共軛高分子熱電材料之研究：側鏈結構調控與離子液體摻雜效應

Abstract

面對全球化能源危機，熱電元件的開發將可使廢熱轉換為電力，為實踐永續能源回收與利用的重要方式。在現今熱電研究當中，有機半導體因其輕量、可溶液製程及具備軟性與可撓性等製備優勢，展現出其應用於穿戴式有機熱電材料潛力。其中，共軛高分子因其可調控的共軛主鏈側鏈結構設計，能有效調整分子內/間作用力、分子鏈段堆疊、能階分布與電荷傳輸特性，進而提升熱電性能。同時，共軛有機高分子與摻雜劑、離子液體及單壁奈米碳管等相容性佳，為有機熱電領域的研究帶來許多發展可能。 本論文探討兩項關於共軛高分子的研究案例。第一部分的專題研究主旨為，使用聚噻吩乙炔（poly(thienylene vinylene)）主鏈並以分支側鏈修飾之共軛高分子，將其和單壁奈米碳管（single-walled carbon nanotubes, SWCNTs）混摻後製備成具熱電效應的奈米級複合材料。此主鏈結構設計具有高度的共平面性與低立體障礙，有助於形成高效率的 π–π 共軛結構。 在本研究中，為了比較烷基分支側鏈（branched alkyl side chains）與硫化烷基分支側鏈（branched alkylthio side chains）對熱電效應的影響，將以兩種新開發的共軛高分子材料進行探討：poly[3,4-bis(2-ethylhexyl)thienylene vinylene]（P3,4EHTV）與 poly[3,4-bis(2-ethylhexylthio)thienylene vinylene]（P3,4EHTTV）。上述共軛高分子與單壁奈米碳管混摻，旨在補償高分子本身不足的載子濃度，並透過強 π–π 交互作用力，形成由共軛高分子鏈緊密包覆於碳管束分散效果良好之複合結構。P3,4EHTTV/SWCNT 奈米複合碳材展現出穩健的分子交互作用，除了來自π–π共軛電子，亦包含由硫化烷基側鏈額外引入的硫–π 作用力，使碳管束尺寸細緻且分布均勻，藉此提升 p 型載子傳輸效率。此外，側鏈中的硫原子有助於能階調控，進一步提升熱電載流子的平均能量。最終，P3,4EHTTV/SWCNT熱電功率因子值高達 363.7 μW m−1 K−2，展現出 PTV 結構設計與硫化烷基側鏈工程對提升熱電性能的影響力。 第二部分的研究聚焦於兩種新設計的電子施體–受體（donor–acceptor）隨機共聚物高分子，其結構由 70% 結晶鍊段（二酮吡咯並吡咯(diketopyrrolopyrrole) 與四噻吩(quarterthiophenes)）與 30% 非晶相「共軛中斷單元」組成。此共軛中斷單元包含軟性的芴（fluorene）-四噻吩或剛性的碳橋併芴（carbon-bridged fused fluorene）結構，藉此加強載流通道上的機械性質，並調控材料的結晶性、電荷遷移率與拉伸延展性。本研究通過氯化鐵摻雜並結合離子液體，與參雜劑反離子進行陰離子的置換，探討共軛高分子與各種摻雜劑之間的交互作用和摻雜機制。本研究中，使用的三種離子液體如下：鋰雙（三氟甲磺醯）亞胺（LiTFSI）、1-乙基-3-甲基咪唑鎓雙（三氟甲磺醯）亞胺（EMIM:TFSI）及1-乙基-3-甲基咪唑鎓四氟硼酸鹽（EMIM:BF4），並探討這些摻雜劑在尺寸、形狀及配位特性上的差異對於高分子相容性的影響。研究發現最佳的聚合物-離子液體組合能維持高分子鏈有序的微觀結構並促進極化子（polaron）穩定性。其中，以LiTFSI進行離子交換的FT薄膜達到3.31 μW m−1 K−2的功率因子，且24小時存放後仍保持3.22 μW m−1 K−2的效能穩定性。此一穩定的熱電傳輸效果歸功於有效的反離子滲透及強化的反離子-極化子耦合。此發現展示了透過共軛中斷單元與摻雜工程開發可拉伸且具穩定載子傳輸之共軛高分子聚合物熱電材料的可行策略。

Thermoelectric (TE) devices enable sustainable energy harvesting by converting waste heat into electricity. Organic semiconductors (OSCs) are promising TE materials due to their light weight, solution processability, and mechanical flexibility. Among them, conjugated polymers stand out for their tunable structures, achieved through tailored backbone design and side chain engineering. These modifications optimize molecular interactions, chain packing, energy levels, and charge transport. Such features enhance their performance and compatibility with dopants, ionic liquids, and conductive fillers like single-walled carbon nanotubes (SWCNTs), making conjugated polymers strong candidates for high-performance organic TE applications. This thesis presents two case studies on conjugated polymers. The first explores branched side chain engineering on a poly(thienylene vinylene) backbone, which offers high coplanarity and low steric hindrance for efficient π–π conjugation. In particular, two newly synthesized polymers, poly[3,4-bis(2-ethylhexyl)thienylene vinylene] (P3,4EHTV) and poly[3,4-bis(2-ethylhexylthio)thienylene vinylene] (P3,4EHTTV), are investigated to unveil the thermoelectric effects contributed by alkyl and alkylthio side chains, respectively. These conjugated polymers are blended with single-walled carbon nanotubes (SWCNTs) to address two goals: enhancing conductivity by compensating for the intrinsic carrier deficiency of OSCs, and promoting strong π–π interactions to form intimate nanocomposite networks, where polymer chains densely wrap around well-dispersed CNT bundles. The P3,4EHTTV/SWCNT nanocomposite exhibits strong molecular interactions not only from π–π stacking but also from sulfur–π interactions introduced by the alkylthio side chains. This leads to uniformly distributed, fine CNT bundles and enhanced p-type carrier transport efficiency. Additionally, the heteroatoms in the side chains help align energy levels, boosting the average carrier energy. As a result, the composite achieves a record-high power factor of 363.7 μW m−1 K−2, underscoring the critical role of PTV design and alkylthio engineering in thermoelectric optimization. The second study focuses on two newly designed donor–acceptor (D–A) random copolymers, composed of 70% crystalline segments (diketopyrrolopyrrole and quarterthiophenes) and 30% amorphous “conjugated breaker” units. The breakers feature either a flexible fluorene-quarterthiophene linkage or a rigid, carbon-bridged fused fluorene structure. These segments are incorporated to improve mechanical property along charge transport pathways while tuning crystallinity, charge mobility, and intrinsic stretchability of the D-A copolymers. The doping mechanism between conjugated polymers and various additives is investigated through FeCl3 doping followed by anion exchange with ionic liquids: lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM:TFSI), and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM:BF4). The compatibility of polymers with these additives, differing in size, shape, and coordination features, was examined. Optimal polymer–ionic liquid combinations preserved ordered microstructures and enhanced polaron stabilization. Notably, LiTFSI-treated FT films achieved a power factor (PF) of 3.31 μW m−1 K−2. These films also exhibited enhanced storage stability, retaining a PF of 3.22 μW m−1 K−2 after 24 hours. This durability is attributed to effective counterion infiltration and strengthened counterion–polaron coupling. These findings highlight a promising strategy for developing stretchable and stable polymer thermoelectric materials through conjugated breaker incorporation and dopant engineering.