共平面性、多元環及側鏈氰基對 Poly(3-hexylthiophene)電子能帶結構的影響

Abstract

本論文是以 stille coupling 合成高分子，從共平面性、主鏈上導入 fuse ring 的 cyclopentadithiophene (CPT)、以及側鏈上修飾強拉電子的氰基(- cyano group，CN group)三方面討論影響 poly(3-hexylthiophene) (P3HT)電子 能階與光學吸收的因素。高分子的分子量在 10000 以上，且在室溫下一般 有機溶劑中具有良好的溶解度。 P3HT 主鏈上的 head-to-tail (H-T)結構比例在 50%~98%之間，受到有規 律的 head-to-head(H-H)結構形成主鏈上的扭曲，形成不同的共平面性。光 學能隙(band gap)與電化學能隙具有同樣的規律，能隙隨共平面性的增加而 下降。隨著 H-H 比例的增加，能隙變大主要來自於最低為填滿分子軌域 (lowest unoccupied molecular oribital ，LUMO)的上升。在主鏈上的扭曲會 使 LUMO 增加 0.2~0.4eV，同時抑制旋轉塗佈薄膜的 π-π stacking。 在 P3HT 主鏈上導入 CPT 結構，形成 CPT 與 3HT 的共聚合物，其能階 與 regioregular P3HT (rr-P3HT) 相近，顯示在主鏈上導入 fuse ring 的 CPT 結 構並沒有對能帶結構產生改變。然而 CPT 結構的共平面性良好，使得高分 子在室溫溶液中形成堆疊(aggregation)。在薄膜狀態下，CPT 系列高分子的 光學吸收並沒有產生紅移，同時看不到 π-π stacking。 在高分子側鏈上修飾強拉電子的氰基，同時拉低最高填滿分子軌域 (Highest Occupied Molecular Orbital，HOMO)及 LUMO 0.2eV，然而並沒有改 變薄膜態的光學吸收與分子堆疊。

IV

Abstract In this thesis, factors affecting the electronic energy levels and optical absorptions of poly(3-hexylthiophene) (P3HT) are systematically studied, including the coplanality, the introduction of fused ring on the P3HT main chain and the strong electron withdrawing cyano (-CN) side group. All the polymers are synthesized via Stille coupling to afford reasonably high molecular weight (> 10,000) and good solubility in common organic solvent at room temperature. Considering the effect of coplanality, P3HT with various head-to-tail (HT) percentages in the main chain ranging from 50% to 98% are synthesized with regularly distribution of the twists (head-to-head, HH, configurations). The optical band gap and the electrochemical band gap are in good agreement, increasing with the decrement of the coplanality. The widen bad gaps are attributed to the elevated lowest unoccupied molecular oribital (LUMO) and the lifts are not proportional to the increment in the HH percentages. A small amount of twists in the main chain would significantly elevate the LUMO by 0.2~0.4 eV, along with the suppression of π-π stacking in the spin-coated thin film. Regarding the effect of fused ring in the main chain, cyclopentadithiophene (CPT) is introduced to the P3HT main chain and the resulting copolymers containing CPT and 3HT exhibited energy levels similar to those of regioregular P3HT(rr-P3HT), suggesting the fused ring in the main chain has no effect on the electronic band structure. However, the strong coplanality of CPT enables the formation of aggregations of CPT based copolymers in solution at room temperature. In comparison with rr-P3HT in solid state, the introduction of CPT would not promote the red absorption; in addition, the π-π stacking is inhibited. The employment of strong electron withdrawing CN side groups simultaneous lower both the highest occupied molecular oribital (HOMO) and LUMO by 0.2eV without significantly altering the optical absorption and the molecular packing in thin films.