高三重態能量矽苯型σ-π共軛高分子電致高效率磷光與電場穩定白光之研究

High Triplet Energy Silylene-Diphenylene σ-π Conjugated Polymers for Highly Efficient Electrophosphorescence and Device Structure Design for Voltage Independent White Light Emission

張耀唐(Yao-Tang Chang)

指導教授 ： 陳壽安

國立清華大學/工學院/化學工程學系/博士(2013年)

摘要

摘 要
相較於無機LED的高亮度與高效率，起步較晚的OLED仍難以與LED競爭，但在固態照明的需求上，由於無機LED為點光源，應用在大面積照明的大多都利用擴散片來改善單點過於刺眼但整面亮度卻不均勻的窘境。然而，OLED除了不需要如無機LED複雜的磊晶製程外，其同時具備了面光源的優勢，也因為有機分子柔軟可捲曲的特性，除了可用於軟性基板的照明，亦提供了方便攜帶的特點。由於OLED使用真空蒸鍍的製程，因此能夠輕易地設計出像是exciton阻擋層或者電荷阻擋層等元件結構設計，但主要缺點卻是高效率元件的結構過於複雜(通常超過5層)而導致良率過低，且蒸鍍製程的速率較低而難以大量生產。而在PLED中，最大的優點是因為能夠進行溼式製程而高速生產大幅降低成本，同時能夠整合不同功能的基團(像是電荷傳遞基團或是發光基團)在同一條高分子鏈上，而使得元件結構得以被大量簡化，排除摻雜系統中長時間操作造成相分離的影響。
現階段，OLED系統亦有嘗試直接使用溼式製程來製作元件，然而其效率相較於真空蒸鍍的系統，大約降低了36%，除了可能是溼式製程難以加入exciton及電荷阻擋層外，主要是因為小分子並不具備良好的成膜性而難以得到平整的發光元件。目前OLED綠光元件的外部量子效率(EQE)大約達到20-22%，而共軛系統的PLED則只有一半的EQE約11%，非共軛系統的PLED如polyvinyl carbazole (PVK)，則達到16-18%，然而在PVK系達到高效率的元件都額外再加入約30%的電子傳輸材料操作時會甚至在幾分鐘內就產生相分離而不適合實際應用。因此，目前PLED最大的挑戰就是元件效率偏低而迫切需要提升。
在本論文中主要分成三個部分：第一部分希望得到高效率且不隨電場改變光色的白光元件以期待能達到良好照明的光源；第二部分藉由分子設計引入電洞傳輸材料TPA與電子傳輸材料OXD來提高載子傳遞效果，並利用主鏈矽苯型的 σ - π 共軛來增加傳導能力，由於這三種不同的基團彼此獨立，而且三重態能量都高於2.9 eV，因此適合用於磷光摻雜的系統；第三部分則針對主鏈矽苯型的 σ - π 共軛高分子側鏈上傳輸基團的末端基影響與物性來作探討。以上三部分，分別以四、五、六等3章論述。
在第四章中，我們使用本實驗室自行開發的材料PFCn6作為中間層，並利用β-PFO與PFO: rubrene分別為藍光與橘光來源，以全溼式製程製作雙發光層的白光元件，由於整個發光元件都是以PFO作為高分子主體結構，因此載子的傳遞完全沒有能障。經由單載子元件的量測與SCLC計算，我們得到PFCn6的電洞與電子傳輸速度，配合實驗室曾經量測過的β-PFO之電洞傳輸速度與文獻測得的PFO電子傳輸速度，我們以time-of-flight的公式計算出當藍光層60 nm與橘光層20 nm時，電洞經過β-PFO及PFCn6到達橘光層的時間與電子經過PFO:rubrene與PFCn6的時間比值R60/20為1.09，相對於藍光層40 nm與橘光層40 nm時的比值R40/40為0.58，其載子更為平衡，也因此不管電場怎麼變化，在60/20 nm系統中都能維持近乎完全不變的Electroluminescence (EL)光譜，其發光元件最大亮度為15695 cd/m2，最大電流效率為5.43 cd/A，CIE座標為(0.32, 0.36)非常接近標準純白光(0.33, 0.33)。
在第五章中，我們引入Ir(ppy)2(acac)作為綠光發光體，因為其HOMO/LUMO為5.2/2.5 eV，剛好介於TPA的HOMO (5.3 eV)與OXD的LUMO (2.4 eV)之間，在Photoluminescence (PL)的量測中，雖然雙極性(bipolar)的高分子都會有exciplex的放光特徵峰，但在摻雜8 wt%的Ir(ppy)2(acac)之後全部PL光譜都只看到綠光，表示exciplex的能量都能夠很有效的轉移給綠光磷光體而沒有太多損失。藉由引入Cl-ITO作為陽極取代常用的PEDOT:PSS，以及適當調整電子傳輸層與發光層的厚度，我們成功在雙極性高分子Si(tOXD)(oTPA)與Si(tOXD)(tTPA)的系統中分別達到80.1 cd/A (EQE 21.2%)與73.5 cd/A (EQE 19.5%)的極高效率，相較於目前PLED綠光EQE 11%，這樣突破性的進展相當令人振奮，再加上僅使用簡單的兩層結構就達到，對於未來的工業化應具有相當的優勢。
在第六章中，我們針對引入tert-butyl基團取代hexyloxy探討對於bipolar材料的影響，發現即使對傳輸基團加入tert-butyl基團，其傳輸速度受到的影響並不多。而由單載子元件測試中，雙極性的高分子其電子數量皆高於電洞數量約2-3個級數，因此我們嘗試加入電洞傳輸/電子阻擋層的PVK，以Ca取代CsF降低電子注入數量，並藉由改變PEDOT:PSS的導電度，達到綠光效率41.6 cd/A。

關鍵字

有機發光二極體 ； 高分子發光二極體 ； 高分子 ； Multilayer ； Solution process ； 高效率

並列摘要

Abstract
Compared to the high brightness and high efficiency of inorganic LED, late developing organic LED (OLED) is difficult to compete with it. However, in solid-state lighting industry, the point lighting source of the former cannot be applied to large area, and requires utilizing a diffuser to improve the problem of harsh in one point and uneven in full plane. On the other hand, OLED possesses natural plane lighting, and is no need to be fabricated by complicated epitaxy process like inorganic LEDs. Because of flexibility of organic molecules, it can be fabricated on flexible substrates and provides suitability for carrying. Furthermore, the use of vacuum deposition for depositing various layers allows flexibility in device structure design for exciton and charge blocking layers to enhance device performance. Normally, an efficient OLED device contains more than five layers. However, the device with large number of layers could lead to lower production yields and is able to be slow fabrication speed. On the contrary, due to the advantages of wet process, PLED is able to be fabricated at higher speed and thus at lower costs. The most important characteristic of PLED is that permits an integration of various functional groups like charge transport moieties and/or emitting species into one polymer chain for simplifying the fabrication process. In the meantime, it not only simplifies the device structure but also excludes possibility of phase separation host-guest systems.
Nowadays, for OLED system, some research groups have attempted to fabricate device using wet process. However, as compared to vacuum deposition process, the efficiency was dropped by 36% resulting from poor film quality. The current state of art for green emission efficiency is in the level of 20-22% in external quantum efficiency (EQE); while green emission of PLED with conjugated polymers is in the level 11%, only half of the above level. The use of non-conjugated polymer (polyvinyl carbazole, PVK) as host with green phosphor as dopant and large amount (about 30%) of electron transport material (PBD) to assist charge balance able to reach EQE at the level of 16-18%. However it suffers from rapid phase separation observable within hours or even minutes and is not suitable for practical use. So far, the most challenging problem in PLED is inadequate efficiency, which is in needs of being promoted immediately.
In this thesis, the contents are divided into three parts giving in chapters 4, 5 and 6. In the first part, we design a highly efficient and voltage independent white light device. In the second part, we design a new series of polymers by introducing hole transport moiety (TPA) and electron transport moiety (OXD) as side arms on the silicone of silylene-diphenylene backbone to enhance its charge transport abilities. Since each unit in the polymer has triplet energy higher than 2.9 eV, we may expect that the polymer has triplet energy of 2.9 eV and is suitable to act as hosts for phosphor dopant. In the third part, we center on the physical properties and device performance affected by end groups at side arms of the transport moieties.
In chapter 4, we utilize our developed material, PFCn6, as an interlayer and introduce β-PFO for blue emission layer and PFO: rubrene for yellow emission layer to obtain stable white light with fully wet process. Due to the whole device host materials are consisted of PFO main chain structure, there are no energy barriers between each layer. Based on the hole mobility of β-PFO, the electron mobility of PFO from literatures and both mobilities of PFCn6 determined using SCLC measurement, we calculate the ratio of travel time (R) for hole to pass through β-PFO and PFCn6 layers to that for electron to pass through PFO: rubrene and PFCn6; for the device R60/20 (60 nm β-PFO and 20 nm PFO: rubrene layers) and R40/40, they are 1.09 and 0.58, respectively. These results show that the times for holes and electrons traveling before recombination in the devices with 60/20 nm (blue/yellow layers) are more balanced than that with 40/40 nm system. Thus, whatever the external electric field varies, the electroluminescences are almost the same, and the maximum brightness of 15695 cd/m2, the maximum luminance efficiency of 5.43 cd/A are achieved. Besides, the CIE coordinates of (0.32, 0.36) are very close to the standard white light of (0.33, 0.33).
In chapter 5, we introduce the phosphor Ir(ppy)2(acac) as green dopant to the proposed bipolar polymers with silylene-diphenylene as backbone, Si(OXD)(TPA), because its HOMO/LUMO levels are 5.2/2.5 eV just lying in between HOMO of TPA (5.3 eV) and LUMO of OXD (2.4 eV). From the photoluminescence (PL) measurement, although the bipolar polymers contain exciplex emission, the films only emit green light after doped 8 wt% Ir(ppy)2(acac), indicating that the exciplex can effectively transfer its energy to green emitter without significant loss. By introducing Cl-ITO as anode (allowing an elimination of PEDOT:PSS layer which is usually used as HTL) and adjusting the thicknesses of ETL and EML, we achieve the high device performance 80.1 cd/A (EQE 21.2%) of Si(tOXD)(oTPA) and 73.5 cd/A (EQE 19.5%) of Si(tOXD)(tTPA), which are extremely high compared to the current reported value of PLED 11% in EQE. This molecular design strategy opens a broad avenue leading toward industrialization of PLED for its two-layer-only device and high performance.
In chapter 6, we focus on the effects of using tert-butyl instead of hexyloxy at the side arms of transport moieties, and we found that even the steric hindrance by the tert-butyl group does not affect much on the charge transport. From single carrier examination, the electron current density is 2-3 magnitude higher than hole current density in the bipolar materials. Therefore, we introduce a PVK layer as HTL/EBL, replace CsF by Ca to reduce electron current, and use high conductivity PEDOT:PSS, the resulting device reaches the efficiency 41.6 cd/A in green emission.

並列關鍵字

OLED ； PLED ； Polymer ； Multilayer ； Solution process ； High efficiency

參考文獻

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