Thermoelectric properties of bismuth telluride and antimony telluride thin films electrodeposited from a non-aqueous solution.

Abstract

Thermoelectric materials have a potential for use in power generation technologies due to their ability to convert heat directly into electricity. The field of thermoelectrics has drawn a significant attention over the last 20-25 years because it is a pollution-free form of energy conversion. Of all the types of thermoelectric materials investigated so far, nanostructured materials have shown the most promise in enhancing performance of thermoelectric devices. This work reports the thermoelectric properties of Bi2Te3 and Sb2Te3 thin films electrodeposited from the weakly coordinating solvent dichloromethane (CH2Cl2). It was found that the oxidation of porous Bi2Te3 films is significant, causing the degradation of its thermoelectric properties. It is shown that the morphology of the film can be improved drastically by applying a short initial nucleation pulse, which generates a large number of nuclei, and then growing the nuclei by pulsed electrodeposition at much lower overpotential. More importantly, this significantly reduces the oxidation of the films as smooth films have smaller surface-to-volume ratio and are less prone to oxidation. X-ray Photo-electron spectroscopy (XPS) shows that those films with Te(O) termination show complete absence of oxygen below the surface layer. A thin film transfer process was developed using polystyrene as a carrier polymer to transfer the films from the conductive TiN to an insulating layer for thermo-electrical characterisation. Temperature dependent Seebeck measurement reveal a room temperature coefficient of -50 µV/K growing to nearly -100 µV/K at 520. The corresponding power factor reaches value of about 90 µW/mK2 at that temperature. It is shown that Sb2Te3 films’ morphology is improved by employing pulsed electrodeposition compared to potentiostatic. As-deposited film exhibits poor crystallinity, crystallising after annealing at 100◦C. The Seebeck coefficient increases with temperature increase reaching a value of 107.5 µV/K at 485 K. The corresponding power factor reaches nearly 100 µW/mK2 at 464 K. Hence, this work demonstrates electrodeposition of Bi2Te3 and Sb2Te3 thin films from dichloromethane, which could be implemented for the fabrication of thin film device for thermo-electric energy conversion.

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ORCID for Kees De Groot: orcid.org/0000-0002-3850-7101

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