Blue Triplet Emitter Research Articles

Correlation of charge trapping and charge transport properties of blue triplet emitters with device performances of blue phosphorescent organic light-emitting diodes

The relationship between driving voltage and charge trapping in blue phosphorescent organic light-emitting diodes was investigated using two blue phosphorescent emitters based on fluorine substituted phenylpyridine ligands. It was demonstrated that electron trapping by dopant reduces electron current density in the emitting layer, while hole trapping had little effect on the hole current density in the emitting layer. It was found that the electron trapping by phosphorescent dopant was the dominant factor for the driving voltage of blue phosphorescent organic light emitting diodes.

A diphenyl ether bridged, high triplet energy host material for blue phosphorescent organic light-emitting diodes

Abstract A carbazole type, thermally stable, high triplet energy host material having a diphenylether linkage, 9,9′-(oxybis([1,1′-biphenyl]-4′,3-diyl))bis(9 H -carbazole), was synthesized and the device performances of blue phosphorescent organic light-emitting diodes were studied. The new host material exhibited a high glass transition temperature of 111 °C and a high triplet energy of 2.73 eV for energy transfer to blue triplet emitter. Blue devices were fabricated using the new host material and a high quantum efficiency of 23.5% was achieved.

Synthesis and Properties of Oxygen-Linked N-Phenylcarbazole Dendrimers

A series of novel oxygen-linked N-phenylcarbazole (NPC) dendritic wedges (3–5) and triphenylamine-centered dendrimers (CBD-G0 to -G2) have been synthesized and extensively studied. The aryl–oxygen–aryl (Ar–O–Ar) linkages were established on 3,6-positions of carbazole and 4-position of N-phenyl group. UV–vis spectra revealed that the Ar–O–Ar linkage would break down the π conjugation and make NPC units manifest their individual absorption moiety. Both steady-state luminescence and fluorescence decay dynamics at room temperature and 77 K in tetrahydrofuran suggested that the transfer of energy from the carbazole wedges to the triphenylamine (TPA) core operates in CBD-G1, so that the luminescence mainly arises from the core unit. The quenching of the emission from carbazole wedges by the TPA core becomes less effective as the generation develops into CBD-G2. The oxidation potential of the NPC derivatives clearly revealed the mesomeric electron-donating effect of the oxygen atom and inductive electron-withdrawing effect of the carbazole unit. The potential gradient could be established on those triphenylamine-centered dendrimers (CBD-G1 and CBD-G2), such that the outer layer is electron-poor and the inner layer is electron-rich. It is remarkable that the HOMO level of the NPC dendrimers is as high as −5.17 eV and triplet energy level is kept above 2.85 eV. These levels of HOMO and triplet energy, together with good thermal stability and compatibility of solution processing, make NPC dendrimers ideal host materials for blue triplet emitters. Using CBD-G2 as the host material, the FIrpic-doped solution-processed light-emitting diodes were fabricated, and maximum luminous efficiency could be achieved at a record high of 24.7 cd/A at 484 cd/m2.

New Solution‐Processable Electron Transport Materials for Highly Efficient Blue Phosphorescent OLEDs

AbstractSeveral new solution‐processable organic semiconductors based on dendritic oligoquinolines were synthesized and were used as electron‐transport and hole‐blocking materials to realize highly efficient blue phosphorescent organic light‐emitting diodes (PhOLEDs). Various substitutions on the quinoline rings while keeping the central meta‐linked tris(quinolin‐2‐yl)benzene gave electron transport materials that combined wide energy gap (>3.3 eV), moderate electron affinity (2.55‐2.8 eV), and deep HOMO energy level (<‐6.08 eV) with electron mobility as high as 3.3 × 10−3 cm2 V−1 s−1. Polymer‐based PhOLEDs with iridium (III) bis(4,6‐(di‐fluorophenyl)pyridinato‐N,C2′)picolinate (FIrpic) blue triplet emitter and solution‐processed oligoquinolines as the electron‐transport layers (ETLs) gave luminous efficiency of 30.5 cd A−1 at a brightness of 4130 cd m−2 with an external quantum efficiency (EQE) of 16.0%. Blue PhOLEDs incorporating solution‐deposited ETLs were over two‐fold more efficient than those containing vacuum‐deposited ETLs. Atomic force microscopy imaging shows that the solution‐deposited oligoquinoline ETLs formed vertically oriented nanopillars and rough surfaces that enable good ETL/cathode contacts, eliminating the need for cathode interfacial materials (LiF, CsF). These solution‐processed blue PhOLEDs have the highest performance observed to date in polymer‐based blue PhOLEDs.

Efficient Low-Driving-Voltage Blue Phosphorescent Homojunction Organic Light-Emitting Devices

A blue phosphorescent p–i–n homojunction organic light-emitting device with a low driving voltage was demonstrated using 4,6-bis[3-(carbazol-9-yl)phenyl] pyrimidine (46DCzPPm) as the bipolar host material and iridium(III) bis[4,6-(difluorophenyl)pyridinato-N,C2'] picolinate as the blue triplet emitter. The device incorporated an n-doping electron-transport layer consisting of Cs2CO3-doped 46DCzPPm and a p-doping hole-transport layer consisting of MoO3-doped 46DCzPPm. A maximum external quantum efficiency of 10.2% and a power efficiency of 27.8 lm W-1 were obtained. At 100 cd m-2, the efficiencies were 9.1% and 21.5 lm W-1, and even at 1000 cd m-2, the efficiencies remained at 7.0% and 12.8 lm W-1, respectively, with an extremely low driving voltage of 3.9 V.

A Bipolar Host Material Containing Triphenylamine and Diphenylphosphoryl‐Substituted Fluorene Units for Highly Efficient Blue Electrophosphorescence

AbstractHighly efficient blue electrophosphorescent organic light‐emitting diodes incorporating a bipolar host, 2,7‐bis(diphenylphosphoryl)‐9‐[4‐(N,N‐diphenylamino)phenyl]‐9‐phenylfluorene (POAPF), doped with a conventional blue triplet emitter, iridium(III) bis[(4,6‐difluoro‐phenyl)pyridinato‐N,C2´]picolinate (FIrpic) are fabricated. The molecular architecture of POAPF features an electron‐donating (p‐type) triphenylamine group and an electron‐accepting (n‐type) 2,7‐bis(diphenyl‐phosphoryl)fluorene segment linked through the sp3‐hybridized C9 position of the fluorene unit. The lack of conjugation between these p‐ and n‐type groups endows POAPF with a triplet energy gap (ET) of 2.75 eV, which is sufficiently high to confine the triplet excitons on the blue‐emitting guest. In addition, the built‐in bipolar functionality facilitates both electron and hole injection. As a result, a POAPF‐based device doped with 7 wt% FIrpic exhibits a very low turn‐on voltage (2.5 V) and high electroluminescence efficiencies (20.6% and 36.7 lm W−1). Even at the practical brightnesses of 100 and 1000 cd m−2, the efficiencies remain high (20.2%/33.8 lm W−1 and 18.8%/24.3 lm W−1, respectively), making POAPF a promising material for use in low‐power‐consumption devices for next‐generation flat‐panel displays and light sources.

11.2: Efficient Deep Blue Triplet Emitters for OLEDs

Cyclometallated iridium carbene complexes are introduced as efficient blue triplet emitters. Quantum mechanical calculations have been used to design and to optimize this class of materials predominantely with respect to color coordinates and luminescence quantum yield. To complete the set of materials required for deep blue OLED devices we engineered suitable host and blocker materials for the use in combination with large triplet energy carbene emitters. These tailor-made materials were applied to develop deep blue electroluminescent devices with excellent efficiency.