Double-emission-layer green phosphorescent OLED based on LiF-doped TPBi as electron transport layer for improving efficiency and operational lifetime

Abstract

We demonstrate an excellent electrophosphorescent organic light-emitting diodes (PHOLEDs) with double-emitting layers (D-EML) and LiF-doped TPBi as electron transport layer (ETL), which has the structure of ITO/NPB (30 nm)/CBP:Ir(ppy)₃ (20 nm)/TPBi:Ir(ppy)₃ (10 nm)/TPBi (10 nm)/TPBi:LiF (40 nm)/LiF (1.2 nm)/Al (150 nm). A peak current efficiency of 40.5 cd/A and a maximum power efficiency of 23.7 lm/W have been achieved, 1.99 and 2.95 times of those of the reference device [ITO/NPB (30 nm)/CBP:Ir(ppy)₃ (30 nm)/TPBi (10 nm)/Alq₃ (40 nm)/LiF (1.2 nm)/Al (150 nm)], respectively. And the device shows better stability than reference device at high current density. Moreover, the operational lifetime is improved, nearly 35 times of that of the reference device. We attribute mainly these improvement to the carriers’ self-balancing character of D-EML OLEDs, less numbers of heterojunction interface and better electron transport property of TPBi:LiF.

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 (η_ext) 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 η_ext 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 (η_A) 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 η_A = 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.