溼式製程應用在藍色磷光有機電致發光元件之介面特性及載子傳輸和光學機制之研究

The interface characteristics, carrier transport, and optical mechanism for phosphorescent organic light-emitting devices in solution process

張永亭(Yung-Ting Chang)

指導教授 ： 吳志毅

共同指導教授 ： 陳錦地(Chin-Ti Chen)

國立臺灣大學/電機資訊學院/電機工程學研究所/博士(2012年)

https://doi.org/10.6342/NTU.2012.00078

摘要

本篇論文主要著重在探討小分子的有機材料應用在溼式製程上之有機電致發光元件。本論文第一部分在於利用小分子主體材料bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane (SimCP2)直接比較目前最常用的高分子主體材料polyvinylcarbazole (PVK)，製作成難度最高的溼式製程單層高效率藍色磷光(FIrpic)元件。將此兩元件最佳化後直接比較效率上的差異，並輔以增光膜(BEF)研究其元件光學出光特性。未貼附增光膜前，實驗結果顯示PVK高分子元件之效率遠勝於SimCP2小分子元件之效率，儘管SimCP2在材料傳輸特性和內部量子效率各方面都勝過PVK。但是在兩個元件都貼附了增光膜後，實驗證實SimCP2小分子元件的效率可以勝過高分子材料，推測原因為高分子和小分子的光學出光特性大不相同之故。小分子材料為等向性(isotropic)的材料，故容易侷限光在元件內部，但是高分子材料為非等向性(anisotropic)的材料，盡管發光體不是主體材料本身是小分子的磷光摻雜材料，但是由於主體材料會改變其放光特性。等向性的材料易產生等向性的電偶極距(isotropic dipole)，而非等向性材料易產生平行的電偶極距(in-plane dipole)。嚴謹的光學模擬驗證等向性的電偶極距確實會侷限住比較多的光，而平行的電偶極距的較不易侷限光，故其高分子元件的出光效率遠高於小分子元件。光學模擬也證實實驗上所看貼附有增光膜的兩個元件，光輸出效率(outcoupling efficiency)，小分子元件的提昇高出高分子元件提昇許多。
要製作高效率的元件除了需要有真空蒸鍍性高的電子注入材料氟化銫(CsF)搭配上鋁(Al)電極(陰極)，會採用1,3-bis[(4-tertbutylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7)或2-(4-biphenyllyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD)，含有噁二氮唑(oxadiazole)的電子傳輸材料。在本論文第二部分中，多加了一個oxadiazole的電子傳輸材料1,3,5-tris[(4ʹ-tert-butylphenyl)-1,3,4-oxadiazolyl]benzene (OXD)，來一起求證含oxadiazole結構之電子傳輸材料之理由。本論文證明了在陰極介面上有含oxadiazole的電子傳輸材料會與氟化銫和鋁有化學反應發生，化學反應為3OXD-7 + 3CsF + Al → 3(Cs+ + OXD-7- )+ AlF3。此發生在oxadiazole與Al金屬上的化學反應，而讓元件的電子注入變的容易許多，使得其電子注入會與沒有oxadiazole的電子傳輸材料的注入差異非常的大。無論是溼式製程的高分子與小分子材料甚至在熱蒸鍍的製程上，此一化學反應都會幫助電子注入，讓本來極度缺乏電子的元件，讓電子和電洞能夠平衡，效率才能因此與熱蒸鍍的有機電致發光元件競爭。利用這樣的方式所設計出小分子濕式製程單層元件，藍色磷光元件效率約為14.7 cd/A，而以這樣子的藍色磷光搭配紅色磷光摻雜物Ir(2-phq)3製作出溼式製程小分子白光單層元件，其效率可達17.3 cd/A。
最後因為溼式製程，主動層需要將所有材料混在一起，但因為混摻的物理機制較為複雜，故我們利用熱蒸鍍方式製作元件，並利用最常見的雙層結構(bi-layer structure)，單一的紅色摻雜物製作出有機電致發光元件。本論文第三部份提出了一個很重要的新型設計元件的方式，利用嚴謹且整合的電光模擬證明此一重要的機制，並且完美的解釋了在實驗上看到的無法解釋之處。本論文證實必須要合適的挑選電子和電洞傳輸層，來調整在元件結構中載子複合最佳位置，將最佳的載子複合位置調整在光學建設性干涉最佳的位置上，才能夠同時達到載子平衡和光學最佳位置。此外本論文也證明了，單獨個別利用電性或光學模擬來最佳化都是不正確的，因為最後最適合放光的位置(載子複合位置)會出現在兩者之間。此一結果非常有助於設計高效率有機電致發光元件，無論是在於溼式製程或是熱蒸鍍製程上。

關鍵字

有機發光元件 ； 高分子發光元件 ； 磷光材料 ； 螢光材料 ； 電性模擬 ； 介面機制 ； 光輸出 ； 光學模擬

並列摘要

This study developed solution-processed single-layer phosphorescent organic light-emitting devices (PHOLEDs) based on a small molecule system. We investigated bis(3,5-di(9H-carbazol-9-yl)phenyl)diphenylsilane (SimCP2), a small molecule, as the host material and applied it in the solution process. All active components, including the host of SimCP2, the ETL of 1,3-bis[(4-tertbutylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7), and the dopant of FIrpic, were mixed in an organic solution for wet processing to fabricate single-layer PHOLEDs. The small molecule device exhibits luminous efficiency as high as 14.7 cd/A (or maximal power efficiency of 8.39 lm/W), which is the highest among similar types of blue PHOLEDs fabricated using a solution process, single-layer, and small molecular host. Using the same solution process for both devices, we conducted a comparison of light out-coupling enhancement with a brightness enhancement film (BEF) between the small molecule SimCP2- and polymer polyvinylcarbazole PVK-based OLEDs. Because of the different nature of dipole orientation (or the anisotropic refractive index environment that alters the emitting light), polymer-based OLEDs trap less light internally than small molecule-based OLEDs, an enhancement of approximately 37% based on ray optics and plane-wave simulation models. Despite superior internal quantum efficiency, the overall light out-coupling efficiency of SimCP2 small molecule-based devices is inferior to that of PVK polymer-based devices without BEF. However, when using BEF for light out-coupling enhancement, the improved ratio in luminous flux and luminous efficiency was 1.64 and 1.57, respectively, which is considerably higher than 1.29 and 1.16, respectively, for PVK-based devices.
We determined the reason for which single-layer OLEDs usually blend 2-(4-biphenyllyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) or OXD-7 (ETL) into EML to improve electron injection, whereas other OLEDs do not. This paper proposes a mechanism in which oxadiazole moiety has a chemical interaction with CsF/Al to boost electron carriers. This mechanism was observed from the device characteristics and UPS and XPS measurements. In addition, the mechanism is suitable for all cases, including the thermal deposition process. We fabricated a high-efficiency two-color white PHOLED with a single-layer solution process. The white PHOLED achieved high luminous efficiency of 17.3 cd/A based on the high efficiency of 14.7 cd/A of a blue PHOLED with a single-layer solution process.
The blending system of EML in a single-layer solution process is complex. To improve the OLED devices, we investigated simple bilayer structures using a thermal deposition process to design the devices accurately. If only one electrical or optical simulation is used to optimize the OLED, it would not achieve the correct inferences, and the simulation results would not correspond to the experimental results. Optimal results were obtained when we integrated the precise electrical and optical simulations simultaneously, and some phenomena in the experimental results were explained. However, the optimized position for recombination emission does not occur at the location of the maximal carrier recombination rate or the location of optimal optical characteristics, but occurs between these positions. Therefore, it is necessary to consider both optical interference and the charge balance effect. In addition, it is advisable to select the adequate ETL and HTL and allow the maximal recombination carrier to fall on the optimal interference position. Our design rules are suitable for all OLEDs, including single-layer solution process PHOLEDs. A device with high efficiency of 8.44 cd/A was achieved by applying this method to design red fluorescent bilayer OLEDs.

並列關鍵字

organic light-emitting devices ； polymer light-emitting devices ； phosphorescence material ； fluorescence material ； light out-coupling ； light extraction ； interface mechanism ； oxadiazole ； electrical simulation ； optical simulation

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