Bioinspired Light-Driven Proton Pump: Engineering Band Alignment of WS2 with PEDOT:PSS and PDINN

Bioinspired two-dimensional (2D) nanofluidic systems for photo-induced ion transport have attracted great attention, as they open a new pathway to enabling light-to-ionic energy conversion. However, there is still a great challenge in achieving a satisfactory performance. It is noticed that organic solar cells (OSCs, light-harvesting device based on photovoltaic effect) commonly require hole/electron transport layer materials (TLMs), PEDOT:PSS (PE) and PDINN (PD), respectively, to promote the energy conversion. Inspired by such a strategy, an artificial proton pump by coupling a nanofluidic system with TLMs is proposed, in which the PE- and PD-functionalized tungsten disulfide (WS2) multilayers construct a heterogeneous membrane, realizing an excellent output power of ≈1.13 nW. The proton transport is fine-regulated due to the TLMs-engineered band structure of WS2. Clearly, the incorporating TLMs of OSCs into 2D nanofluidic systems offers a feasible and promising approach for band edge engineering and promoting the light-to-ionic energy conversion.

Thermoelectric Characteristics of Self-Supporting WSe2-Nanosheet/PEDOT-Nanowire Composite Films

Conducting polymer poly(3,4-ethylenedioxythiophene) nanowires (PEDOT NWs) were synthesized by a modified self-assembled micellar soft-template method, followed by fabrication by vacuum filtration of self-supporting exfoliated WSe₂-nanosheet (NS)/PEDOT-NW composite films. The results showed that as the mass fractions of WSe₂ NSs increased from 0 to 20 wt % in the composite films, the electrical conductivity of the samples decreased from ∼1700 to ∼400 S cm^-1, and the Seebeck coefficient increased from 12.3 to 23.1 μV K^-1 at 300 K. A room-temperature power factor of 44.5 μW m^-1 K^-2 was achieved at 300 K for the sample containing 5 wt % WSe₂ NSs, and a power factor of 67.3 μW m^-1 K^-2 was obtained at 380 K. The composite film containing 5 wt % WSe₂ NSs was mechanically flexible, as shown by its resistance change ratio of 7.1% after bending for 500 cycles at a bending radius of 4 mm. A flexible thermoelectric (TE) power generator containing four TE legs could generate an output power of 52.1 nW at a temperature difference of 28.5 K, corresponding to a power density of ∼0.33 W/m². This work demonstrates that the fabrication of inorganic nanosheet/organic nanowire TE composites is an approach to improve the TE properties of conducting polymers.

Electronic and surface modulation of 2D MoS2nanosheets for an enhancement on flexible thermoelectric property

Two-dimensional materials have potential applications for flexible thermoelectric materials because of their excellent mechanical and unique electronic transport properties. Here we present a functionalization method by a Lewis acid-base reaction to modulate atomic structure and electronic properties at surface of the MoS₂nanosheets. By AlCl₃solution doping, the lone pair electronics from S atoms would enter into the empty orbitals of Al³⁺ions, which made the Fermi level of the 1T phase MoS₂move towards valence band, achieving a 1.8-fold enhancement of the thermoelectric power factor. Meanwhile, benefiting from the chemical welding effect of Al³⁺ions, the mechanical flexibility of the nanosheets restacking has been improved. We fabricate a wearable thermoelectric wristband based on this improved MoS₂nanosheets and achieved 5 mV voltage output when contacting with human body. We think this method makes most of the transition metal chalcogenides have great potential to harvest human body heat for supplying wearable electronic devices due to their similar molecular structure.

Ink-jet printing and drop-casting deposition of 2H-phase SnSe2and WSe2nanoflake assemblies for thermoelectric applications

The development of simple, scalable, and cost-effective methods to prepare Van der Waals materials for thermoelectric applications is a timely research field, whose potential and possibilities are still largely unexplored. In this work, we present a systematic study of ink-jet printing and drop-casting deposition of 2H phase SnSe₂and WSe₂nanoflake assemblies, obtained by liquid phase exfoliation, and their characterization in terms of electronic and thermoelectric properties. The choice of optimal annealing temperature and time is crucial for preserving phase purity and stoichiometry and for removing dry residues of ink solvents at inter-flake boundaries, while maximizing the sintering of nanoflakes. An additional pressing is beneficial to improve nanoflake orientation and packing, thus enhancing electric conductivity. In nanoflake assemblies deposited by drop casting and pressed at 1 GPa, we obtained thermoelectric power factors at room temperature up to 2.2 × 10^-4mW m^-1K^-2for SnSe₂and up to 3.0 × 10^-4mW m^-1K^-2for WSe₂.

Wearable Thermoelectric Materials and Devices for Self-Powered Electronic Systems

The emergence of artificial intelligence and the Internet of Things has led to a growing demand for wearable and maintenance-free power sources. The continual push toward lower operating voltages and power consumption in modern integrated circuits has made the development of devices powered by body heat finally feasible. In this context, thermoelectric (TE) materials have emerged as promising candidates for the effective conversion of body heat into electricity to power wearable devices without being limited by environmental conditions. Driven by rapid advances in processing technology and the performance of TE materials over the past two decades, wearable thermoelectric generators (WTEGs) have gradually become more flexible and stretchable so that they can be used on complex and dynamic surfaces. In this review, the functional materials, processing techniques, and strategies for the device design of different types of WTEGs are comprehensively covered. Wearable self-powered systems based on WTEGs are summarized, including multi-function TE modules, hybrid energy harvesting, and all-in-one energy devices. Challenges in organic TE materials, interfacial engineering, and assessments of device performance are discussed, and suggestions for future developments in the area are provided. This review will promote the rapid implementation of wearable TE materials and devices in self-powered electronic systems.

Thermal and Photo Sensing Capabilities of Mono- and Few-Layer Thick Transition Metal Dichalcogenides

Two-dimensional (2D) materials have shown promise in various optical and electrical applications. Among these materials, semiconducting transition metal dichalcogenides (TMDs) have been heavily studied recently for their photodetection and thermoelectric properties. The recent progress in fabrication, defect engineering, doping, and heterostructure design has shown vast improvements in response time and sensitivity, which can be applied to both contact-based (thermocouple), and non-contact (photodetector) thermal sensing applications. These improvements have allowed the possibility of cost-effective and tunable thermal sensors for novel applications, such as broadband photodetectors, ultrafast detectors, and high thermoelectric figures of merit. In this review, we summarize the properties arisen in works that focus on the respective qualities of TMD-based photodetectors and thermocouples, with a focus on their optical, electrical, and thermoelectric capabilities for using them in sensing and detection.

2D and 3D nanostructuring strategies for thermoelectric materials

Thermoelectric materials have attracted increased research attention as the implementation of various nanostructures has potential to improve both their performance and applicability. A traditional limitation of thermoelectric performance in bulk materials is the interconnected nature of the individual parameters (for example, it is difficult to decrease thermal conductivity while maintaining electrical conductivity), but through the rational design of nanoscale structures, it is possible to decouple these relationships and greatly enhance the performance. For 2D strategies, newly investigated materials such as graphene, transition metal dichalcogenides, black phosphorus, etc. are attractive thanks to not only their unique thermoelectric properties, but also potential advantages in ease of processing, flexibility, and lack of rare or toxic constituent elements. For 3D strategies, the use of induced porosity, assembly of various nanostructures, and nanoscale lithography all offer specific advantages over bulk materials of the same chemical composition, most notably decreased thermal conductivity due to phonon scattering and enhanced Seebeck coefficient due to energy filtering. In this review, a general summary of the popular techniques and strategies for 2D and 3D thermoelectric materials will be provided, along with suggestions for future research directions based on the observed trends.

Flexible Thermoelectric Materials and Generators: Challenges and Innovations

The urgent need for ecofriendly, stable, long-lifetime power sources is driving the booming market for miniaturized and integrated electronics, including wearable and medical implantable devices. Flexible thermoelectric materials and devices are receiving increasing attention, due to their capability to convert heat into electricity directly by conformably attaching them onto heat sources. Polymer-based flexible thermoelectric materials are particularly fascinating because of their intrinsic flexibility, affordability, and low toxicity. There are other promising alternatives including inorganic-based flexible thermoelectrics that have high energy-conversion efficiency, large power output, and stability at relatively high temperature. Herein, the state-of-the-art in the development of flexible thermoelectric materials and devices is summarized, including exploring the fundamentals behind the performance of flexible thermoelectric materials and devices by relating materials chemistry and physics to properties. By taking insights from carrier and phonon transport, the limitations of high-performance flexible thermoelectric materials and the underlying mechanisms associated with each optimization strategy are highlighted. Finally, the remaining challenges in flexible thermoelectric materials are discussed in conclusion, and suggestions and a framework to guide future development are provided, which may pave the way for a bright future for flexible thermoelectric devices in the energy market.

One-step Synthesis and Enhanced Thermoelectric Properties of Polymer-Quantum Dot Composite Films

Conventional syntheses of polymer-inorganic composite thermoelectric materials suffer major problems such as inhomogeneity, large particle size, and oxidation that result in ineffective loading. Now a one-step synthesis can be used to fabricate high-quality small-sized anions codoped poly(3,4-ethylenedioxythiophene):dodecylbenzenesulfonate/Cl-tellurium (PEDOT:DBSA/Cl-Te) composite films using a series of novel Te^IV -based oxidants. The synchronized production of PEDOT and Te results in thick and homogeneous films containing evenly distributed and well-protected Te quantum dots. Owing to the heavily doped crystalline polymer matrix as well as the <5 nm unoxidized Te quantum dot loading, at low Te concentrations as 2.1-5.8 wt %, the films exhibits high power factors of about 100 μW m^-1  K^-2 , which is 50 % higher compared to a pure PEDOT:DBSA film.

Low-Cost Black Phosphorus Nanofillers for Improved Thermoelectric Performance in PEDOT:PSS Composite Films

In recent years, two-dimensional black phosphorus (BP) has seen a surge of research because of its unique optical, electronic, and chemical properties. BP has also received interest as a potential thermoelectric material because of its high Seebeck coefficient and excellent charge mobility, but further development is limited by the high cost and poor scalability of traditional BP synthesis techniques. In this work, high-quality BP is synthesized using a low-cost method and utilized in a PEDOT:PSS film to create the first ever BP composite thermoelectric material. The thermoelectric properties are found to be greatly enhanced after the BP addition, with the power factor of the film, with 2 wt % BP (36.2 μW m^-1 K^-2) representing a 109% improvement over the pure PEDOT:PSS film (17.3 μW m^-1 K^-2). A simultaneous increase of mobility and decrease of the carrier concentration is found to occur with the increasing BP wt %, which allows for both Seebeck coefficient and electrical conductivity to be increased. These results show the potential of this low-cost BP for use in energy devices.

Hydrothermal synthesis of stable metallic 1T phase WS2 nanosheets for thermoelectric application

Two-dimensional materials have gained great attention as a promising thermoelectric (TE) material due to their unique density of state with confined electrons and holes. Here, we synthesized 1T phase tungsten disulfide (WS₂) nanosheets with high TE performance via the hydrothermal method. Flexible WS₂ nanosheets restacked thin films were fabricated by employing the vacuum filtration technique. The measured electrical conductivity was 45 S cm^-1 with a Seebeck coefficient of +30 μV K^-1 at room temperature, indicating a p-type characteristic. Furthermore, the TE performance could be further improved by thermal annealing treatment. It was found the electrical conductivity could be enhanced 2.7 times without sacrificing the Seebeck coefficient, resulting in the power factor of 9.40 μW m^-1 K^-2. Moreover, such 1T phase WS₂ nanosheets possess high phase stability since the TE properties maintained constant at least half one year in the air atmosphere. Notably, other kinds of 1T phase transitional metal dichalcogenides (TMDCs) with excellent TE performance also could be imitated by using the procedure in this work. Finally, we believe a variety of materials based on 1T phase TMDCs nanosheets have great potential as candidate for future TE applications.