- 學位論文
利用支鏈依附密度以及主鏈曲度控制予體－受體共軛寡共聚物薄膜結晶方向
Using Side‐chain Attachment Density and Backbone Curvature to Regulate D–A Co-oligomer Crystalline Orientation in Thin Film
摘要
導電高分子為目前被廣泛研究應用在光電元件中的重要材料，而導電高分子之結晶方向為決定元件內電荷載子遷移率以及元件表現之關鍵，因此控制導電高分子之結晶方向十分重要。依據運作原理，導電高分子之結晶方向在有機場效型電晶體以及有機太陽能電池若分別以側向以及面向結構排列，將有助益於提升其表現。然而控制導電高分子結晶方向之背後機制至今尚不得而知，且目前仍少有相關文獻能夠透過系統性分子設計即能控制結晶方向。此研究中我們由噻吩並異靛藍素為受體、寡噻吩為予體，並由分子設計角度著手，合成出八種交替共軛寡共聚物poly(thienoisoindigo-alt-(xoctylthiophene)n，PTInT-xC8。其中TI為噻吩並異靛藍素、n為予體內噻吩數目、T為噻吩、x為噻吩上正辛烷基 (C8) 數目；再藉以：(1) 增減寡噻吩上正辛烷基數目 (x) 改變支鏈依附密度以及 (2) 增減予體中噻吩數目 (n) 改變主鏈曲度兩項要素依序控制此系列寡共聚物薄膜結晶方向。 我們成功地分別以Stille偶聯反應以及直接芳香化縮聚法合成出此系列寡共聚物，並以核磁共振儀做化學結構鑑定、基質輔助雷射脫附游離飛行時間式質譜儀分析其分子量、低略角廣角X光散射儀分析薄膜結晶結構、紫外光－可見光－近紅外光吸收光譜儀分析光電性質，以及循環伏安儀測量能階位置，並使用熱重分析儀和差示掃描量熱分析儀用作熱性質分析。我們也以密度泛函理論詳細對此八種寡共聚物建構模型做分子模擬計算。研究結果發現：(1) 當噻吩數目 (n) 維持不變，但逐漸增加予體噻吩上支鏈個數 (x) 後，除了分子模擬結果顯示其主鏈將大幅扭轉，吸收光譜中也可觀察到明顯之電子由予體之HOMO躍遷至予體的LUMO之吸收訊號峰 (λ1) 相對強度增加、分子內電荷轉移吸收峰 (λ2) 相對強度下降。由低略角廣角X光散射儀分析其薄膜內結晶形態也指出：寡共聚物鏈位於 (010) 方向之π-π堆疊距離 (dπ-π) 也將被拉大，如PTI2T-0C8、PTI2T-2C8其dπ-π分別為3.57 Å以及4.83 Å，PTI2T-4C8其dπ-π則被導入之支鏈大幅拉大，故幾乎無法觀察到明顯訊號。(2) 在固定予體噻吩上之支鏈個數 (x) 後，逐漸增加噻吩個數 (n) 將會使得主鏈扭轉自由度增加、進而減少π-共軛長度而使得能隙增加。如PTI2T-2C8、PTI3T-2C8及PTI4T-2C8其Egsol分別為1.00、1.18以及1.25 eV，Egfilm則分別為0.87、1.14及1.20 eV。另外GIWAXS研究也指出PTI3T之層狀堆疊距離 (d(100)) 也較PTI2T及PTI4T系列寡共聚物來得大，這是由於予體內不同噻吩個數 (n) 將會使其主鏈擁有不同構形 (n = 2以及4時為中心對稱，n = 3 時則為軸對稱)。最後在結晶方向方面，我們發現支鏈會大幅影響了兩兩分子鏈之間堆疊，若同時配合不同主鏈曲度將會對其結晶排列方式產生影響：當主鏈以中心對稱構形 (n = 2或4) 以及低支鏈依附密度 (x = 0) 下，增強了寡共聚物站立基板能力與分子間π-π作用力，故結晶方向以側向為主；隨著支鏈依附密度增加 (x ≧ 2) 其側向分布將逐漸減少而轉換至面向。若主鏈以軸對稱 (n = 3) 使其以「鋸齒形」存在，則隨著支鏈依附密度 (x) 不同其結晶方向將有極大之變化。當支鏈依附密度低時 (x = 0) 其仍為側向結構，但一旦支鏈數目增加 (x = 2) 其結晶方向轉換為具極高規整性之面向排列；但更高之支鏈數目 (x = 4) 則破壞了此寡共聚物鏈堆疊，使得其成為一非晶質結構材料。在循環伏安儀分析中我們也發現了本系列寡共聚物具明顯氧化峰值，故其將可勝任做為一p型半導體材料。在熱性質分析中也發現其為一熱穩定性材料，部分材料甚至其熱裂解溫度高達390°C。 此研究結果成功地提供從分子設計的角度做出所欲結晶方向導電高分子之路徑，並預期其應用在適當光電元件中會有極佳表現。
並列摘要
Conducting polymers are important materials in the fabrication of optoelectronic devices. The orientation of the polymer crystalline is critical to achieve high charge carrier mobility and performance in the devices. In general, the edge-on orientation is preferred for utilities of organic field-effect transistors, and the face-on orientation is desired for applications in organic solar cells. However, the detail mechanisms behind the cause of favored crystal orientation of conducting polymers are not clearly understood currently. The systematic studies of molecular design and synthesis for the desired crystal orientation are relatively few present in the literatures. In this research, we design and synthesize eight different co-oligomers, poly(thienoisoindigo-alt-(xoctylthiophene)n, PTInT-xC8, where thienoisoindigo unit (TI) acting as an acceptor is conjugated with thiophene (T) as a donor; n is the number of thiophene in a donor segment, and xC8 denotes the number of n-octyl substituted on thiophenes in a repeat unit. We could tailor the crystal orientation by varying two parameters: (1) side-chain attachment density by changing the number of n-octyl substituents (x) on the thiophenes, and (2) the backbone curvature by changing the number of thiophene (n). The co-oligomers were synthesized successfully via either Stille coupling reaction or direct arylation polycondensation, and were characterized by nuclear magnetic resonance spectroscopy (NMR) for chemical structures, matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF) for molecular weights, grazing-incidence wide angle X-ray scattering (GIWAXS) for crystalline structures, ultraviolet－visible－near infrared absorption spectroscopy (UV－Vis－NIR) for optical properties and cyclic voltammetry (CV) for band gap determination, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) for thermal properties. The differences among 8 co-oligomers are also studied in details by modeling the molecules by the density functional theory (DFT) calclations. The results indicate that: (1) increasing the number of substituted n-octyl chains (x) on the donor segment of thiophenes while the number of thiophenes (n) is fixed, the polymer backbone will be twisted dramatically as shown by molecular modeling. From the absorption spectra, the relative absorption peak intensity of electrons excited from donor’s HOMO to donor’s LUMO (λ1) is increased while the one symbolized intramolecular transfer (λ2) is decreased. The GIWAXS results also indicate that the π-π stacking distances (dπ-π) in the direction of (010) increase. For example, the dπ-π of PTI2T-0C8 and PTI2T-2C8 are 3.57 Å and 4.83 Å, respectively; the π-π interaction of PTI2T-4C8 is weakened enough that no diffraction peaks could be defined. (2) Increasing the number of thiophene (n) on donor segment while fixing the number of substituted n-octyl chains (x) on thiophenes will distort the backbone due to the increasing freedom of torsion, which shortens the π-conjugation length, and the band gap is increased accordingly. For example, Egsol of PTI2T-2C8, PTI3T-2C8 and PTI4T-2C8 is 1.00, 1.18 and 1.25 eV, respectively; Egfilm is 0.87, 1.14 and 1.20 eV, respectively. The GIWAXS study shows the lamellar distances (d(100)) of PTI3T series are larger than those series of PTI2T and PTI4T. The results are due to the comformation differences between the odd and even number of thiophene (n) in donor segment on the backbone. The even number n gives a centrosymmetric structure whereas the odd number exhibits an axisymmetric configuration. Meanwhile, the steric hindrance of side-chain also influences the packing of polymer backbones. Thus, the centrosymmetric polymers (n = 2 or 4) exhibit the edge-on crystal orientation of long-range order due to the strong π-π interaction. Their orientation will be gradually changed to face-on orientation when the number of side-chains (x) on thiophenes equal to 2 or larger. The change of orientation is far more dramatic for axisymmetric co-oligomerss when the number of side-chains is changed due to the the nonlinear characteristic of backbone which are not suitable for standing on substrate. The edge-on packing is observed for co-oligomers (PTI3T-0C8) without side-chain attached on thiophenes. However, the packing orientation becomes well-ordered face-on with two side-chains added onto the donor segment of thiophenes, and amorphous structure is obtained with adding two more side-chains. The CV results also reveal that most of the PTInT could be used as a p-type semi-conductor materials, and part of the co-oligomers are quiet stable at high temperature with the decomposition temperature as high as 390°C. The outcomes of this research provide pathways for designing and synthesizing conducting polymers with desired crystal orientation for specific optoelectronic device applications.