脈衝雷射沉積氯摻雜硫化鉍/銅奈米粒子複合薄膜於熱電轉換之應用

Abstract

熱電材料能將熱能與電能互為轉換，是極具應用潛力之能源材料。在眾多熱電材料中，碲化鉛(PbTe)為一出色之中溫型熱電材料，具有可應用程度之熱電轉換效率(熱電優值ZT~1.2)，然而碲與鉛此二元素均具有毒性，基於環境友善的理念，開發碲化鉛的替代熱電材料成為迫切之研究重點。本研究特別選定硫化鉍(Bi2S3)用以替代碲化鉛，係因其具有相對為高之Seebeck係數(102−103 μVK-1)。然而，偏低之導電率(<102 S/cm)使其熱電優值僅達0.2，遠低於可應用的標準(ZT~1)，嚴重限制硫化鉍之應用性。本研究期藉由基板溫度之調控，促使硫化鉍薄膜之硫組成汽化，最終化學成份偏離計量比，用以改善硫化鉍之導電率。此外，藉由摻雜氯化鉍(BiCl3)，使氯離子取代硫離子的位置，用以提升硫化鉍薄膜之導電率。更進一步設計雙光束雷射獨立剝蝕氯摻雜硫化鉍與銅靶，製備新穎氯摻雜硫化鉍/銅奈米粒子之多層週期性複合薄膜，期藉此達到在不顯著影響Seebeck係數的前提下，達到進一步改善導電率，乃至熱電優值的目標。 結果發現，於SiO2/Si絕緣基板上成功製備高結晶性奈米柱狀晶結構硫化鉍薄膜，其具有高載子遷移率與載子濃度，導電率高達638 Scm-1，Seebeck係數為- 453 μVK-1，功率因子高達137 μWcm-1K-1，約為現今最佳硫化鉍薄膜(酸浴法製備)的四倍(29 μWcm-1K-1)。而摻雜氯化鉍於Seebeck係數-522 μVK-1之本質硫化鉍薄膜，其導電率從170 Scm-1提升至450 Scm-1。然而，氯摻雜改善導電率的同時，Seebeck係數亦隨之下降(從-522 μVK-1降至-221 μVK-1)。為此，於氯摻雜硫化鉍薄膜內導入奈米異質結構，利用雙靶雷射個別沉積，製備氯摻雜硫化鉍/銅奈米粒子多層週期性複合薄膜，薄膜內部有銅與鉍之奈米異質結構生成，不僅能提升其導電率(715 Scm-1)，亦能維持其Seebeck係數(- 413 μVK-1)，使得其功率因子高達122 μWcm-1K-1，相較本質硫化鉍薄膜(49 μWcm-1K-1)提升兩倍。

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).