旋塗式有機主動層薄膜電晶體的製程改善與可靠度分析

Abstract

近年來，有機顯示器成為市場上的前瞻技術，利用有機發光二極體和有機薄膜電晶體已經可以製作出低成本、可彎曲、全彩的平面顯示器。本論文以“bottom contact”的結構，SiO2為絕緣層，利用旋塗的方式成長有機薄膜，成功地製作出以poly(3-hexylthiophene)、簡稱P3HT為有機材料的有機薄膜電晶體。 在第二章當中，我們利用二甲苯和氯仿這兩種不同的溶劑來溶解P3HT，可發現當P3HT的重量百分濃度高於0.3%時，就無法完全溶解於二甲苯中，並留下許多微小顆粒，而氯仿則能夠溶解高濃度的P3HT (> 2 wt%)，顯示氯仿對於P3HT而言是一種極佳的溶劑。此外，我們發現以氯仿為溶劑所製作出來的有機薄膜電晶體能夠有效地抑制異常的閘極漏電流，當P3HT的重量百分濃度超過0.8%時，會有明顯的bulk漏電流現象產生，以0.3%之P3HT製成的薄膜電晶體則可以獲得最低的表面粗糙度以及較佳的元件特性。 在第三章當中，有機薄膜電晶體分別經由不同時間的真空處理、氧氣處理、氮氣處理、泡水處理或閘極偏壓stress；由於氧氣是P3HT的摻雜物之一，經由氧氣處理之有機薄膜電晶體的臨限電壓值與漏電流會隨著氧氣處理時間而大幅增加，氮氣和真空處理都可以降低P3HT薄膜內的氧含量，故可有效改善元件特性；至於泡水處理則對於有機薄膜電晶體的特性不會產生明顯的影響。在閘極施加正的偏壓stress會使P3HT內部的耦極重新排列，並造成臨限電壓的正偏移，閘極負偏壓stress則會使臨限電壓往負方向偏移。 在第四章當中，我們利用鉑、金、鎳和鈦來當作有機薄膜電晶體之源極和汲極的電極材料，由於鉑跟金的功函數大於P3HT，兩者都能夠和P3HT形成較佳的歐姆接觸，以鎳為源極/汲極電極材料的電晶體，則在低汲極電壓下產生crowding effect。我們亦改變附著層金屬和接觸層金屬的厚度比例來觀察其對電性的影響，但是實驗發現接觸電阻受其影響的程度非常小。

Recently, active matrix organic diode displays (AMOLEDs) become the most advanced technology in the market; organic light-emitting diodes (OLEDs) and organic thin film transistors (OTFTs) enable the fabrication of low-coat, flexible, full color flat panel displays. In this paper, organic thin film transistors based on poly (3-hexylthiophene) (P3HT) with the “bottom contact”structure, SiO2 as insulating layer, organic active layer grown with spin-coating have successfully been demonstrated. In chapter 2, we used two kinds of solvents, xylene and chloroform, to dissolve P3HT. While weight percentage is as high as 0.3%, the P3HT cannot completely be dissolved in xylene and then many clusters of undissolved P3HT powder is observed. However, chloroform can dissolve high weight percentage of P3HT (> 2 wt%). Therefore, it is proved that chloroform is a good solvent for P3HT. Furthermore, we observed that the anomalous gate leakage current was suppressed by fabricating OTFTs with chloroform solution. As the weight percentage of P3HT is above 0.8%, it would cause obvious bulk leakage current. In summary, the P3HT OTFTs fabricated by 0.3% chloroform solution can acquire the lowest surface roughness and the better performance of devices than others. In chapter 3, the OTFTs are treated with vacuum treatment, O2 treatment, N2 treatment, immersed in water or gate bias stress for different time. Since oxygen is a kind of dopant for P3HT, threshold voltage and leakage current of the OTFTs drastically increase with O2 treatment time. Vacuum and N2 treatments can be used to recover some of the lost performance through vacuum-induced expulsion of absorbed oxygen. As regards the OTFTs being immersed in water, the performance of electrical characteristics would not be affected. When a positive bias was applied to the gate electrode, it would lead to the dipole moment arranging in the P3HT polymer and positive threshold voltage shift. Negative gate bias stress causes negative threshold voltage shift. In chapter 4, we use Pt, Au, Ni or Ti as S/D contact materials of OTFTs. Because the work function of Pt and Au are larger than the work function of P3HT, they can form better ohmic contact with P3HT than others. Nevertheless, it was observed that the crowding effect was occurred at the small drain bias for Ni as S/D contact material of OTFTs. Secondly, we adjusted the thickness ratio of adhesion/contact metals, such as Ti/Pt and Ti/Au. From experiment results, it was observed that contact resistance is weakly dependent on the thickness ratio of adhesion/contact metals.