Control over conductivity and carrier type (electrons and holes) defines semiconductors. A primary approach to target carrier concentrations involves introducing a small population of aliovalent impurity dopant atoms. In a combined synthetic and computational study, we assess impurity doping by introducing $\mathrm{Bi}$ and $\mathrm{Sn}$ into the prototype 2D Ruddlesden-Popper hybrid perovskite phenylethylammonium lead iodide (${\mathrm{PEA}}_2{\mathrm{Pb}\mathrm{I}}_4$). Experimentally, we demonstrate that $\mathrm{Bi}$ and $\mathrm{Sn}$ can achieve n- and p-type doping, respectively, but the doping efficiency is low. Simulations show that $\mathrm{Bi}$ introduces a deep defect energy level (∼0.5 eV below the conduction band minimum) that contributes to the low doping efficiency, but, to reproduce the low doping efficiency observed experimentally, an acceptor level must also be present that limits n-type doping. Experiments find that $\mathrm{Sn}$ achieves p-dopant behavior and simulations suggest that this occurs through the additional oxidation of $\mathrm{Sn}$ defects. We also study how substitutional $\mathrm{Bi}$ incorporation can be controlled by tuning the electrochemical environment during synthesis. First-principles impurity doping simulations can be challenging; typical dopant concentrations constitute less than 0.01% of the atoms, necessitating large supercells, while a high level of theory is needed to capture the electronic levels. We demonstrate simulations of complex defect-containing unit cells that include up to 3383 atoms, employing spin-orbit coupled hybrid density functional theory. While p- and n-type behavior can be achieved with $\mathrm{Sn}$ and $\mathrm{Bi}$, simulations and experiments provide concrete directions where future efforts must be focused to achieve higher doping efficiency.

A forced oscillation event in power grids refers to a state where malfunctioning or abnormally operating equipment causes persisting periodic disturbances in the system. While power grids are designed to damp most perturbations during standard operations, some of them can excite normal modes of the system and cause significant energy transfers across the system, creating large oscillations thousands of miles away from the source. Localization of the source of such disturbances remains an outstanding challenge due to a limited knowledge of the system parameters outside of the zone of responsibility of system operators. Here, we propose a new method for locating the source of forced oscillations that addresses this challenge by performing a simultaneous dynamic model identification using a principled maximum likelihood approach. We illustrate the validity of the algorithm on a variety of examples where forcing leads to resonance conditions in the system dynamics. Our results establish that an accurate knowledge of system parameters is not required for a successful inference of the source and frequency of a forced oscillation. We anticipate that our method will find a broader application in general dynamical systems that can be well described by their linearized dynamics over short periods of time.

The effects of neutron irradiation on materials are often interpreted in terms of atomic recoils, initiated by neutron impacts and producing crystal lattice defects. In addition, there is a remarkable two-step process, strongly pronounced in the medium-weight and heavy elements. This process involves the generation of energetic $\gamma$ photons in nonelastic collisions of neutrons with atomic nuclei, achieved via capture and inelastic reactions. Subsequently, high-energy electrons are excited through the scattering of $\gamma$ photons by the atomic electrons. We derive and validate equations enabling a fast and robust evaluation of photon and electron fluxes produced by the neutrons in the bulk of materials. The two-step $n$-$\gamma$-$e$ scattering creates a nonequilibrium dynamically fluctuating steady-state population of high-energy electrons, with the spectra of photon and electron energies extending well into the mega-electron-volt range. This stimulates vacancy diffusion through electron-triggered atomic recoils, primarily involving vacancy-impurity dissociation, even if thermal activation is ineffective. Tungsten converts the energy of fusion or fission neutrons into a flux of $\gamma$ radiation at the conversion efficiency approaching 99%, with implications for structural materials, superconductors, and insulators, as well as phenomena like corrosion, and helium and hydrogen isotope retention.

Oscillatory behavior in diffusive systems has recently attracted significant interest because of the possibility of extending the properties of resonators to transport phenomena. In this context, it is shown that internally heated and thermally coupled countercurrent flows display heat oscillations that are controllable by tuning the mutual rates at which such flows move. The combined effect of conduction and advection causes heat to circulate in the system, oscillating back and forth between the channels containing the flows. This oscillation allows heat in the system to be recycled, which is demonstrated to have practical applications, such as notably increasing the energy efficiency of solar-driven water desalination. In contrast to typical frequency-dependent oscillators, the resonance condition of flow-driven systems is determined by the fluid flow rates. While it is shown that a two-channel heat oscillator displays a resonance when the flow rates are equal and opposite, a complete understanding of these systems, especially when such heat oscillators are coupled and feature multiple channels, is still an open problem. Here, we investigate the fundamental properties of flow-driven resonant oscillators and introduce a figure of merit that completely quantifies the performance of coupled heat oscillators up to $N$-channel stacked configurations. Using this figure of merit, we determine the ideal configuration of flow rates to optimize a system with any number of channels. Interestingly, it is found that stacking multiple heat oscillators can increase the overall heat transfer for a fixed input power. The results of this work expand our fundamental understanding of flow-driven heat oscillators with implications for their application to real-world problems involving heat recovery, exchange, and accumulation.

Future highly renewable energy systems might require substantial storage deployment. At the current stage, the technology portfolio of dominant storage options is limited to pumped-hydro storage and $\mathrm{Li}$-ion batteries. It is uncertain which storage design will be able to compete with these options. Considering Europe as a case study, we derive the cost and efficiency requirements of a generic storage technology, which we refer to as storage-X, to be deployed in the cost-optimal system. This is performed while including existing pumped-hydro facilities and accounting for the competition from stationary $\mathrm{Li}$-ion batteries, flexible generation technology, and flexible demand in a highly renewable sector-coupled energy system. Based on a sample space of 724 storage configurations, we show that energy capacity cost and discharge efficiency largely determine the optimal storage deployment, in agreement with previous studies. Here, we show that charge capacity cost is also important due to its impact on renewable curtailment. A significant deployment of storage-X in a cost-optimal system requires (a) discharge efficiency of at least 95%, (b) discharge efficiency of at least 50% together with low energy capacity cost (10 $\text{€}$/kWh), or (c) discharge efficiency of at least 25% with very low energy capacity cost (2 $\text{€}$/kWh). Comparing our findings with seven emerging technologies reveals that none of them fulfill these requirements. Thermal energy storage is, however, on the verge of qualifying due to its low energy capacity cost and concurrent low charge capacity cost. Exploring the space of storage designs reveals that system cost reduction from storage-X deployment can reach 9% at its best, but this requires high round-trip efficiency ($\mathrm{RTE}\ge 90\mathrm{\%}$) and low charge capacity cost (35 $\text{€}$/kW).

Diamine-appended metal-organic frameworks are ultraporous materials exhibiting selective and cooperative ${\mathrm{CO}}_2$ adsorption mechanisms, leading to chemically tunable step-shaped isotherms and isobars that enable a large fraction of their full ${\mathrm{CO}}_2$ capacity to be captured or released with a modest change in temperature or pressure. Although progress has been made in elucidating carbon capture phenomena in this system, its thermal properties are poorly understood. Here, we develop density-functional-theory-derived neural network potentials for amine-appended ${\mathrm{Mg}}_2$(dobpdc), metal-organic frameworks made up of $\mathrm{Mg}$ cations and dobpdc [${\mathrm{dobpdc}}^{4-}$ = 4,4’-dihydroxy-(1,1’-biphenyl)-3,3’-dicarboxylic acid] linkers. These potentials are constructed with an active learning approach where their accuracy and transferability are improved through iterative generation of training datasets based on molecular dynamics. Our potentials predict adsorption energy, mechanical properties, vibrational and thermal properties with and without ${\mathrm{CO}}_2$, reaching ab initio accuracy at a fraction of the computational cost. We compute the temperature-dependent heat capacity and lattice thermal expansion of ${\mathrm{Mg}}_2$(dobpdc), with and without amine appendages and with and without ${\mathrm{CO}}_2$, quantitatively capturing measured trends where available and explaining their molecular origins. Furthermore, we show that these potentials can be incorporated into a simulated annealing approach to identify starting structures for energy minimization needed to compute accurate binding enthalpies of these and other complex systems. Our density-functional-theory-derived neural network potentials explain the temperature evolution of the structure and properties of an important class of metal-organic frameworks at molecular length scales, while providing a necessary foundation for future studies of the chemical dynamics of carbon capture.

With high photovoltaic efficiencies, low production costs, and long-term stability of single junction cells, $\mathrm{Cd}\mathrm{Te}/\mathrm{Si}$ four-terminal tandem solar cells are promising devices for surpassing single junction $\mathrm{Si}$ solar cell efficiency limits. High sub-band-gap transmission in the top junction is crucial to reach high efficiencies. Here, we study the impact of surface roughness on transmission using experiments and simulations, showing that the as-deposited texture both increases backscattering and parasitic absorption compared to flat surfaces due to electric field concentration in mesoscale surface features. Adding the ethylene vinyl acetate interlayer increases transmittance by reducing the index contrast at the back transparent conductive oxide. We show that these roughness-induced losses can be overcome by including high index optical coatings as additional interlayers, resulting in increased transmission through the $\mathrm{Cd}\mathrm{Te}$ cell and $\mathrm{Si}$ bottom cell efficiency that is comparable to a flat reference device.

This article presents a simple geometrical approach to visualize the hydrodynamic properties of wave energy converters (WECs), in terms of wave reflection, transmission, and absorption, and how those properties are governed by the WEC control parameters. The problem is modeled as an array of periodic rows of WECs parallel to the wave front, which is representative of WEC farms located along the shoreline, and allows for a straightforward two-dimensional analysis of energy fluxes. The WECs are assumed to be symmetric with respect to the plane perpendicular to the wave propagation direction, and they operate in a single degree of freedom. Under those assumptions, fundamental hydrodynamic relationships allow the WEC operation at a given frequency to be mapped to a single complex number, $\hat{T}$, which represents the WEC (complex) transmission coefficient, located in a circle with center $1/2$ and radius $1/2$. The WEC hydrodynamic and control parameters (added mass, stiffness, damping) govern the precise location of $\hat{T}$ within the circle. The distance of $\hat{T}$ to the center of the circle determines the hydrodynamic efficiency, between 0 (when $\hat{T}$ is on the circle border) and $1/2$ (which is achieved when $\hat{T}$ is at the center of the circle). Therefore, the representation of $\hat{T}$ provides an immediate insight into the balance between reflection, transmission, and absorption. The proposed unified representation reflects, in a didactic way, some fundamental wave energy concepts, common to all WECs, such as the Haskind relationship, or the impedance matching condition for optimal wave power absorption. Two numerical examples illustrate how the locus of $\hat{T}$, across a prescribed frequency range, provides a distinctive “signature” specific to the WEC geometry, mode of operation, and control strategy. Finally, the proposed representation shows many similarities to the Smith chart and, as such, is but one additional analogy between wave energy conversion and electrical engineering.

Bose-Einstein condensation of excitons, in which excitons condense into a single coherent quantum state, known as an exciton condensate, enables frictionless energy transfer, but typically occurs under extreme conditions in highly ordered materials, such as graphene double layers. In contrast, photosynthetic light-harvesting complexes demonstrate extremely efficient transfer of energy in disordered systems under ambient conditions. Here, we establish a link between the two phenomena by investigating the potential for exciton-condensate-like amplification of energy transport in room-temperature light harvesting. Using a model of the Fenna-Matthews-Olson complex and accounting for intrachromophore electron correlation explicitly through the addition of multiple sites to the individual chromophores, we observe amplification of the exciton population in the particle-hole reduced density matrix through an exciton-condensate-like mechanism. The exciton-condensate-like amplification evolves with the dynamics of exciton transfer, and the nature of amplification is influenced by intra- and interchromophore entanglement, as well as the initial excitation model and number of sites per chromophore. Tuning intrachromophore coupling also increases the rate of exciton transfer with a maximum enhancement of nearly $100\mathrm{\%}$. The research provides fundamental connections between exciton condensation and exciton transport in light-harvesting complexes with potential applications for harnessing the exciton-condensate-like mechanism to enhance energy transfer in synthetic systems and create new materials capable of highly efficient energy transfer.

The development of a highly selective, membrane-based ion separation technology could significantly improve the sustainability and energy efficiency of water treatment technologies and emerging applications, such as electrochemical ${\mathrm{CO}}_2$ reduction, extraction of valuable metals from seawater, and battery recycling. In this work, we show through computational modeling that an electronic flashing ratchet mechanism can be used for high-precision ion separation. The suggested ratchet-based ion pumps utilize a unique feature of electronic ratchets, frequency-dependent current reversal, to drive ions with the same charge, but different diffusion coefficient, in opposite directions. The model shows that ions whose diffusion coefficients differ by as little as 1% can be separated by driving them in opposite directions with a velocity difference as high as 1.2 mm/s. Since the pumping properties of the ratchet are determined by a time-varying electric input signal, the proposed ion pumps could be instrumental in realizing an efficient, large-scale, and fit-for-purpose system for selective ion separation. Examples of ratchet-driven systems for lithium extraction from seawater, lead removal from drinking water, and water desalination are discussed and analyzed.

With the increasing amount of available data from simulations and experiments, research for the development of data-driven models for wind-farm power prediction has increased significantly. While the data-driven models can successfully predict the power of a wind farm with similar characteristics as those in the training ensemble, they generally do not have a high degree of flexibility for extrapolation to unseen cases in contrast to the physics-based models. In this paper, we focus on data-driven models with improved interpretability and generalizability levels that can predict the performance of turbines in wind farms. To prepare the datasets, several cases are defined based on the layouts of operational wind farms, and massive computational fluid dynamics simulations are performed. The extreme gradient boosting algorithm is used afterward to build models, which have turbine-level geometric inputs in combination with the efficiency from physics-based models as the features. After training, to analyze the models’ capability in generalization, their predictions for the unseen cases with different operating conditions, inflow turbulence levels, and wind-farm layouts are compared to the Park model and an empirical-analytical Gaussian wake model. Results show that the physics-guided machine-learning models outperform both physics-based models, showing a high degree of generalizability, and that the machine is not sensitive to the choice of the physics-based model.

The iodine vacancy ($V_{\mathrm{I}}$) has frequently been discussed as a strong nonradiative recombination center in halide perovskites. This proposition was mainly based on the presence of charge-state transition levels in the band gap, as found in early first-principles calculations. In this work, we perform accurate hybrid-density-functional calculations for $V_{\mathrm{I}}$ in ${\mathrm{Cs}\mathrm{Pb}\mathrm{I}}_3$, ${\mathrm{Cs}\mathrm{Sn}\mathrm{I}}_3$, and ${\mathrm{Cs}\mathrm{Ge}\mathrm{I}}_3$ and find that $V_{\mathrm{I}}$ does not have any transition levels in the band gap in ${\mathrm{Cs}\mathrm{Pb}\mathrm{I}}_3$, in contrast to the results from calculations based on semilocal functionals. The iodine vacancy $V_{\mathrm{I}}$ does introduce levels in the band gap in ${\mathrm{Cs}\mathrm{Sn}\mathrm{I}}_3$ and ${\mathrm{Cs}\mathrm{Ge}\mathrm{I}}_3$, but our explicitly computed nonradiative capture coefficients demonstrate that $V_{\mathrm{I}}$ has a negligible impact on nonradiative recombination. Our study corrects a misunderstanding of the role of $V_{\mathrm{I}}$ in the iodide-based perovskites, and shifts the focus toward identifying and mitigating actual recombination centers in order to further improve the optoelectronic performance.

Conventional lithium-ion batteries, and many next-generation technologies, rely on organic electrolytes with multiple solvents to achieve the desired physicochemical and interfacial properties. The complex interplay between these properties can often be elucidated via the coordination environment of the cation. We develop a theory for the coordination shell of cations in nonaqueous solvent mixtures that can be applied with high fidelity, up to extremely high salt concentrations. Our theory can naturally explain simulation and experimental values of cation solvation in “classical” nonaqueous electrolytes. Moreover, we utilize our theory to understand general design principles of emerging classes of nonaqueous electrolyte mixtures, such as high entropy electrolytes. It is hoped that this theory provides a systematic framework to understand simulations and experiments that engineer the solvation structure and ionic associations of concentrated nonaqueous electrolytes.

The bulk photovoltaic effect (BPVE) leads to directed photocurrents and photovoltages in bulk materials. Unlike photovoltages in $p$-$n$ junction solar cells that are limited by carrier recombination to values below the band-gap energy of the absorbing material, BPVE photovoltages have been shown to greatly exceed the band-gap energy. Therefore, the BPVE is not subject to the Shockley-Queisser limit for sunlight to electricity conversion in single-junction solar cells and experimental claims of efficiencies beyond this limit have been made. Here, we show that BPVE energy conversion efficiencies are, in practice, orders of magnitude below the Shockley-Queisser limit of single-junction solar cells and are subject to different, more stringent limits. The name BPVE stands for two different fundamental effects: the shift current and the injection current. In both of these, the voltage bias necessary to produce electrical energy accelerates both intrinsic and photogenerated carriers. We discuss how energy conservation alone fundamentally limits the BPVE to a band-gap-dependent value that exceeds the Shockley-Queisser limit only for very small band gaps. Yet, small band-gap materials have a large number of intrinsic carriers, leading to high conductivity that suppresses the photovoltage. We discuss further how slightly more stringent fundamental limits for injection (ballistic) currents may be derived from the trade-off between high resistivity, needed for a high voltage, and long ballistic transport length, needed for a high current. We also explain how erroneous experimental and theoretical claims of high efficiency have arisen. Finally, we calculate the energy conversion efficiency for an example two-dimensional (2D) material that has been suggested as a candidate material for high-efficiency BPVE-based solar cells and show that the efficiency is very similar to the efficiency of known 3D materials.

The desire to increase the energy density of stoichiometric layered $\mathrm{Li}$TM${\mathrm{O}}_2$ (TM = 3d transition metal) cathode materials has promoted investigation into their properties at high states of charge. Although there is increasing evidence for pronounced oxygen participation in the charge compensation mechanism, questions remain whether this is true $\mathrm{O}$-redox, as observed in $\mathrm{Li}$-excess cathodes. Through a high-resolution $\mathrm{O}$ K-edge resonant inelastic x-ray spectroscopy (RIXS) study of the $\mathrm{Mn}$-free $\mathrm{Ni}$-rich layered oxide ${\mathrm{Li}\mathrm{Ni}}_{0.98}{\mathrm{W}}_{0.02}{\mathrm{O}}_2$, we demonstrate that the same oxidized oxygen environment exists in both $\mathrm{Li}$-excess and non-$\mathrm{Li}$-excess systems. The observation of identical RIXS loss features in both classes of compounds is remarkable given the differences in their crystallographic structure and delithiation pathways. This lack of a specific structural motif reveals the importance of electron correlation in the charge compensation mechanism for these systems and indicates how a better description of charge compensation in layered oxides is required to understand anionic redox for energy storage.

Diffusion lengths are abundantly mentioned as a key enabler of high performance in lead-halide perovskite solar cells based on the implicit assumption that diffusion lengths are directly connected to efficient collection of charge carriers. While long diffusion lengths are indeed an important factor for highly efficient solar cells, they are a poor descriptor of efficient collection in the presence of undoped and low-mobility transport layers. Especially, the presence of low permittivity and organic electron- and hole-transport layers that are predominantly used in p-i-n-type perovskite solar cells can lead to substantial concentrations of electrons and holes within the perovskite absorber, as evidenced by measuring the photoluminescence at short circuit. These high carrier concentrations can lead to the seemingly odd situation in which charge collection at short circuit can be inefficient, even though the diffusion length substantially exceeds the absorber thickness. We use experiments, as well as analytical and numerical modeling, to define the requirements for both absorber and contact layers to enable efficient collection in perovskite solar cells.

The market quest for fast-charging, safe, long-lasting, and performant batteries drives the exploration of new energy storage materials, but also promotes fundamental investigations of materials already widely used. Presently, renewed interest in anode materials is observed—primarily graphite electrodes for lithium-ion batteries. Here, we focus on the upper limit of lithium intercalation in the morphologically quasi-ideal highly oriented pyrolytic graphite, with a ${\mathrm{Li}\mathrm{C}}_6$ stoichiometry corresponding to nominally 100% state of charge. We prepare a sample by immersion in liquid lithium at ambient pressure and investigate it by static ${}^7\mathrm{Li}$ nuclear magnetic resonance (NMR). We resolve unexpected signatures of superdense intercalation compounds, ${\mathrm{Li}\mathrm{C}}_{6-x}$. These have been ruled out for decades, since the highest geometrically accessible composition, ${\mathrm{Li}\mathrm{C}}_2$, can only be prepared under high pressure. We thus challenge the widespread notion that any additional intercalation beyond ${\mathrm{Li}\mathrm{C}}_6$ is insignificant under ambient conditions. We monitor the sample upon calendaric ageing and employ ab initio calculations to rationalize the NMR results. Computed relative stabilities of different superdense configurations reveal that non-negligible overintercalation does proceed spontaneously beyond the currently accepted capacity limit. The associated capacity gain is not worth pushing graphitic battery anodes beyond the ${\mathrm{Li}\mathrm{C}}_6$ limit in practical applications; rather these findings carry more fundamental implications. Neglecting overintercalation may hinder the correct interpretation of experimental observations, as well as the correct design of computational models, in investigations of performance-critical phenomena, as it is likely to play a crucial role in the onset mechanism of lithium plating and dendrite formation in real battery materials.

We consider bioinspired ring systems as photovoltaic circuits to explore the advantage of “optical ratcheting,” a process whereby the arrangement of coupled optical dipoles enables delocalized excitonic states that are protected against radiative decay whilst permitting the absorption of further photons. We explore how the performance of a ratcheting antenna scales with system size when excitons are incoherently or coherently extracted from the antenna to an associated trap site. In both instances we also move to the polaron frame in order to more closely model realistic systems where the coupling to vibrational modes can generally be assumed to be strong. We find a multifaceted and nuanced dependence of the predicted performance on an interplay between geometrical arrangement, extraction mechanism, and vibrational coupling strength. Certain regimes support substantial performance improvements in the power extracted per site.

Carrier effective mass is a central parameter in solid-state physics. It is a measure of the strength of the coupling between a carrier and excitations arising from its surrounding medium, and features prominently in transport and optical calculations. Experimental techniques employed to determine it are steady-state ones, and so are unable to detect any change in the effective mass after a strong perturbation to the system, e.g., strong optical excitation. By combining time-resolved terahertz spectroscopy and transient absorption spectroscopy, on a mixed-cation mixed-halide perovskite thin film, we observe a large and long-lived photoinduced enhancement of the carrier effective mass, and from it deduce a twofold increase of the carrier-phonon coupling constant, giving evidence of polaron formation. Our work demonstrates a new approach to track the strength and ultrafast lifetimes of photoinduced carrier-boson interactions down to picosecond timescales that can be applied to a wide range of solid-state systems.

Battery technologies have evolved rapidly over the past decade, including the advent of solid-state batteries. In this time, it has become apparent that thermal management is paramount for device operation and lifetime. However, the fundamental importance of the thermal properties of materials, such as thermal conductivity, in engineering design and mitigating the risk of catastrophic failure is yet to be fully understood. This Perspective aims to provide motivation for the fields of thermal transport and ionic transport to join forces to understand heat transport for better battery design, especially in light of solid-state batteries. From the basic characterization of thermal conductivity in bulk materials to considering the full complexity of battery composites during electrochemical cycling, there are many potential directions for fundamental and applied investigations. We anticipate that studying heat transport in battery materials has the added benefit of extending the design space to other functional devices. The difficulty in controlling heat transport in solid-state energy devices, including microelectronics, batteries, and thermoelectrics, is often a limiting factor in improving device performance. Especially in batteries, not only can excessive heat cause degradation that leads to a loss of charge capacity over time, but thermal runaway can occur when the battery overheats to catastrophic failure. Thus, understanding heat evolution and thermal transport in batteries is an important step to improve lifetime and safety. It is from this perspective that we provide the motivation for the importance of bringing together the fields of thermal transport and battery research, particularly to study solid-state batteries, which epitomize the overall complexity of battery systems and require a state-of-the-art understanding of thermal transport mechanisms. Here, we identify the basic and applied scientific directions that may prove fruitful for the next generation of battery thermal management.