利用新穎方法提昇有機薄膜電晶體元件之性質

Novel Methods of Performance Enhancement for OTFT

葉哲源(Je-Yuan Yeh)

指導教授 ： 蔣 見 超

國立中正大學/工學院/化學工程學系/博士(2014年)

摘要

在1930年，場效電晶體(Field-effect Transistor；FET)的概念首次被Lilienfeld所提出，且第一顆無機金屬氧化物半導體FET(MOSFET)是在1960年被Kahng和Atalla兩人製造出來，相較於無機電晶體，有機薄膜電晶體在1970年時才被提出來討論。 在1983年F.Ebisawa等人是以聚乙炔(Polyacetylene)為主動層所製造出來的有機薄膜電晶，自此之後，有機薄膜電晶體才被大量的研究。 但是，由於有機薄膜電晶體相對於單晶無機半導體其遷移率(Mobility)較低，從這點可以很明顯的看出有機薄膜電晶體並不適合被應用在需要高切換速度的元件上，不過有機薄膜電晶體具有製作溫度低(~180℃)的優點，因此可被應用在可撓性(Flexible)的塑膠基板上，而且其製作成本相對也較低，因此有機薄膜電晶體(Organic Thin Film Transistor；OTFT)可以應用於電子書、電子紙或電子識別證的主動式矩陣驅動中，另外也可以應用在顯示器方面，形成主動元件，所以有機薄膜電晶體在學術與商業應用上均有很大的潛力存在。 本論文共分為五個章節，第一章為緒論，內容包括共軛高分子和有機薄膜電晶體的簡介，第二章則是以高立體規則度聚(三-己基噻吩) (rr-Poly(3-Hexylthiophene)；rr-P3HT)高分子當作主動層(Active Layer)，並將rr-P3HT做末端官能化改質置換成一級醇，藉此利用醇類的官能基能與親水性的SiO2基版產生氫鍵以利親油性的rr-P3HT高分子可以規則排列在親水性的SiO2基版上，以便提升元件的電性。其結果顯示出相較於沒有官能化的rr-P3HT，有官能化的rr-P3HT(P3HT-OH)其OTFT元件的開關電流比(On/Off ratio)提升了114%，電洞遷移率提升12.4%，啟動電壓降低23.3%和表面粗糙度降低了30.1% ，但因為在rr-P3HT與SiO2基版中間並沒有較優良的介電層情況下造成了元件的漏電流(Leakage Current)與啟動電壓(Threshold Voltage; Vt)過大的缺點，因此在第三章中我們利用具有穩定形式電荷(Stable Formal Charge)結構的SAM (Self-assembled Monolayers; 3-Azidopropyltriethoxysilane)來修飾SiO2表面，並且為了提升更高的元件電性，我們同時也將主動層(rr-P3HT)換成電性更佳的有機小分子五環素(Pentacene)，藉此提升更高的元件電性，以改善第二章元件的缺點。 在文獻回顧中，絕大部分的介電層均使用自我組裝層(Self-Assembled Monolayer)來修飾SiO2基版，其目的可降低元件漏電流、降低啟動電壓和增加五環素的排列更趨向於高規則性以利提升元件性質，但所有的自我組裝層都不具有穩定形式電荷(Stable Formal Charge)的結構。因為曾經有人證實具有該結構的材料可增進太陽能電池的光電性質，故在第三章中我們利用3-疊氮丙基三乙氧基矽烷(3-Azidopropyltriethoxysilane)當作新的OTFT介電層材料，並使用化學鍵結的方式來修飾SiO2的基版表面，最後再與一般常見的烷基類矽烷(Alkyl Siliane)做比較，其結果顯示有疊氮修飾後的元件能夠大幅降低元件的漏電流和Off current，藉此提升元件的開關電流比(On/Off ratio)，除此之外，並沒有大幅提昇元件的其它電性，原因在於3-疊氮丙基三乙氧基矽烷並沒有大幅提昇介電層應該具有的高介電常數(Dielectric Constant; K)，且文獻顯示出若使用矽烷類來修飾基版表面時，最大的限制為基版表面必須要有可做化學反應的羥基(Hydroxyl Group)，換句話說，矽烷類的SAM只能用在SiO2的基版，若要用在其它基版(如ITO glass或可撓式塑膠基版)時則不行。 為了一次解決界電層受限於基版種類的問題，因此在第四章中，我們將開發出所有基版均可使用且必須具有高介電常數的介電材料。 文獻顯示出以ITO為基版時，大部分均使用高分子當介電層材料(如聚乙烯苯酚Poly(Vinyl Phenol) ; PVP)，原因在於高分子可以經由旋轉塗佈(Spin Coating)的方式製作在各種不同的基版上。 雖然PVP為常用的高分子介電材料，不過因PVP的介電係數並不高(K<4)，且文獻顯示以高分子當介電材料時，其膜厚通常要高於300nm並且需要較大的啟動電壓(>60V)，如此方可降低Pin-Holes效應，因此在第四章中我們合成出新穎的具有高介電常數(K>4)的高分子介電材料，同時最低膜厚能夠減低(＜158nm)以降低啟動電壓(＜1V)，且有優良的OTFT元件性質。我們選用壓克力系列為主的高分子Poly(MMA-co-HEMA-g-AA)當做新的介電材料(因壓克力系列的高分子介電常數為高分子中最高的)，最後再與一般文獻交聯的PMMA介電層做OTFT電性比較，其結果顯示出在相同的介電層厚度下我們的高分子具有極高的介電常數(K=4.2)，優於所有文獻的PMMA(K~3.9)，且厚度也小於300nm (158nm)，這樣所呈現出的OTFT元件性質(μ = 0.057max (or 0.031avg) cm2s-1v-1, On/Off = 3.2x104, Vt = -0.57 V, 158 nm, Ci = 23.52 nFcm-2 和 K= 4.2)極佳，探討其原因在於我們所開發出的新穎高分子介電材料在電洞場中(Electron-hole Field)產生了意想不到的離子極化(Ionic Polarization)結構，因此造成極高的介電常數，所以才會呈現出優良的OTFT電性。另外，新開發的高分子介電材料其成膜的溫度極低(140℃)、合成的方法極為簡單和成膜後透明度極佳(因PMMA具有壓克力玻璃之稱)等優點，所以我們所開發的新穎高分子是非常適合當作OTFT的介電層，預計未來必將被廣泛使用，並取代舊有的高分子介電材料。 最後第五章則是介紹本論文中實驗所需的原理、設備與儀器。 總而言之，由第二章到第四章的實驗過程中我們學習到若要將有機薄膜電晶體的元件性質提升必須要提高主動層的分子排列、降低介電層的厚度和提高界電層的介電常數，如此方可大幅提升有機薄膜電晶體的元件性質。

關鍵字

多種極化結構 ； 穩定形式電荷 ； 自我組裝單分分子層 ； 疊氮基 ； 末端官能化羥乙基 ； 聚(3-己烷基噻吩) ； 有機薄膜電晶體 ； 聚(壓克力)閘極介電層 ； 高介電常數 ； 超低啟動電壓

並列摘要

Field effect transistors (FETs) were first proposed by Lilienfeld in 1930, and then the first inorganic metal-oxide-semiconductor FET (MOSFET) was fabricated by Kahng and Atalla in 1960. In 1983, F.Ebisawa et al. used polyacetylene as a novel organic active layer material to produce the first organic field-effect transistor (or organic thin film transistor; OTFT). After that time, OTFT have been widely studied. Nonetheless, OTFT are not suitable for applications requiring high switching speed due to their lower mobility and lower On/Off ratio than inorganic thin film transistors (e.g. single-crystal inorganic thin film transistors). However, advantages of OTFT are many, such as a low- temperature production(~ 180 ℃), lightweight, low cost, solution processability, simple packaging, compatibility with flexible plastic substrates and potential applicability to flexible device display…etc. Therefore, OTFT can be widely used to make e-books, e-paper, electronic identification cards for active matrix driving, sensors, flexible circuits, radio-frequency identification (RFID) devices and display operation…etc. In the future, OTFT will have even greater applications in academic and industry.

My dissertation is divided into five chapters. In chapter one, the introduction of conjugated polymer and OTFT is provided. In chapter two, the synthesized hydroxyethyl-terminated poly (3-hexylthiophene) (P3HT-OH) is used as a novel active layer material for fabrication OTFT. The hydroxyl group creates the hydrogen bounding between the hydrophobic P3HT polymer chain and the hydrophilic surface of SiO2 wafer, and this leads to regulated polymer chains on the surface of SiO2 wafer and enhances OTFT properties. Compared against the hydrophobic P3HT without hydroxyl group, hydroxyl groups at a 7.5% weight content lead to more chain regularity when polymer is bonded to SiO2 wafer surface and thus enhance the performance of OTFT device, such as an 114.2% increase in On/Off ratio, an 12.4% increase in mobility, a 23.3% decrease in threshold voltage and a 30.1% decrease in surface roughness. However, in a architecture (Si/SiO2/P3HT-OH) without any extra dielectric layer between P3HT and SiO2, many disadvantages appear, such as large leakage current and threshold voltage.
To eliminate those disadvantages, in chapter three, we used a self-assembled monolayer, 3-azidopropyltriethoxysilane as a novel dielectric layer to modify the interface between the SiO2 and active layer, and replace simultaneously P3HT-OH with pentacene. Compared to the commonly used alkyl siliane C18 dielectric, 3-azidopropyltriethoxysilane which possesses stable formal charges is far more effective in increasing the On/Off ratio of OTFT device with an improvement of nearly three orders of magnitude. However, this novel SAM does not significantly enhance the other electrical properties except the On/Off ratio duo to the lower dielectric constant of 3-azidopropyltriethoxysilane than alkyl silane C18. Moreover, literatures show the use of silane as a modifying dielectric material is limited to SiO2 surface because the SiO2 surface has reactive hydroxyl groups. In other words, silane types of SAM can not be used in ITO or flexible substrates. In order to fix such problems, we will develop a new dielectric material with a high dielectric constant for all substrates in next chapter. Literatures show that polymeric dielectric layer materials (such as polyethylene phenol Poly (Vinyl Phenol); PVP) was used in ITO substrate because the polymer can be fabricated via spin-coating method on ITO glass. Although PVP is a commonly used polymeric dielectric material, its dielectric constant of PVP is not high (K <4), and literatures show that minimum thickness of polymeric dielectric layer is usually higher than 300nm and a higher operating voltage, 60V are needed for lowering the Pin-Holes effect. In order for OTFT devices to have a dielectric layer with high K (K>4.0)、small minimum thickness (< 300nm) and an ultra-low operating voltage ( < 1V), herein, in chapter four, we aim to design and synthesize an acrylic-based and multiply-polarized dielectric polymer material (because acrylics has the highest dielectric constant than all other polymers).
In chapter four, we report a novel crosslinked polyacrylic dielectric material, poly(MMA-co-HEMA-g-AA), which is made via a crosslinking esterification between a specifically synthesized poly(MMA-co-HEMA) and poly(acrylic acid). When used in an ultra-low voltage (operating voltage Vt <1V) pentacene-based organic thin-film transistor (OTFT) as a Gate dielectric, poly(MMA-co-HEMA-g-AA) results in remarkable device performance and exhibits a higher dielectric constant (K=4.2) than those traditionally used crosslinked PMMA (K<3.9; Vt >1V) because of its multiple polarization structures, such as the dipole polarization from ester group of MMA and HEMA, and the ionic polarization from carboxylic acid group of AA. Moreover, the crosslinking esterification temperature for forming poly(MMA-co- HEMA-g-AA) (140℃) is lower than the glass transition temperature (Tg = 185℃) of most widely used plastic substrates, including poly(ether sulfones), and this is very advantageous for being a Gate dielectric. Furthermore, poly(MMA-co-HEMA-g- AA), being an acrylic (refractive index 1.4914 at 587.6 nm), transmits up to 92% of visible light (at 3 mm thickness) and reflects about 4% from its surface, and thus are more suitable than other polymers for fabrication of highly transparent OTFT devices, e.g. electronic papers.

Overall, the experimental procedures from the chapter two to chapter four show that the dielectric constant and regulated active layer play very important roles in improving the performance of organic thin film transistors.

並列關鍵字

Multiply-polarized Structure ； Stable Formal Charges ； Self-Assembled Monolayer ； Azido ； Hydroxyethyl-terminated ； Poly(3-hexylthiophene) ； Organic Thin Film Transistors ； Polyacrylic Gate Dielectric ； High Dielectric Constant ； Ultra-low Voltage

參考文獻

[3] R. L. Greene, B. R. Street, L. J. Suter, Phys. Rev. Lett., 34 (1975) 577.

[12] C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater., 11 (2001) 15.

[14] J. L. Segura, N. Martin, J. Mater. Chem., 10 (2000) 2403.

[17] Q. B. Pei, G. Yu, C. Zhang, Y. Yang, A. J. Heeger, Science, 269 (1995) 1086

[23] G. Horowitz, M. E. Hajlaoui, Adv. Mater., 12 (2000) 1046.