Recent advancement in the performance of solar cells by incorporating transition metal dichalcogenides as counter electrode and photoabsorber

Two‐dimensional (2D) transition metal dichalcogenides (TMDCs) architectures have revealed fascinating characteristics such as direct band gap, strong light absorption, and novel electrochemical properties, which make them promising materials for photovoltaic applications. The review focuses on (1) the study of electrochemical and photovoltaic properties of TMDCs thereby using them as counter electrodes (CEs) in dye‐sensitized solar cells (DSSCs) and (2) analyzing the light absorption and charge transport performance of TMDCs heterostructures with different 3D materials. We have further investigated different materials in combination with TMDCs such as reduced graphene oxide nanocomposite, graphene flakes, and molybdenum as CEs in DSSCs. Conventionally, platinum (Pt) is used as a CE material for DSSCs that displays excellent catalytic activity and high electrical conductivity but due to the high cost and scarcity of Pt limits the large‐scale production. Therefore, the excellent electrochemical properties and cost‐effectiveness of TMDCs make them promising contender to replace Pt as CEs in DSSCs. Additionally, the photovoltaic properties of TMDCs and their heterostructures with various materials such as silicon, gallium arsenide, indium phosphate, tungsten disulfide, boron nitride, and organic polymers are reviewed. TMDCs are also investigated as hole transport layer (HTL) and electron transport layer (ETL) with various organic polymers such as P3HT, PCBM, PEDOT:PSS, PTB7, and spiro‐OmeTAD for organic and perovskite‐based solar cells (SCs). The utilization of TMDCs as CEs and photoabsorbers enhances the power conversion efficiency (PCE) to generate cost‐effective and high performance SC devices that can be exploit for future technological applications.     

Thermal Transport Properties of Black Phosphorus: A Topical Review

Black phosphorus (BP) is a layered direct bandgap semiconductor. Few-layer BP, an emergent and promising two-dimensional semiconductor, has attracted increasing interest for novel electronic, photonic, and energy conversion applications due to its unique tunable direct bandgap and anisotropic in-plane transport properties. Understanding the thermal transport properties is critical for thermal management of electronics and thermoelectric applications. In this review, we cover the thermal transport properties of single-layer, few-layer, and bulk BP from most recent theoretical and experimental work, with a particular focus on the anisotropic in-plane thermal conductivity.     

Exploring the Extremes of Heat Conduction in Anisotropic Materials

Anisotropic solids possess thermal conductivities ranging from among the highest found in nature, as in the in-plane thermal conductivity of graphite, to the lowest, as in the cross-plane thermal conductivity of disordered layered crystals. Though these extremes of thermal conductivity make anisotropic materials attractive for diverse applications such as thermal management and thermal insulation, the microscopic physics of heat conduction in these materials remain poorly understood. In this review article, we discuss the recent advances in our understanding of thermal phonon transport in anisotropic solids obtained using new theoretical, computational, and experimental tools.     

Nanostructural thermoelectric materials and their performance

In this review, an attempt was made to introduce the traditional concepts and materials in thermoelectric application and the recent development in searching high-performance thermoelectric materials. Due to the use of nanostructural engineering, thermoelectric materials with a high figure of merit are designed, leading to their blooming application in the energy field. One dimensional nanotubes and nanoribbons, two-dimensional planner structures, nanocomposites, and heterostructures were summarized. In addition, the state-of-the-art theoretical calculation in the prediction of thermoelectric materials was also reviewed, including the molecular dynamics (MD), Boltzmann transport equation, and non-equilibrium Green’s function. The combination of experimental fabrication and first-principles prediction significantly promotes the discovery of new promising candidates in the thermoelectric field.     

High performance solid-state thermoelectric energy conversion via inorganic metal halide perovskites under tailored mechanical deformation

Solid-state thermoelectric energy conversion devices attract broad research interests because of their great promises in waste heat recycling, space power generation, deep water power generation, and temperature control, but the search for essential thermoelectric materials with high performance still remains a great challenge. As an emerging low cost, solution-processed thermoelectric material, inorganic metal halide perovskites CsPb(I_1–xBr_x)₃ under mechanical deformation is systematically investigated using the first-principle calculations and the Boltzmann transport theory. It is demonstrated that halogen mixing and mechanical deformation are efficient methods to tailor electronic structures and charge transport properties in CsPb(I_1–xBr_x)₃ synergistically. Halogen mixing leads to band splitting and anisotropic charge transport due to symmetry-breaking-induced intrinsic strains. Such band splitting reconstructs the band edge and can decrease the charge carrier effective mass, leading to excellent charge transport properties. Mechanical deformation can further push the orbital energies apart from each other in a more controllable manner, surpassing the impact from intrinsic strains. Both anisotropic charge transport properties andZT values are sensitive to the direction and magnitude of strain, showing a wide range of variation from 20% to 400% (with a ZT value of up to 1.85) compared with unstrained cases. The power generation efficiency of the thermoelectric device can reach as high as approximately 12% using mixed halide perovskites under tailored mechanical deformation when the heat-source is at 500 K and the cold side is maintained at 300 K, surpassing the performance of many existing bulk thermoelectric materials.     

Prediction of the thermal conductivity anisotropy of Si nanofilms. Results of several numerical methods

The purpose of this work is to predict the in-plane and cross-plane thermal properties of crystalline silicon films. Several thicknesses from 20 nm to 6 μm and mean temperatures between 20 and 500 K have been investigated. Heat transport properties in silicon films have been studied through three different techniques: a semi-analytical method based upon the Kinetic Theory, a deterministic solution of the Boltzmann Transfer Equation (BTE) through the Discrete Ordinate Method and a statistical handling of the BTE by means of Monte Carlo Method. Each technique requires a model for the bulk material dispersion curves and the collision times of the different scattering processes. The three techniques have been validated through their correct prediction of silicon bulk thermal conductivity. Comparisons with in-plane thermal conductivity calculations and measurements have been also discussed. Thus, the cross-plane thermal conduction properties have been predicted. The expected temperature and thickness variations of the thermal conduction properties have been observed: the cross-plane thermal conduction appears to be less efficient than the in-plane thermal conduction, which proves that a significant anisotropy exists.     

Inorganic thermoelectric materials: A review

Thermoelectric generator, which converts heat into electrical energy, has great potential to power portable devices. Nevertheless, the efficiency of a thermoelectric generator suffers due to inefficient thermoelectric material performance. In the last two decades, the performance of inorganic thermoelectric materials has been significantly advanced through rigorous efforts and novel techniques. In this review, major issues and recent advancements that are associated with the efficiency of inorganic thermoelectric materials are encapsulated. In addition, miscellaneous optimization strategies, such as band engineering, energy filtering, modulation doping, and low dimensional materials to improve the performance of inorganic thermoelectric materials are reported. The methodological reviews and analyses showed that all these techniques have significantly enhanced the Seebeck coefficient, electrical conductivity, and reduced the thermal conductivity, consequently, improved ZT value to 2.42, 2.6, and 1.85 for near-room, medium, and high temperature inorganic thermoelectric material, respectively. Moreover, this review also focuses on the performance of silicon nanowires and their common fabrication techniques, which have the potential for thermoelectric power generation. Finally, the key outcomes along with future directions from this review are discussed at the end of this article.     

Thermal transport in individual thermoelectric nanowires: a review

Abstract

Extensive research efforts have been devoted to nanowires because of their novel electronic, optical and thermoelectric properties due to spatial confinement in two dimensions. Among various fields, nanowires have been of interest in the thermoelectric community not only for their novel thermoelectric properties but also for their ease of use in fundamental scientific studies as the physics learned using nanowires can be applied in bulk thermoelectric nanocomposites. In this paper, we limit our discussion to experimental thermal transport in thermoelectric nanowires such as Bi–Te, Pb–Te and Si–Ge nanowires. After reviewing the reasons why nanowires are of interest in the thermoelectric community, we discuss various synthesis methods and thermal transport measurements. Next, we evaluated how thermal transport in nanowires is affected by various scattering mechanisms such as phonon boundary scattering, alloy scattering, etc. We also discuss a recent study concerning how the surface roughness affects phonon transport. This article is useful to gain insight into how to manage thermal transport in various applications.     

Current progress on 3D graphene-based photocatalysts: From synthesis to photocatalytic hydrogen production

Hydrogen (H₂) is considered an alternative energy carrier for future clean energy systems in many applications. The three-dimensional (3D) graphene is one of the promising candidates for various applications especially in photocatalytic H₂ production due to its high electron conductivity, mechanical stability, fast electron transfer, and large surface area. Exploring the changes in the physical properties from different dimensionalities can be interesting because a 3D structure may improve the photocatalytic efficiency in terms of enhancing the light adsorption, increasing the accessible active surface, and improving the charge transport. Graphene can act as an electron acceptor and cocatalyst, and combining the graphene with metal oxides, transition metal dichalcogenides, or other semiconducting materials can enhance the photocatalytic activity of composites. Therefore, the synthesis, characterization, mechanism, and performance of the 3D graphene-based photocatalyst in the photocatalytic H₂ production are comprehensively discussed. The current progress and future challenges in the H₂ generation is also discussed in this review.     

氧化钛基半导体热电材料的研究进展

热电材料—即实现热能和电能之间直接相互转换的一类功能材料,提供了一种制冷或发电的新方法—在解决能源和环境危机问题上正在扮演越来越重要的角色。传统的三维材料中,由于几个决定热电性能的关键物理参数相互关联,使得现有热电材料很难获得较高热电优值（ZT）。金属氧化物热电材料由于其良好的耐高温性能,是中高温区使用的理想候选者。如果能提高氧化钛基化合物的热电优值,那么氧化钛基化合物将是一类非常优秀的热电材料,因为其不仅具有优良的化学稳定性和热稳定性,而且原材料丰富、不含有毒元素以及制备工艺简单。纳米化能显著降低材料的热导率,是最近二十年提高热电性能的一条主要途径。同时,通过界面和化学组成调控增加与电学性能相关的功率因子也是一种继续提高热电性能的重要方法。本文综述了我们近期对氧化钛基热电材料的研究成果,包括对钛酸盐纳米管较大赛贝克（Seebeck）系数的实验发现,提出利用一维纳米材料独特的空心结构和纳米管层状特殊构造,将两个相关联的物理参数（热导率和电导率）分别调控;通过合成氧化钛基纳米复合材料,研究界面对载流子和声子散射的作用,提出通过载流子能量过滤效应提高其热电性能;采用尿素燃烧法和高温烧结等方法合成具有纳米结构和化学组成调控的氧化钛基化合物,认识化学组成以及界面对声电输运的作用规律;最后介绍能显著提高热电材料功率因子的载流子非对称迁移的理论。     

Electronic structure and assessment of thermoelectric performance of TiCoSb

Abstract

First principles band structure calculations coupled with the Boltzmann transport theory are used to study the thermoelectric properties in TiCoSb under pressure. The density of states and band structure were studied in detail. The thermoelectric power, electrical conductivity and electronic thermal conductivity were analysed using the Boltzmann transport equation with the assumption of the constant relaxation time approximation and the rigid band model. The enhancement of the thermoelectric properties of TiCoSb by adjusting pressure is predicted.     

Recent progress about graphene for chemical energy storage applications

石墨烯独特的二维空间结构使其具有优异的导电性能,力学性能以及超大的比表面积,被认为是颇具潜力的新型储能材料,是目前储能研究的热点之一.本文综述了石墨烯在储氢,超级电容器,锂离子电池,锂硫电池以及锂-空气电池等化学储能领域中的应用,探讨了不同制备方法对其性能的影响.石墨烯以其特殊的空间结构而成为极具前景的储氢材料,同时与其它材料复合后形成三维导电网络结构而提高电极材料的电化学性能,还可以缓冲电极材料在循环过程中的体积变化,有效提升储能材料的循环寿命.通过优化复合材料的微观结构,将进一步提高其电化学性能.本文最后就石墨烯在储能应用中的关键问题进行了简要分析.     

Emerging 2D-Nanostructured materials for electrochemical and sensing Application-A review

The discovery of novel 2D-monoelemental materials with extraordinary physical, mechanical, thermal, optical and electronic properties has predicted many potential applications in various areas of technology based on the grounds of advanced sciences. The monoelemental two dimensional (2D) materials arouse a tremendous attention in different areas of science due to their unique properties and extensive applications. The 2D nanomaterials like Borophene and Bismuthene have emerged as effective nanomaterials due to their unique properties including large surface area, structural anisotropy, tunable band gap, and high carrier mobility. They are attracting increasing research interest in electronics, optoelectronics, and catalysis and also in energy storage and energy conversion applications. These materials are massively studied under theoretical approaches but many of its physical characteristics have still to be analyzed experimentally. This review article gives a detail theoretical and experimental information about the 2D-nanostructured Borophene and Bismuthene materials including their synthesis techniques, properties and also analyzed their advantages and disadvantages over each other. Further we performed an overview of the status of Borophene and Bismuthene in electrochemical and sensing applications including batteries, sensors, catalysis, and gas storage devices. Furthermore, we present our insight into the challenges, future perspective and opportunities, which would hopefully shed light on the great potential of this ever-expanding field. The nanomaterials like borophene and bismuthene have emerged as effective alternatives to graphene with excellent electrochemical properties finding potential applications in detecting and sensing devices. It is established that Borophene and Bismuthene find a large area of applications in developing conductors for electric and thermal appliances. Borophene has demonstrated incredible flexibility and high structural anisotropy and it is a material massively studied by theoretical approaches. However, many of its physical characteristics have still to be realized experimentally. In this review, we present a brief survey on preparation methods of 2D-nanostructured materials Borophene and Bismuthene. Also, an overview of the applications’ status of Borophene in electrochemical area, batteries, sensors, and catalysis and gas storage devices is covered along with an assessment on 2D-nanostructured Bismuthene being an extremely efficient electrocatalyst. It is described that Bismuthene shows a modest cyclability for Li-ion batteries, Na-ion batteries and K-ion batteries. Bismuthene is typically obtained through a low-cost liquid exfoliation synthesis method, for batteries and other energy conversion devices. In the synthesis of 2D-nanomaterials, the removal of dissolved chemicals from the reaction solutions and improving the device efficiency are still challenging. Herein, electrochemical and sensing applications of 2D-nanostructured materials, along with the advantages and disadvantages are comprehensively reviewed.     

Facile synthesis of three‐dimensional graphene networks by magnetron sputtering for supercapacitor electrode

Three‐dimensional (3D) graphene network deposition on Ni foam without any conductive agents and polymer binders was successfully synthesized by radio frequency magnetron sputtering at low temperature. The direct and close contact between graphene and Ni foam is beneficial to the enhanced conductivity of the electrode, as well as the improvement of ion diffusion/transport into the electrode. As a result, the 3D graphene network deposition on Ni foam electrode delivered a high specific capacitance of 122.0 F g⁻¹ at 1.0 A g⁻¹ and excellent cycle stability with capacitance retention of 99.0% after 1000 charge–discharge cycles. The work shows a new way to facile synthesis of 3D graphene network for various applications in the future. Copyright © 2016 John Wiley & Sons, Ltd.     

Compact energy storage: Opportunities and challenges of graphene for supercapacitors

碳纳米储能材料发展迅速,质量容量性能不断刷新。但通常碳纳米材料的密度较低,导致其体积比容量有限,在很多时候很难将材料水平上的优异性能反映到最终的器件上。发展高体积能量密度储能材料,在器件水平上实现致密储能,对推动储能材料和器件的实用化至关重要。作为其它sp2碳质材料的基本结构单元和一种柔性二维材料,石墨烯通过组装可以实现纳米结构致密化,在致密储能方面具有先天优势。本文以石墨烯在超级电容器中的应用为主,分别从材料、电极、器件3个层次讨论了实用化储能器件的设计原则,梳理了高体积能量密度碳基储能材料的研究进展,重点介绍了高体积容量碳电极材料的致密化设计理念,强调了从器件角度考虑储能材料设计的重要性,并对致密储能面临的机遇和挑战作了分析。     

Thermal transport in organic/inorganic composites

Composite materials, which consist of organic and inorganic components, are widely used in various fields because of their excellent mechanical properties, resistance to corrosion, low-cost fabrication, etc. Thermal properties of organic/inorganic composites play a crucial role in some applications such as thermal interface materials for micro-electronic packaging, nano-porous materials for sensor development, thermal insulators for aerospace, and high-performance thermoelectric materials for power generation and refrigeration. In the past few years, many studies have been conducted to reveal the physical mechanism of thermal transport in organic/inorganic composite materials in order to stimulate their practical applications. In this paper, the theoretical and experimental progresses in this field are reviewed. Besides, main factors affecting the thermal conductivity of organic/inorganic composites are discussed, including the intrinsic properties of organic matrix and inorganic fillers, topological structure of composites, loading volume fraction, and the interfacial thermal resistance between fillers and organic matrix.     

Impacts of cone-structured interface and aperiodicity on nanoscale thermal transport in Si/Ge superlattices

Si/Ge superlattices are promising thermoelectric materials to convert thermal energy into electric power. The nanoscale thermal transport in Si/Ge superlattices is investigated via molecular dynamics (MD) simulation in this short communication. The impact of Si and Ge interface on the cross-plane thermal conductivity reduction in the Si/Ge superlattices is studied by designing cone-structured interface and aperiodicity between the Si and Ge layers. The temperature difference between the left and right sides of the Si/Ge superlattices is set up for nonequilibrium MD simulation. The spatial distribution of temperature is recorded to examine whether the steady-state has been reached. As a crucial factor to quantify thermal transport, the temporal evolution of heat flux flowing through Si/Ge superlattices is calculated. Compared with the even interface, the cone-structured interface contributes remarkable resistance to the thermal transport, whereas the aperiodic arrangement of Si and Ge layers with unequal thicknesses has a marginal influence on the reduction of effective thermal conductivity. The interface with divergent cone-structure shows the most excellent performance of all the simulated cases, which brings a 33% reduction of the average thermal conductivity to the other Si/Ge superlattices with even, convergent cone-structured interfaces and aperiodic arrangements. The design of divergent cone-structured interface sheds promising light on enhancing the thermoelectric efficiency of Si/Ge based materials.     

Largely reduced cross-plane thermal conductivity of nanoporous In0.1Ga0.9N thin films directly grown by metal organic chemical vapor deposition

In recent year, nanoporous Si thin films have been widely studied for their potential applications in thermoelectrics, in which high thermoelectric performance can be obtained by combining both the dramatically reduced lattice thermal conductivity and bulk-like electrical properties. Along this line, a high thermoelectric figure of merit (ZT) is also anticipated for other nanoporous thin films, whose bulk counterparts possess superior electrical properties but also high lattice thermal conductivities. Numerous thermoelectric studies have been carried out on Si-based nanoporous thin films, whereas cost-effective nitrides and oxides are not systematically studied for similar thermoelectric benefits. In this work, the cross-plane thermal conductivities of nanoporous In_0.1Ga_0.9N thin films with varied porous patterns were measured with the time-domain thermoreflectance technique. These alloys are suggested to have better electrical properties than conventional Si_xGe_1−x alloys; however, a high ZT is hindered by their intrinsically high lattice thermal conductivity, which can be addressed by introducing nanopores to scatter phonons. In contrast to previous studies using dry-etched nanopores with amorphous pore edges, the measured nanoporous thin films of this work are directly grown on a patterned sapphire substrate to minimize the structural damage by dry etching. This removes the uncertainty in the phonon transport analysis due to amorphous pore edges. Based on the measurement results, remarkable phonon size effects can be found for a thin film with periodic 300-nm-diameter pores of different patterns. This indicates that a significant amount of heat inside these alloys is still carried by phonons with ~300 nm or longer mean free paths. Our studies provide important guidance for ZT enhancement in alloys of nitrides and similar oxides.     

Role of graphene and transition metal dichalcogenides as hole transport layer and counter electrode in solar cells

Photovoltaic (PV) technology got much attention in the past few decades in developing advanced and environment friendly solar cells (SCs). However, high cost, unstable nature, and low efficiency are major limitations towards commercialization of SCs. To overcome the issues, two-dimensional materials (2DMs) have been exploited due to low cost, high catalytic activity, fast charge separation, and better electrochemical performance. The review emphasis on (a) the electrochemical performance of graphene and transition metal dichalcogenides (TMDCs) as a hole transport layer (HTL) in SCs and (b) to explore low-cost and effective counter electrode (CE) based on graphene and TMDCs for dye-sensitized solar cell (DSSC). The review presents a comparative analysis of 2DMs as HTL and CE to attain highly efficient and low-cost PV devices. Multiple combinations of the material with graphene, graphene oxide (GO), reduced graphene oxide (rGO), tungsten disulfide (WS₂), molybdenum disulfide (MoS₂) as HTL, and CE material in PV cells are discussed and comparatively analyzed. Numerous strategies are briefly discussed to enhance the efficiency of SCs by utilizing graphene and TMDCs based HTL and CEs. The review focuses on the recent progress in developing low-cost and highly efficient PV devices by using 2DMs. Our study reveals that GO/PEDOT:PSS demonstrate a maximum power conversion efficiency (PCE) of 13.1% when fabricated at different revolutions. Moreover, our statistical analysis unveils that efficiency of the cell can be enhanced by optimizing the layer thickness, which provide a route to develop highly efficient and better performance SCs that can be exploited for future commercial applications.     

In-plane Thermal Conductivity Measurement with Nanosecond Grating Imaging Technique

We develop a nanosecond grating imaging (NGI) technique to measure in-plane thermal transport properties in bulk and thin-film samples. Based on nanosecond time-domain thermoreflectance (ns-TDTR), NGI incorporates a photomask with periodic metal strips patterned on a transparent dielectric substrate to generate grating images of pump and probe lasers on the sample surface, which induces heat conduction along both cross- and in-plane directions. Analytical and numerical models have been developed to extract thermal conductivities in both bulk and thin-film samples from NGI measurements. This newly developed technique is used to determine thickness-dependent in-plane thermal conductivities (κ_x) in Cu nano-films, which agree well with the electron thermal conductivity values converted from four-point electrical conductivity measurements using the Wiedemamn–Franz law, as well as previously reported experimental values. The κ_x measured with NGI in an 8 nm x 8 nm GaAs/AlAs superlattice (SL) is about 10.2 W/m?K, larger than the cross-plane thermal conductivity (8.8 W/m?K), indicating the anisotropic thermal transport in the SL structure. The uncertainty of the measured κ_x is about 25% in the Cu film and less than 5% in SL. Sensitivity analysis suggests that, with the careful selection of proper substrate and interface resistance, the uncertainty of κ_x in Cu nano-films can be as low as 5%, showing the potential of the NGI technique to determine κ_x in thin films with improved accuracy. By simply installing a photomask into ns-TDTR, NGI provides a convenient, fast, and cost-effective method to measure the in-plane thermal conductivities in a wide range of structures and materials.