Ultrathin Crystalline Silicon Nano and Micro Membranes with High Areal Density for Low-Cost Flexible Electronics

Ultrathin crystalline silicon is widely used as an active material for high-performance, flexible, and stretchable electronics, from simple passive and active components to complex integrated circuits, due to its excellent electrical and mechanical properties. However, in contrast to conventional silicon wafer-based devices, ultrathin crystalline silicon-based electronics require an expensive and rather complicated fabrication process. Although silicon-on-insulator (SOI) wafers are commonly used to obtain a single layer of crystalline silicon, they are costly and difficult to process. Therefore, as an alternative to SOI wafers-based thin layers, here, a simple transfer method is proposed for printing ultrathin multiple crystalline silicon sheets with thicknesses between 300 nm to 13 µm and high areal density (>90%) from a single mother wafer. Theoretically, the silicon nano/micro membrane can be generated until the mother wafer is completely consumed. In addition, the electronic applications of silicon membranes are successfully demonstrated through the fabrication of a flexible solar cell and flexible NMOS transistor arrays.

Synthesis and structural characterization of the new Zintl phases Eu10Mn6Bi12 and Yb10Zn6Sb12

Two new ternary compounds, Eu₁₀Mn₆Bi₁₂ and Yb₁₀Zn₆Sb₁₂, were synthesized and structurally characterized. The synthesis was achieved either through reactions in sealed niobium tubes or in alumina crucibles by combining the elements in excess molten Sb. Their structures were elucidated using single-crystal X-ray diffraction, and they were determined to crystallize in the orthorhombic space group Cmmm (no. 65) with the Eu₁₀Cd₆Bi₁₂ structure type. Akin to the archetype phase, both Mn and Zn sites contain about 25% of vacancies. The anionic substructure of the title phases can be described as [M₆Pn₁₂] (M = Zn, Mn; Pn = Sb, Bi) double layers composed of the corner and edge-sharing [MPn₄] tetrahedra, linked by [Pn₂]^4- dumbbells. Eu²⁺/Yb²⁺ cations fill the space between the layers, with the valence electron counts adhering closely to the Zintl-Klemm rules, i.e., both Eu₁₀Mn₆Bi₁₂ and Yb₁₀Zn₆Sb₁₂ are expected to be valence-precise compounds. Analysis of the electronic structure and transport properties of Yb₁₀Zn₆Sb₁₂ indicate semimetallic behavior with relatively low Seebeck coefficient and resistivity that slightly decreases as a function of temperature.

Recent Progress in Designing Thermoelectric Metal-Organic Frameworks

Thermoelectrics that enable direct heat-electricity conversion possess unique advantages for green and renewable energy revolution and have received rapidly growing attention in the past decade. Among various thermoelectric materials, metal-organic frameworks (MOFs) with intrinsic high porosity and tunable physical/chemical properties are emerging as a promising class of materials that have been demonstrated to exhibit many unique merits for thermoelectric applications. Their structural topologies and thermoelectric properties can be facilely regulated by precisely selecting and arranging metal centers and organic ligands. Besides, a large variety of guest molecules can be incorporated within their pores, giving rise to novel avenues of raising energy-conversion efficiency. This review focuses on the recent advances in designing conductive MOFs and MOF-based composites for thermoelectric applications. It first introduces the fundamental thermoelectric parameters and the underlying regulation mechanisms specifically effective for MOFs, then summarizes the related studies conducted in recent years, where the structural design strategies of tuning thermoelectric properties are demonstrated and discussed. In the final part, conclusions and perspectives with the envision of an outlook for this promising area are presented.

Impact of noble-gas filler atoms on the lattice thermal conductivity of CoSb3skutterudites: first-principles modelling

We present a systematic first-principles modelling study of the structural dynamics and thermal transport in CoSb₃skutterudites with a series of noble-gas filler atoms. Filling with chemically-inert atoms provides an idealised model for isolating the effects of the fillers from the impact of redox changes to the host electronic structure. A range of analysis techniques are proposed to estimate the filler rattling frequencies, to quantify the separate impacts of the fillers on the phonon group velocities and lifetimes, and to show how changes to the phonon spectra and interaction strengths lead to suppressed lifetimes. The noble-gas fillers are found to reduce the thermal conductivity of the CoSb₃framework by up to 15% primarily by suppressing the group velocities of low-lying optic modes. The filler rattling frequencies are determined by a detailed balance of increasing atomic mass and stronger interactions with the framework, and are found to be a good predictor of the impact on the heat transport. Lowering the rattling frequency below ∼1.5 THz by selecting heavy fillers that interact weakly with the framework is predicted to lead to a much larger suppression of the thermal transport, by inducing avoided crossings in the acoustic-mode dispersion and facilitating enhanced scattering and a consequent large reduction in phonon lifetimes. Approximate rattling frequencies determined from the harmonic force constants may therefore provide a useful metric for selecting filler atoms to optimise the thermal transport in skutterudites and other cage compounds such as clathrates.

Effect of iodine doping on the electrical, thermal and mechanical properties of SnSe for thermoelectric applications

We report the evolution of the thermoelectric and mechanical properties of n-type SnSe obtained by iodine doping at the Se site. The thermoelectric performance of n-type SnSe is detailed in the temperature range starting from 150 K ≤ T ≤ 700 K. The power factor of 0.25% iodine doped SnSe is found to be 0.33 mW m-1 K-2 at 700 K, comparable to that of the other monovalent doped n-type SnSe. The temperature-dependent electrical conductivity of the undoped and iodine doped SnSe samples is corroborated by using the adiabatic small polaron hopping model. A very low value of thermal conductivity, 0.62 W m-1 K-1, is obtained at 300 K and is comparable to that of SnSe single crystals. The low thermal conductivity of n-type polycrystalline SnSe is understood by taking into account the anharmonic phonon vibrations induced by the incorporation of heavy iodine atoms at the Se sites as well as the structural hierarchy of the compound. Besides, iodine doping is found to improve the reduced Young's modulus and hardness values of SnSe, which is highly desirable for thermoelectric device applications.

Enhancing Thermoelectric Performance of Polyaniline/Single-Walled Carbon Nanotube Composites via Dimethyl Sulfoxide-Mediated Electropolymerization

The fabrication of flexible high-performance organic/inorganic thermoelectric (TE) composite films has been a hot spot for researchers in recent years. In this work, dynamic 3-phase interfacial electropolymerization of aniline, together with physical mixing with single-walled carbon nanotubes (SWCNTs), was adopted to prepare polyaniline/SWCNT (PANI/SWCNT) TE composites. The dimethyl sulfoxide (DMSO) added into the electrochemical polymerization system affords strong capability in improving the TE performance of composite films. Moreover, varying loadings of SWCNTs can also conveniently tune the TE performance of composites. Hence, the resultant composites afford the highest power factor (PF) of 236.4 ± 5.9 μW m^-1 K^-2 at room temperature. This work demonstrates that the introduction of DMSO into the electrolyte and the electrochemical polymerization are highly effective in fabricating high-performance PANI/SWCNT TE composites.

Generation of multi-dimensional defect structures for synergetic engineering of hole and phonon transport: enhanced thermoelectric performance in Sb and Cu co-doped GeTe

The thermoelectric performance of GeTe can be enhanced by Sb/Cu codoping due to the generation of complex defect structures.

Enhanced Output Performance of All-Solution-Processed Organic Thermoelectrics: Spray Printing and Interface Engineering

We report two organocompatible strategies to enhance the output performance of all-solution-processed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thermoelectric generators (TEGs): introducing an additive spray printing process and functionalized polymer interlayers to reduce the module resistance. The spray printing enabled the deposition of 1-μm-thick PEDOT:PSS layers with a high degree of design freedom, resulting in a significantly reduced sheet resistance of 16 Ω sq^-1 that is closely related to the thermoelectric output performance. Also, by inserting an ultrathin silane-terminated polystyrene (PS) interlayer between the PEDOT:PSS thermoelectric layers and inkjet-printed Ag interconnects selectively, the contact resistivity extracted by the transmission line method was reduced from 6.02 × 10^-2 to 2.77 × 10^-2 Ω cm². We found that the PS interlayers behaved as a thin tunneling layer, which facilitated the carrier injection from the inkjet-printed Ag electrodes into the PEDOT:PSS films by field emission with an effectively lowered energy barrier. The activation energy was also extracted using the Richardson equation, resulting in a reduction of 2.59 ± 0.04 meV after the PS treatment. Scalable plastic-compatible processability and selective interface engineering enabled to demonstrate the flexible 74-leg PEDOT:PSS TEGs exhibiting the open-circuit voltage of 9.21 mV and the output power of 2.23 nW at a temperature difference of 10 K.

Enhanced Thermoelectric Cooling through Introduction of Material Anisotropy in Transverse Thermoelectric Composites

Transverse thermoelectric materials can achieve appreciable cooling power with minimal space requirement. Among all types of material candidates for transverse thermoelectric applications, composite materials have the best cooling performance. In this study, anisotropic material properties were applied to the component phase of transverse thermoelectric composites. A mathematical model was established for predicting the performance of fibrous transverse thermoelectric composites with anisotropic components. The mathematical model was then validated by finite element analysis. The thermoelectric performance of three types of composites are presented, each with the same set of component materials. For each type of component, both anisotropic single-crystal and isotropic polycrystal material properties were applied. The results showed that the cooling capacity of the system was improved by introducing material anisotropy in the component phase of composite. The results also indicated that the orientation of the anisotropic component's property axis, the anisotropic characteristic of a material, will significantly influence the thermoelectric performance of the composite. For a composite material consisting of Copper fiber and Bi2Te3 matrix, the maximum cooling capacity can vary as much as 50% at 300 K depending on the property axis alignment of Bi2Te3 in the composite. The composite with Copper and anisotropic SnSe single crystal had a 51% improvement in the maximum cooling capacity compared to the composite made of Copper and isotropic SnSe polycrystals.

One-Dimensional Nanostructure Engineering of Conducting Polymers for Thermoelectric Applications

The past few decades have witnessed considerable progress of conducting polymer-based organic thermoelectric materials due to their significant advantages over the traditional inorganic materials. The nanostructure engineering and performance investigation of these conducting polymers for thermoelectric applications have received considerable interest but have not been well documented. This review gives an outline of the synthesis of various one-dimensional (1D) structured conducting polymers as well as the strategies for hybridization with other nanomaterials or polymers. The thermoelectric performance enhancement of these materials in association with the unique morphologies and structures are discussed. Finally, perspectives and suggestions for the future research based on these interesting nanostructuring methodologies for improvement of thermoelectric materials are also presented.