超分子化學應用於有機薄膜電晶體設計與製造

Abstract

近年來，奈米材料被應用於許多領域中，本論文旨在探討高分子半導體在有機薄膜電晶體(OTFTs)中的場效載子遷移率對於電荷載子傳輸行為的影響。由於有機半導體層研究的演進，使有機薄膜電晶體的研發方向朝向發展新的高分子半導體材料或是在製程上改良以製造出性能優越的元件。 本研究中，我們導入超分子化學(supramolecular chemistry)的概念合成在有機薄膜電晶體結構裡的電雙層材料與高分子半導體，並藉由探討高分子的自組裝行為提升有機薄膜電晶體元件效率。在第四章節中，我們直接利用薗頭耦合催化反應(Sonogashira coupling)成功合成出一系列的高分子電雙層材料。如1,3,5,7-tetrabromoadamantane; 1,3,5,7-tetrachloroadamanatane; 1,3,5,7-tetraiodoadamantane and 1,3,5,7-tetrauraciladamantane (AdUr4)。當這些材料能夠在室溫下形成具有高度的穩定性的薄膜時，能夠有效降低有機薄膜電晶體漏電流現象，可進一步被導入以聚(3-己基噻吩)(Poly(3-hexylthiophene, P3HT)為主p-型有機薄膜電晶體結構裡的閘極介電材料。我們同時也成功地製備以AdUr4為介電材料，聚醯亞胺(Polyimide，PI)為基板之可繞式有機薄膜電晶體。我們藉由掠角繞射法(Grazing Incidence X-Ray Diffraction, GIXRD)和原子力顯微鏡（Atomic Force Microscope, AFM）深入探討AdUr4具有誘導P3HT側鏈的分子軸方向，P3HT側鏈分子軸方向正是有機薄膜電晶體的電荷傳導方向。然而在此可繞式元件上，我們量測到相當優良的開/關電流比值 104 與高場效載子遷移率0.15 cm2 V-1 s-1。 有機薄膜電晶體是由有機共軛高分子或寡分子材料做為其主動層而製備出的電晶體，與傳統之無機電晶體比較，有機薄膜電晶體具有在低溫下製作的優勢。在第五章節中，我們以薗頭耦合催化反應(Sonogashira coupling)成功地製備出碗形的寡分子材料CbzTPAU2 (Mw = 2169)。藉由超分子化學的導入，CbzTPAU2端基的尿嘧啶(Uracil, U)基團會以四單元自組裝成碗形分子。由於鏈段聚集喜好溶劑的程度不同，分子間氫鍵作用力的大小也不盡相同，當CbzTPAU2在形成碗形超分子結構時，是以中心 2,7-二取代的咔唑(2,7-disubstituted carbazole)端基，以分子間氫鍵作用力的方式連接TPAU2上的笨環化合物(1,4-bis(decyloxy)-2,5-diethynylbenzene)。為了深入探討此材料在奈米階段性的自組裝行為。在AFM得知，溶劑極性的不同會影響到氫鍵之平衡常數，進而影響到氫鍵平衡常數與自身氫鍵常數比例。在此章節最後，我們也將此寡分子材料CbzTPAU2導入p-型有機薄膜電晶體結構中的電荷輸送層，在此寡分子材料我們也量測到相當優良的開/關電流比值103 與高場效載子遷移率0.167 cm2 V-1 s-1。 有機/高分子材料在電子元件的應用漸受歡迎，主因為其製程相對容易，且對於相關材料性質的了解也日益增加，可以預見這種新科技將會是未來研究的主軸。在第六章節中，我們使用有機材料和簡單的溶液製程，製作可撓性金屬-介電層-金屬(MIM)電容，以及有機薄膜電晶體。在本章節的第一部分，我們利用製作可撓性金屬-介電層-金屬電容，探討有機材料聚丙烯腈(Polyacrylonitrile, PAN)薄膜，以及聚苯乙烯(Polystyrene, PS)和共聚物P123(Pluronic® P123 Block Copolymer Surfactant)的混合薄膜，作為介電材料在應用上的潛力。我們使用溶膠-凝膠(sol-gel)法、旋轉塗布(spin coating)的技術製作，沉積聚丙烯腈薄膜以及聚苯乙烯/P123的混合薄膜。對於聚丙烯腈薄膜，我們測試了幾種不同製程條件，並取得最好的製程參數。而聚苯乙烯/P123的混合薄膜則是沉積在聚丙烯腈薄膜之上，作為緩衝區。緩衝區的作用是為了防止作為通道層的有機半導體材料與高介電材料聚丙烯腈的直接接觸，並且提供一個能量較低的表面能。研究結果顯示此種雙層(在聚丙烯腈薄膜上再沉積聚苯乙烯/P123的混合薄膜)的介電薄膜相對於單層聚丙烯腈薄膜，能有更加優異的電性以及可靠性。傅里葉轉換紅外光譜(FT-IR)以及接觸角(Contact angle)的量測顯示了這些有機材料的特性。經過仔細研究聚丙烯腈薄膜和聚苯乙烯/P123混合薄膜的濃度後我們把這種雙層介電材料應用於有機薄膜電晶體的絕緣層。本篇研究的第二部分著重於利用溶液製程的DH4T(α,ω-dihexylquaterthiophene)可撓式有機薄膜電晶體，並使用第一部分研究所製作的雙層介電材料。有機薄膜電晶體是製作在可撓式的聚亞醯胺基板上，以利測量在撓取下的電性表現。利用滴落塗布法(drop casting)沉積DH4T有機半導體層，並且加熱至90 ℃熱退火30分鐘，能得到最佳的電性。開/關電流比值(ON/OFF ratio)可達到103，載子移動率(Mobility)可達到10-2，與其他已發表的研究結果相當。在不同金屬的上電極的比較中，我們發現銀和金作為上電極會得到比較良好的電性。在不同程度的可撓性測試中，發現了載子的躍遷機制(hopping)對彎曲的元件電性的影響。最後進行了可靠性的測試，研究結果顯示有機薄膜電晶體經過5天後電性會有所惡化，但仍可以藉由重新加熱至90℃的方式，使得電性回復。 本篇論文最後，我們將角蛋白(Keratin protein)；一種直接從雞羽毛直接萃取出新穎生物可降解介電材料，經TEM，FT-IR，AFM結構鑑定後，直接以使用溶膠-凝膠(sol-gel)法、旋轉塗布(spin coating)的技術，結合角蛋白製備P3HT製作有機薄膜電晶體。為了更深一步探討角蛋白的結構並了解到角蛋白是如何增強有機薄膜電晶體的效率，從結構中，我們發現到高含量的-版型(-sheet)角蛋白具有非常有良的絕緣特性，能夠進一步促使P3HT的結晶形成約150 奈米寬的奈米帶。我們將角蛋白導入在沉積300奈米厚的SiO2作為閘極氧化層的矽基板上，發現到載子移動率達到0.20 cm2 V‒1 s-1，元件效率更是優良於於P3HT材料或是其他高分子材料的有機薄膜電晶體。我們在AFM中觀察導入角蛋白時，P3HT會形成約5奈米長度奈米晶鬚，而這些高分子晶鬚恰巧是載子傳遞的方向，正是能夠獲得較良好有機薄膜電晶體效率的主要原因。

Most recent work in organic thin film transistors (OTFTs) has been focused on improving the field-effect mobility of charge carriers in semiconducting polymers. The materials used in fabrication of OTFTs and the device design play a significant role in realizing their required electrical performance. The study of new methods to further enhance their electrical performance is essential for their adoption in practical applications. In this direction, this thesis presents a detailed study which explains how supramolecular chemistry concepts can be intermingled with electronics for fabrication of electronic devices with better electrical performance. Initial research has been focused on the development of new dielectric and semiconductor materials. Apart from this the semiconductor-dielectric interface study is also very important for the optimum performance of OTFTs. Herein, in Chapter 4, the detailed synthesis of a whole new family of dielectric materials which are 1,3,5,7-tetrabromoadamantane; 1,3,5,7-tetrachloroadamanatane; 1,3,5,7-tetraiodoadamantane and 1,3,5,7-tetrauraciladamantane (AdUr4) has been reported. The unique ability of these molecules to undergo supramolecular thin film formation at room temperature was analysed for their potential use as an insulator in organic electronic devices. Owing to the good leakage current density property shown by AdUr4 dielectric material it was further employed as a gate dielectric in regioregular poly(3-hexylthiophene), (P3HT) based OTFT. This OTFT device which was fabricated on flexible polyimide (PI) plastic substrate has showed good on/off current ratio (e.g., 2.18 × 104) and high mobility (e.g., 0.15 cm2 V-1 s-1). The semiconductor‒dielectric interface study, has revealed that AdUr4 gate dielectric layer has guided the P3HT molecular chain domains to undergo edge-on orientation, which is the charge transport direction in OTFTs. In this process, the grazing incidence X-ray diffraction (GI-XRD) analysis and AFM study was also employed. For the designing and development of organic electronic devices, the main focus is particularly on the synthesis of new organic semiconductors and dielectric materials. Molecular engineering is another effective strategy, in this direction which has been explored successfully in the Chapter 5, through synthesis of a π-conjugated oligomer CbzTPAU2, with Mw = 2169. This bow shaped oligomer with its core unit made from 2,7-disubstituted carbazole which further has been connected to its end-terminal unit TPAU2 by 1,4-bis(decyloxy)-2,5-diethynylbenzene. The presence of Uracil moiety on end terminals of CbzTPAU2 has triggered the self-assembly of CbzTPAU2 molecules through knitting up of each these single units through four Uracil-Uracil intermolecular hydrogen bonds (U---U) per CbzTPAU2 unit. AFM study was employed to explore the directionality of hydrogen bonding. Further, the effect of solvent polarity on the stability of U---U bonding in CbzTPAU2 oligomers has also been reported here in this study. The potential of these self-assembled CbzTPAU2 oligomers when explored as charge transporting layer in OTFTs has shown p-type behaviour. The OTFT device bottom-gate, top-contact when fabricated on the heavily doped n-type Si wafer with SiO2 as gate dielectric (200 nm) has shown good on/off ratio 3.43 × 103 and with average hole mobility of 0.167 cm2 V-1 s-1. In Chapter 6, the potential of a new bilayer gate dielectric material, which was composed of Polystyrene (PS), Pluronic P123 Block Copolymer Surfactant (P123) and Polyacrylonitrile (PAN) was illustrated through fabrication of metal insulator metal (MIM) capacitor devices and OTFTs. The conditions for fabrication of PAN and PS-P123 as a bilayer dielectric material was optimized before employing it further as a gate dielectric in OTFTs. Simple solution processable techniques were applied to deposit PAN and PS-P123 as a bilayer dielectric layer on PI substrates. Contact angle study was further performed to explore the surface property of this bilayer polymer gate dielectric material. The importance of low k ~ 2.8 dielectric PS-P123 layer has prevented the direct contact between the organic semiconducting layer and high k ~ 5.5 dielectric PAN. This new bilayer dielectric has a k value of 3.7 intermediate to that of PS-P123 and PAN dielectric has successfully acted as a buffer layer. The OTFT devices based on α,ω-dihexylquaterthiophene (DH4T) incorporated with this bilayer dielectric, has demonstrated a hole mobility of 1.37 × 10-2 and on/off current ratio of 103 which is one of the good values as reported before. Several bending conditions were applied, to explore the charge carrier hopping mechanism involved in deterioration of electrical properties of these OTFTs. Additionally, the electrical performance of OTFTs, which were kept in open atmosphere for five days, can be interestingly recovered by means of re-baking them respectively at 90 oC. Further, Chapter 7 reports first time the application of Keratin protein, which is a new biodegradable dielectric material in a direction towards the advancement of solution-processed OTFTs based on P3HT. This so called new biomaterial, is successfully extracted from chicken feathers with its detailed structural analysis with Transmission Electron Microscopy (TEM), AFM and FTIR. This deep in through into the Keratin protein structure, is accomplished to put forward a detailed study about, how a penultimate dielectric layer controls the morphology of semiconducting layer in OTFTs and hence dictates their performance. The high content of -sheet structure in Keratin has provided it good insulating properties and further has guided the P3HT polymer chains to self-assemble and hence form 2 dimensional P3HT nanoribbons of large crystallite size (150 nm). Owing to this the carrier mobility of such OTFTs, 0.20 cm2 V‒1 s-1 in the saturation regime is obtained. It is found to be better than that of P3HT OTFTs based on SiO2 and other common polymer gate dielectrics. This is because in the later case, P3HT forms different architecture, 1D nanowiskers of 5 nm size only as explored by AFM. The biodegradable nature of Keratin protein is explored by providing it as feed to fishes. The water compatible nature of Keratin has further helped to overcome the issue of washing away of the dielectric layer during fabrication of OTFTs by sol-gel method.