電激發光高分子半導體元件之奈米尺度下電荷載子注入及傳遞性質研究

Studies on Nanoscale Charge Injection and Transport Properties for Electroluminescence Polymer Semiconductor Devices

陳明志(Ming-Chih Chen)

指導教授 ： 陳壽安

國立清華大學/工學院/化學工程學系/博士(2008年)

摘要

近幾年來，高分子發光二極體(polymer light-emitting diode, PLED)引起廣泛的注意及探討是因其具有商業化應用及科學研究的價值。然而，PLED元件效能及穩定度仍然需要進一步的改善才能達到商業化的價值。本研究主要分成三個部份。第一部份是探討銦錫氧化物(ITO)薄膜內氧缺乏的程度對其PLED元件效能的影響。最後兩個部份討論共軛高分子薄膜中奈米尺度下電荷載子傳遞分佈均勻度的特性、局部的共軛高分子薄膜形態(morphology)及其相對應的PLED元件效能的相互關係。
我們使用射頻磁控濺鍍機沉積ITO陽極薄膜於不同氧流量的條件下且研究其相對應PLED的元件效能。在不同氧流量的條件下，ITO薄膜內會產生不同程度的氧缺乏。藉由霍爾效應、XRD、導電性原子力顯微術及元件效能的量測，我們發現在ITO薄膜內氧缺乏的程度會明顯影響元件的效能且也是元件漏電流的來源之一。若使用較低的氧流量來沉積ITO陽極薄膜，則PLED元件會產生較大的漏電流。這結果可歸因於ITO薄膜內有較大程度的氧缺乏。若使用最佳化的氧流量來沉積ITO陽極薄膜，則增加的元件效能(元件結構為 ITO/MEH-PPV (100 nm)/Ca/Al)可以歸納為兩個原因: MEH-PPV元件有較低的漏電流及較平衡的電洞及電子通量。
使用超高真空導電性原子力顯微術量測MEH-PPV薄膜奈米尺度的電流分佈，我們發現選擇一個適當的溶劑或混合溶劑溶解MEH-PPV，可產生空間均勻分佈的電荷載子傳遞形態及抑制高導電區域的數量，進而最佳化PLED的元件效能(因為可降低元件的漏電流)。進一步藉由GI-WAXS、UV-vis及元件效能的量測，我們發現MEH-PPV薄膜中局部規則結構[完整堆疊鏈(well-pack chains)及集聚體(aggregates)] 的空間分佈若不均勻則會產生電荷載子傳遞穿過薄膜(charge transport across the film)的穿透路徑(percolated pathways for charge transport across the film)。因此電流將會優先通過這些穿透路徑，在MEH-PPV薄膜內產生高導電區域，進而引導出其相對應的PLED元件產生較大的漏電流，因此降低PLED的元件效能。所以MEH-PPV薄膜內的高導電區域起源於局部規則結構的空間不均勻分佈，此現象是除了載子遷移率、載子陷阱及集聚體、氧分子和電洞偏極子(hole polarons)淬息激子等等因素外，影響MEH-PPV元件效率的重要因素之一。
使用導電性原子力顯微術分析PFO薄膜裡β-phase的奈米尺度電流分佈的特性。β-phase的形成可使電洞電流增加以及電子電流減少，使得電子與電洞在元件的流通量比pristine PFO更加平衡，進而提升元件的效率。然而過量的β-phase亦會形成較多的高電流區域，這些區域可能產生電荷載子的穿透路徑，進而增加元件的漏電流，因此降低元件的效能。因此，適量的β-phase形成，則能最佳化PFO元件的效能。
Polymer light-emitting diode (PLED) has drawn great attention in recent years because of its scientific importance and potentially commercial applications. However, the stability and performance of PLED are in needs of further improvement for commercial application. This study is divided into three parts. The first part is the influence of oxygen deficiency in the indium tin oxide (ITO) on the performance of PLED. The last two parts discuss a correlation among homogeneity of nanoscale charge transport distribution in conjugated polymer thin films, local morphology and performance of the corresponding PLED devices.
We investigated effects of oxygen deficiency in the indium tin oxide (ITO) on the performance of poly(2-methoxy-5-(2’-ethylhexyloxy)- 1,4-phenylenevinylene) (MEH-PPV)-based polymer light-emitting diodes, in which the ITO anode was deposited by radio frequency magnetron sputtering at different oxygen flow rates. By Hall-effect, X-ray diffraction (XRD), conducting atomic microscopy (CAFM), and brightness-current density-electric field measurements, we found that the degree of oxygen deficiency in the ITO films can affect the device performance significantly and is a source of current leakage. A larger leakage current is observed in the devices with ITO anodes deposited at lower oxygen flow rates, which may be associated with a higher degree of oxygen deficiency in the ITO films. At the optimal oxygen flow rate, the leakage current of devices can be reduced and the balance between hole and electron fluxes can be promoted in the MEH-PPV layer to improve device efficiency.
From the results of ultra high vacuum-conducting atomic force microscope (UHV-CAFM) measurement on MEH-PPV thin films, we found that the morphology with spatially homogeneous charge transport and suppressed coverage of highly conducting regions can be achieved by a proper choice of solvent or mixed solvent for the polymer, by which the performance of PLED can be optimized (due to a reduction of leakage current). By Grazing-Incidence Wide-Angle X-ray Scattering (GI-WAXS), UV-vis spectroscopy, and brightness-current density-electric field measurements, we found that the spatially inhomogeneous distribution of the ordered structures (well-pack chains and aggregates) in MEH-PPV thin film offer percolated pathways for charge transport across the film. Thus, current can preferentially pass through these percolated pathways, which can result in highly conducting regions in the thin film and lead to a larger current leakage and therefore lower device efficiency in the corresponding polymer light-emitting diodes. Thus, the highly conducting regions in MEH-PPV thin film originate from a spatially inhomogeneous distribution of ordered structures, which is one of the important factors (such as charge mobility, charge traps and exciton quenching by interchain interaction, molecular oxygen or hole polarons, etc.) that affect the efficiency of PLED.
Conducting atomic force microscopy is used to map the nanodomain current distribution for β-phase PFO thin films. The enhancement of efficiency results from the better balance of hole and electron fluxes due to the increase of hole flux and the decrease of electron flux with increasing β-phase content. However, excess content of β-phase content could offer percolated pathways for charge transport across the PFO film. Thus, current can preferentially pass through these percolated pathways, which can result in highly current regions in the thin film and lead to a larger current leakage and therefore lower device efficiency in the corresponding polymer light-emitting diodes. Thus, the optimal content of β-phase can optimize the device efficiency (due to a reduction of leakage current).

關鍵字

超高真空導電性原子力顯微術 ； 高分子發光二極體 ； 銦錫氧化物 ； 氧缺乏 ； 漏電流 ； 共軛高分子 ； 集聚體 ； 完整堆疊鏈

並列摘要

無資料

並列關鍵字

ultra high vacuum-conducting atomic force microscope ； polymer light-emitting diodes ； indium tin oxide ； oxygen deficiency ； leakage current ； Conjugated polymer ； aggregates ； well-pack chains

參考文獻

[1]J. C. W. Chien, “Polyacetylene: Chemistry, Physics, and Material Science”, Academic Press, Orlando (1984).

[9]C. Kittel, “Introduction to Solid State Physics”, 6th edition, John Wiley & Son, Singapore (1986).

[11]D. A. Skoog, D. M. West, F. J. Holler, “Fundamentals of Analytical Chemistry”, 5th edition, Saunders College Publishing (1988).

[12]N. J. Turro, “Modern Molecular Photochemistry”, Sausalito, Carlifonia, University Science Books (1991).

[5]E. G. J. Staring, R. C. J. E. Demandt, D. Brun, D. L. J. Rikken, Y. A. R. R. Kessener, A. H. J. Venhuizen, M. M. F. van Knippenberg, and M. Bouwmans, Synth. Met. 1995, 71, 2179.