Organic-based flexible thermoelectric generators: From materials to devices

Abstract

There is a rapidly growing demand for self-powered technologies in wearable electronic devices that can be integrated with the human body to accomplish various functions. Flexible thermoelectric generators produce electricity from temperature differences and demonstrate huge potential as a power source for these devices. Organic materials have attracted extensive attention in the field of flexible thermoelectric generators due to their unique benefits such as lower prices, small weight, material abundance, convenient solution processability, inherent low thermal conductivity, and intrinsically high flexibility. Although noticeable breakthroughs have been made in obtaining high-performance organic thermoelectric materials, the best ZT achieved from them is still far behind that of inorganic counterparts such as BiTe-based alloys at near room temperatures. In this paper, we overview major advanced research on organic-based flexible thermoelectric generators, starting with the basic principles of thermoelectricity and description of thermal and electrical transport interrelationships. Then, we give a comprehensive appraisal of the state-of-the-art in organic-based thermoelectric materials, their advantages and challenges for thermoelectric applications. It is followed by a critical discussion on several strategies to enhance the thermoelectric properties of these materials. Later, we explain different materials’ fabrication methods and compare their technical features. Furthermore, we demonstrate different types of structural designs for flexible organic-based thermoelectric generators, their fabrication methods, and several practical strategies to improve their performance. Finally, we summarize practical solutions to tackle the challenges that organic-based flexible thermoelectric power generation is facing to be established as a key technology to be utilized in next-generation wearable electronic devices. 1. Thermoelectric technology A temperature difference directly converts to electrical voltage and vice versa through the thermoelectric (TE) effect [1–3]. Three separate phenomena are included in thermoelectricity: the Seebeck, the Peltier, and the Thomson phenomena [2,3]. The Seebeck effect is named in honor of Thomas Johann Seebeck who in 1821 discovered that a voltage is created by a circuit made from two different metals with junctions held at different temperatures [2,3]. The Peltier effect results when electric current passes through a circuit of two different metals and leads to the release or absorption of heat at the junctions [2,3]. The Thomson effect refers to absorption or release of thermal energy in the presence of an electric field and temperature difference in a material [2,3]. The TE generator (TEG) is a promising technology to produce electricity from a temperature difference via the Seebeck effect. TEGs have simple configurations, no moving parts, can be operated maintenancefree for thousands of hours, can be adapted for different temperature ranges, and offer the potential to produce electrical energy from waste thermal energy [1,3]. The TEG comprises pand n-type semiconducting materials (as legs) which are commonly assembled electrically in series and thermally in parallel. Flexible TEGs have attracted extensive attention because they provide a platform to harvest thermal energy from complex surfaces, including the human body [4–6]. For wearable electronics in particular, flexible TEGs must possess superior flexibility and good durability, and be lightweight, comfortable, low cost, and safe [7–9]. Inorganic chalcogenide TE materials including Bi2Te3 [10], PbTe [11], Sb2Te3 [12], and SnSe [13] in the form of bulk, thin film, low-dimensional, and hetero-structure have been developed as high performance TE candidates. However, their applications in flexible TEGs are limited by several obstacles such as high price, use of rare elements, inconvenient fabrication processes, and mechanical rigidity [14]. Several strategies have been introduced to develop flexible TEGs based on inorganic TE materials: (1) fabrication of low-dimensional TE * Corresponding author. E-mail address: pakdela@tcd.ie (A. Pakdel).