載子控制層強化有機發光二極體之研究

Study of Carrier-Controlling Layer Enhanced Organic Light-Emitting Diodes

沈仕旻(Shih-Ming Shen)

指導教授 ： 周卓煇

國立清華大學/工學院/材料科學工程學系/博士(2011年)

https://doi.org/10.6843/NTHU.2011.00324

摘要

本研究利用載子控制層，包含電子侷限層、電洞傳輸層、以及電洞調制層等，來研製高效率與超高演色性有機發光二極體（Organic Light- Emitting Diode, OLED），所製備之元件，可分為三部分作探討。
第一部分，本研究利用電子侷限層，來提升載子再結合率，研製出高效率磷光橘紅光有機發光二極體，在此研究中，比較了(1)未使用電子侷限層，與(2)使用2,7-bis(carbazo-9-yl)-9,9-ditolyfluorene (Spiro-2CBP) ，以及(3)使用di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane (TAPC) 為電子侷限層之元件，結果發現，使用TAPC之元件，在亮度100 cd/m2下，效率由39.5 cd/A提升至46.2 cd/A，在1,000 cd/m2下，則由34.5 cd/A提升至44.8 cd/A；效率提升可歸因於TAPC有效阻擋電子於發光層中，進而提高載子再結合機率；同時，TAPC兼具了阻擋部分電洞注入發光層之功能，使得元件在發光層中達載子注入平衡，而獲得更高之發光效率。
第二部份，本研究利用具高發光效率之天藍光與紅光染料，搭配具高能量轉移效率之主客體，製作一螢光白光OLED，在亮度100與1,000 cd/m2下，能量效率為分別為11.9與9.1 lm/W，在加入第二電洞傳輸層N,N’-bis-(1-naphthyl)-N,N’-diphenyl-1,10-biphenyl- 4-4’-diamine (NPB)後，在100 cd/m2下，元件效率提升59 %至18.9 lm/W，在1,000 cd/m2下，更大幅提升81 %至16.5 lm/W；這可歸因於NPB的加入，形成一電洞注入能障，有效阻擋部份電洞進入發光層中，達到載子注入平衡，進而提升元件效率，而在高亮度下(等同於高電壓、高電流密度下)，NPB的電洞阻擋效果更加顯著，使得元件效率提升幅度更為明顯。
在第三部份，本研究研製出一具有超高演色性(color-rendering index, CRI)之白光OLED，元件在100 cd/m2下，CRI 達98，效率為8.3 lm/W，在1,000 cd/m2下，CRI為96，效率為5.2 lm/W；此超高演色性可歸因於使用可構成全波段光譜之染料，以及加入適當厚度之電洞調制層1,3,5-trisN-phenylbenzimidazol-2-ylbenzene於兩白光發光層之間，電洞調制層可將電洞適當地分配在兩發光層中，使所有染料分子有效且恰當地放光，獲得一近乎全波段的光譜，相較之下，未使用電洞調制層之元件，因電洞過度集中於單一發光層，所得演色性僅73；值得一提的是，此超高演色性OLED之元件效率，是目前演色性超過95的元件中最高的，而在100 cd/m2到5,000 cd/m2間，均維持大於96的超高演色性，且非常貼近日光曲線(daylight locus)，使其具有潛力成為高品質照明光源。
總括而言，本研究主要貢獻為藉由載子控制層的使用，大幅提升白光與橘紅光OLED元件之能量與電流效率，以及研製出超高演色性OLED元件。而在超高演色性元件的研究中，更是使用以往只被用來作為電子傳輸層之材料1,3,5-trisN-phenylbenzimidazol-2-ylbenzene，來作為關鍵的電洞調制層，這打破了以往元件設計的基本觀念，也表示任何材料皆可有多種功能，這將由其放置在元件內之位置來決定，且往往有預料外的效果，值得有機電子領域的研究人員作為參考。

關鍵字

有機發光二極體 ； 載子控制層 ； 高效率 ； 超高演色性 ； 載子再結合

並列摘要

In the study, we incorporate carrier-controlling layers, including electron-blocking layer (EBL), hole-transporting layer (HTL) and hole-modulating layer (HML), to develop and fabricate highly efficient and very-high color rendering index (CRI) organic light-emitting diodes (OLEDs). The investigated devices are shown as follow.
In Part I, we demonstrate an efficient orange-red phosphorescent OLED using a novel host, 2,7-bis(carbazo-9-yl)-9,9-ditolyfluorene, doped with tris(2-phenylquinoline) iridium(III), as well as a thin EBL of 1,1-bis-(4-bis(4-methylphenyl)-aminophenyl)-cyclohexane (TAPC) is deposited prior to the emissive layer. The resulting device exhibits a current efficiency of 44.8 cd/A at 1,000 cd/m2. This high efficiency may be attributed to the adoption of the host, which favors the injection of holes, as well as the emissive-layer architecture enabling excitons to form on the host and hence favoring efficient energy-transfer from host to guest. The TAPC layer helps to modulate excessive holes to be injected into the emissive layer and to confine the electrons, which would in turn balance the injection of both carriers and improve the device efficiency, either at low or high voltages.
In Part II, we demonstrated the use of double HTLs, poly(3,4-ethylene-dioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS) and N,N’-bis-(1-naphthyl)-N,N’-diphenyl-1,10-biphenyl-4-4’-diamine (NPB), to modify the hole injection characteristics, to balance the injection of carriers, and consequently to improve the device efficiency. With the addition of a 7.5 nm second HTL (NPB), the resultant power-efficiency at 100 cd/m2 was increased from 11.9 to 18.9 lm/W, an improvement of 59%. The improvement was even more marked at 1,000 cd/m2, i.e. that the power-efficiency was increased from 9.1 to 16.5 lm/W, an improvement of 81%. The marked efficiency improvement may be attributed to a better balance of carrier-injection in the desired emissive zone since the addition of the NPB layer in between the first HTL and the EML may have effectively reduced the injection of excessive holes into the EML due to the relatively high energy-barrier to holes, which was 0.5 eV, at the interface of the two HTLs.
In Part III, we demonstrate an efficient very-high CRI OLED with a CRI of 98 and an efficacy of 8.3 lm/W at 100 cd/m2, or a CRI of 96 and an 5.2 lm/W at 1,000 cd/m2. The very high CRI may be attributed to the successful deposition and emission of the two full-spectrum complementary white emissive layers, especially as a thin HML is inserted in between to regulate the injection of carriers. Without the interlayer, the resultant CRI drops to 73 and efficacy to 3.6 lm/W at 1,000 cd/m2. The employment of the carrier regulating layer also helps disperse the injected carriers, leading recombination to occur in a wider area and hence a higher efficiency. Moreover, it is worth mentioning that the CRI remained around 96 to 98 throughout the brightness range over which we investigated. The CIE coordinates of the very high CRI OLED also fall within the pure-white region and are very close to daylight (Planckian) locus, indicating the device to possess the characteristics required for high-quality illumination.
In summary, the main contribution of this study is to markedly increase the power and current efficiencies of white and orange-red OLEDs and to fabricate a efficient very-high CRI OLED by using carrier-controlling layers. In the study of very-high CRI OLED, we use 1,3,5-trisN-phenylbenzimidazol-2-ylbenzene, a general electron-transporting layer, as the hole-modulating layer. This is a novel concept on the design of device structure. This also implies that any materials will have unanticipated functions if we use it at a different position in device. This is a very useful concept for the researchers in the field of organic electronics.

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