Carrier Transport and Recombination Mechanism in Blue Phosphorescent Organic Light-Emitting Diode with Hosts Consisting of Cabazole- and Triazole-Moiety

Abstract

In this study, we demonstrated a blue phosphorescent organic light-emitting diode (BPOLED) based on a host with two carbazole and one trizole (2CbzTAZ) moiety, 9,9′-(2-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)-1,3-phenylene)bis(9H-carbazole), that exhibits bipolar transport characteristics. Compared with the devices with a carbazole host (N,N’-dicarbazolyl-3,5-benzene, (mCP)), triazole host (3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole, (TAZ)), or a physical mixture of mCP:TAZ, which exhibit hole, electron, and bipolar transport characteristics, respectively, the BPOLED with the bipolar 2CbzTAZ host exhibited the lowest driving voltage (6.55 V at 10 mA/cm 2 ), the highest efficiencies (maximum current efficiency of 52.25 cd/A and external quantum efficiency of 23.89%), and the lowest efficiency roll-off, when doped with bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic) as blue phosphor. From analyses of light leakage of the emission spectra of electroluminescence, transient electroluminescence, and partially doped OLEDs, it was found that the recombination zone was well confined inside the emitting layer and the recombination rate was most efficient in a 2CbzTAZ-based OLED. For the other cases using mCP, TAZ, and mCP:TAZ as hosts, electrons and holes transported with different routes that resulted in carrier accumulation on different organic molecules and lowered the recombination rate.

Bistriazoles with a Biphenyl Core Derivative as an Electron-Favorable Bipolar Host of Efficient Blue Phosphorescent Organic Light-Emitting Diodes

“… 17 − 20 Well-known benchmark bipolar host materials mCP and mCBP have good hole-transporting characteristics and also demonstrate good efficiency performance with an EQE around 20% for the blue phosphorescent OLED. 10 , 21 However, typically, such a host material exhibited higher hole mobility than the electron one, which results in carrier imbalance and reduces the electroluminescence efficiency. Donor–acceptor-type bipolar organic materials were synthesized as the host of blue phosphorescent OLED for improving the efficiency and reducing the efficiency roll-off by connecting the hole-transporting unit (e.g., carbazole moiety) and electron-transporting unit (e.g., cyano, triazole, phosphine oxide, benzimidazole, and oxadiazole moieties).…”

“…Donor–acceptor-type bipolar organic materials were synthesized as the host of blue phosphorescent OLED for improving the efficiency and reducing the efficiency roll-off by connecting the hole-transporting unit (e.g., carbazole moiety) and electron-transporting unit (e.g., cyano, triazole, phosphine oxide, benzimidazole, and oxadiazole moieties). 21 − 26 Many bipolar host materials such as mCP, mCBP, mNBICz, UGH2, CbzBiz, DPEPO, and so on were reported with the high-efficiency OLEDs. 10 , 27 − 31 Specifically, most of these bipolar hosts exhibited a faster intrinsic hole mobility compared to the intrinsic electron one.…”

“…The green phosphorescent OLED showed a good performance with a CE of 73.6 cd/A, PE of 72.2 lm/W, and EQE of 21.8%. 37 In our previous works, the pristine TAZ was employed as a blue host doped with an FIrpic emitter to achieve an efficient phosphorescent OLED with a CE of 45.9 cd/A and EQE of 20.2%; 21 the C2 chirality of the biphenyl core was also demonstrated to allow the stacking of electron-transporting moieties (e.g., oxadiazole) and to make the through-space interactions feasible. 38 Hence, in this study, BTBP was synthesized, combining the viewpoint of our above two reports, and employed as the host for blue phosphorescent OLED, which showed efficient device performances.…”

Fluorene–Triphenylamine-Based Bipolar Materials: Fluorescent Emitter and Host for Yellow Phosphorescent OLEDs

“…4,4 -bis(N-carbazolyl)-1,1 -biphenyl (CBP), 1,3-bis(N-carbazolyl)benzene (mCP), and 4,4 -cyclohexyli denebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) are well-known HT type host materials, but they do not show stable thermal properties. Tris(8-hydroxyquinoline)aluminum (Alq 3 ) and Bis [2-(diphenylphosphino)phenyl] ether oxide (DPEPO) are ET type host materials that are used in phosphorescent and TADF OLEDs [37][38][39][40][41][42][43][44][45][46][47][48][49]. Host materials are essential energy suppliers to the dopant material that also prevent energy flow back from the dopant.…”

A Brief Minireview of poly-triazole: Alkyne and Azide Substrate Selective, Metal-catalyst Design

“…9,10 It doesn't even need for special separation, purification, accounting for alkyne groups coordination with copper, so as to improving the reactivity of alkynyl. 11,12 Along with high regioselectivity, carbon atoms are coordinated on the electron cloud density change without end group of copper ion catalytic acetylene and azide. 13,14 In most cases, the 3-dipole ring addition reaction is not regionally selective and the reaction is slow.…”