論共軛分子的階級結構對有機場效電晶體性能影響

Abstract

在本論文中，我們首先透過分子設計討論共軛分子的結構-性能關係，接著探討結晶多型態與晶體缺陷對共軛分子的電荷遷移率影響，到透過濕式製程單晶生技術，控制晶體形貌以符合 有機場效電晶體 (organic field transistor；OFET) 元件的應用，以縱向角度探討影響材料電荷遷移率的因子。 首先，在分子尺度中，階梯型共軛高分子的分子結構具有較佳的電荷傳遞特性，但實際研究中，這類分子的電荷遷移率卻偏低，所以在第一部份研究，我們將七元環并苯的 DTCC 單元與 1T 、 3T、 BDT、 TT 連接單元相接，形成 PDTCC 交替高分子，透過光電與 X 光繞射分析，PDTCC-3T 的固態結構具有最規則的分子排列與較強的分子間電子耦合，由於 DTCC 與 3T 為軸對稱分子，因此共聚物的高分子主鏈屬於直線型，而當 DTCC 和具有中心對稱的單元 (BDT 與 TT) 相接時，彎取型的共軛高分子較難以形成規則的固態結構，雖然 PDTCC-1T 一樣為直線型分子，但 1T 長度過短，因此無法降低烷基側鏈的立體障礙，造成 π-π 堆疊結構被破壞，在此分子系列中，PDTCC-3T 具有最高的電洞遷移率：µh = 0.0136 cm2/Vs。第二部份研究，我們嘗試透過物理硬化的方式，提升共軛高分子主鏈的共平面性與剛硬性，我們將雙鍵單元引入 PVTh4FBT 共軛高分子，並且探討烷基側鏈在主鏈上的取代位置與密度對其固態結構的影響，從光電與繞射分析可知P1高分子具有最佳的共平面性與剛硬性，因此能引導出較規則的晶體排列與分子取向性，並且 P1 的 µh = 0.26 cm2/Vs。 第三部份研究：溶劑在濕式製程中扮演很重要的角色，為了探討溶劑效應對C60 結晶行為與電子遷移率 (µe) 的影響，首先利用 PDMS 輔助長晶法 (PAC) 生長出 C60 晶體陣列，在形貌與結構分析中發現從不同的溶劑類型生長出 C60晶體陣列有不一樣的結晶習慣與溶劑化晶體結構，進一步以熱退火處理移除溶劑分子，因此 CS2 條件下的 C60 ¬晶體陣列有最高的 µe = 1.70 cm2/Vs。第四部份研究，我們利用PAC法生長 TIPS-PEN 晶體陣列，從低沸點溶液長出的TIPS-PEN 晶體透過 GIXD與電子繞射分析，發現晶體的長軸有晶格失配與低角度晶界生成，由於在低沸點的溶液環境下，TIPS-PEN 晶體生長製程時間較短，所以分子沿著晶體長軸方向排列時，容易產生出晶格失配並且造成µh 下降，而當提高沸點時，增加 TIPS-PEN 的生長製程時間，因此 TIPS-PEN 晶體長軸方向上的晶格失配與晶體缺陷能有效降低，所以 TIPS-PEN在甲苯條件下具有高的 µh = 2.0 cm2/Vs。第五部分研究: C60 分子在分子尺度下具有良好的光電特性，因此在分子自組裝領域中，C60 單元為突出的結構單元 (building block)。因此在此研究，我們探究新穎的熱致型 C60 液晶分子 Cn-azo-C8-C60 分子 (n = 4, 7, 8, 9, 12)，此系列分子，由 C60 單元和不同末端烷基取代的偶氮苯，並且透過八個碳的亞甲基鏈將 C60 單元與偶氮苯相接形成C60 液晶分子，在熔融態以下，此系列的 C60 液晶分子形成側鏈層與C60層交替的層狀液晶結構，並且我們成功透過偶氮苯側鏈層的熔融與結晶機制控制液晶結構中 C60 堆疊的層數，當末端烷基鏈數 n < 9 時，由於側鏈無法結晶，使得 C60 液晶分子自組裝成四層C60 堆疊的層狀結構，當 n = 9, 12，側鏈層在常溫下形成有序結構，因此 C9 和 C12 分子自組裝成兩層 C60 堆疊的層狀結構進一步當側鏈單元熔融時，C9 和 C12 分子則會進一步組裝成四層與三層的層狀液晶結構，在未來將進一步探討此系列材料在有機場校電晶體的電荷傳遞性能。

In this study, we give the systematic study on the charge transport properties of conjugated molecules from its structure-properties relationship to the influences of polymorphism and crystal defects on the charge transports of crystal arrays. First, the roles of the comonomeric units in reaching high hole mobility (µh) of copolymers containing a heptacyclic arene unit (DTCC) were investigated in this study. A series of four DTCC-based alternating copolymers, PDTCC-1T, PDTCC-3T, PDTCC-BDT, and PDTCC-TT, were synthesized from the copolymerizations between DTCC and comonomeric units including thiophene (1T), terthiophene (3T), benzodithiophene (BDT) and thienothiophene (TT) units. Optoelectronic and 2D-WAXD studies revealed that strong electronic interaction and ordered solid-state structure were only observed in PDTCC-3T. It is attributed to the combination of two axisymmertric units, DTCC and 3T, linearized the polymer backbone, leading to a compact solid-state packing and high µh = 0.0136 cm2/Vs. Furthermore, the short axisymmeritric 1T although results in a linear backbone of PDTCC-1T, comparing to 3T, it is too short to effectively reduce interchain steric hindrance caused by the side chains on DTCC. Thus, effective π−π stacking is hindered in PDTCC-1T, resulting in low µh. Second, we used the physical rigidification to enhance the backbone coplanarity and rigidity of conjugated polymers. With the introduction of vinylene to the PVTh4FBT backbone, a series of three PVTh4FBT polymers containing different alkyl side chain placement was synthesized. The thermochromic behaviors and DFT calculations indicated that the backbone coplanarity and rigidity of the PVTh4FBT polymers can be effectively modulated by adjusting the side chain position and density. Higher ordered and better oriented edge-on lamellar packing was formed by P1, which possesses the most rigid and planar backbone among the three polymers. Thus, P1 delivered the highest µh = 0.26 cm2/Vs among the three analogues Third, the solvent effects on the C60 crystallization behavior and electron mobility (µe) were discussed. Using the PDMS–assisted crystallization method, the C60 crystal arrays were grown from CS2, m-xylene and ODCB solutions. In the morphological analysis and structural characterization, C60 crystal arrays from different solvent conditions have distinct crystal habits and solvated lattice structures. After the thermal treatment, the solvents were removed from C60 solvated lattice structures. Thus, after the thermal treatment, C60 crystal arrays (CS2) with better morphology and non-solvated lattice structure gives highest µe = 1.70 cm2/Vs. Fourth, in order to study the relationship of crystal-growth rate and crystal defects, the TIPS-PEN crystal arrays were prepared from low and high Tb solvents using PAC method. In the diffraction analysis, lattice mis-orientation and low angle boundaries were found in the long axis of crystal arrays from low Tb solvents, dichloromethane and CS2 whih lead to the higher crystal-growth rates. Using high Tb solvents, toluene and chlorobenzene, the lattice mis-orientation in the long axis of crystal arrays can be reduced effectively along with lower crystal-growth rate. Therefore, among the solvent conditons, TIPS-PEN crystal arrays from toluene solution have the highest µh = 2.0 cm2/Vs. In the final study, we successfully characterize the self-assembly structures of a family of new thermotropic C60 liquid crystals: Cn-azo-C8-C60 (n=4, 7, 8, 9, 12). Since C60 molecule performs excellent electrical and optical properties in the molecular scale, it is an ideal candidate as the building block in the field of molecular self-assembly. Cn-azo-C8-C60 are composed of three segments: 1. C60 unit; 2. azobenzene segment with long alkoxyl chains (n=4, 7, 8, 9, 12) and 3. octa-methylene chain as a flexible spacer connecting the other segments. Cn-azo-C8-C60 molecules are self-orangized into the semetic lamella structures with C60 layer sandwiched by side-chain layers. Furthermore, the number of C60 layer in the lamella superlattice is successfully controlled by phase behavior of azobenzene segment chains. For n= 4-8, due to shorter length azobenzene segment chains, the fullerene molecules are assembled to quadruple-layer two-dimensional (2D) fullerene crystals sandwiched between alkyl-chain layers, driven by π- π interaction. In contrast, for n = 9, 12, The competition between crystallization of azobenzene segment chains and π- π interaction leads C9 and C12 molecules to form double-layer 2D fullerene semetic liquid crystals. Once azobenzene segment chains become isotropic under heat, π- π interaction takes over the formation of the structures, and causes C9 and C12 to pack into quadruple-layer and triple-layer structures respectively. In the future, the charge transport property of Cn-azo-C8-C60 will be further examined to discuss how the layer numbers of C60 influence the charge transport property。