Ultrahigh Conversion Efficiency Research Articles

Lattice Regularization by Manipulating Over-Stoichiometric Defects Yields High-Performance (Bi,Sb)2Te3 Thermoelectrics.

Bi2Te3-based alloys have historically dominated the commercial sector of near-ambient-temperature thermoelectric technology. However, the massive intrinsic defects form the "donor-like" effect and affect the transport properties of Bi2-xSbxTe3 significantly. Here, it is demonstrated that the over-stoichiometric Sb fills Te vacancies and weakens the defect scattering, resulting in a desirable carrier mobility. The boost-generated antisite defects also compensate for the extra hole carries. Combined with dilute Cu doping, the global microstructural modulation is synergistically promoted, characterized by Sb coherent nanoprecipitates and high-density twins. Benefitting from the decoupled electrical-thermal transport, the peak ZT is improved to ≈1.50 at 350 K, with an average ZT of 1.25 from 300 to 500 K. The further designed and integrated 17-pair power generators exhibit ultrahigh conversion efficiency, reaching 6.7% under a 200 K temperature gradient, and show excellent operational stability. These achievements hold great potential for advancing Bi2Te3-based power generators in low-grade waste heat recovery.

Realizing Ultrahigh Conversion Efficiency of ≈9.0% in YbCd2Sb2/Mg3Sb2 Zintl Module for Thermoelectric Power Generation.

Recently, YbCd2Sb2-based Zintl compounds have been widely investigated owing to their extraordinary thermoelectric (TE) performance. However, its p orbitals of anions that determined the valence band structure are split due to crystal field splitting that provides a good platform for band manipulation by doping/alloying and, more importantly, the YbCd2Sb2-based device has yet to be reported. In this work, single-phase YbCd1.5Zn0.5Sb2 is successfully obtained through precise chemical composition control. Then, YbMg2Sb2-alloying increases the cationic vacancy defect formation energy and further optimizes carrier concentration. Moreover, the band structure of YbCd1.5Zn0.5Sb2 is subtly manipulated, and the underlying mechanism is experimentally explored. Combined with the reduced lattice thermal conductivity, a high peak ZT value of ∼1.43 at 700K is obtained for YbCd1.425Zn0.475Mg0.1Sb2. Subsequently, choosing Fe90Sb10 as the diffusion barrier layer and adopting the transient liquid phase bonding technique, for the first time, it is demonstrated that YbCd2Sb2/Mg3(Sb, Bi)2 TE module with an ultrahigh conversion efficiency of ≈9.0% at a heat difference of 430K. More importantly, this module displays good thermal stability. This work paves the way for YbCd2Sb2 materials and devices in mid-temperature heat recovery.

Ultra-broadband and ultra-high conversion efficiency difference frequency generation in a dual-layer thin-film LiNbO3 waveguide with reversed ferroelectric domain orientations

Although there is a great breakthrough in second harmonic generation based on the birefringence and quasi phase matching (QPM) concepts, the development of an efficient difference frequency generation (DFG) based on modal phase matching (MPM) is still in its infancy at present. In this paper, we propose an ultra-broadband and ultra-high conversion efficiency mid-infrared DFG on the basis of MPM in a dual-layer waveguide consisting of two reversed poled LiNbO3 thin-films on sapphire. The ultra-high conversion efficiency is ascribed to significantly enhanced nonlinear interaction realized by making use of spatial profile of optical nonlinearity of LN crystal produced by two reversed ferroelectric domain orientations in the dual-layer waveguide. The proposed DFG device has been modeled on the basis of a 0.98 μm pump wave and a 1.34–1.44 μm signal wave, the frequency difference of which generates a mid-infrared idler wave centered at 3.32 μm wavelength. The results show that the device exhibits excellent DFG performance, including an ultra-high nonlinear conversion efficiency 434.6 % W−1 cm−2 and an ultra-broad tuning band 340 nm meanwhile, which far exceed the performance of conventional MPM devices, even the QPM ones.

A General Strategy for Developing Ultrasensitive "Transistor-Like" Thermochromic Fluorescent Materials for Multilevel Information Encryption.

Thermochromic fluorescent materials (TFMs) exhibit great potential in information encryption applications but are limited by low thermosensitivity, poor color tunability, and a wide temperature-responsive range. Herein, a novel strategy for constructing highly sensitive TFMs with tunable emission (450-650nm) toward multilevel information encryption is proposed, which employs polarity-sensitive fluorophores with donor-acceptor-donor (D-A-D) type structures as emitters and long-chain alkanes as thermosensitive loading matrixes. The structure-function relationships between the performance of TFMs and the structures of both fluorescent emitters and phase-change molecules are systematically studied. Benefiting from the above design, the obtained TFMs exhibit over 9500-fold fluorescence enhancement toward the temperature change, as well as ultrahigh relative temperature sensitivity up to 80%K-1 , which are first confirmed. Thanks to the superior transducing performance, the above-prepared TFMs can be further developed as information-storage platforms within a relatively narrow interval of temperature variation, including temperature-dominated multicolored information display and multilevel information encryption. This work will not only provide a novel perspective for designing superior TFMs for information encryption but also bring inspiration to the design and preparation of other response-switching-type fluorescent probes with ultrahigh conversion efficiency.

Ultra-Wideband OAM Mode Generator Based on a Helical Grating Written in a Graded-Index Few-Mode Fiber

An ultra-wideband orbital-angular-momentum (OAM) mode generator has been proposed and demonstrated both theoretically and experimentally, which is based on a helical long period fiber grating (HLPG) but written in a graded-index few-mode fiber (GIFMF). Unlike all the HLPG-based broadband OAM generators reported to date, where the utilized HLPG (fabricated only in step-index fibers) all were operated at wavelengths very close to the dispersion-turning point (DTP), the GIFMF-based HLPG proposed in this study is, however, operated at wavelengths far from its DTP, which provides a new and robust method enabling to produce a ultra-wideband OAM mode generator with an ultra-high conversion-efficiency and yet with a great flexibility in controlling the central wavelength of itself. As a typical example, the first-order OAM mode generator with a central wavelength of ∼1510 nm, a bandwidth of ∼235 nm, and a conversion-efficiency larger than 99% (∼20.7 dB) has been successfully obtained. To the best of our knowledge, this is the first report for practical demonstration of the GIFMF-based HLPG.

Engineering salt-rejecting solar evaporator from naturally hierarchical tree root for sufficient clean water production

Solar desalination has been recognized as one of the most promising technologies for solving current freshwater scarcity. However, enhancement of the photothermal conversion efficiency and prohibiting salt crystallization on the top light-absorbing surface are the two major critical challenges for developing highly-efficient and stable solar evaporators. Here, we have discovered that tree roots, often as processing waste when utilizing woods, can serve as a high-efficient solar evaporation device for continuous desalination. The naturally occurring hierarchical structure of tree roots possesses multilevel longitudinal channels from micrometer to millimeter-scale and interconnected porous microstructures, which allows excellent water transport and multidirectional salt exchange, exhibiting a superior anti-salt-accumulation capability even in 21% brine. Together with a mountain-shaped hydrophobic absorber surface obtained from the in situ synthesized Fe3O4 nanoparticles, the designed tree root-based solar evaporator demonstrates an evaporation rate of 1.64 kg m−2 h−1 and ultra-high conversion efficiency of 96% at 1 solar irradiation, rendering it as one of the best performing wood-based solar evaporators. Furthermore, such a high-efficacy, techno-economic and long-term stable seawater desalination platform provided by the architectures of multi-level channel-array in tree root will inspire material scientists and engineers to tailor structurally similar materials from building blocks of broader selections.

(Invited) Advanced Concepts for Ultra-High Conversion Efficiency of Solar Photons into Photovoltaics and Solar Fuels Based on Semiconductor Nanostructures and Molecular Singlet Fissionajn

In order to utilize solar power for the production of solar electricity and solar fuels on a global scale, it is necessary to develop solar photon conversion systems that have an appropriate combination of very high efficiency (delivered watts/m2) and low capital cost ($/m2). One potential promising approach to attain high conversion efficiencies well above the Shockley-Queisser limit of 33% is to utilize the unique properties of semiconductor nanostructures (viz., quantum dots/rods/films) and singlet fission chromophores to control the relaxation dynamics of photogenerated carriers and photochemical excited states, respectively. The former process creates enhanced photocurrent through efficient photogenerated electron-hole pair multiplication (electron-hole pairs exist as excitons in size-quantized nanostructures), or enhanced photopotential through hot carrier transport, transfer, and extraction processes. The latter process of singlet fission generates two triplets from an excited singlet state. These fundamental dynamics in various bulk and nanoscale semiconductors and in molecular chromophores have been studied using transient absorption, photoluminescence, photocurrent and THz spectroscopy with fs to ns time resolution. The predictions that the generation of more than one electron-hole pair or exciton per absorbed photon would be an efficient process in nanostructures, and via singlet fission, have been confirmed in different classes of materials, molecules, and their architectures in photon conversion devices. Selected aspects of this work will be summarized and recent advances will be discussed, including the very remarkable and beneficial theoretical effects of combining MEG or singlet fission with solar concentration and controlling the thicknesses of the photoactive layers in a two-photon photoelectrosynthesis cell structure for solar fuels.

Design of on-chip mid-IR frequency comb with ultra-low power pump in near-IR.

Broadband mid-infrared frequency combs are of particular interest to mid-infrared spectroscopy due to their ruler-like precise discrete comb teeth. However, the state-of-the-art mid-infrared frequency combs are usually limited to low integration level and high pump power as a result of the conventional way of mid-infrared frequency comb generation--producing a near-infrared frequency comb first and then convert it to mid-infrared regime through a nonlinear process. Here, we theoretically investigate two integrated designs for generating mid-infrared frequency combs with ultra-low power pump based on the lithium-niobate on insulator (LNOI) platform. Utilizing periodically poled lithium-niobate (PPLN) waveguides and microring electro-optic phase modulators, we switch the conventional order of comb generation and nonlinear conversion. This paradigm shift significantly improves the conversion efficiency of mid-infrared frequency comb generation and obviates the need for femtosecond lasers. Our theoretical results predict that a broadband mid-infrared frequency comb around 4.3 µm with nanowatt-power-level comb teeth can be produced from continuous-wave (CW) inputs whose power is lower than 5 mW with an ultra-high conversion efficiency above 1800 %/W. Our designs of mid-infrared frequency comb have high controllability, flexibility and integration level, enabling the miniaturization of mid-infrared spectrometers.

Ultrahigh conversion efficiency of betavoltaic cell using diamond pn junction

A betavoltaic cell, which directly converts beta particles into energy, is composed of a junction diode and a beta-emitting source. Because the cells can deliver electricity over a long operation life ranging from several years to a decade, they are promising devices for applications in remote locations such as outer space, deserts, and underground areas. Herein, we report efficient energy conversion using a diamond pn junction. We characterized the betavoltaic performance under electron-beam irradiation using scanning electron microscopy and observed an open-circuit voltage of 4.26 V, a fill factor of 0.85, and a semiconductor conversion efficiency of 28%. These are the best values reported thus far for betavoltaic cells. The efficiency is close to the theoretical Shockley–Queisser efficiency limit for betavoltaic cells.

Structure Architecting for Salt-Rejecting Solar Interfacial Desalination to Achieve High-Performance Evaporation With In Situ Energy Generation.

The past few years have witnessed a rapid development of solar‐driven interfacial evaporation, a promising technology for low‐cost water desalination. As of today, solar‐to‐steam conversion efficiencies close to 100% or even beyond the limit are becoming increasingly achievable in virtue of unique photothermal materials and structures. Herein, the cutting‐edge approaches are summarized, and their mechanisms for photothermal structure architecting are uncovered in order to achieve ultrahigh conversion efficiency. Design principles to enhance evaporation performance and currently available salt‐rejection strategies for long‐term desalination are systematically investigated. The guidelines to utilize every component in solar desalination systems for simultaneous in situ energy generation are also revealed. Finally, opportunities and challenges for future works in this field are also discussed and concluded.

Experimental Demonstration of a 98.8% Efficient Isolated DC–DC GaN Converter

This paper demonstrates the utilization of gallium nitride (GaN) devices to achieve an ultrahigh efficiency in an isolated dc–dc converter. This paper presents the design and implementation of high-efficiency magnetics necessary to realize such an ultrahigh efficiency converter. Synchronous rectification is also implemented to further improve the efficiency. Compared to an equivalent silicon MOSFET, GaN devices have low output charge, very low gate drive losses, and zero reverse recovery losses, this makes GaN devices more suitable for power converters to achieve high efficiency. A complete analytical loss modeling for an isolated full bridge dc–dc converter is discussed in this paper. Furthermore, the experimental demonstration of ultrahigh efficiency in a 2.4-kW isolated GaN converter is presented. Ultrahigh conversion efficiency allows the realization of a very compact dc–dc converter with a very small or no heat sink. The prototype converter has a power density of 7 kW/L and the measured efficiency is above 98.5% over a wide range of output power. The maximum measured efficiency of the converter is 98.8% at 50% of full load.

Microwave linear polarization rotator in a bilayered chiral metasurface based on strong asymmetric transmission

We propose and study a kind of bilayered chiral metasurface (BCM) composed of complementary L-shaped resonators with a lossless dielectric spacer that can realize linear polarization rotation with ultrahigh conversion efficiency. We present a theoretical analysis of the BCM with specific chiral geometry that enables asymmetric transmission for linear polarization only. Numerical results show that the proposed metasurface has dual-band asymmetric transmission with nearly 100% cross-polarization conversion efficiency when the loss is ignored. More importantly, depending on the incident direction, only one of the cross-polarization transmissions can approach unity while all the remaining transmissions are close to zero. As a result, nearly perfect linear polarization rotation is achieved for a particular polarization direction. We further show that the working frequency and the bandwidth of the proposed BCM can be tuned by adjusting the geometric size and spatial arrangement of the unit cell.

Silicon heterojunction solar cells with electron selective TiOx contact

Silicon solar cells featuring carrier selective contacts have been demonstrated to reach ultra-high conversion efficiency. In this work, the electron-selective contact characteristics of ultrathin TiOx films deposited by atomic layer deposition on silicon are investigated via simultaneous consideration of the surface passivation quality and the contact resistivity. Thin TiOx films are demonstrated to provide not only good passivation to silicon surfaces, but also allow a relative low contact resistivity at the TiOx/Si heterojunction. A maximum implied open-circuit voltage (iVoc) of ~703mV is achieved with the passivation of a 4.5nm TiOx film, and a relatively low contact resistivity of (~0.25Ωcm2 is obtained at the TiOx/n-Si heterojunction simultaneously. N-type silicon solar cell with the champion efficiency of 20.5% is achieved by the implementation of a full-area electron-selective TiOx contacts. A simulated efficiency of up to 23.7% is achieved on the n-type solar cell with a full-area TiOx contact. The efficient, low cost electron-transporting/hole-blocking TiOx layer enables the fabrication of high efficiency silicon solar cells with a simplified process flow.

Ultrahigh efficiencies in vertical epitaxial heterostructure architectures

Optical to electrical power converting semiconductor devices were achieved with breakthrough performance by designing a Vertical Epitaxial Heterostructure Architecture. The devices are featuring modeled and measured conversion efficiencies greater than 65%. The ultrahigh conversion efficiencies were obtained by monolithically integrating several thin GaAs photovoltaic junctions tailored with submicron absorption thicknesses and grown in a single crystal by epitaxy. The heterostructures that were engineered with a number N of such ultrathin junctions yielded an optimal external quantum efficiencies approaching 100%/N. The heterostructures are capable of output voltages that are multiple times larger than the corresponding photovoltage of the input light. The individual nanoscale junctions are each generating up to ∼1.2 V of output voltage when illuminated in the infrared. We compare the optoelectronic properties of phototransducers prepared with designs having 5 to 12 junctions and that are exhibiting voltage outputs between >5 V and >14 V.

Organic–Inorganic Halide Perovskites: Perspectives for Silicon-Based Tandem Solar Cells

We investigate the efficiency potential of organic-inorganic halide perovskite/crystalline silicon tandem solar cells, a new class of photovoltaic devices targeting long-term cost reductions by ultrahigh conversion efficiencies. Methyl ammonium lead triiodide perovskite solar cells are particularly interesting as the top cell in Si-based tandem devices due to their suitable band gap, high photovoltage, and low sub-bandgap absorption. We derive optical models for a perovskite/Si tandem cell with Lambertian light trapping in the perovskite top cell, as well as for a top cell in the single pass limit. We find that unlike for other thin-film device architectures, light trapping is not required for the triiodide perovskite/Si tandem to reach matched top and bottom cell currents. While a Lambertian top cell could be employed in a four-terminal tandem, a top cell in the single pass limit enables a current-matched monolithic device with realistic top cell thicknesses. We calculate a limiting efficiency of 35.67% for an ideal (no parasitic absorption, ideal contacts) monolithic tandem, assuming a top cell open-circuit voltage of 1100 mV.

Third generation photovoltaics: Ultra‐high conversion efficiency at low cost

AbstractSince the early days of terrestrial photovoltaics, a common perception has been that ‘first generation’ silicon wafer‐based solar cells eventually would be replaced by a ‘second generation’ of lower cost thin‐film technology, probably also involving a different semiconductor. Historically, cadmium sulphide, amorphous silicon, copper indium diselenide, cadmium telluride and now thin‐film polycrystalline silicon have been regarded as key thin‐film candidates. Any mature solar cell technology seems likely to evolve to the stage where costs are dominated by those of the constituent materials, be they silicon wafers or glass sheet. It is argued, therefore, that photovoltaics is likely to evolve, in its most mature form, to a ‘third generation’ of high‐efficiency thin‐film technology. By high efficiency, what is meant is energy conversion values double or triple the 15–20% range presently targeted, closer to the thermodynamic limit of 93%. Tandem cells are the best known of such high‐efficiency approaches, where efficiency can be increased merely by adding more cells of different bandgap to a cell stack, at the expense of increased complexity and spectral sensitivity. However, a range of other more ‘paralleled’ approaches offer similar efficiency to an infinite stack of tandem cells. These options are reviewed together with possible approaches for practical implementation, likely to become more feasible with the evolution of materials technology over the next two decades. Copyright © 2001 John Wiley & Sons, Ltd.