高效率有機電激發光元件之研究

Abstract

在這論文中我們製作高效率串聯式有機二極體元件，以連接層整合兩個綠光與白光元件於一垂直結構。我們也製作一種p-i-n OLED，包含了p-型摻雜層使用無機氧化物WO3，摻雜電洞傳輸層2-TNATA。 製作高效率串聯式有機二極體元件之結構使用ITO/CuPc/NPB/C545T:Alq3/Alq3/Mg:Alq3/WO3/NPB/C545T:Alq3 / Alq3/LiF/Al。我們發現串聯式元件的發光效率可以被WO3的厚度所控制。串聯式元件的WO3薄膜厚度為4 nm，在電流密度20 mA/cm2時發光效率可以達到34.9 cd/A，幾乎是傳統元件(ITO/CuPc/NPB/C545T:Alq3/ Alq3/LiF/Al)的四倍，比較所有的研究報告這種倍增效應是從未被發現，其外部量子效應約為8.8%，以及擁有飽和的綠光顏色(CIEx = 0.33，CIEy = 0.64)，是很好的螢光染料摻雜有機發光元件。我們也證明電子注入層Mg:Alq3對於增強發光效率是必需的。此研究結果可以來證實串聯式元件是一種高發光效率且高生命期的有機二極體元件。 兩種串聯式白光有機的發光二極體(OLEDs)，使用Mg:Alq3/WO3連接層連接。當分別的藍色與黃光的OLEDs的串聯時, (CIE)對視角是很敏感，然而連結兩發出白色的OLEDs的串聯元件則變化較少。在串聯元件WO3厚度為最佳5nm時,亮度效率是單個元件的三倍。包括兩白色螢光的OLEDs 可以產生最大的效率22 cd/A，並且在最初亮度100 cd/m 2的半衰期是超過80，000 h。 我們也成功地使用連接層Mg:Alq3/WO3來串聯兩個元件，並探討了串聯式結構的光電特性，因此將探討Mg:Alq3/WO3之光電特性，並製作一種測試元件，量測其電特性，深入討論連接層的物理機制。我們發現連接層是p-n介面，穿遂物理機制是Fowler Nordheim穿遂模型。我們也將使用p-n介面製作穿遂式有機發光元件(TJOLED)，此種穿遂介面可以增長元件壽命，提升電洞注入。我們論證了Mg:Alq3/WO3異質接面，在逆向偏壓時是一種Fowler Nordheim tunneling模型扮演著歐姆接觸的功能，這種穿遂介面可以改善元件的電洞注入，由ITO注入NPB的效率，也可以增長元件的壽命，這種原因也許歸因於ITO與NPB介面之間的位障降低及粗糙度變好導致更均勻的介面電場分佈。 最後，我們製作一種p-i-n OLED，包含了無機氧化物WO3，與電洞傳輸層2-TNATA共蒸鍍來形成p-型摻雜層，並以tris(8-quinolinolato)aluminium (Alq3)當作發光層來討論光電特性，目的在於取代高揮發性的F4-TCNQ。除此之外，也將藉由改變摻雜濃度探討2-TNATA:WO3 層的特性與功能。我們製作一種p-i-n OLED，包含了p-型摻雜層與n-型摻雜層來提高載子的注入， Alq3 發光元件在100 cd/m2 時電壓為3.1 V，發光功率效率可以達到3.5 lm/W。我們以導納頻譜分析NPB- Alq3 雙異質接面有機發光二極體中的NPB摻雜入tungsten oxide (WO3)後的電特性影響。由變溫導納頻譜的量測分析得知，將WO3的摻雜濃度由0到16%可以增加NPB層中的電洞濃度由1.97×1014 到 1.90×1017 cm-3，並且同時降低NPB層中電阻的活化能由0.354到0.176 eV。由以上的實驗數據可得知攙雜WO3之候會造成ITO與NPB界面的能帶彎曲度增加，藉由此介面所產生的narrow depletion region使得電洞更易於從ITO經由穿遂進入NPB層中。此現象得以解釋改善電洞注入特性的原因。由實驗數據並且得到NPB層隨WO3參雜濃度的增加，可以有效的降低電阻，進而得到降低元件中歐姆損耗的現象。

In this thesis, we report on the fabrication of multilayer organic light emitting diodes (OLEDs) with high electroluminescent (EL) yield by integrating two units of green and whit emissive devices in series. We also demonstrate p-i-n organic light-emitting diodes (OLEDs) incorporating a p-doped transport layer which comprises tungsten oxide (WO3) and 4,4',4''-tris(N-(2-naphthyl)- N-phenyl-amino)triphenylamine (2-TNATA). The architecture of the multilayer OLEDs used in the experiment is ITO/CuPc/NPB/C545T:Alq3/Mg:Alq3/WO3/NPB/C545T:Alq3/Alq3/LiF/Al. We fund the efficiency of the two-unit device can be controllable by the thickness of WO3. The two-unit device with 4 nm WO3 produces the highest luminance efficiency of 34.9 cd/A at 20 mA/cm2, which is around thrice of that of the controlled single-unit device (ITO/CuPc/NPB/C545T:Alq3/ Alq3/LiF/Al). Compared to research reported to data, the “amplification effect” discovered in our device is a rather unexpected result. The external quantum efficiency of 8.8%, with a near saturated Commission Internationale d’Eclairage coordinates (CIEx = 0.33, CIEy = 0.64), is one of the best ever reported for a fluorescent dye-doped OLEDs. We also demonstrate the electron injection layer of Mg:Alq3 is a necessary component for the enhancement of EL efficiency. These results may prove to be an effective method to enhance the efficiency as well as the lifetime of current OLEDs. Two types of tandem organic light-emitting diodes (OLEDs) with white-light emission have been developed by using Mg:Alq3/WO3 as the interconnecting layer. While the Commission Internationale d’Eclairage (CIE) coordinates of the tandem device with individual blue and yellow-emitting OLEDs was sensitive to the viewing angle and the operating time, tandem device connecting two white-emitting OLEDs was considerably less. At an optimal WO3 thickness of 5 nm, the tandem 2-unit device produced thrice higher luminance efficiency than that expected of a single-unit device. A maximum efficiency of 22 cd/A was achieved by the tandem device comprising two white-fluorescent OLEDs, and the projected half-life under the initial luminance of 100 cd/m2 was over 80,000 h. We also demonstrate enhanced hole-injection and lower driving voltage in vacuum-deposited organic light-emitting diodes (OLEDs) with a tunneling junction composed of the Mg:Alq3/WO3 layer. We propose the laminated Mg:Alq3/WO3/NPB functions as a Fowler-Nordheim tunneling junction, which leads to efficient carrier injection by tunneling and improves the electric contact between ITO and NPB. The improvement of operational stability in TJ-OLED may be attributed to the distribution of applied field in interface or the reduction in the contact resistance of ITO and NPB interface and its planarization . Finally, we demonstrate p-i-n organic light-emitting diodes (OLEDs) incorporating a p-doped transport layer which comprises tungsten oxide (WO3) and 4,4',4''-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine (2-TNATA) to replace the volatile F4-TCNQ. We propose the 2-TNATA:WO3 composition functions as a p-doping layer which significantly improves hole-injection and conductivity of the device that leads to the fabrication of Alq3 based p-i-n OLEDs with long lifetime, low driving voltage (3.1 V), and high power efficiency (3.5 lm/W) at 100 cd/m2. The effect of tungsten oxide (WO3) incorporation into the 1,4- bis[N-(1-naphthyl)-N′-phenylamino]−4,4′ diamine (NPB) layer is investigated in NPB-Alq3 heterojunction organic light-emitting diodes. The admittance spectroscopy studies show that increasing the WO3 concentration from 0 to 16 % can increase the hole concentration of NBP layer from 1.97e14 to 1.90e17 cm-3 and decrease the activation energy of the resistance of the NPB layer from 0.354 to 0.176 eV. Thus, this incorporation reduces the ohmic loss and increases the band bending in the NBP layer near the interface, resulting in an improved hole injection via tunneling through a narrow depletion region.