藉由調整載子傳送基團的空間與能量順序設計高效率有機電致發光材料

Design of Highly Efficient Organic Electroluminescent Materials via Finely Tuning the Energetic and Spatial Sequence of Charge Transporting Moieties

黃智偉(Chih-Wei Huang)

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

摘要
本研究主要為藉由調整載子傳送基團的空間與能量順序設計高效率有機電致發光材料。而其可分為四個部分。
第一部分，在開發藍光共軛高分子方面，我們首先提出了在單一高分子上整合了具有階梯式(漸進式)能階的概念，利用在高分子側鏈接上兩種電洞傳輸基團，而這兩種電洞傳輸基團的游離能相對於主鏈是漸進式的。用此方法設計出的高分子克服了大部分藍光發光分子所具有電洞注入困難的缺點。並且伴隨著兩性載子電流的平衡以及相接近的載子傳輸速率。以上幾種優點使得此類高分子發光元件有超高的外部量子效率，藍綠光效率為7.53% (13.10 cd/A) 亮度為 14,000 cd/m2而深藍光效率為 4.54% (4.57 cd/A) 亮度為 22,000 cd/m2。並且，經由此設計方式的高分子也很適合當成一個發光主體材料。經由混摻了螢光客體小分子後，以TPA-Cz-sPF 混摻了 2wt% 4-(Di-p-toylamino)-4’-[(di-p-tolyamino)styryl]stilbene (DPAVB) 的元件具有更高的效率17.80 cd/A (15.12 lm/W) 亮度22,000 cd/m2以及超低的啟動電壓2.8 V。上述的效率為現有藍光高分子中最高者。
第二部分，我們也利用了兩種設計的高分子spiro-diEHPF 和spiro-TPA50-diEHPF當成主體材料。此兩種材料具有相當高的載子傳輸速率，約在10-3 ~ 10-4 cm2/Vs 之間。在混摻了螢光客體小分子後，spiro-diEHPF 混摻了 rubrene 的元件具有相當好的表現，白光效率為 3.5 cd/A，亮度為 36,000 cd/m2而黃光效率為 9 cd/A 亮度為 72,000 cd/m2。spiro-TPA50-diEHPF 混摻了 rubrene 的元件具有更好的表現，白光效率為 9 cd/A，亮度為 56,000 cd/m2而黃光效率為 14 cd/A 亮度為 72,000 cd/m2。而且，這些元件的電致發光光譜隨著操作電壓並沒有很大的漂移。
第三部分，我們設計了一系列新穎性的銥配合物，藉由設計電荷傳輸基團的空間與能量順序其具有電荷收集的能力(在此定義為有效收集電荷並且能夠引導這些電荷傳遞至發光核心以增加電荷利用率)。在引入了triphenylamine (當成收集電洞)與carbazole (當成傳遞的橋梁)之後不僅僅能夠有效的在發光層中收集大量的電洞並且能有有效的利用漸進式能階引導這些電洞進入發光的核心 (然而只修飾上TPA 或者Cz基團的銥配合物只顯示出強烈的電荷捕捉能力而沒有引導能力)。除此之外，這樣的改質也使得銥配合物與主體材料之間有更佳的化學相容性並且增加了發光量子效率。此系列中單層元件(含有7 wt.-% 銥配合物、30 wt-% 電子傳輸材料以及PVK)具有超高的外部量子效率，藍綠光效率為10.87 % (20.11 cd/A), 13.3 lm/W，並且具有很低的操作電壓3.9V。這些效率優於其他文獻中利用直接塗佈的藍光磷光元件，說明了引進收集電荷天線的結構來增加電洞傳遞至發光核心的優點。
此外，我們也將此天線收集的概念延伸來設計藍光共軛高分子。將漸進式能階當成一個功能性的模組，經由非共軛方式接於高分子主鏈。設計出的高分子展現出相當優異的電荷收及能力。此類高分子具有整齊、大量的漸進式能階電荷傳輸基團，不僅能夠有效的促進電洞注入而且也可以吸引這些電洞傳遞至主鏈。實驗結果顯示這些整齊、大量的漸進式能階電荷傳輸基團的高分子相較於任意排列的漸進式能階電荷傳輸基團的高分子具有更強的電洞注入能力與引導效果。此類高分子發光元件有高的外部量子效率，深藍光效率更往上提升為5%亮度為 12,000 cd/m2。此分子設計的概念提供了新的思維來開發共軛高分子，並且能夠被延伸至有機光電的其他領域。
第四部分，我們開發了了另一系列新穎性的直接塗佈的磷光材料。利用非共軛的方式同時引入了電子與電洞傳輸基團至發光核心的周圍而形成樹狀體，這樣的設計概念有許多優點。包含了相當有效的電子電洞傳遞能力、保持發光核心的發光光色、簡單製備、高的溶解度以及好的成膜能力。在沒有額外的混摻或蒸鍍了電子傳輸材料下，此樹狀體單層的元件就具有相當不錯的發光能力。黃綠光效率為 23.3 cd/A，亮度為 8223 cd/m2。這說明了此種設計概念的優勢。

Abstract
The study mainly discloses how to design highly efficient electroluminescent materials by finely tuning the energetic and spatial sequence of charge transporting moieties. This research can be divided into four parts.
Firstly, for the development of the blue electroluminescent conjugated polymers, we proposed integrated gradient ionization potentials idea in single polymer for the first time, to design highly efficient blue electroluminescent polymers by grafting dual hole transporting groups with graded ionization potential relative to that of its backbone. The polymers overcome the difficulty of hole injection which generally occurs in blue emitters. Together with balanced bipolar carrier fluxes and comparable carrier mobilities of the polymer, these advantages result in ultra high external quantum efficiency 7.53% (13.10 cd/A) and 4.54% (4.57 cd/A) with high luminance 14,000 cd/m2 and 22,000 cd/m2 for blue and deep blue emitting devices, respectively. Furthermore, the designed poly-spiros also demonstrate their feasibilities for being a host. By blending with fluorescent dopants, the blue-greenish device based on TPA-Cz-sPF doped with 2wt% 4-(Di-p-toylamino)-4’-[(di-p-tolyamino)styryl]stilbene (DPAVB) exhibits a luminous efficiency 17.80 cd/A (15.12 lm/W), high brightness of 22,000 cd/m2 in performance as reflected in Von at 2.8 V. The present efficiency is the highest one among blue PLEDs.
Secondly, we used two designed polyspirofluorenes, spiro-diEHPF and spiro-TPA50-diEHPF, which exhibit rather high charge mobilities in the order of 10-3 ~ 10-4 cm2/Vs. The light-emitting devices with rubrene-doped spiro-diEHPF show maximal luminance of 36,000 and 70,000 cd/m2 with maximal efficiency 3.5 and 9 cd/A for the white and yellow emissions, respectively. For rubrene doped triphenylamine-grafted polymer, spiro-TPA50-diEHPF, the maximal luminance and efficiency are 56,000 cd/m2 and 9 cd/A for white-emitting devices, and 72,000 cd/m2 and 14 cd/A for yellow emission. Furthermore, the electroluminescence profiles of the investigated devices show nearly independent of the applied voltages.
Thirdly, we design novel blue iridium complexes with charge antenna characteristics (defined as capable of collecting charges effectively and also guiding these charges into desired emission core to increase charge utilization) by proper grafting dual hole-transporting groups with graded ionization potentials relative to that of emission core as well as finely adjusting their spatial arrangement. An incorporation of the dual transport groups, triphenylamine (as hole receiver) and carbazole (as transporting bridge), that meets this requirement not only exhibits excellent antenna characteristic which can collect holes in the matrix efficiently but also provide gradient levels to allow a guiding of hole hopping toward the emission core (whereas the TPA only or Cz only modified Ir-complex exhibit no guiding capability but merely strong charge trapping). In addition, such grafting results in an excellent chemical compatibility of the dopant with the host polymer and enhanced photoluminescence quantum efficiency. Single layer devices with the light-emitting layer comprised of the present Ir-complexes 7 wt.-%, the electron transporting material 1,3-bis(4-tert-butylphenyl-1,3,4-oxadiazoyl)phenylene (OXD-7) 30 wt-% in polyvinylcarbazole (PVK) show sky blue emission with CIE (0.14, 0.27) and the performance of external quantum, power efficiency and low turn-on voltage of 10.87 % (20.11 cd/A), 13.3 lm/W and 3.9V, respectively. These efficiencies are superior to those reported using solution processed blue phosphorescent OLEDs, revealing the advantages of using charge antenna structure to increase chances for hole hopping into the emission core.
Further, we apply such design concept for blue emitting conjugated polymers. By grafting “graded HTMs” onto the backbone with nonconjugated linkage, the present poly-spiros exhibit charge harvesting antenna properties for collecting as well as guiding holes into the polymer backbone. With this well ordered, astronomical “gradient HTMs”, the designed polymers not only facilitate hole injection near the LEP/anode interface but also exhibit charge-harvesting antenna characteristics that can guide holes in the bulk to the main chain. Preliminary results show that well ordered, astronomical “gradient HTMs” functionalized poly-spiros not only exhibit much more stronger hole injection but also more effective hole guiding capabilities than the random distributed counterparts. Single layer LED achieves high efficiency of 5% and high luminance of 12,000 cd/m2, revealing the present molecular design is a powerful methodology. The present study provides a new concept for designing conjugated polymers and can be extended to the science of molecular electronics.
Fourthly, we also develop a novel design concept for solution processable phosphorescent materials. By grafting bipolar transporting groups onto the phosphorescent core with nonconjugated linkage exhibits many attractive properties for application in LEDs such as providing effective bipolar charge transport, retaining desire emission color, ease of synthesis, high solubility and good film forming ability. Simple dendrimer LED achieves high efficiency of 23.3 cd/A (7.27%) and high luminance of 8223 cd/m2 without introducing additional electron transporting materials, revealing the present molecular design is a powerful methodology.

關鍵字

發光二極體 ； 高分子 ； 電致發光 ； 銥配合物

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