Processing and doping of thick polymer active layers for flexible organic thermoelectric modules
Graphical abstract
Introduction
Organic thermoelectric generators (OTEGs) replace the traditional inorganic active materials in thermoelectric generators (TEGs) with organic compounds and have advantages such as low thermal conductivity, light weight, and high flexibility that enable the potential to easily cover large heat exchanger surfaces of any shape and size [1]. Organic materials also offer the prospect of low-temperature solution processing, which could enable the roll-to-roll mass printing of large-area, integrated modules resulting in cost reduction [2]. Thus, OTEGs are poised to become versatile distributed power sources in the near future with applications from wearable electronics to power supplies for mobile devices and distributed sensor networks [3].
The performance of TEGs is often compared based on the figure of merit ZT = σS2T/κ. For high performance with an average temperature between the hot and cold surfaces of T, high electrical conductivity (σ) and Seebeck coefficient (S) are required while maintaining a low thermal conductivity (κ) [4]. Pristine or lowly-doped organic semiconductors can have high S [5], [6] and some of the lowest κ among known materials, but generally suffer from low σ. Because of trade-offs between S, σ, and κ, controlling the carrier density in the active materials to increase σ while maintaining high S and low κ is key for optimizing and improving OTEG performance [1], [7], [8], [9], [10]. In particular, reducing the decrease in S with increasing σ is of great importance since many reported organic thermoelectric materials already have low κ of less than 0.5, which is attributed to poor phonon transport in the amorphous layers [10].
Conducting polymers are good candidates for OTEGs because the carrier density can be controlled by modifying the chemical oxidation levels [8]. In fact, recent progress using poly(3,4-ethylenedioxythiophene) (PEDOT) has yielded remarkably high thermoelectric performance comparable to that of inorganic materials because of a low κ (0.2–0.4 Wm−1 K−1) and high σ (more than several hundred Scm−1) [8], [11], [12], [13]. Furthermore, conducting polymers can be soluble in many common organic solvents and are viscoelastic, making them suitable for the mass-production of flexible OTEGs [3].
An important consideration for the design of flexible OTEGs is having the longest thermal and electrical conduction paths (i.e., the thickest active layer) possible while maintaining flexibility to obtain the largest temperature gradient and highest thermoelectric voltage. However, the organic thin films used in common organic devices such as light-emitting diodes [14], field-effect transistors [15], and solar cells [16] usually have thicknesses of several tens to a few hundred nanometers (nano-films). To increase power output, fabrication and processing methods for films with micrometer-scale thicknesses (micro-films) must be developed. In particular, the control of carrier density by doping the films is important for optimizing device performance [4]. While doping processes for OTEGs have been widely tested and studied in nano-films [6], [8], [11], [17], [18], [19], [20], [21], few investigations of the doping of micro-film OTEGs exist.
In addition to the development of individual devices, the connection of many devices, in series electrically and in parallel thermally, to create modules is necessary to get practically useful thermoelectric voltages [1], [4]. A typical thermoelectric module consists of a series of alternately connected p-type and n-type elements forming a π-shape that must be balanced to account for differences in S, σ, and κ and avoid power losses [1], [4]. Inorganic thermoelectric modules are generally fabricated by the assembly of individual pieces of the inorganic materials cut from a slab, resulting in expensive and rigid devices with a limited number of existing industrial and natural environments to which they can be easily applied [22].
To overcome these drawbacks, flexible organic thermoelectric modules with the elements simultaneously fabricated directly onto the substrate by solution processing are highly desired [2], [22], [23], [24]. However, the scarcity of good candidates for organic n-type materials and the complexity of the module design makes the adoption of a π-leg structure difficult [23], [25]. Alternatively, the design and fabrication can be simplified by using a uni-leg architecture, in which the top electrode is directly connected to the bottom electrode of the adjacent device, enabling the generation of power by using only one type of thermoelectric material (p-type or n-type) [2], [26]. For both types of modules, ink-jet and screen printing have been studied as solution processes to realize the mass production of large-area modules [23], [25], [26]. However, viscosity and solvent limitations make ink-jet printing slow for fabricating thick active layers [25], [26], and controlling the crystal orientation of deposited materials is difficult by screen printing. Thus, alternative fabrication techniques are desirable.
In this paper, the effects of two different dopants on the properties of nano- and micro-films of the polymer poly(3-hexylthiophene-2,5-diyl) (P3HT) fabricated by solution processing, using spin coating and drop casting with drying in a solvent vapor environment, respectively, are studied to realize flexible, solution-processed OTEGs based on micro-films. The polymer P3HT (Fig. 1A) was chosen because it is a commonly used p-type semiconductor polymer and is soluble in many organic solvents. As dopants, the well-known electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ, Fig. 1B) and the Lewis acid iron(III) p-toluenesulfonate hexahydrate (Fe3+-tos3·6H2O, Fig. 1C) were used. The effects of doping concentration on thermoelectric properties, performance, and air-stability of individual OTEGs are investigated and compared for both dopants in nano- and micro-films.
Furthermore, we develop a fabrication method for flexible uni-leg OTE modules using a photo-etching technique to directly pattern a micrometer-thick active layer into individual elements without the need for alignment and assembly of the elements. The thermoelectric properties of the modules are investigated in detail to better understand module design principles. This solution processing method is a step toward roll-to-roll mass production that could significantly reduce the complexity, time, and cost of thermoelectric module production compared to conventional processes such as ink-jet and screen printing.
Section snippets
Experimental
Regio-regular P3HT (MERCK, regio-regularity > 94.7%, molecular weight of 34 kg/mol), F4-TCNQ (TCI), and Fe3+-tos3·6H2O (Sigma–Aldrich) were purchased commercially and used without further purification. The substrates and bottom electrodes were prepared as follows. The glass substrates (700 μm thick) were first immersed in a Piranha solution (hydrogen peroxide: sulfuric acid vol. ratio of 1:3) in a petri dish heated at 125 °C for 30 min. Next, the substrates were cleaned by ultrasonication in
Effect of doping on thermoelectric properties
Generally, pristine organic materials have very low σ because of their low carrier mobilities and densities, which limits their ZT̅ and performance in OTEGs [27]. To increase σ, organic materials are often doped with compounds that generate charge carriers by inducing charge transfer (CT) [28]. In this study, the polymer films were chemically doped using solution processing by dipping the films into solutions containing F4-TCNQ or Fe3+-tos3·6H2O. as dopants [27]. The presence and strength of CT
Conclusions
In this study, Fe3+-tos3·6H2O was shown to be more effective than F4-TCNQ for the chemical doping of P3HT micro-films fabricated by solution processing. Although F4-TCNQ doping of nano-films could be used to increase σ and power factor, σ remained low in micro-films even when increasing the doping concentration, which was attributed to poor diffusion into the films based on XRD and stability measurements. Furthermore, the lifetime dramatically increases for the micro-film doped with Fe3+-tos3·6H
Acknowledgments
This research was supported in part by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST Program).
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