Asymmetrical-Dendronized TADF Emitters for Efficient Non-doped Solution-Processed OLEDs by Eliminating Degenerate Excited States and Creating Solely Thermal Equilibrium Routes

The mechanism of thermally activated delayed fluorescence (TADF) in dendrimers is not clear. We report that fully-conjugated or fully-nonconjugated structures cause unwanted degenerate excited states due to multiple identical dendrons, which limit their TADF efficiency. We have synthesized asymmetrical "half-dendronized" and "half-dendronized-half-encapsulated" emitters. By eliminating degenerate excited states, the triplet locally excited state is ≥0.3 eV above the lowest triplet charge-transfer state, assuring a solely thermal equilibrium route for an effective spin-flip process. The isolated encapsulating tricarbazole unit can protect the TADF unit, reducing nonradiative decay and enhancing TADF performance. Non-doped solution-processed devices reach a high external quantum efficiency (EQEmax ) of 24.0 % (65.9 cd A-1 , 59.2 lm W-1 ) with CIE coordinates of (0.24, 0.45) with a low efficiency roll-off and EQEs of 23.6 % and 21.3 % at 100 and 500 cd m-2 .

Cyclometalated Ir(III) complexes as potential electron acceptors for organic solar cells

Cyclometalated iridium(iii) complexes have been investigated as promising electron donor (D) materials in organic solar cells (OSCs) due to their unique octahedral configuration for optimized morphology and their significantly long lifetimes potentially for enhanced exciton dissociation. However, the application as electron acceptor (A) materials has never been reported. In order to fill this blank, herein, two cyclometalated heteroleptic Ir complexes, TRIr and 2TRIr, based on electron donating-accepting type organic ligands with different π-conjugation lengths are reported as electron acceptor materials in comparison with their corresponding main organic ligands. The two Ir complexes exhibit suitable HOMO/LUMO energy levels of -5.55/-3.47 eV and -5.44/-3.48 eV, which are ∼0.1 eV higher in the HOMO and ∼0.15 eV deeper in the LUMO than the TR and 2TR ligands, respectively. 2TRIr with extended ligand π-conjugation displays a poor triplet feature, while TRIr demonstrates obvious metal-to-ligand charge transfer (MLCT) transition absorption, with a triplet component photoluminescence (PL) lifetime of 85 ns in neat films. When blended with PBDB-T in bulk heterojunction (BHJ) OSCs, the power conversion efficiencies (PCEs) are 2-3 times higher than their relevant ligands, with values of 1.20% and 1.62% for TRIr and 2TRIr, and 0.58% and 0.47% for the TR and 2TR ligand-based devices, respectively. TRIr and 2TRIr based active layer blends exhibit poorer hole and electron mobilities, whereas compared with their relatively linear planar ligands, both of the two octahedral Ir complexes exhibit an optimized surface morphology for less bimolecular recombination and more efficient exciton dissociation, thus contributing to improved photovoltaic performance.

Solution-Processible Blue Fluorescent Dendrimers with Carbazole/Diphenylamine Hybrid Dendrons for Power-Efficient Organic Light-Emitting Diodes

Two blue fluorescent dendrimers named PVAC2 and PVACA have been newly synthesized and investigated, where the carbazole/diphenylamine hybrid dendron is adopted instead of oligocarbazole. Compared with the reference dendrimer PVCt3, the emission maxima of PVAC2 and PVACA are found to be red-shifted accompanied by a slight reduction of the photoluminescence quantum yield in films. Most importantly, the highest occupied molecular orbital level is elevated from -5.35 eV of PVCt3 to -5.20 eV of PVAC2 and -4.95 eV of PVACA. Because of the favored hole injection, the turn-on voltage is accordingly decreased from 3.6 to 3.2 and 2.6 V. The value of PVACA is even lower than the theoretical limit of 2.78 V. In addition, PVAC2 exhibited the best nondoped device performance, showing a nearly doubled power efficiency of 4.80 lm/W relative to PVCt3 (2.37 lm/W). The results clearly indicate that dendron engineering is also a promising strategy to develop solution-processible blue fluorescent dendrimers capable of being used for power-efficient organic light-emitting diodes.

Thiphenylmethane based structural fragments as building blocks towards solution-processable heteroleptic iridium(iii) complexes for OLED use

Functionalization with 1,1,1-triphenylmethylpentane groups can be used for heteroleptic iridium(iii) complexes in order to obtain solution-processable OLED emitters.

Multinuclear Iridium Complex Encapsulated by Oligocarbazole Dendrons for Enhanced Nondoped Device Efficiency

A dendritic multinuclear Ir complex, namely Cz-3IrB-IrG, has been designed and synthesized by introducing the second-generation oligocarbazole dendrons into its periphery. Because of the characteristic encapsulation, the intermolecular interactions could be effectively alleviated to prevent the unwanted triplet-triplet annihilation stemmed from the outer blue Ir complexes. Compared with 3IrB-IrG in the absence of dendrons, the film photoluminescence quantum yield of Cz-3IrB-IrG is greatly increased from 0.46 to 0.82 together with a small blue-shifted emission from 524 to 520 nm. On the basis of Cz-3IrB-IrG as the emitting layer alone, the nondoped device realizes a promising luminous efficiency of 40.9 cd/A (12.0%), much higher than that of 3IrB-IrG (32.6 cd/A, 9.7%). The obtained improvement clearly indicates that further dendronization toward multinuclear Ir complex will provide an alternative strategy to construct highly efficient phosphors used for nondoped phosphorescent organic light-emitting diodes.

Tetranuclear Iridium Complex with a Self-Host Feature for High-Efficiency Nondoped Phosphorescent Organic Light-Emitting Diodes

A tetranuclear Ir complex 3IrB-IrG was newly designed and synthesized with one green-emitting complex as the core, whose periphery is encapsulated by three blue-emitting complexes via a nonconjugated linkage. In the case of such a multinuclear system, a self-host feature can be formed, showing negligible electron communication, efficient outside-in energy transfer, and unique shielding effect. When using 3IrB-IrG as the emitting layer in the absence of host, the corresponding nondoped device reveals a state-of-art luminous efficiency of 32.6 cd/A (34.2 lm/W, 9.7%) as well as CIE coordinates of (0.38, 0.58). The performance significantly outperforms that of the bare mononuclear complex IrG (8.4 cd/A, 9.2 lm/W, 2.5%), highlighting the potential of self-host multinuclear complexes to realize high-efficiency nondoped PhOLEDs for the first time.

Novel bluish green benzimidazole-based iridium(iii) complexes for highly efficient phosphorescent organic light-emitting diodes

Fluorinated benzimidazole-based Ir(iii) complexes were developed to regulate the emitting color and improve the luminous efficiency of Ir(iii) phosphors.

Dendron engineering in self-host blue iridium dendrimers towards low-voltage-driving and power-efficient nondoped electrophosphorescent devices

Low-voltage-driving and power-efficient nondoped electrophosphorescent devices have been realized by increasing the dendron's HOMO energy level to favor effective hole injection and promote exciton formation.

Self-Host Blue-Emitting Iridium Dendrimer Containing Bipolar Dendrons for Nondoped Electrophosphorescent Devices with Superior High-Brightness Performance

A novel self-host blue-emitting iridium dendrimer, namely, B-CzPO, has been designed and synthesized via a postdendronization route, where a bipolar carbazole/triphenylphosphine oxide hybrid is selected as the peripheral dendron instead of the p-type oligocarbazole used in unipolar analogue B-CzG2. This structural modification can render B-CzPO with more balanced charge transportation relative to that of B-CzG2. As a result of the significantly reduced efficiency roll-off, the nondoped phosphorescent organic light-emitting diodes (PhOLEDs) of B-CzPO show a superior high-brightness performance, revealing a luminous efficiency of 21.2, 16.1, and 10.5 cd/A at 1000, 5000, and 10 000 cd/m², respectively. Compared with that of B-CzG2 (i.e., 7.8 cd/A @5000 cd/m²), more than doubled high-brightness performance is achieved for B-CzPO. The results indicate that the design of self-host phosphorescent dendrimers with a bipolar feature will be a promising strategy to develop efficient nondoped PhOLEDs suitable for high-brightness applications including general illumination and micro displays.

Highly Efficient Cuprous Complexes with Thermally Activated Delayed Fluorescence for Solution-Processed Organic Light-Emitting Devices

Two mononuclear cuprous complexes [Cu(PNNA)(POP)]BF4 (1) and [Cu(PNNA)(Xantphos)]BF4 (2) (PNNA = 9,9-dimethyl-10-(6-(3-phenyl-1H-pyrazol-1-yl)pyridin-3-yl)-9,10-dihydroacridine, POP = bis[2-(dipenylphosphino)phenyl]ether, Xantphos =4,5-bis(diphenylphosphino)-9,9-dimethylxanthene), with intense bluish-green luminescence based on a new diimine ligand were designed and synthesized. Their structural, electrochemical, and photophysical properties were characterized by single-crystal X-ray analysis, cyclic voltammetry, temperature dependence of spectroscopy, time-dependent emission spectroscopy, etc. The complexes exhibit high photoluminescence quantum yields in doped films (up to 74.6%) at room temperature. Thermally activated delayed fluorescence based on intraligand charge transfer was observed by grafting a strong electron-donor moiety, 9,9-dimethylacridan, on the diimine ligand, which is supported by the density functional theory calculations on two complexes. Highly efficient solution-processed OLEDs based on these two complexes were fabricated, among which the electroluminescent device using 2 as dopant shows a peak external quantum efficiency of 7.42%, a peak current efficiency of 20.24 cd/A, and a maximum brightness of 5579 cd/m(2).

Bis-Tridentate Ir(III) Complexes with Nearly Unitary RGB Phosphorescence and Organic Light-Emitting Diodes with External Quantum Efficiency Exceeding 31%

A new class of neutral bis-tridentate Ir(III) metal complexes that show nearly unitary red, green, and blue emissions in solution is prepared and employed for the fabrication of both monochrome and white-emitting organic light-emitting diodes, among which a green device gives external quantum efficiency exceeding 31%.

Enhanced Electron Affinity and Exciton Confinement in Exciplex-Type Host: Power Efficient Solution-Processed Blue Phosphorescent OLEDs with Low Turn-on Voltage

A benzimidazole/phosphine oxide hybrid 1,3,5-tris(1-(4-(diphenylphosphoryl)phenyl)-1H-benzo[d]imidazol-2-yl)benzene (TPOB) was newly designed and synthesized as the electron-transporting component to form an exciplex-type host with the conventional hole-transporting material tris(4-carbazoyl-9-ylphenyl)amine (TCTA). Because of the enhanced triplet energy and electron affinity of TPOB, the energy leakage from exciplex-state to the constituting molecule was eliminated. Using energy transfer from exciplex-state, solution-processed blue phosphorescent organic light-emitting diodes (PHOLEDs) achieved an extremely low turn-on voltage of 2.8 V and impressively high power efficiency of 22 lm W(-1). In addition, the efficiency roll-off was very small even at luminance up to 10 000 cd m(-2), which suggested the balanced charge transfer in the emission layer. This study demonstrated that molecular modulation was an effective way to develop efficient exciplex-type host for high performanced PHOLEDs.

Synthesis and electrophosphorescence of novel heteroleptic iridium complexes based on thiazole-containing ligands

Novel heteroleptic iridium complexes containing aromatic-ring-fused-thiazole in cyclometalating ligands are developed and exhibit a maximum external quantum efficiency of 22.2% in orange phosphorescent organic light-emitting diodes.

Single molecular tuning of the charge balance in blue-emitting iridium dendrimers for efficient nondoped solution-processed phosphorescent OLEDs

Charge balance modulation at a single dendritic molecule is demonstrated to be far superior to physical blending due to the elimination of phase segregation, leading to improved EQEs and negligible efficiency roll-off.

Curing temperature reduction and performance improvement of solution-processable hole-transporting materials for phosphorescent OLEDs by manipulation of cross-linking functionalities and core structures

Novel solution-processable hole-transporting materials for OLEDs capable of 150 °C thermal curing could be prepared by manipulation of cross-linking functionalities.

Electrochemical synthesis of electrochromic polycarbazole films from N-phenyl-3,6-bis(N-carbazolyl)carbazoles

Electroactive and electrochromic polycarbazole films were directly prepared on electrodes from N-phenyl-3,6-bis(N-carbazolyl)carbazoles by carbazole-based electrochemical oxidative coupling.