Progress in Measurement of Thermoelectric Properties of Micro/Nano Thermoelectric Materials: A Critical Review

Wearable thermoelectric generators (WTEGs) can incessantly convert body heat into electricity to power electronics. However, the low efficiency of thermoelectric materials, tiny terminal temperature difference, rigidity, and neglecting optimization of lateral heat transfer preclude WTEGs from broad utilization. In this review, we aim to comprehensively summarize the state-of-the-art strategies for the realization of flexibility and high normalized power density in thermoelectric generators by establishing the links among materials, TE performance, and advanced design of WTEGs (structure, heatsinks, thermal regulation, thermal analysis, etc.) based on inorganic bulk TE materials. Each section starts with a concise summary of its fundamentals and carefully selected examples. In the end, we point out the controversies, challenges, and outlooks toward the future development of wearable thermoelectric devices and potential applications. Overall, this review will serve to help materials scientists, electronic engineers, particularly students and young researchers, in selecting suitable thermoelectric devices and potential applications.

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Review on Wearable Thermoelectric Generators: From Devices to Applications

For organic memristors, non-zero-crossing current-voltage (I-V) curves are often observed, which can be attributed to capacitive effects. If the conversion between the capacitance-dominated state (CDS) and the memristance-dominated state (MDS) can be realized in a controllable manner, more device functions can be obtained. In this work, a two-terminal memristor using a common organic dye, azobenzene (AZB), as the active layer was prepared. It is found that as the applied voltage gradually increases, the device can transition from CDS to MDS. In the low voltage range (<1 V), the device is in CDS, and the capacitance is significantly increased by ∼104 compared to the theoretical value. In the high voltage range (>1 V), the device is in MDS, achieving an HRS (high resistance state)/LRS (low resistance state) resistance ratio of ∼104, and the logic operations are achieved. Through the analysis of the I-V curve, energy diagram of the materials, and computer simulation results, the mechanisms of CDS, MDS, and their conversion are proposed. This work provides an in-depth understanding of the working mechanism of organic memristors and demonstrates the potential of AZB-based organic memristors for information storage and logic display applications.

Voltage-Controlled Conversion from CDS to MDS in an Azobenzene-Based Organic Memristor for Information Storage and Logic Operations.

There is an urgent need for alternative compact technologies that can derive and store energy from the sun, especially the large amount of solar heat that is not effectively used for power generation. Here, we report a combination of solution- and neat-film-based molecular solar thermal (MOST) systems, where solar energy can be stored as chemical energy and released as heat, with microfabricated thermoelectric generators to produce electricity when solar radiation is not available. The photophysical properties of two MOST couples are characterized both in liquid with a catalytical cycling setup and in a phase-interconvertible neat film. Their suitable photophysical properties let us combine them individually with a microelectromechanical ultrathin thermoelectric chip to use the stored solar energy for electrical power generation. The generator can produce, as a proof of concept, a power output of up to 0.1 nW (power output per unit volume up to 1.3 W m−3). Our results demonstrate that such a molecular thermal power generation system has a high potential to store and transfer solar power into electricity and is thus potentially independent of geographical restrictions. 

Chip-scale solar thermal electrical power generation

Nanostructure engineering is a key strategy for tailoring properties in the fields of batteries, solar cells, thermoelectrics, and so on. Limited by grain coarsening, however, the nanostructure effect gradually degrades during the materials’ manufacturing and in‐service period. Herein, a strategy of cleavage‐fracture for grain shrinking is developed in a Pb0.98Sb0.02Te sample during sintering, and the grain size remains stable after repeated tests. Moreover, the initial grain boundary is filled by fractured slender grains and enriched by dislocations, evolving into a hierarchical grain‐boundary structure. The lattice thermal conductivity (klat) is greatly reduced to approach the amorphous limit. As a result, a record‐high ZT value of ≈1.9 is obtained at 815 K in the n‐type Pb0.98Sb0.02Te sample and a decent efficiency of 6.7% in thermoelectric device. This strategy for grain shrinking will shed light on the application of nanostructure engineering under high temperature and extreme conditions in other material systems.

Dramatic Enhancement of Thermoelectric Performance in PbTe by Unconventional Grain Shrinking in the Sintering Process

Thermoelectric converter: Strategies from materials to device application

Passive radiative cooling is an emerging energy-free pathway that radiates the emission of heat from the earth to cold space (3 K) through an atmosphere transparent window (7–14 µm). Recently, this technology has attracted great interest in residential and industrial applications. However, critical challenges remain for 24-h radiative cooling due to the unwanted solar heating effect in the daytime. The complex manufacturing process and high price also result in a cost barrier for large-scale applications. Here, an easy-prepared and cost-effective radiative cooler (RC) was processed which consists of polydimethylsiloxane (PDMS) and SiO2 microspheres based on cellulose nanofibers (filter paper) with a backing silver (Ag) mirror for mass production. This whitish hybrid material exhibited high reflectance of 91% in the AM 1.5 solar spectrum (0.2–2.5 µm) and high thermal emissivity of 90% in the atmospheric window, which was enhanced by the randomized multiphase interfaces. Moreover, it can accomplish 24-h continuous outdoor cooling with an efficient average temperature reduction of 5 °C, especially 6.5 °C in the nighttime. Overall, this work provided a new approach to promote 24-h radiative cooling as a next-generation green energy technology. 

Cost effective 24-h radiative cooler with multiphase interface enhanced solar scattering and thermal emission

Photothermal conversion is an environmentally friendly process that harvests energy from the sun and has been attracting growing research interest in recent years. However, nanostructured strategies to improve light capture performance deserve further development, and the application of solar heating effects for clean energy needs to be explored. Herein, a multiscale nanomaterial was prepared by in situ polymerizing the polyaniline (PANI) nanoparticles into porous anodic aluminum oxide (AAO) membrane. As a result, the as-prepared PANI-AAO shows broadband solar absorption and provides a platform for efficient photothermal conversion. What is more, we introduced a typical thermoelectricity generator (TEG) with excellent output performance and combined it with PANI-AAO to prepare a solar thermoelectric generator (s-TEG). The s-TEG harvests solar energy and converts it into electricity, showing an outstanding power generation capability in outdoor conditions. Thus, the nanostructured broadband solar absorber and the integrated solar thermoelectric generator offer a promising candidate for a sustainable and green energy source in the future.

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Nanostructured Broadband Solar Absorber for Effective Photothermal Conversion and Electricity Generation

Environmental energy source is abundant, inexhaustible, ubiquitous, and free. However, harvesting thermal energy from the environment to generate uninterrupted electricity is still challenging. Herein, a power device to simultaneously harvest energy from the sun and cold space based on a microfabricated thermoelectric generator (TEG) integrated with a solar absorber (SA) and radiative cooling emitter (RCE) is reported. Nano‐channel arrays structure is introduced in SA to achieve high broadband light absorption (≈96%) over the entire solar spectrum. Then, a typical RCE is fabricated to demonstrate the great potential of cooperating with SA to create a continuous temperature difference for thermal energy harvesting. Furthermore, a TEG sandwiched between SA and RCE can convert the thermal energy into electricity, which is proved by a chip‐integrated micro‐TEG experimentally. What is more, two self‐generation power devices are designed, and the power generation of the reverse structure demo device (r‐TEG) is 130% of the forward one (f‐TEG) in the daytime and 260% in the nighttime. The results demonstrate a renewable and sustainable thermodynamic green resource on chips for power generation independent of time and geographical restrictions, which is vital for promoting the sun and cold space as viable energy sources beyond traditional technologies.

Shuai Zhang

Papers

Thermoelectric converter: Strategies from materials to device application

Cost effective 24-h radiative cooler with multiphase interface enhanced solar scattering and thermal emission

Nanostructured Broadband Solar Absorber for Effective Photothermal Conversion and Electricity Generation

Power Generation on Chips: Harvesting Energy From the Sun and Cold Space

Bipolar Resistive Switching in the Ag/Sb2Te3/Pt Heterojunction