Pulsed Laser Deposition of BiCl3 Doped Bi2S3/Cu Nanoparticle Composite Films for Thermoelectric Applications

Abstract

Thermoelectric (TE) materials capable of directly converting heat into electricity have become a highly potential category in energy materials and are expected to tackle the energy issue of the future. Among a variety of TE materials, lead telluride (PbTe) has been recognized as an outstanding mid-temperature TE material with an applicable TE figure of merit (ZT) of ~1.2. However, due to the toxicity of both Pb and Te, development of alternative mid-temperature TE materials is urgently required. Bismuth sulfide (Bi2S3) is specially selected in the present study as the most promising candidate for alternating PbTe mainly because of its potentially high Seebeck coefficient (102−103 μVK-1). Unfortunately, the generally poor electrical conductivity (<102 S/cm) and the resulting low ZT (~0.2) of Bi2S3 severely restrict its applicability. In this work, by controlling the substrate temperature, the volatility of sulfur would lead the compositional deviation from the chemical stoichiometry of Bi2S3 and the electrical conductivity of the sulfur-vacancy introduced Bi2S3-x would thus be improved. Similarly, BiCl3 was also considered and applied as a donor dopant to improve the electrical conductivity. Furthermore, a series of novel BiCl3 doped Bi2S3-Cu nanoparticles layer-by-layer composite films were designed and fabricated through the newly built dual-beam pulsed laser deposition for improving the electric conductivity without seriously sacrificing the Seebeck coefficient. The prepared highly crystalline intrinsic Bi2S3 films on insulated SiO2/Si substrates exhibit a very high electric conductivity of 638 Scm-1 evidently due to the simultaneously enhanced carrier concentration and mobility, and the Seebeck coefficient of - 453 μVK-1. The resulting power factor is 137 μWcm-1K-1 which is four times higher than the reported highest value (29 μWcm-1K-1) from the chemically prepared Bi2S3 so far. The introduction of BiCl3 into Bi2S3 indeed leads to an enhancement of the electrical conductivity from 170 Scm-1 (before BiCl3 doping) to 450 Scm-1 whereas the Seebeck coefficient decreases from -522 μVK-1 to -221 μVK-1. On the sake of enhancing the electricity conductivity and Seebeck coefficient simultaneously, the Cu nanoparticles were introduced into the BiCl3 doped Bi2S3 film. The heterostructure of Cl doped Bi2S3/Cu played an important role in obtaining the high electrical conductivity of 715 Scm-1, Seebeck coefficient of -413 μVK-1 and the power factor of 122 μWcm-1K-1 which is twice higher than those of the intrinsic Bi2S3 film (49 μWcm-1K-1).