研究新穎溶液製程介電層並應用於可撓式有機薄膜電晶體

Abstract

近期有機材料在電子產品中的應用漸受歡迎，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。在本篇研究中，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(OTFT)。 在本篇研究的第一部分，我們利用製作可撓性金屬-介電層-金屬(MIM)電容，探討有機材料聚丙烯腈(PAN, Polyacrylonitrile)薄膜，以及聚苯乙烯(PS, Polystyrene)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以及接觸角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(polyimide)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 OC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 OC的方式，使得電性回復。

Organic thin film transistors (OTFTs) are an important class of electronic devices owing to their easy processability, light weight and flexibility. They also offer a better understanding of the material properties as used in it through structure property relationship study. It can be foreseen that this new technology can play a leading role in the future studies for commercialization of electronic devices based on OTFTs. In this study, we have utilized the organic materials to fabricate flexible devices namely, metal-insulator-metal (MIM) capacitor and organic thin-film transistor (OTFT) by simple solution process. In this study, we have analyzed the potential of dielectric material PAN (Polyacrylonitrile) and PS (Polystyrene)-P123 (Pluronic® P123 Block Copolymer Surfactant) through fabrication of metal insulator metal capacitor devices. Sol gel process and spin coating techniques are applied to deposit PAN and PS-P123 dielectric layer. We have demonstrated several parameters of PAN layer fabrication conditions like solubility, concentration and annealing. After extracting best conditions from this part, we drop casted a PS-P123 layer penultimate to the PAN layer which has acted as a buffer region. This bilayer dielectric strategy has prevented the direct contact between organic semiconductor and the high k dielectric PAN, and provided a lower surface energy. The capacitor with bilayer structure have attained better electrical performance and reliability as compared to their monolayer counterparts. FTIR and contact angle analysis study has further revealed other interesting properties of the polymer dielectric materials. The conditions for fabrication of PAN and PS-P123 based bilayer dielectric was optimized before employing it further as a gate dielectric in OTFT based on DH4T (α,ω-dihexylquaterthiophene) as an active semiconducting layer. The OTFTs devices are fabricated on the flexible polyimide (PI) substrate. The best electrical performance was acquired by drop-casting the DH4T solution on PI substrate followed by annealing at 90 oC for 30 min. The ON/OFF ratio can attain 103, and the mobility is at the order of 10-2, which is comparable to the reported researches. Different top contact metals electrode are also tested, and the Ag and Au are found to be the better candidate. Several bending situation are applied, to explore the charge carrier hopping mechanism involved in deterioration of electrical properties of OTFTs. Finally, reliability experiments are conducted, which revealed that the electrical performance of OTFTs was deteriorated after five days, but the electric property can be recovered by means of re-baking the devices at the temperature of 90 oC.