batteries can offer exceedingly high energy densities for a range of emerging applications related to aviation and electric vehicles. However, reversible operation of reservoir-free cells is plagued by a range of degradation mechanisms. The morphology of lithium metal film and subsequent evolution during operation can be highly variable and is dependent on the type of solid electrolyte, current collector, and operating conditions (current density, temperature, pressure, etc.). Here we evaluate lithium metal vertical and horizontal growth mechanisms at an argyrodite solid electrolyte–stainless steel interface at various temperatures from 25 to $80{}^{\circ }\mathrm{C}$. Combining confocal imaging and mesoscale modeling, we demonstrate how lithium metal island agglomeration is accelerated at elevated temperatures and facilitates greater horizontal growth of lithium metal. Maintaining high active material contact (e.g., horizontal growth) is critical for the formation of reversible lithium metal anodes in reservoir-free cells.

Diffusion of $\mathrm{Se}$ from the $\mathrm{Cd}\mathrm{Se}$ window layer into the $\mathrm{Cd}\mathrm{Te}$ absorber improves the short circuit current density by narrowing the band gap and increasing the carrier lifetime. Thicker $\mathrm{Cd}\mathrm{Se}$ layers, however, show a dramatic loss in photocurrent collection due to $\mathrm{Se}$ over-alloying. Electron microscopy investigations show that this decrease in performance is due to the formation of small grains (∼783 nm average diameter), which exhibit grain boundary porosity in the $\mathrm{Se}$ inter-diffusion region. The larger grain boundary area and void free surfaces give rise to higher levels of nonradiative recombination, and therefore, a lower photocurrent. It is proposed that the small grain size is due to a drag force exerted by segregated $\mathrm{Se}$ solute atoms on a moving grain boundary, while faster $\mathrm{Se}$ diffusion along the grain boundaries results in vacancy build up and porosity due to the Kirkendall effect. The results indicate that the device processing conditions must be carefully controlled such that the negative effects of $\mathrm{Se}$ alloying (i.e., smaller grains, Kirkendall voids) do not undermine its benefits.

Though electrochemical impedance spectroscopy (EIS) has been widely used in mechanistic investigations of electrocatalytic reactions, the correlation between the shape of an EIS diagram and the electrocatalytic activity is largely unclear; the complexity of electrocatalytic reactions in terms of reaction mechanisms and influencing factors casts an air of pessimism over the existence and, if it exists, the stability of such a correlation. Nevertheless, understanding the correlation can help select reaction mechanisms and electrical circuit models in data analysis, use the EIS shape as a descriptor of electrocatalytic activity, and detect side reactions if the measured shapes deviate from the theoretical prediction based on the main reaction. Herein, the problem is tackled by a systematic mathematical analysis of firstly single-adsorbate reactions and then more complicated reactions involving multiple adsorbates. A complete regime diagram of all possible EIS shapes of single-adsorbate reactions is provided, navigating the subsequent analysis in the multidimensional parametric space. For single-adsorbate reactions involving two steps with identical transfer coefficients of 0.5 in the absence of lateral adsorbate interactions, several rigorous remarks on the EIS shape are made. Unfortunately, many of them require modifications when different transfer coefficients and lateral interactions are considered. Nevertheless, several trends in EIS shapes are robust against complexities of reaction mechanisms and variations in reaction parameters, which also receive experimental evidence collected from the literature. The theoretical analysis presented here may trigger interest in finding a correlation between the impedance shape and the electrocatalytic activity across a wide range of reactions.

Metal-halide perovskites (MHPs) are attracting considerable interest for optoelectronic applications, with ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{Br}}_6$ one of the main contenders among lead-free systems. ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{Br}}_6$ crystallizes in a nominally double-perovskite structure, but exhibits a soft lattice with large atomic fluctuations characteristic of MHPs. While crucial to understand electron-phonon and phonon-phonon couplings, the spatiotemporal correlations of these fluctuations remain largely unknown. Here, we reveal these correlations using comprehensive neutron and x-ray scattering measurements on ${\mathrm{Cs}}_2{\mathrm{Ag}\mathrm{Bi}\mathrm{Br}}_6$ single crystals, complemented with first-principles simulations augmented with machine-learned neural-network potentials. We report the discovery of an unexpected complex modulated ground-state structure containing several hundred atoms, arising from a soft-phonon instability of the low-temperature tetragonal phase. Further, our experiments and simulations both reveal extensive correlated two-dimensional fluctuations of $\mathrm{Br}$ octahedra at finite temperature, arising from soft optic phonons that are strongly broadened by anhamonicity, reflecting very shallow potential wells. These results provide new insights into the atomic structure and fluctuations in MHPs, critical to understand and control their thermal and optoelectronic properties.

Selenium is experiencing renewed interest as a promising candidate for the wide bandgap photoabsorber in tandem solar cells. However, despite the potential of selenium-based tandems to surpass the theoretical efficiency limit of single-junction devices, such a device has never been demonstrated. In this study, we present the first monolithically integrated selenium/silicon tandem solar cell. Guided by device simulations, we investigate various carrier-selective contact materials and achieve encouraging results, including an open-circuit voltage of $V_{\mathrm{oc}}=1.68\mathrm{V}$ from suns-$V_{\mathrm{oc}}$ measurements. The high open-circuit voltage positions selenium/silicon tandem solar cells as serious contenders to the industrially dominant single-junction technologies. Furthermore, we quantify a pseudo fill factor of more than 80% using injection-level-dependent open-circuit voltage measurements, indicating that a significant fraction of the photovoltaic losses can be attributed to parasitic series resistance. This work provides valuable insights into the key challenges that need to be addressed for realizing higher efficiency selenium/silicon tandem solar cells.

Understanding the elastic mechanical behavior of layered $\mathrm{Li}({\mathrm{Ni}}_x{\mathrm{Mn}}_y{\mathrm{Co}}_z){\mathrm{O}}_2$ (NMC) cathode materials is critical for developing strategies to address their prominent cracking issues and mitigate the mechanical degradation of the cathode of lithium-ion batteries (LIBs). So far, a systematic and in-depth investigation into the elastic mechanical properties of NMC materials is still lacking. Hence, both the isotropic and anisotropic mechanical properties, including elastic constants, Young’s modulus, shear modulus, bulk modulus, compressive modulus, linear compressibility, Poisson’s ratio, Pugh’s ratio, Cauchy pressure, and Kube’s log-Euclidean and universal elastic anisotropy indexes, of seven NMC materials with different compositions are systematically studied here through first-principles calculations. The bulk modulus is proved to be an isotropic mechanical quantity, and its proper relationship with the anisotropic linear compressibility is revealed. Then, we investigate how the isotropic and anisotropic mechanical properties of NMC materials can be tuned by the compositions of $\mathrm{Ni}$, $\mathrm{Mn}$, and $\mathrm{Co}$. It is found that the elastic moduli of NMC materials decrease as the $\mathrm{Ni}$ content increases, and $\mathrm{Mn}$ plays a positive role in not only enhancing the elastic moduli but also in reducing the mechanical anisotropy of NMC materials. Through a quantitative chemical bond analysis, we find that the reason for such a dependence of the elastic moduli on the transition metal (TM) compositions of NMC materials lies in the difference in the strengths of TM–O bonds, and the positive effect of $\mathrm{Mn}$ mainly stems from the contribution of the ionic bonding part of the $\mathrm{Mn}-\mathrm{O}$ bond due to the high average valence state (+4) of $\mathrm{Mn}$ ions; this highlights the benefits of doping with other high-valence-state ions for the promotion of the mechanical properties of NMC materials. Besides, the applicability, physical meaning, and proper use of Pugh’s ratio and the Cauchy pressure in metal-oxides like NMC materials, as well as their relations with Poisson’s ratio, are clarified. The analysis of the relative ductility and brittleness of NMC materials with different compositions via Pugh’s ratio and the isotropic Poisson's ratio reveals a negative correlation between the relative ductility and the magnitude of the modulus. Cauchy pressures of layered NMC materials are found to be directional, with an intralayer negative value but an interlayer positive value, which predicts the significant covalent bonding character of the TM–O bonds, but a nearly ionic dominated bonding character of the $\mathrm{Li}-\mathrm{O}$ bonds, and this qualitative result has been further proved by a quantitative chemical bond analysis. Furthermore, an in-depth investigation of the three-dimensional spatial distribution characteristics of anisotropic mechanical properties of NMC materials is conducted. It is found that, first, the distributions of directions of the extreme values of various anisotropic mechanical properties all exhibit an approximate trigonal symmetry to some extent; second, the anisotropy is most prominent in the $\{2\overline{1}0\}$ crystal plane family; third, the $(104)$ plane and $[\overline{4}\overline{8}1]$ direction are, respectively, prone to be the cleavage plane and cleavage direction, along which the microcracks and even mechanical failures may be susceptible to occur due to stress concentration. The findings presented in this study will provide guidance for the design and development of robust and reliable LIBs.

Materials discovery lays the foundation for many technological advancements. The prediction and discovery of new materials are not simple tasks. Here, we outline some basic principles of solid-state chemistry, which might help to advance both, and discuss pitfalls and challenges in materials discovery. Using the recent work of Szymanski et al. [Nature 624, 86 (2023)], which reported the autonomous discovery of 43 novel materials, as an example, we discuss problems that can arise in unsupervised materials discovery and hope that by addressing these, autonomous materials discovery can be brought closer to reality. We discuss all 43 synthetic products and point out four common shortfalls in the analysis. These errors unfortunately lead to the conclusion that no new materials have been discovered in that work. We conclude that there are two important points of improvement that require future work from the community, as follows. (i) Automated Rietveld analysis of powder x-ray diffraction data is not yet reliable. Future improvement of such, and the development of a reliable artificial-intelligence-based tool for Rietveld fitting, would be very helpful, not only for autonomous materials discovery but also for the community in general. (ii) We find that disorder in materials is often neglected in predictions. The predicted compounds investigated herein have all their elemental components located on distinct crystallographic positions but in reality, elements can share crystallographic sites, resulting in higher-symmetry space groups and—very often—known alloys or solid solutions. This error might be related to the difficulty of modeling disorder in a computationally economical way and needs to be addressed both by computational and experimental material scientists. We find that two thirds of the claimed successful materials in Szymanski et al. are likely to be known compositionally disordered versions of the predicted ordered compounds. We highlight important issues in materials discovery, computational chemistry, and autonomous interpretation of x-ray diffraction. We discuss concepts of materials discovery from an experimentalist point of view, which we hope will be helpful for the community to further advance this important new aspect of our field.

The drive towards renewable energy generation is causing fundamental changes in both the structure and dynamics of power grids. Their topology is becoming increasingly decentralized due to distributed, embedded generation, and the emergence of microgrids. Grid dynamics are being impacted by decreasing inertia, as conventional generators with massive spinning cores are replaced by dc renewable sources. This leads to a risk of destabilization and places an upper limit on the volume of renewable power sources that can be installed. A wide variety of different control schemes have been proposed to overcome this problem. Such schemes fall into two broad categories: so-called “grid-following” controllers that seek to match output ac power with grid frequency, and “grid-forming” systems that seek to boost grid stability. The latter frequently work by providing synthetic inertia, enabling dc renewable sources to emulate conventional generators. This paper uses the master stability function methodology to analyze the stability of synchrony in microgrids of arbitrary size and containing arbitrary control systems. This approach provides a powerful and computationally efficient framework in which to benchmark the impact of any number of renewable sources on grid stability and thereby to support microgrid design strategies. The method is demonstrated by computing stability bounds for two different grid-forming systems, providing bounds on the feasible number of generators that can be accommodated. In addition, we contrast our results with predictions from a simplistic but widely used phase-oscillator model, finding that such descriptions significantly over estimate the grid stability properties.

We establish an analytical criterion for stable electrodeposition by combining electrochemistry and mechanics. In situ experiments combined with molecular simulations are performed to validate the robustness of the theoretical predictions. Our analysis shows that stable electrodeposition is intimately linked with the interfacial defect size, fracture toughness, and molar volume of solid electrolytes. We find an exponential scaling law between the critical current density and the interfacial defect size, and the inherent softening in strength of grain boundaries lowers the critical current density in polycrystalline solid electrolytes. Modeling and analyses provide roadmaps to design solid electrolytes with metallic anodes for achieving stable electrodeposition.

A simple theory of tribovoltaics is proposed by using a quantum mechanical model of energy release due to sliding-induced bonding between the surfaces of a $p$-doped semiconductor and a $n$-doped semiconductor. The energy release in forming a bond may lead to the excitation of electron-hole pairs at the $p$-$n$ semiconductor interface if the released energy is higher than the effective band gap at the semiconductor interface. An expression for the generated current as a function of the relative sliding speed between the $p$ and $n$ sides is suggested and used to model current transport by solving the complete set of drift-diffusion equations with appropriate boundary and initial conditions. Analytical results are obtained and verified numerically using the comsol finite-element-method software. It is shown that since the typical time period associated with periodic sliding is many orders of magnitude higher than the carrier lifetimes, the time-dependent variations in the electron and hole concentrations and the current density follow the time variation of the sliding speed. Since the electron-hole pair generation occurs near the semiconductor interface only, the current density is shown to be constant as a function of position even if the sliding speed changes in time.