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Driven by an ever-growing demand for environmentally benign cooling systems, the past decade has witnessed the booming development in the field of electrocaloric (EC) cooling technology, which is considered as a promising solid-state cooling approach. Multilayer ceramic chip capacitors (MLCCs) represent the optimum structure for EC cooling elements because of large breakdown strengths, low driving voltages, and high macroscopic volumes of active EC materials. However, fundamental relationships between the geometric parameters of MLCCs and the EC coefficient are less understood. In this study, 0.92Pb(Mg1/3Nb2/3)O3-0.08PbTiO3 (PMN-PT) MLCCs with controlled configurations, such as active/inactive layer thickness, number of layers, and active volume ratio, were fabricated, and their EC performance was evaluated. The electric properties of the MLCCs are confirmed to be closely related to the geometric structure, which influences not only the heat flow but also the internal stress, resulting in the variability of EC performance and reliability/breakdown strength. The internal stress arises due to the residual thermal stress originating from the densification-related shrinkage, thermal expansion mismatch during the sintering, and clamping stress arising from the inactive area due to the large strain from the active area under a high electric field. The geometric structure-based stress distribution and the magnitude of stress on the active layers in MLCCs were determined by finite element modeling (FEM) and correlated with the experimental EC coefficients. The results reveal that a low inactive volume percentage is beneficial toward increasing the breakdown field and enhancement of EC performance because of reduced clamping stress on active EC material.

Summary Sustainably recycling photovoltaic (PV) products at the end of their life cycle is essential for reducing environmental impact and saving costs on raw materials. Halide perovskite PVs currently suffer from a short lifespan, and recycling and remaking could equivalently extend their life cycle. Here, we report an effective and rapid “one-key reset” recycling paradigm to collect all the functional layer components from a used PV device at a high material extraction efficiency. A remade device incorporating all the layers, including recycled indium tin oxide/tin oxide substrate, perovskite, spiro-OMeTAD, and gold, exhibits an average power conversion efficiency of over 20%. This finding that the recycled device exhibits identical performance compared with the original, while providing enormous savings in cost, time, and energy, could lead to a recycling route that will dramatically lower the levelized cost of energy of perovskite PVs and enhance their competitiveness in future energy markets.

Magnetoelectric voltage tunable inductors (VTIs) offer a new paradigm for power electronics circuit design. Here, air-gapped VTIs are demonstrated exhibiting a large inductance tunability of 220% with applied fields under 20 kVcm−1 and operational stability up to 5 MHz that covers the full frequency range of the state-of-the-art power electronics. The design of air-gapped VTI comprises of two C-shaped ferrite cores, one permeability-variable magnetic flux valve (MFV) in between C-shaped cores, and slotted air gap between cores and MFV. The tunability of VTIs is achieved through the electric field modulation of permeability in the MFV based on the magnetoelectric effect. There is a tradeoff between the tunability and saturation current. The introduction of air gap significantly reduces the tunability but provides methodology toward increasing the saturation current and power handling capability due to the increased reluctance. The inductance and tunability reduces by 50% with increase in the air-gap width from 0.07 mm to infinity (condition with no C-shaped cores). Phase field simulations demonstrate that the air gap affects the magnetization, permeability, and tunability of VTIs by tailoring the demagnetization field that further influences the magnetic domain rotation process. VTIs with increased frequency range and saturation current will strengthen the continued development of tunable power electronics.

Enhanced pyroelectric response is achieved via domain engineering from [001] grain-oriented, tetragonal-phase, lead-free 0.2(2/3K0.5Bi0.5TiO3-1/3BaTiO3)-0.8Na0.5Bi0.5TiO3 (KBT-BT-NBT) ceramics prepared by a templated grain growth method. The [001] crystallographic orientation leads to large polarization in tetragonal symmetry; therefore, texturing along this direction is employed to enhance the pyroelectricity. X-ray diffraction analysis revealed a Lotgering factor (degree of texturing) of 93 % along the [001] crystallographic direction. The textured KBT-BT-NBT lead-free ceramics showed comparable pyroelectric figures of merit to those of lead-based ferroelectric materials at room temperature (RT). In addition to the enhanced pyroelectric response at RT, an enormous enhancement in the pyroelectric response (from 1750 to 90,900 μC m−2 K−1) was achieved at the depolarization temperature because of the sharp ferroelectric to antiferroelectric phase transition owing to coherent 180° domain switching. These results will motivate the development of a wide range of lead-free pyroelectric devices, such as thermal sensors and infra-red detectors.

Black orthorhombic (B-γ) CsSnI3 with low toxicity and excellent optoelectronic properties is a promising candidate for perovskite solar cell (PSC). However, the performance of the B-γ CsSnI3-based ...

Fully inorganic black orthorhombic (B-γ) CsSnI3 has become a promising candidate for perovskite solar cell (PSC) thanks to its low toxicity and decently high theoretical power conversion efficiency (PCE). However, so far, the reported PCE of the B-γ CsSnI3 PSC is still not comparable with its lead-based or organotin-based counterparts. Herein, a mixed electron transport layer (ETL) composed of ZnO nanoparticles (NPs) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is incorporated into inverted B-γ CsSnI3 PSCs. The mixed ETL exhibits the merits of both ZnO and PCBM. The highest PCE of 6.08% was recorded for the PSC with mixed ZnO-PCBM ETL, which is 34.2% higher than that of the device with plain PCBM ETL (PCE of 4.53%) and 28.8% superior to that of plain ZnO ETL-based device (PCE of 4.72%). Meanwhile, the mixed ZnO-PCBM ETL-based PSC retained 71% of its initial PCE under inert conditions at room temperature after 60 days of storage and maintained 67% PCE after 20 days of storage under ambient air at 30% relative humidity and room temperature.

The energy available in the ambient vibrations, magnetic fields, and sunlight can be simultaneously or independently harvested using universal architecture. The universal harvester design is shown to effectively convert ambient magnetic fields, vibration, and light into electricity. The architecture is composed of a perovskite solar cell integrated onto a magnetoelectric composite cantilever beam. The efficiency of the large-area perovskite solar cell is shown to reach 15.74% (cell area is1100% larger than traditional perovskite solar cells) by selecting glass/indium tin oxide (ITO) as the cathode that reduces the charge recombination. The magnetoelectric composite beam is designed to include the effect of the mass and volume of the solar cell on power generation. Results demonstrate that universal energy harvester can simultaneously capture vibration, magnetic fields, and solar irradiation to provide an ultrahigh-power density of 18.6 mW/cm

Surface defects have been a key constraint for perovskite photovoltaics. Herein, 1,3-dimethyl-3-imidazolium hexafluorophosphate (DMIMPF6 ) ionic liquid (IL) is adopted to passivate the surface of a formamidinium-cesium lead iodide perovskite (Cs0.08 FA0.92 PbI3 ) and also reduce the energy barrier between the perovskite and hole transport layer. Theoretical simulations and experimental results demonstrate that Pb-cluster and Pb-I antisite defects can be effectively passivated by [DMIM]+ bonding with the Pb2+ ion on the perovskite surface, leading to significantly suppressed non-radiative recombination. As a result, the solar cell efficiency was increased to 23.25 % from 21.09 %. Meanwhile, the DMIMPF6 -treated perovskite device demonstrated long-term stability because the hydrophobic DMIMPF6 layer blocked moisture permeation.

Piezoelectric transformers (PTs) have been conventionally used in step-up high-voltage low-power applications such as liquid crystal display (LCD) backlighting for portable electronic devices. PTs have also been considered for step-down applications such as battery chargers for portable devices. In these applications, adapting the output impedance of the PT to meet the requirements of lower output voltages and higher output currents implies the design of PTs with larger number of output layers. Furthermore, these step-down applications require efficient operation of PT under a variety of output load conditions to meet the conditions of battery charging. Thus, strategies to adapt the PT to the variation of the output load are being pursued. In this article, a novel tunable piezoelectric transformer (TPT) for ac–dc and dc–dc high-power converters is presented. This TPT is designed to operate in the radial mode and is fabricated using cofired multilayer process. By introducing control layers into the design, the TPT is shown to provide desired features that include adjustable frequency response and flexible transfer ratio. Fabricated TPT exhibits high efficiency over 96% with an output power capability of 40 W when operating under its nominal load conditions.

Control of magnetic permeability through electric field in magnetoelectric materials promises to create novel voltage tunable inductors (VTIs). VTIs synthesized using co-fired ceramic processing exhibit many advantages over traditional epoxy bonding method, but the internal residual stress in co-fired VTIs resulting from thermal expansion mismatch hinders a full exploitation of the tunability of permeability. To find the optimal condition for high tunability of co-fired VTIs, domain-level phase field modeling and computer simulation are employed to study co-fired magnetoelectric composites comprising NiZn ferrite and PZT. Two key factors important toward increasing the inductor tunability are systematically investigated: intrinsic magnetocrystalline anisotropy of the ferrite material and internal residual stress caused by the co-firing process. The simulations indicate that in order to achieve a large tunability, the tuned permeability should be confined within the linear region of the reciprocal of susceptibility and stress. Additionally, both magnetocrystalline anisotropy and residual stress should be as small as possible. These results provide a design strategy for realizing high-tunability co-fired VTIs.

Strongly coupled electronic and thermal transport behavior in thermoelectric (TE) materials has limited their figure of merit (zT). Here we provide breakthrough in decoupling TE parameters in n-type (Hf0.6Zr0.4)NiSn0.99Sb0.01 half-Heusler (hH) alloys through multi-scale nanocomposite architecture comprising of tungsten nanoinclusions. The tungsten nanoparticles not only assist electron injection, thereby improving electrical conductivity, but also enhance the Seebeck coefficient through energy filtering effect. The microstructure comprises of disordered phases with varying size of microstructural features, which assists in effective scattering of heat-carrying phonons over diverse mean-free-path ranges. Cumulatively, these effects are shown to result in outstanding thermoelectric performance of zTmax ∼ 1.4 at 773 K and zTavg ∼ 0.93 between 300 and 973 K. Using this material, a TE generator is demonstrated, which exhibits high power density of 13.93 W cm−2 and conversion efficiency of 10.7% under ΔT = 674 K. The fundamental material design principle for TE nanocomposites demonstrated here can be generalized and extended to other TE systems.

We report electrical and thermal properties of perovskite-type Ba(Cu1/3Nb2/3)O3 (BCN) and Sr(Cu1/3Nb2/3)O3 (SCN). The BCN and SCN ceramics were synthesized by using the conventional solid state (CS...

Summary Conventional processing of the halide perovskite usually utilizes ionic precursor solutions containing mobile precursor ions in a high-boiling-point solvent. The assembly of these disordered ions into low-entropic crystals needs to overcome the large thermodynamic energy barrier. Traditional processes rely on high-temperature annealing and complex anti-solvent treatments to overcome this barrier, but they remain highly uncontrollable. Here, we report a novel nonionic and low-entropic MA(MMA)nPbI3 ink for fast perovskite crystallization with superior {110} preferred orientation and a high Lotgering factor. And the ink-based films exhibit an extraordinary long carrier diffusion length of 4.6 μm at the single-crystal level. With facial one-step processing, the corresponding photovoltaic cell exhibits a record efficiency of 21.8% based on the MAPbI3-system. Scaling of active area from 0.096 to 0.5 cm2 did not deteriorate the cell performance as the efficiency remained over 21%. These results demonstrate the potential of this nonionic ink for the industrial transition of perovskite photovoltaic technology.

Thermoelectric power generation is a reliable energy harvesting technique for directly converting heat into electricity. Recent studies have reported the thermal-to-electrical energy conversion efficiency of thermoelectric generators (TEGs) up to 11% under laboratory settings. However, the practical efficiency of TEGs deployed under real environments is still not more than a few percent. In this study, we provide fundamental insight on the operation of TEGs in realistic environments by illustrating the combinatory effect of thermoelectric material properties, device boundary conditions, and environmental thermal resistivity on TEG performance in conjunction with the module parameters. Using numerical and experimental studies, we demonstrate the existence of a critical heat transfer coefficient that dramatically affects the design and performance of TEGs. Results provide a set of concrete design criteria for developing efficient TEGs that meet the metrics for field deployments. High-performance TEGs demonstrated in this study generated up to 28% higher power and 162% higher power per unit mass of thermoelectric materials as compared to the commercial module deployed for low-grade waste heat recovery. This advancement in understanding the TEG operation will have a transformative impact on the development of scalable thermal energy harvesters and in realizing their practical targets for efficiency, power density, and total output power.

The solution processability of photoactive halide perovskites differentiates them from traditional inorganic semiconducting materials that require multiple post-processing steps such as thermal/vacuum/blowing- and solvent-assisted treatment. Here we report a technical breakthrough of isothermally crystallizing high-quality perovskite films at room-temperature (RT) without the necessity of any post-processing. This process takes advantage of our discovery of a metastable intermediate of lower-dimensionality formed by amine-assisted crystallographic lattice expansion from an initial three-dimensional perovskite. Using in situ optoelectrical/chemical and ex situ structural characterizations, a detailed understanding of the low-dimensional metastable intermediate is developed. In conjunction with the metastable intermediate, the rapid evaporation of the solvent and amine facilitates ultra-fast crystallization at RT within seconds. This RT rapidly synthesized perovskite film exhibits a carrier diffusion length of 2.9 μm and {00} preferred orientation with an ultrahigh Lotgering factor of 97%. These films are highly compatible to conventional or inverted devices, demonstrating 22.3% and 23.1% power conversion efficiencies, respectively.

Lower-dimensional quasi-two-dimensional (quasi-2D) halide perovskites have emerged as promising building blocks for multiple optoelectronic applications due to their superior photophysical properties. Recently, there has been a research focus on the terrace edge states (TES) in quasi-2D perovskites, which are thought to provide hypothetical highways to transport the excited states and thus give a new insight into boosting relevant device performance. Nevertheless, there is neither direct evidence of the electronic facets nor an in-depth understanding of these newly observed nontrivial TES. Here, we are the first to directly visualize the highly charged concentrated TES by means of a charge gradient microscopy (CGM) technique and elucidate the nature of TES through a combination of microscopic characterizations, including high-resolution transmission electron microscopy, Kelvin probe force microscopy and confocal fluorescence microscopy coupled with a first-principles density functional theory (DFT) calculation. It is shown that TES of quasi-2D perovskites are highly conductive (in distinct contrast to the insulating flat terrace region) and display a high Fermi-level and small forbidden bandwidth, which is attributed to the unique electron orbitals of the Pb atoms at the terrace edges. This distinctive conductivity of TES is of great importance in distinguishing them from bulk physical properties and inspiring novel nanoscale electronic applications, such as tip-based data storage and triboelectric nanogenerators.

Cd doping and metallic Ag additives in Ca3Co4O9+δ polycrystalline materials are shown to result in improved thermoelectric (TE) transport properties. Carrier concentration and mobility were optimized through the combination of doping and compositional modulation approaches. The formation of filiform Ag nanoinclusions between the interlayers and grain boundaries enhances the anisotropic carrier transport, leading to higher carrier mobility. A spin entropy enhancement due to the change of the net valence of Co induced by Cd substitution on the Ca site was confirmed by X-ray photoelectron spectroscopy. High carrier mobility and enhanced spin entropy results in higher electrical conductivity and Seebeck coefficient, leading to the increase of the power factor. In conjunction, mass fluctuation between Cd and Ca on the same crystal site along with the increase of metallic Ag nanoinclusions effectively lowers thermal conductivity. Consequently, the figure-of-merit, zT, has been improved to 0.31 at 950 K for 10 wt % Ag-modified Ca2.9Cd0.1Co4O9+δ specimen, which is a significant improvement compared to the pristine material. This dual-mode control of electron and phonon transport by including Ag additives and Cd doping offers an approach for tuning the correlated TE parameters.

Thermoelectric generators (TEGs) can convert body heat into electricity, thereby providing a continuous power source for wearable and implantable devices. For wearables, the low fill factor (area occupied by legs over the TEG base area) TEG modules are relevant as they provide large thermal gradient across the legs and require less material, which reduces the cost and weight. However, TEGs with a fill factor below 15% suffer from reduced mechanical robustness; consequently, commercial modules are usually fabricated with a fill factor in the range of 25-50%. In this study, TEG modules with a low and high fill factor are demonstrated and their performance is compared in harvesting body heat. Fabricated modules demonstrate ∼80% output power enhancement as compared to commercially available designs, resulting in high power density of up to 35 μW/cm2 in a steady state. This enhanced power is achieved by using two-third less thermoelectric materials in comparison to commercial modules. These results will advance the ongoing development of wearable devices by providing a consistent high specific power density source.

Thermoelectric generators (TEGs) exploiting the Seebeck effect provide a promising solution for waste heat recovery. Among the large number of thermoelectric (TE) materials, half-Heusler (hH) alloys are leading candidates for medium- to high-temperature power generation applications. However, the fundamental challenge in this field has been inhomogeneous material properties at large wafer diameters, insufficient power output from the modules, and rigid form factors of TE modules. This has restricted the transition of TEGs in practical applications for over three decades. Here, we successfully demonstrate large diameter wafers with uniform TE properties, high-power conformal hH TE modules for high-temperature application, and their direct integration on flue gas platforms, such as cylindrical tubes, to form large area flexible TEGs. This new conformal architecture design provides a breakthrough toward medium-/high-temperature TEGs over the conventional BiTe- and polymer-based flexible TEG design. A variable fill factor and greater flexibility due to the conformal design result in higher device performance as compared to conventional rigid TEG devices. Modules with 72-couple hH legs exhibit a device high-power-density of 3.13 W cm-2 and a total output power of 56.6 W under a temperature difference of 570 °C. These results provide a promising pathway toward widespread utilization of thermoelectric technology into the waste heat recovery application and will have a significant impact on the development of practical thermal to electrical converters.

Magnetoelectric (ME) transducers, comprising of layered magnetostrictive and piezoelectric materials, are more efficient than inductive coils in converting low-frequency magnetic fields into electric fields, particularly in applications that require miniaturized devices such as biomedical implants. Therefore, ME transducers are an attractive candidate for wireless power transfer (WPT) using low-frequency magnetic fields, which are less harmful to the human body and can penetrate easily through different lossy media. The literature lacks a comprehensive study on the ME transducer as a power receiver in a WPT link. This paper studies the impact of different ME design parameters on the WPT link performance. An accurate analytical model of the ME transducer, operating in the longitudinal-transverse mode, is presented, describing both temporal and spatial deformations. Nine ME transducers with different sizes (ME volume: 5-150 mm3) were fabricated with Galfenol and PZT-5A as magnetostrictive and piezoelectric layers, respectively. Through the modeling and measurement of these ME transducers, the effects of the ME transducer dimension, DC bias magnetic field, loading (RL), and operation frequency on the resonance frequency, quality factor, and received power (PL) of the ME transducer are determined. In measurements, a 150 mm3 ME transducer achieved > 10-fold higher PL for a wide RL range of 500 to 1 M at 95.5 kHz, compared to an optimized coil with comparable size and operation frequency.

In this work we present PL and time resolved PL (TRPL) measurements of three of these materials: BA2PbI4, BA2MA1Pb2I7, and BA2CuCI4 where BA2 represents (CH3(CH2)3NH3)2, and MA: CH3NH3. Both BA2PbI4 and BA2CuCI4 have a single layer of perovskite material separated by an organic cation layer while BA2MA1Pb2I7 has two atomic layers of perovskite. Our observations indicate the existence of both free and trapped excitons in these systems. Additionally, BA2PbI4 displays two sets of peaks for both trapped and free excitons that evolve with temperature, indicating that as the temperature is reduced the system begins, but does not complete, a phase change from a tetragonal to an orthorhombic crystal lattice. Our result provides new insights on the low temperature behavior of this phase transition, as well as exploring the exciton spectra as a function of both temperature and magnetic field. This material is based upon work supported by the Air Force Office of Scientific Research under awar

Deoxyribonucleic acid (DNA) has been recently recognized as hole transport material apart from its well-known generic role. The promising long-range hole transport capability in DNA make it potenti...

Conventional ceramic based piezoelectric materials are brittle, which restricts their use in energy harvesting where flexibility is required. Polymer counterparts are flexible but exhibit comparati...

One major challenge in the commercialization of halide perovskite solar cells is the device instability against moisture. Efficient perovskite solar cells usually incorporate the ionic dopants within the charge transfer material to secure their electrical conductivity. Typical ionic dopants are hygroscopic, resulting in significant moisture adsorption and accelerated perovskite degradation. Herein, we report a nonionic dopant, Sc3N@C80, consisting of a hydrophobic fullerene cage that encapsulates the metal salt to achieve a moisture resistive and highly electrically conductive hole transfer layer (HTL). The direct electronic transaction between Sc3N@C80 and spiro-OMeTAD renders a drastically improved conductivity and a lower Fermi-level of the HTL to minimize the Schottky barrier. The hydrophobicity of Sc3N@C80 also decreases the moisture wettability. Perovskite solar cells using the Sc3N@C80-doped HTL exhibit an efficiency surge from 18.15 to 20.77% (champion cell exhibiting 21.09%) and improved device s...

The Ca3Co4O9 (CCO) with layered structure has been considered as a potential candidate for high-temperature thermal energy harvesting application. However, CCO’s layered structure imparts anisotropy in transport properties, which results in anisotropic thermoelectric performance. So far, limited attempt has been made to understand the anisotropic thermoelectric performance of CCO, which often results in erroneous estimation of the thermoelectric response. Here, we fabricated highly textured CCO/x wt % Ag (x = 0, 1, 3, 5) nanoinclusion composites using the spark plasma sintering (SPS) technique and systematically investigated correlation between microstructure and anisotropic thermoelectric properties. The thermoelectric response was measured along both in-plane and out-of-plane directions (perpendicular and parallel to the pressure axis). We developed a two-step SPS method to achieve enhanced degree of texturing and increased electrical conductivity along ab-planes. The addition of Ag nanoinclusions was f...

Flexible perovskite solar cells have attracted widespread research effort because of their potential in portable electronics. The efficiency has exceeded 18 % owing to the high‐quality perovskite film achieved by various low‐temperature fabrication methods and matching of the interface and electrode materials. This Review focuses on recent progress in flexible perovskite solar cells concerning low‐temperature fabrication methods to improve the properties of perovskite films, such as full coverage, uniform morphology, and good crystallinity; demonstrated interface layers used in flexible perovskite solar cells, considering key figures‐of‐merit such as high transmittance, high carrier mobility, suitable band gap, and easy fabrication via low‐temperature methods; flexible transparent electrode materials developed to enhance the mechanical stability of the devices; mechanical and long‐term environmental stability; an outlook of flexible perovskite solar cells in portable electronic devices; and perspectives of commercialization for flexible perovskite solar cells based on cost.

Piezoelectric coefficients are constrained by the intrinsic crystal structure of the constituent material. Here we describe design and manufacturing routes to previously inaccessible classes of piezoelectric materials that have arbitrary piezoelectric coefficient tensors. Our scheme is based on the manipulation of electric displacement maps from families of structural cell patterns. We implement our designs by additively manufacturing free-form, perovskite-based piezoelectric nanocomposites with complex three-dimensional architectures. The resulting voltage response of the activated piezoelectric metamaterials at a given mode can be selectively suppressed, reversed or enhanced with applied stress. Additionally, these electromechanical metamaterials achieve high specific piezoelectric constants and tailorable flexibility using only a fraction of their parent materials. This strategy may be applied to create the next generation of intelligent infrastructure, able to perform a variety of structural and functional tasks, including simultaneous impact absorption and monitoring, three-dimensional pressure mapping and directionality detection. Piezoelectrics convert force into electrical charge, and vice versa, but the coefficients that determine piezoelectric behaviour are constrained by crystal structure. Here, metamaterials are 3D printed that show arbitrary piezoelectric coefficients.

The filling fraction limitation (FFL) in n-type CoSb3 skutterudites is far below that of p-type (Fe,Co)Sb3-based skutterudites, and it is critical to increase FFL for accomplishing high thermoelect...

The desire for multifunctional devices has driven significant research toward exploring multiferroics, where the coupling between electric, magnetic, optical, and structural order parameters can provide new functionality. While BiFeO3 is a well studied multiferroic, recent research has shown that the addition of BaTiO3 can improve the material properties. In this study, we perform time resolved differential reflectivity measurements of (1 − x)BaTiO3–(x)BiFeO3, with x = 0.725 epitaxial films grown on a lanthanum strontium manganite (LSMO) layer with a strontium titanate (STO) substrate, and BaTiO3–BiFeO3 nano-rods grown on Pt/Si. Our time resolved measurements can be used to probe dynamical properties on picosecond time scales. Information obtained is vital towards the design and understanding of high speed devices that incorporate multiferroic materials. We find that the contribution to the transient reflectivity response of a BaTiO3–BiFeO3 film due to carrier dynamics can be quantitatively explained by the diffusion of the photoexcited carriers away from the surface and that the ambipolar diffusion constant is below 1–2 cm2 s−1. At low temperature and for thicker films, in addition to a response from carrier dynamics, we also observed oscillations in the transient reflectivity with periods of 30–37 ps. We attribute these oscillations to propagating coherent acoustic phonons in the films. In the BaTiO3–BiFeO3 nano-rods samples only, we observed an additional oscillation with frequency in the range of 8 GHz. This is close to a theoretically predicted magnon frequency, but the strength of this oscillation only had a very weak magnetic field dependence. Another explanation for this feature could be multiple reflections of the acoustic phonons at the Pt interfaces (due to the large acoustic impedance mismatch).

High-purity monocrystalline silicon has a long history in the development of photovoltaics; so far, it has dominant applications in modern computers with its profound implementations in transistors and chips. The success of silicon has shown that monocrystalline wafers/thin films of semiconducting materials with superior electronic properties are a good platform for optoelectronic and electronic applications. Recently, the newly emerging semiconducting materials of halide perovskites (HPs) have attracted considerable attention owing to their continuing success in high-efficiency solar cells. The demonstrated optoelectronic properties of HPs indicate that it could be a promising alternative to the silicon-based semiconducting industry. However, the prerequisite of high-efficiency devices is the material accessibility of monocrystalline HPs (mono-HPs), as per the lessons learned from monocrystalline silicon. Current HPs-based technologies, in terms of research areas such as solar cells, photodetectors, light-emitting diodes (LEDs), lasers, and transistors, suffer a bottleneck in manufacturing mono-HP wafers/thin-film materials; hence, exciting results involving mono-HP devices are absent. State-of-the-art optoelectronic HP-based devices are exclusively built using polycrystalline thin films, which are limited in their performance due to issues such as grain-boundary defects, large trap density, and inhomogeneous charge transport. However, these issues can be resolved by utilizing mono-HPs. In this review, we provide in-depth analyses and discussions on the potential of mono-HPs in photovoltaics and transistor applications, and we present the remaining challenges, as well as promising research strategies, to provide a direction for future programs.

The power conversion efficiencies of perovskite solar cells (PSCs) have reached 23.3% recently, rivaling those of established photovoltaic technologies. For PSCs to be commercially competitive, one of the important challenges is to overcome the limitations of small area and excessive material waste from spin-coating. Electrospray printing is a scalable and roll-to-roll compatible method with high material utilization rate. Here, we report an all electrospray printing process for PSCs in ambient air below 150 °C. Strategies for successful electrospray printing of PSCs include formulating the precursor inks with solvents of low vapor pressures and judicial choice of droplet flight time, as well as tailoring the wetting property of the substrate to suppress coffee ring effects. Implementation of these strategies leads to pin-hole free, smooth and uniform perovskite layer, hole transport layer and electron transport layer. The power conversion efficiency of the all electrospray printed devices reaches up to 15.0%, which is the highest to date for fully printed PSCs using mainstream printing methods in air without significant material waste.

Environmentally friendly NaNbO3 capacitors have a great potential for applications in pulsed-discharge and power conditioning electronic systems because of their AFE-like hysteresis behavior, high saturation polarization and low mass. Here, we demonstrate (0.96 − x)NaNbO3–0.04CaZrO3–xBi0.5Na0.5TiO3 (abbreviated as NN-CZ-xBNT) capacitors with high energy storage density (Wrec) and efficiency (η). The performances of capacitors were tuned by the composition induced relaxor behavior and grain refinement which resulted in reduction of hysteresis and increase in the breakdown strength (BDS). Two-step sintering is found to be effective for reducing the grain size of MLCCs to 3 μm, which is 86% smaller as compared to grain size obtained by conventional solid-state sintering (22 μm). Small grain size significantly reduces the leakage current and losses and increases the BDS, yielding excellent Wrec = 3.7 J cm−3 and η = 82.1% in the NN-0.04CZ-0.16BNT multilayered capacitors. Results provide a systematic strategy for enhancing the energy storage density and efficiency of dielectric capacitors through the combination of dielectric relaxation and grain size refinement.

Simultaneously achieving high efficiency and high durability in perovskite solar cells is a critical step toward the commercialization of this technology. Inverted perovskite photovoltaic (IP‐PV) cells incorporating robust and low levelized‐cost‐of‐energy (LCOE) buffer layers are supposed to be a promising solution to this target. However, insufficient inventory of materials for back‐electrode buffers substantially limits the development of IP‐PV. Herein, a composite consisting of 1D cation‐doped TiO2 brookite nanorod (NR) embedded by 0D fullerene is investigated as a top modification buffer for IP‐PV. The cathode buffer is constructed by introducing fullerene to fill the interstitial space of the TiO2 NR matrix. Meanwhile, cations of transition metal Co or Fe are doped into the TiO2 NR to further tune the electronic property. Such a top buffer exhibits multifold advantages, including improved film uniformity, enhanced electron extraction and transfer ability, better energy level matching with perovskite, and stronger moisture resistance. Correspondingly, the resultant IP‐PV displays an efficiency exceeding 22% with a 22‐fold prolonged working lifetime. The strategy not only provides an essential addition to the material inventory for top electron buffers by introducing the 0D:1D composite concept, but also opens a new avenue to optimize perovskite PVs with desirable properties.

Even though ZnO is commonly used as the ETL in the perovskite solar cell (PSC), the reactivity of perovskite deposited thereupon limits its performance. Herein, an ethylene diamine tetraacetic acid‐complexed ZnO (E‐ZnO) is successfully developed as a significantly improved electron selective layer (ESLs) in perovskite device. It is found that E‐ZnO exhibits higher electron mobility and better matched energy level with perovskite compared to ZnO. In addition, in order to eliminate the proton transfer reaction at the ZnO/perovskite interface, a high quality perovskite film fabrication process that requires neither annealing nor antisolvent is developed. By taking advantages of both E‐ZnO and the new process, the highest efficiency of 20.39% is obtained for PSCs based on E‐ZnO. Moreover, the efficiency of unencapsulated PSCs with E‐ZnO retains 95% of its initial value exposed in an ambient atmosphere after 3604 h. This work provides a feasible path toward high performance of PSCs, and it is believed that the present work will facilitate transition of perovskite photovoltaics in flexible and tandem devices since the annealing‐ and antisolvent‐free technology., The highest efficiency of 20.39% is obtained for perovskite solar cells based on modified ZnO without annealing and antisolvent process. The perovskite devices exhibit unprecedented environmental stability owing to efficient decreased organic ligands on ZnO surface.

High temperature waste heat recovery has gained tremendous interest to generate useful electricity while reducing the harmful impact on the environment. Thermoelectric (TE) solid-state materials enable direct conversion of heat into electricity with high efficiency, thereby offering a practical solution for waste heat recovery. Half-Heusler (hH) alloys are the leading TE materials for medium to high temperature applications, as they exhibit a high figure of merit and mechanical strength at temperatures as high as 973 K. Here we investigate the most promising hH alloys represented as MNiSn, MCoSb, and NbFeSb systems (M = Hf, Zr, and Ti) and provide fundamental understanding of their in-air thermal stability at high temperatures under realistic operating conditions required for energy generation. The understanding of oxidation resistance of TE materials is crucial for their practical deployment in extreme environments without vacuum sealing. The n-type MNiSn and p-type NbFeSb compounds are found to exhibit excellent oxidation resistance at a high temperature of 873 K. The oxidation resistance is enhanced through the presence of an intermetallic Ni-Sn layer for MNiSn and Nb-TiO

Perovskite nanocrystals have attracted worldwide attention due to their outstanding optical versatility, high photoluminescence quantum yields, and facile synthesis. In this review, we firstly revisit the synthetic methods for perovskite nanocrystals (PNCs), including hot injection, anion exchange, solvothermal reaction, etc. In the meantime, we discuss effects of the different synthetic methods on the properties of PNCs, including the crystal size, emission spectral feature, quantum yield, etc., followed by several optimizing strategies. Finally, lasing and display applications of these PNCs in combination with liquid crystal materials are discussed thoroughly. Outlooks on the challenges and opportunities of these nanocrystalline materials in terms of adjunct applications with liquid crystals have been presented at the end, which are highly promising for next-generation light emission applications.

Summary Thermoelectric generators (TEGs) offer cost-effective and sustainable solid-state energy conversion mechanism from wasted heat into useful electrical power. Thermoelectric (TE) materials based upon bismuth telluride (BiTe) systems are widely utilized in applications ranging from energy generation to sensing to cooling. There is demand for BiTe materials with high figure of merit (zT) and TEG modules with high conversion efficiency over intermediate temperatures (25°C–250°C). Here we provide fundamental breakthrough in design of BiTe-based TE materials and utilize them to demonstrate modules with outstanding conversion efficiency of 8%, which is 40% higher compared with state-of-the-art commercial modules. The average zT of 1.08 for p-type and 0.84 for n-type bismuth telluride alloys is obtained between 25 and 250°C. The significant enhancement in zT is achieved through compositional and defect engineering in both p- and n-type materials. The high conversion efficiency accelerates the transition of TEGs for waste heat recovery., Graphical Abstract, Highlights • Significant improvement in design of bismuth telluride alloys is demonstrated • High peak and average zT are obtained in both p- and n-type bismuth telluride alloys • Thermoelectric generator with conversion efficiency of 8% is fabricated • High efficiency accelerates the transition of TEGs for waste heat recovery, Energy Engineering; Materials Science; Energy Materials

Ferroic perovskite oxides remain a major topic of interest in material science and condensed matter physics. Their versatile structure can accommodate many transition metal ions leading to a variety of useful properties, such as, among others, ferroelectricity. Several mechanisms behind the ferroelectricity of perovskites have been proposed. Using advanced x-ray scattering techniques and 3D modeling, the increased piezoelectric response of the exemplary sodium-potassium niobate ferroics is shown to arise from the presence of local fluctuations in the tilt pattern of the constituent niobium-oxygen octahedra. The result implies that the search for perovskite ferroics with increased functionality need not necessarily concentrate on systems exhibiting a morphotropic boundary and/or distinct polar nanodomains. Inducing local structural disorder in a controlled manner, e.g. through measured metal ion substitution, may become worthwhile.

Bismuth tungstate (Bi2WO6) micro/nanoparticles have been synthesized by a facile solvothermal route. The XRD pattern confirms the orthorhombic phase of russellite Bi2WO6 material. The sample BWO prepared by solvothermal route at 180°C gives rise to slightly elongatedshapewith an average particle size of about 9.91 nm, whereas the sample BWO-600 which has been annealed at 600°C for 2 hrs possess a change in the morphology to nanoplates whose border length measures between 0.4 µm to 1.0 µm. The vibrational modes of Bi-O and W-O have been confirmed by exploring the Fourier transform infra-red spectrum (FTIR). The optical absorption properties of the samples were studies using UV-visible diffuse reflectance spectroscopy. From the UV-DRS spectrum, it is revealed that the absorption band edge of the annealed BWO-600 sample shows a blue shift when compared with the sample BWO. The optical energy band gap of BWO and BWO-600 is found to be 3.16 eV and 2.92 eV respectively. The higher band gap value observed in BWO is attributed to the quantum confinement effect due to the formation of smaller sized particles.

Zinc ferrite nanoparticles were synthesized effectively by sol gel method and were annealed at 500°C. The sample was characterized by various techniques. Powder XRD, FTIR and FESEM studies were utilized to evaluate the phase structure and surface morphology. XRD revealed single phase spinel cubic structure of zinc ferrite. Light absorption and optical properties were deduced by UV-visible spectrophotometer. Magnetic properties of the sample at room temperature was deduced by vibrating sample magnetometer. The hysteresis loop depicted the ferromagnetic behavior of the sample.

Broadband mechanical energy harvesting implies stable output power over a wide range of source frequency. Here we present a cost-effective solution towards achieving broadband response by designing a magnetically coupled piezoelectric energy harvester array that exhibits a large power density of 243 μW/cm3 g2 at natural frequency and bandwidth of more than 30 Hz under 1 g acceleration. The magnetically coupled piezoelectric energy harvester array exhibits dual modes of energy harvesting, responding to both stray magnetic field as well as ambient vibrations, and is found to exhibit the output power density of 36.5 μW/cm3 Oe2 at 79.5 Hz under the ambient magnetic field while maintaining the broadband nature. The magnetically coupled piezoelectric energy harvester array was demonstrated to harvest continuous power from a rotary pump vibration, an automobile engine vibration and a parasitic magnetic field surrounding a cable of an electric kettle. These demonstrations suggest that the magnetically coupled piezoelectric energy harvester array could serve the role of a standalone power source for wireless sensor nodes and small electronic devices.

Oxide thermoelectric materials are nontoxic, chemically and thermally stable in oxidizing environments, cost-effective, and comparatively simpler to synthesize. However, thermoelectric oxides exhibit comparatively lower figure of merit (ZT) than that of metallic alloy counterparts. In this study, nanoscale texturing and interface engineering were utilized for enhancing the thermoelectric performance of oxide polycrystalline Ca3Co4O9 materials, which were synthesized using conventional sintering and spark plasma sintering (SPS) techniques. Results demonstrated that nanoscale platelets (having layered structure with nanoscale spacing) and metallic inclusions provide effective scattering of phonons, resulting in lower thermal conductivity and higher ZT. Thermoelectric measurement direction was found to have a significant effect on the magnitude of ZT because of the strong anisotropy in the transport properties induced by the layered nanostructure. The peak ZT value for the Ca2.85Lu0.15Co3.95Ga0.05O9 specimen measured along both perpendicular and parallel directions with respect to the SPS pressure axis is found be 0.16 at 630 °C and 0.04 at 580 °C, respectively. The peak ZT of 0.25 at 670 °C was observed for the spark plasma-sintered Ca2.95Ag0.05Co4O9 sample. The estimated output power of 2.15 W was obtained for the full size model, showing high-temperature thermoelectric applicability of this nanostructured material without significant oxidation.

Even though the mesoporous-type perovskite solar cell (PSC) is known for high efficiency, its planar-type counterpart exhibits lower efficiency and hysteretic response. Herein, we report success in suppressing hysteresis and record efficiency for planar-type devices using EDTA-complexed tin oxide (SnO2) electron-transport layer. The Fermi level of EDTA-complexed SnO2 is better matched with the conduction band of perovskite, leading to high open-circuit voltage. Its electron mobility is about three times larger than that of the SnO2. The record power conversion efficiency of planar-type PSCs with EDTA-complexed SnO2 increases to 21.60% (certified at 21.52% by Newport) with negligible hysteresis. Meanwhile, the low-temperature processed EDTA-complexed SnO2 enables 18.28% efficiency for a flexible device. Moreover, the unsealed PSCs with EDTA-complexed SnO2 degrade only by 8% exposed in an ambient atmosphere after 2880 h, and only by 14% after 120 h under irradiation at 100 mW cm−2. The development of high efficiency planar-type perovskite solar cell has been lagging behind the mesoporous-type counterpart. Here Yang et al. modify the oxide based electron transporting layer with organic acid and obtain planar-type cells with high certified efficiency of 21.5% and decent stability.

Thermoelectric skutterudite materials have been widely investigated for their potential application in mid-temperature waste heat recovery that has not been efficiently utilized A large amount of research has focused on developing materials with a high thermoelectric figure of merit (zT). However, the translation of material properties to device performance has limited success. Here, we demonstrate single-filling n-type Yb0.25Fe0.25Co3.75Sb12 and multi-filling La0.7Ti0.1Ga0.1Fe2.7Co1.3Sb12 skutterudites with a maximum zT of ~1.3 at 740 K and ~0.97 at 760 K. The peak zT of skutterudites usually occurs above 800 K, but, as shown here, the shift in peak zT to lower temperatures is beneficial for enhancing conversion efficiency at a lower hot-side temperature. In this work, we have demonstrated that the Fe-substitution significantly reduces the thermal conductivity of n-type skutterudite, closer to p-type skutterudite thermal conductivity, resulting in a module that is more compatible to operate at elevated temperatures. A uni-couple skutterudite module was fabricated using a molybdenum electrode and Ga&ndash, Sn liquid metal as the thermal interface material. A conversion efficiency of 7.27% at a low temperature gradient of 366 K was achieved, which is among the highest efficiencies reported in the literature at this temperature gradient. These results highlight that peak zT shift and optimized module design can improve conversion efficiency of thermoelectric modules at a low temperature gradient.

In the emerging era of Internet of Things (IoT), power sources for wireless sensor nodes in conjunction with efficient and secure wireless data transfer are required. Energy harvesting technologies are promising solution toward meeting the requirements for sustainable power sources for the IoT. In this review, we focus on approaches for harvesting stray vibrations and magnetic field due to their abundance in the environment. Piezoelectric materials and piezoelectric–magnetostrictive [magnetoelectric (ME)] composites can be used to harvest vibration and magnetic field, respectively. Currently, such harvesters use modified lead zirconate titanate (or lead-based) piezoelectric materials and ME composites. However, environmental concerns and government regulations require the development of a suitable lead-free replacement for lead-based piezoelectric materials. In the past decade, several lead-free piezoelectric compositions have been developed and demonstrated with promising piezoelectric response. This paper reviews the significant results reported on lead-free piezoelectric materials with respect to high-density energy harvesting, covering novel processing techniques for improving the piezoelectric response and temperature stability. The review of the state-of-the-art studies on vibration and magnetic field harvesting is provided and the results are used to discuss various strategies for designing high-performance energy harvesting devices.