濕式製程應用於磷光有機電致發光元件效能提升和物理特性之研究

Abstract

本篇論文的主要研究著重於利用小分子有機材料當作主體材料應用在溼式製程之磷光有機電致發光元件(phospohorescent organic light-emitting device； PHOLED)。我們將分別先測試出藍色磷光元件的最佳化結構後(極簡化結構(單層)和多層的結構)，以藍色磷光元件為基底，再進一步測試白光磷光元件。 本篇論文分為三大部份探討，第一部部份將之分為兩主題；前半部乃最佳化極簡化的單層結構和後半部為改善陰極介面的電子注入，以達到載子平衡以提高元件的效率。我們所使用的有機主體材料為小分子的Tris(4-carbazoyl-9-ylphenyl)amine (TCTA)；此一主體材料為具有相當高的電洞遷移率特性、合適的能隙，並且較高的三重態能階(T1)，因此TCTA適合當作藍色磷光的客體材料Bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic)之主體材料(電洞傳輸材料)。電子傳輸材料2,2'-(1,3-Phenylene)bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7)的加入發光層內，以利於平衡TCTA的高電洞遷移率來平衡電洞和電子，讓複合的激子可以最大，且可以讓放光位置遠離陰極，減少激子淬熄的發生。極簡化結構且高效率實為溼式製程之有機電致發光元件的最終目標，元件結構為ITO/PEDOT:PSS/EML/CsF/Al。極簡化結構的藍色磷光最高效率可達16.6 cd/A和 9 lm/W。而後半部乃探討在陰極介面處的電子注入之修飾改善和物理機制，一般有機電致發光元件，絕大部分材料之電洞的遷移率遠大過電子的遷移率，在溼式製程之中此問題更為明顯。為了改善此問題，我們在陰極部分加入鹼土族金屬鋇(Ba)與氟化銫(CsF)進行比較。鋇為極易氧化的金屬且具有較高的活性和功函數，在有機電致發光元件中對陰極電子注入方面將會有很大的提升，實驗結果也證明使用鋇金屬當做電子注入層取代氟化銫可以大幅提高效率，此一藍色磷光最高效率可達18 cd/A(增幅8.5%)和11.5 lm/W(27.8%)。 第二部份將之分為三個主題；乃探討高效率的雙層結構、多層結構和使用雙主體材料的藍色磷光有機電致發光元件。為了更進一步提升OLED效率，我們將極簡化結構改變成雙層結構，將發光層內原本主體材料(TCTA)和電子傳輸(OXD-7)材料中的電子傳輸層材料獨立出來，並用載子遷移率和三重態能階更高的3,3'-[5'-[3-(3-Pyridinyl)phenyl][1,1':3',1'-terphenyl]-3,3'-diyl]bispyridine (TmPyPb)取代OXD-7。並再重新最佳化藍色磷光有機發光元件，重新調整藍色磷光摻雜物FIrpic的濃度，以及調整每一層的厚度和元件的總厚度達到適當的共振腔長度，以達出光最佳化(micro-cavity effect)和載子平衡(charge balance)。我們開發出的以小分子有機材料為主體材料之高效率的藍色磷光有機發光元件，不論單層或雙層，其元件效能甚至優於其他作相似研究的團隊。最高效率可達30.1 cd/A和 18.45 lm/W，比單層提高 76.4 %。另外我們嘗試了結構較為複雜的多層結構及雙主體材料系統有機發光元件，也可達到穩定的元件效能，未來可以再進一步研究，來突破雙層的最高效率。 第三部份為利用前面測試出來的極簡化結構(單層)和雙層藍色磷光元件為基底，製作白光磷光元件。藉由摻雜橘紅色的磷光材料Tris(2-phenylquinoline)iridium(III) (Ir(2-phq)3)製作白光元件，調整紅色磷光材料及藍色磷光材料的相對比例，並最佳化此極簡化和多層的白光元件。極簡化的白光元件，白光元件最佳效率為 12.5 cd/A和 4.7 lm/W， EQE為 2.2 %，CIE座標為(0.32, 0.41)。雙層結構的白光元件，白光元件最佳效率為30 cd/A和23 lm/W， EQE為 11.6 %，CIE座標為(0.30, 0.42)。

In this study, the small molecule of organic materials would be applied to solution process for phosphorescent organic light-emitting device (PHOLED). The blue PHOLED of two structures, extremely simplified structure (single layer) and multilayer structure, would be optimized for the base of white PHOLED. Our thesis is separated into three parts. In the front section of first part, we first demonstrate that how to optimize the extremely simplified structures step by step. In the latter section of first part, the device performance could be further improved by increasing the electron injection ability to reach more charge balance. The small molecule materials of Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) that we choose is widely used as host material of blue phosphorescent system due to quite fast hole mobility, suitable energy bandgap, and high triplet state energy and the popular blue phosphorescent dye Bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) would be used in our research. For the extremely simplified structure, the electron transport material 2,2'-(1,3-Phenylene)bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7) would be mixed into the emissive layer and then the excitons will be greatly increased due to improving electron injection and transport to reach more charge balance. In addition, the recombination zone will be kept far away from the cathode to prevent the excitons quenching effect. Extremely simplified structure OLED having high efficiency is the objective of solution processed OLED and the device structure is ITO/PEDOT:PSS/EML/CsF/Al. For our experiment, the maximum efficiency in our blue PHOLED is achieved 17 cd/A and 9 lm/W. In the latter section of first part, we modify electron injection at the interface of cathode to investigate the interface of physical mechanism. For most of organic materials, the hole mobility is usually quite higher than electron mobility, and it would result in the device having series charge imbalance, especially occurring solution process. To solve such a problem, we utilize alkali metal Barium (Ba) in replace of CsF at the interface of cathode. The metal Ba has high work function and strong activity, and hence it easily cases oxidation-reduction reaction with other materials. This is why we use Ba to improve the electron injection of the device. The experiment results are also proved our concention and the device performance is enhanced again in comparison of CsF. Consequently, the maximum luminous and power efficiency of 18 cd/A (increasing 8.5%) and 11.5 lm/W (27.8%) are achieved. In the second part of the research with including three topics, the double layer structure, multilayer structure and co-host system are investigated for achieving higher efficiency PHOLED. In order to improve the PHOLED performance further, the double layer structure PHOLED would be used to replace extremely simplified structure. Therefore, the electron transport layer are separated from emissive layer to substitute for mixing OXD-7 into the emissive layer and the electron transport material TmPyPb, which contains higher electron mobility and T1, are evaporated after emissive layer. By re-tuning the FIrpic doping ratio and the device structure, we could optimize the device performance due to better charge balance and optimal micro-cavity effect. No matter what single layer or double layer structures that we developed, our blue PHOLEDs performances are greatly higher than other groups. The maximum luminous of 30 cd/A and power efficiency of 18.45 lm/W are achieved and 76.4% enhancement is attained in comparison with single layered PHOLED. Moreover, we also devise triple layer structure and co-host system OLED even though it does not achieve the optimal performance. We will work on these topics in the future for pursuing higher PHOLED efficiency. In the third part of the research, we fabricated white PHOLED of single layer and double layer structure according to the high efficiency bases of blue PHOLED, that we devised in the previous part. The red phosphorescent dye Tris(2-phenylquinoline)iridium(III) (Ir(2-phq)3) would be used in our experiment. By tuning the relative ratios of red and blue dopant materials, we could optimize the white PHOLED. Therefore, the maximum luminous efficiency of 12.5 cd/A, power efficiency of 4.7 lm/W, and maximum external quantum efficiency (EQE) of 2.2%, CIE coordinate of (0.32, 0.41) are achieved for the single layered structure of white PHOLED and the maximum luminous efficiency of 30 cd/A, power efficiency of 23 lm/W, and EQE of 11.6%, and CIE coordinate of (0.30, 0.42) are achieved for the double layered structure of white PHOLED.