Double-emission-layer green phosphorescent OLED based on LiF-doped TPBi as electron transport layer for improving efficiency and operational lifetime
Highlights
► LiF-doped TPBi as electron transport layer. ► Electrophosphorescent organic light-emitting diodes based on double-emitting layers. ► The efficiency and operational lifetime are remarkably improved. ► The device shows better stability at high current density.
Introduction
Phosphorescent organic light emitting diodes (PHOLEDs) have received considerable attention due to their highly efficient emission compared with fluorescent OLEDs [1], [2], [3], [4]. In theory, PHOLEDs have demonstrated internal quantum efficiency of 100% through radiative recombination of both singlet and triplet excitons [5], [6], [7]. Both the efficiency and lifetime of OLEDs are very critical parameters for practical application. In previous studies [2], [8], an external quantum efficiency (ηext) of 8% was reported using a light-emitting layer (EML) consisting of 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) host doped with tris(2-phenylpyridine) iridium(III) (Ir(ppy)3). In 2002, Zhou et al. [9] introduced the double-emission-layer (D-EML) concept to device design so as to lead to the ηext of about 12.6%, a significant improvement compared with conventional single-emission layer (S-EML) OLEDs. The D-EML structure can make charges throughout the whole EML to increase the charge balancing inside the active layers, so it can significantly improve efficiency and decrease efficiency loss at high current density. The electrical doping in charge transport layers has been proved to be a very effective way for improving efficiency and lifetime because of the increased conductivity [10], [11], [12]. To dope semiconductors, typically strong electron acceptors such as F4-TCNQ are used as p-dopants [13], [14] and electron donors such as alkaline metals as n-dopants [15], [16], [17]. D’Andrade et al. [18] had proved that Li:BPhen to form a mixed electron transport layer (ETL) can increase the operation lifetime. Choudhury et al. [19] demonstrated LiF-doped Alq3 as ETL can improve device efficiency and reliability. Up to now, the manufacturing high-efficiency, long-lifetime and low roll-off PHOLEDs are still hotspot [20], [21], [22].
Raymond et al. [23] had reported a series of PHOLEDs through employing different material as hole blocking layer (HBL). The results showed that using bis-(2-methyl-8-quinolinolato)-4-(phenyl-phenolato) aluminum-(III) [BAlq] as HBL, the PHOLED has the longest lifetime (about 10 000 h at an initial luminance L0 = 500 cd/m2), but lower the efficiency (the maximal current efficiency (ηA) is 19.0 cd/A). In contrast, using 2,2′,2′′-(1, 3, 5-benzenetriyl) tris-[1-phenyl-1H- benzimidazole] (TPBi) instead of BAlq as HBL, the efficiency of PHOLEDs is improved to ηA = 25.3 cd/A, but its lifetime is the shortest (only 70 h under encapsulation). In this paper, we designed a PHOLED with D-EML and n-doped ETL to decrease heterojunction interface number so as to get longer lifetime and higher efficiency, in which CBP and TPBi were used as the hosts, Ir(ppy)3 as phosphorescent dopant and LiF-doped TPBi as ETL. We also fabricated a S-EML PHOLED as a reference, where CBP was used as the host, Ir(ppy)3 as phosphorescent dopant, TPBi as HBL and tris (8-hydroxyquinoline) aluminum (Alq3) as ETL, which is similar to the device reported by Raymond et al. [23]. The D-EML PHOLED with n-doped ETL revealed high current/power efficiency and long lifetime, which are 1.99/2.95 and 35 times of those of the reference device, respectively. Moreover, it shows better stability than reference device at high current density.
Section snippets
Experimental details
In our experiment, a series of OLEDs were fabricated. The ITO-coated glass substrates with a sheet resistance of about 10 Ω/sq were used as anodes. The OLEDs were made by following process. First, ITO-coated glasses were cleaned successively using acetone and deionized water in an ultrasonic bath, dried in drying cabinet, and then irradiated by ultraviolet light. Second, the films of N,N′-bis(1-naphthyl)-N,N′- diphenyl-1,1′- biphenyl-4,4′-diamine (NPB), CBP:Ir(ppy)3, TPBi:Ir(ppy)3, TPBi, Alq3
Results and discussion
The EL spectra of devices A–D are shown in Fig. 2. The emission peak wavelengths are all 516 nm and the Commission International de l’Eclairage (CIE) (1931) color coordinates are all (0.31, 0.60), exclusively originating from the triplet excited state emission of the phosphor Ir(ppy)3. The lack of emission from either hosts or blocking layers indicates complete energy transfer from the hosts to the dopant and efficient confinement of charge carriers within the emission layers.
The Fig. 3 shows L–J
Conclusions
In this work, we demonstrate a highly efficient green POLED with D-EML and TPBi:LiF as ETL. The peak current efficiency is 40.5 cd/A and the maximum power efficiency is 23.7 lm/W, 1.99 and 2.95 times of those of the reference device A, respectively. And the device shows better stability at high current density. Moreover, the operational lifetime is also improved, nearly 35 times of the reference device A. We attribute these improvement mainly to the carriers’ self-balancing character of D-EML
Acknowledgments
This research work was financially supported by National Natural Scientific Foundation of China (60976018, 21071108), Program for Changjiang Scholar and Innovative Research Team in University (IRT0972), and Shanxi Natural Scientific Foundation (2008011008, 2010021023-2).
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