A Facile Method to Improve Adhesion Between CNT Films and Substrates for Better Thermoelectric Performance

Doping methods are common applied to improve the electrical conductivity of CNT films or change the type of CNT films (p-type to n-type) to construct devices with p-n legs. However, the beneficial effect of increasing electrical conductivity, by a common p-type dopant of FeCl₃, will disappear after a heat treatment at 150 °C for 1 h (necessary for activating the self-folding). Moreover, CNT films doped with FeCl₃, had a poor adhesion to the electrode substrate for device fabrication. Therefore, it would be desirable to find new additives that can promote CNT films to achieve better adhesion and higher electrical conductivity simultaneously, without increasing the thermal conductivity. PEDOT:PSS is a common conductive polymer that is widely applied in organic solar cells, TE devices and stretchable devices. It has low thermal conductivity, favourable environmental stability and flexibility but relatively low electrical conductivity. Strategies like adding organic polar solvents (e.g. DMSO, ethylene glycol (EG) or acid treatment (e.g. formic acid (FA), sulfuric acid (H₂SO₄) have been used to improve PEDOT:PSS electrical conductivity.<br/>In this work, PEDOT:PSS-doped CNT films were fabricated by drop casting PEDOT:PSS solutions on the surface of CNT films. p-p conjugate interactions between PEDOT:PSS and CNT are beneficial to improve the TE properties (e.g. electrical conductivity) of the composites. In addition, PEDOT:PSS has been previously reported to act as an adhesion promoter between organic semiconductors (e.g. P3HT) and certain substrates (e.g. UV/ozone-activated polydimethylsiloxane (PDMS), fullerene films). A new self-folded TE device based on CNT/PEDOT:PSS films has been fabricated. The open circuit voltage and output power of this device were 93 mV and 85 µW at a temperature gradient of 80 K, higher than the device without adding PEDOT:PSS (86 mV and 42 µW).<br/>This work provides a new and simple approach to achieve better TE performance by exploring the role of PEDOT:PSS as a dopant and adhesion promoter for CNT films, and offers inspirations for fabrication and applications of new flexible p-type TE materials, for instance in the wearable electronics field.