The world is currently going through an unprecedented energy crisis. Climate change and the need to reduce CO2 emissions are forcing us compelling us to seek alternative energy sources.

We must also reduce energy losses such as heat losses, given that most of the energy generated by conversion is partly lost in the form of exothermic energy. As a result, much of the generated energy is dissipated as heat. Further research is, therefore, essential to capture and reconvert this wasted heat back into electrical energy. For this reason, converting heat into electricity is now a key research area.

Thermoelectric modules have attracted considerable interest in recent years due to their great potential to generate electricity from temperature differences in devices and laptops. Thermoelectric devices convert thermal energy into electrical energy using the Seebeck effect. 

These devices are based on the properties of certain materials that transform heat into an electric voltage when subjected to a range of temperatures. Conversely, upon the application of voltage to these devices, one side heats up while the other side cools down. Electrons move from the hotter end of the material to the colder end, thus creating positive and negative electrodes and electrical voltage. This process, known as the Peltier-Seebeck effect, is fully reversible and is a property of a very limited range of materials.

Image 1: Estimated energy consumed in the US in 2018 Source: Desjardins, J. (2019). “Visualizing US Energy Use in One Giant Chart.”

In thermoelectric devices, the primary materials employed p- and n-type semiconductors. The most commonly used semiconductor materials in these devices are bismuth telluride, antimony telluride and silicon-germanium which have either been heavily doped (n-type) or lightly doped (p-type). The fundamental challenge with creating efficient thermoelectric materials lies in their need to be very good at conducting electricity while minimizing heat conduction.

The vast majority of these materials are made of metal alloys. Certain metal alloys, such as silicon-germanium compounds, can also be employed for the fabrication of thermoelectric devices. The best performers are currently epitaxial multilayer structures based on Sb2Te3/Bi2Te3, as described in the article “Thin-film thermoelectric devices with high room-temperature figures of merit” (R. Venkatasubramanian et al., 2001), and quantum dot superlattices based on the inclusion of PbTeSe on a PbTe matrix, which are discussed in “Quantum Dot Superlattice Thermoelectric Materials and Devices” (T. C. Harman et al., 2002).

In terms of structures, the different kinds of thermoelectric devices include solid-state cells. This module should mainly have heat dissipation devices such as radiators and extended surfaces. Other cells under development are hybrid with one component being solid and the other printed. Further research is ongoing over other potential hybrids that are currently being studied to optimize the cylindrical or flat surface where the thermoelectric device is applied.

Image 2: Hybrid Thermoelectric Device Source: Ecoinventos, 2022, “Dispositivos termoeléctricos flexibles que permiten la captura de energía de la piel humana”

One example is described in the article, “Enhanced Electrical Transport Properties via Defect Control for Screen-Printed Bi2Te3 Films over a Wide Temperature Range” (Jingjing Feng et al., 2020). This paper discusses screen-printed thin-film thermoelectric devices, which are still in the early stages of development mainly due to the limited performance of screen-printed thermoelectric devices and, especially, poor electrical transport properties.

A high-performance screen-printed Bi2Te3 thin film is designed and prepared with the introduction of tellurium-based excessive nano-soldering (Te-NS) to simultaneously create the conduction channel and perform defect control. the promoted carrier migration causes the electrical conductivity to increase by a factor of 7, with a power factor of 4.65 W cm-1 K-2. Meanwhile, the defect formation mechanism in the screen-printed Bi2Te3 film after the introduction of Te-NS was also studied in-depth, and the bipolar conduction was reduced by increased generation of TeBi and/or more suppression of BiTe′, resulting in a postponed temperature of the maximum Seebeck coefficient. 

Therefore, the large engineering power factor is achieved with excellent temperature linearity, indicating a possibility of screen-printed film application in a large temperature region. An open-circuit voltage of 11.34 mV and maximum output power of 27.1 μW at a temperature gradient of 105 K has been achieved over a wide temperature range from 303 to 478 K. This study provides a theoretical and practical basis for improving the performance of screen-printed thermoelectric lamps and devices.

Image 3: Thermoelectric device with Te-NS nano-soldering Source: Jingjing Feng et al., 2020, “Enhanced Electrical Transport Properties via Defect Control for Screen-Printed Bi2Te3 Films over a Wide Temperature Range”

However, most of the materials currently used are highly polluting, which is why research is being done to identify alternative solutions with less polluting materials that can be recycled and whose efficiency is close to that of materials with a higher performance level (the Seebeck coefficient of bismuth telluride is 300 μV/K). 

As an alternative to traditionally used materials, research lines have been opened and the first developments made based on conductive polymers such as PEDOT: PSS, an intrinsic conductive polymer that has aroused much curiosity since the discovery of its thermoelectric functions. It may be used to create new thermoelectric devices combining it with carbon-based nanomaterials such as carbon nanotubes (CNTs) and single-walled carbon nanotubes (SWCNTs), as well as metal alloys and organic compounds. 

In the case of the n-type single-walled carbon nanotubes (SWCNTs) mentioned above, the paper “Dual-Type Flexible-Film Thermoelectric Generators Using All-Carbon Nanotube Films” (Ryota Konagayay et al., 2023), discusses the increased performance of this material. The dual-type flexible-film thermoelectric generator (DFTEG) has reached an output voltage of 40 mV and a maximum power of 891 nW for a temperature difference of 25 K between the cold and client side of the module. Low yields have been achieved compared to those obtained with traditional Te, Bi and Ge materials, but this is a promising result for the development of thermoelectric modules with non-polluting materials.

Image 4. Schematic diagram of DFTEG manufacturing process Source: Ryota Konagayay and Masayuki Takashiri, 2023, “Dual-Type Flexible-Film Thermoelectric Generators Using All-Carbon Nanotube Films”

Therefore, the new generation thermoelectric devices, characterized by their sustainability, flexibility, and utilization of lightweight materials, have many potential applications for the self-supply of electricity. Within the realm of electronics, these devices could be used to power wearable devices such as smart watches and fitness trackers. In the field of sensor technology, these devices could be used to power temperature, pressure and humidity sensors.

As thermoelectric technology continues to advance, new materials and manufacturing methods are expected to be developed to improve the efficiency and scalability of these devices, and solutions for greater efficiency and energy use should be found.