Alq3與BAlq之電流傳輸分析與不同濃度WO3摻雜入NPB之電性影響

Abstract

本論文主要利用半導體上的電性量測技術與理論來探討雙層型異質界面有機發光元件，首先對電子傳輸層中所使用的Alq3與Balq這兩種有機材料作大範圍溫度的電流-電壓量測，由實驗中得知有機材料的高阻值特性與電流傳輸機制是和固態理論中半絕緣性的電流傳輸機制是相符的，所以我們利用半絕緣性電流傳輸理論中的SCL電流傳輸公式得到Alq3與BAlq的缺陷能階與缺陷濃度分別為0.145eV、1.89*1018cm-3與0.201eV、4*1018cm-3，並且由實驗數據發現BAlq隨電場變化的程度是大於Alq3。隨之我們再對元件作電容-電壓與電容-頻率量測，我們成功的運用等效電路模型分析雙層型異質界面元件中的電洞傳輸層，並且由實驗中得知在ITO表面以電漿氧氣處理過後可以有效改善電洞由ITO注入NPB的效率。接著探討WO3在串聯式OLED元件中的影響以及將WO3摻雜進NPB後形成p-type的特性變化。在串聯式OLED元件中導通的原因是由於WO3與NPB作用形成p-type與Alq3跟Mg作用形成的p-type之間的p-n介面在外加電場下載子經由穿遂(tunneling)效應所造成。接著利用變溫導納頻譜探討不同濃度下WO3摻雜在NPB的p-type材料的電特性，由實驗得知將WO3摻雜入NPB中可以有效的降低NPB的電阻，並且隨著摻雜濃度的增加（0%∼16.7%）NPB的活化能隨之變小(0.337∼0.176eV)，我們認為在NPB中存在一個缺陷能階為0.337EV而費米能階也被高濃度的缺陷固定在缺陷能接的位置上，隨著摻雜濃度的增加費米能街則巷價帶的位置移動。最後由實驗數據再經由理論計算可以得到各個摻雜濃度下的電洞濃度與電洞遷移率，而電洞濃度隨著摻雜的增加而大量的增加，電洞遷移率則隨著摻雜濃度的增加而略為減少。

In this thesis, the electrical properties of organic heterojunction light-emitting devices were investigated. We measured the current-voltage (I-V) characteristics of electron transport layer (HTL) Alq3 and hole blacking layer (HBL) BAlq an over a wide range of temperatures, We found that the mechanisms of the current transport are consistent with a trap charge limited (TCL) model. From the model, a trap at 0.145 eV with a concentration of 1.89*1018cm-3 and a trap at 0.201 eV with a concentration of 4*1018cm-3 are obtain in Alq3 and BAlq, respectively. In addition, BAlq is shown to have a higher mobility dependence on an electric field than Alq3. We analyzed the capacitance-frequency (C-F), capacitance-voltage (C-V), and admittance spectroscopy of hole transport layer (HTL) using equivalent circuit model. The results of C-V and C-F measurements show an O2 plasma treatment on ITO surface can enhance the efficiency of the hole injection. In multilayer OLEDs, the WO3/Mg:Alq3 layer is shown to play the important role; WO3 reacts with NPB to form a p-type layer and Mg reacts with Alq3 to form a n-type layer. The electrons in a n-type layer and the holes in a p-type layer are injected separately into emitting layers by Fowler Nordheim tunneling when an electric field is applied. Finally, we discuss the effect of WO3 incorporation on the electric properties of a p-type NPB by temperature-dependent admittance spectroscopy. We found that doping NPB with WO3 can decrease the resistance of NPB and improve the efficiency of hole injection from anode ITO. Furthermore, increasing the concentration of WO3 from 0% to 16.7% can decrease the activation energy (Ea) of NPB from 0.337eV to 0.176eV. We speculate that NPB contains a trap level at 0.337 eV above the valance band, and Fermi level is pinned to the trap level. From simple calculation we obtain the hole concentration and mobility in terms of the WO3 incorporation; the hole concentration increases and the mobility decreases with increasing the WO3 concentration.