INVESTIGATION OF THE THERMOELECTRIC PROPERTIES OF PEDOT:PSS/Bi0.5Sb1.5Te3 COMPOSITES AND THE PRINTING OF FLEXIBLE THERMOELECTRIC GENERATORS

Abstract

The increasing demand for sustainable power sources in wearable electronic devices has spurred the exploration of innovative energy harvesting technologies. In this context, flexible thermoelectric (TE) materials and generators (TEGs) show promise in converting body heat into electrical power. Recent research has concentrated on developing flexible TEGs using organic and polymeric semiconducting materials. One material of particular interest is Poly (3,4- ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) due to its flexibility, nontoxicity, easy processability, high electrical conductivity, and low thermal conductivity. However, a comprehensive review of literature indicates that hybridization with inorganic materials, optimization of doping levels, and structural modifications to enhance their TE conversion efficiency is still an ongoing area of investigation, and a comprehensive study encompassing all these approaches is lacking. To fabricate such flexible devices, printing methods have emerged as promising strategies. Although there has been extensive exploration of printing techniques in recent years, there is a limited number of studies on the development of high-performance flexible TEGs made by spray printing of organic/inorganic thin film composites. This study aimed to bridge the knowledge gaps by developing flexible thin film TEGs using PEDOT:PSS/Bi0.5Sb1.5Te3 (BST) composite thin films through a spray printing technique. Initially, we introduced the design, fabrication, and evaluation of a fully automatic system, capable of simultaneous measurement of the Seebeck voltage and electrical resistance in thin/thick film and bulk samples within a wide range of electrical resistances, lateral dimensions, and thicknesses. The measurement principle is based on analysing the current-voltage (I-V) characteristic of the samples. The systems performance was rigorously assessed by measuring the electrical properties of a pure platinum standard sample, a bulk (Bi, Sb)2Te3 sample, and a PEDOT:PSS thin film sample under both steady-state and quasi-steady-state conditions. The obtained results were consistent with those acquired by state-of-the-art commercial systems and reported in the literature, confirming the accuracy and reliability of the developed system. In the next stage, the effect of BST particle size and concentration in the PEDOT:PSS matrix was investigated, with the intention to optimize the power factor. Then, the optimal composite, exhibiting a maximum power factor at room temperature, underwent various single and sequential post-treatments using secondary dopants (DMSO, EG, and H2SO4) and chemical dedoping agents (NaOH). While a post-treatment with 0.5 M H2SO4 increased the power factor 240 times compared to the pristine thin film composite, a sequential post-treatment with 0.5 M H2SO4 and 0.1 M NaOH resulted in an 864-fold increase of the power factor at room temperature. Furthermore, temperature-dependent electrical conductivity and Seebeck coefficient analyses revealed that charge carrier transport mechanism changed from three-dimensional variable range hopping in pristine samples to metallic conduction in post-treated samples. In the next stage, the optimal composite was used for spray printing, and the effect of substrate temperature on the microstructural and TE properties was examined. The mechanical stability and flexibility of the printed films were also evaluated, demonstrating their suitability for wearable electronic applications. Furthermore, flexible composite thin films were fabricated using TE ink pre-treated by a secondary dopant (DMSO) and a chemical dedoping agent (NaOH) for structural modification and tailoring the doping levels. Additionally, a flexible TEG made of 40 TE legs was fabricated using the spray printing of pre-treated TE ink, and the electrical properties were characterized. Finally, as a proof of concept, the application of the printed flexible TEG was demonstrated by attaching it to the human body and monitoring the generated voltage from the skin heat. This study explores the relationship between particle engineering, post-treatment strategies, printing parameter and the resulting microstructural, morphological, and TE properties of flexible PEDOT:PSS/ BST composite thin films and TEGs. Overall, the findings offer critical insights into optimizing flexible TE materials and TEGs for efficient energy harvesting in wearable electronics.