以阻抗對電壓特徵曲線分析有機發光二極體載子傳輸及起始電壓特性

本論文主要研究使用電流及阻抗對電壓特徵曲線來探討有機發光二極體元件內部特性，包含元件的起始電壓、載子傳輸層中的載子遷移率、以及陰極金屬或電子注入層擴散距離。第一部分中，起始電壓定義為將電流取對數後在對電壓圖上的上升起始點，自起始電壓後元件開始導通電流。實驗結果發現有機發光二極體的起始電壓由電洞傳輸層的最高填滿軌域與電子傳輸層的最低未填滿軌域能階差決定，此能階差需考慮兩有機層間的真空能階位移。實驗中以紫外光光電子能譜分析儀觀察有機材料介面的實際能階圖。此外，在元件導通前的阻抗轉折為另一研究主題，此阻抗轉折與兩載子傳輸層間的介面電荷相關。實驗結果顯示僅當電洞傳輸層中的電洞遷移率遠大於電子傳輸層中的電子遷移率時才會有此阻抗轉折發生，反之，當兩載子遷移率相差不大，阻抗僅在元件導通時下降。接著，將阻抗轉折處的電壓定義為轉折電壓，並探討轉折電壓與元件結構的關係。一般而言，轉折電壓由電子傳輸層厚度及介面電荷密度決定，但實驗發現在電子注入層的存在下，轉折電壓會往正向偏壓平移。由轉折電壓與電子傳輸層厚度關係推測，該平移是由於電子注入層或陰極金屬材料擴散進入電子傳輸層造成。根據以上推論，阻抗對電壓特徵曲線可用來初步比較兩載子傳輸層的載子遷移率關係並且以非破壞性的方式觀察元件內陰極材料擴散的情形。

關鍵字

有機發光二極體；阻抗對電壓曲線；起始電壓；載子遷移率；陰極擴散長度

並列摘要

The electrical properties of organic light emitting diodes (OLEDs), including the turn-on voltage, carrier mobility, and diffusion length of cathode materials, are investigated by current density-voltage (J-V) and impedance-voltage (Z-V) characteristics in this dissertation. First, the turn-on voltage, which is determined by the voltage where log J exhibits a sharp rise and the current starts to flow through the device, is demonstrated to be close to the difference between the highest occupied molecular orbital (HOMO) of the hole transport layer (HTL) and the lowest unoccupied molecular orbital (LUMO) of the electron transport layer (ETL), taking into consideration of the vacuum level shift analyzed by ultra violet photoelectron spectroscopy (UPS). Then the additional impedance transition before the device turns on is examined in the devices with several combinations of the carrier transport layers, and the results indicate that the impedance transition occurs only in the devices with the hole mobility in the HTL much greater than the electron mobility in the ETL. In addition, the further investigations of the transition voltages, defined as the voltage where the impedance drops to a lower value before the turn-on voltage, imply that the transition voltages shift to the positive voltage in the devices with the electron injection layer, which is inferred to correlate closely with the diffusion length of the cathode materials. Based on these assumptions, the Z-V characteristic is proposed as the potential methods to initially compare the carrier mobilities in the carrier transport layers and nondestructively observe the cathode diffusion length in the OLEDs.

並列關鍵字

OLED ； Z-V characteristics ； turn-on voltage ； carrier mobility ； cathode diffusion length

參考文獻

Chapter 1

1. Pope, M., H.P. Kallmann, and P. Magnante, Electroluminescence in Organic Crystals. The Journal of Chemical Physics, 1963. 38(8): p. 2042-2043.

2. Helfrich, W. and W.G. Schneider, Recombination Radiation in Anthracene Crystals. Physical Review Letters, 1965. 14(7): p. 229-231.

3. Tang, C.W. and S.A. VanSlyke, Organic electroluminescent diodes. Applied Physics Letters, 1987. 51(12): p. 913-915.

4. So, F., J. Kido, and P. Burrows, Organic Light-Emitting Devices for Solid-State Lighting. MRS Bulletin, 2008. 33: p. 663-669.

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以阻抗對電壓特徵曲線分析有機發光二極體載子傳輸及起始電壓特性

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