調節共軛聚合物的鏈間／鏈內相互作用及其對場效電晶體性能的影響

Abstract

共軛高分子的發現可以追溯到約五十年前發生的一個美麗的錯誤，由於計算錯誤，過量的添加劑被加入了聚乙炔聚合物中，而聚乙炔也展現出了前所未有的銀色光澤。從那時起，材料科學便進入了一個嶄新的世代。與金屬相比，共軛聚合物具有良好的溶解性、質地輕盈和可拉伸性等優勢，使得共軛高分子成為大規模溶液加工製程和可穿戴設備的首選材料。近年來，共軛高分子已在有機場效電晶體（OFETs）、有機太陽能電池（OSCs）和有機發光二極體（OLEDs）等光電領域取得許多的突破，其中，由於場效電晶體可以進一步應用於放大器及記憶體中，受到許多矚目。因此，為了滿足發展有機場效電晶體的多功能特性，科學家們付出了相當大的努力合成具有高遷移率的N型高分子材料，如：二酮吡咯並吡咯（DPP）和苯並二呋喃二酮基寡（對苯乙烯）（BDOPV）為主的共軛高分子。 除了共軛單體的創新設計外，側鏈工程也被證明能有效增加高分子的溶解度與調節其分子間距及相互作用以獲得更高的載子遷移率。有鑒於此，科學家們開始在共軛主幹上引入不對稱的側鏈設計。延續此概念，我們在DPP高分子上引入了線性烷基鏈、碳矽烷鏈和末端矽氧烷鏈，形成不同的側鏈組合。碳矽烷鏈和末端矽氧烷的組合成功地誘導出異向堆疊型態，提高了聚合物的遷移率和可拉伸性。另一方面，線性烷基鏈和矽氧烷鏈的相容性則不如前面的組合好，於是無法提供高分子理想的遷移率和可拉伸性，此結果顯示側鏈設計可調控分子間的相互作用進而影響高分子的性能。 除了側鏈工程以外，非共價鍵的形成也可調節分子間和分子內的相互作用。分子內的非共價鍵可有效地讓高分子主鏈平面化，提供更好的堆疊型態更有效地傳輸電子。我們因此設計了一系列由BDOPV單元和吲哚二硫酚（IDT）單元組成的隨機聚合共聚物，並研究其分子內的相互作用在共面性中所扮演的角色。結果顯示，由於氧原子和氫原子中的電子親和力差異，分子內的非共價鍵有效地固定了芳香族主鏈，形成了類梯形的高分子。在氧－氫吸引力的相互作用下，高分子鏈以更緊密、更牢固地方式堆疊，有利於分子間的軌域重疊和電子／電洞傳輸。相反地，BDOPV單體的旋轉角和IDT單體的剛硬結構不能夠完美地相互配合，以至於分子鏈的排列遇到障礙，證明了分子內相互作用會嚴重影響聚合物鏈的結構和構型。

The discovery of conjugated polymers can be traced back to a beautiful mistake made some fifty years ago. Due to a miscalculation, an excessive amount of additive was added to polyacetylene and the polymer exhibited an unprecedented silver lustre. Since then, materials science entered a new era. The solubility, lightness and stretchability of conjugated polymers compared to metals have made them the material of choice for large-scale solution processing and wearable devices. In recent yours, conjugated polymers have made many breakthroughs in the field of optoelectronic applications such as organic field effect transistors (OFETs), organic solar cells (OSCs) and organic light-emitting diodes (OLEDs). Among these, FETs received lots of attention because they can be further used in amplifiers and memories. Therefore, in order to satisfy the need to develop the multifunctional properties of FETs, considerable efforts have been made to synthesize N-type polymers with high mobility, such as diketopyrrolopyrrole (DPP)- and benzodifurandione-based oligo(p-phenylene vinylene) (BDOPV)-based conjugated polymers. In addition to innovative designs for conjugated monomers, side chain engineering has also been shown to be effective in increasing the solubility of polymers and modulating their intermolecular distances and interactions to achieve higher mobilities. In response to this, scientists have started to introduce asymmetric side chain designs on the conjugated backbone. Continuing with this concept, we herein introduce linear alkyl chain, carbosilane chain and siloxane-terminated chain to DPP polymers, forming different combinations of side chains. The combination of carbonsilane chain and siloxane-terminated chain successfully induces bimodal stacking patterns that enhance the mobility and stretchability of the polymer. On the other hand, the linear alkyl chain and siloxane chain are not as compatible as the previous combination and therefore do not provide the desired mobility and stretchability of the polymer. This result suggests that the side chain design can modulate intermolecular interactions and thus influence the properties of conjugated polymers. In addition to side chain engineering, the formation of non-covalent bonds can also regulate inter- and intra-molecular interactions. Intramolecular non-covalent bonding can effectively planarize polymer backbones, providing better stacking patterns for more efficient electron transport. We have therefore designed a series of random copolymers consisting of BDOPV and indacenodithiophene (IDT) units and investigate the role of intramolecular interactions in coplanarity. The results show that due to the difference in electron affinity between the oxygen and hydrogen atoms, the intramolecular non-covalent bonding effectively immobilizes the aromatic backbone, forming a ladder-type polymer. Under the O-H intramolecular interaction, the polymer chains stack in a tighter and stronger manner, facilitating intermolecular orbital domain overlap and electron/hole transport. Conversely, the rotation angle of BDOPV and the rigid structure of IDT do not perfectly match each other, to the extent that the alignment of the molecular chains encounters obstacles, demonstrating that intramolecular interactions can seriously affect the structures and configurations of polymer chains.