Shumeng Wang

Self-host blue-emitting iridium dendrimer with carbazole dendrons: nondoped phosphorescent organic light-emitting diodes.

A blue-emitting iridium dendrimer, namely B-G2, has been successfully designed and synthesized with a secondgeneration oligocarbazole as the dendron, which is covalently attached to the emissive tris[2-(2,4-difluorophenyl)-pyridyl]iridium(III) core through a nonconjugated link to form an efficient self-host system in one dendrimer. Unlike small molecular phosphors and other phosphorescent dendrimers, B-G2 shows a continuous enhancement in the device efficiency with increasing doping concentration. When using neat B-G2 as the emitting layer, the nondoped device is achieved without loss in efficiency, thus giving a state-of-art EQE as high as 15.3% (31.3 cdA1, 28.9 lmW1) along with CIE coordinates of (0.16, 0.29).

Solution-Processed Phosphorescent Organic Light-Emitting Diodes with Ultralow Driving Voltage and Very High Power Efficiency

To realize power efficient solution-processed phosphorescent organic light-emitting diodes (s-PhOLEDs), the corresponding high driving voltage issue should be well solved. To solve it, efforts have been devoted to the exploitation of novel host or interfacial materials. However, the issues of charge trapping of phosphor and/or charge injection barrier are still serious, largely restraining the power efficiency (PE) levels. Herein, with the utilization of an exciplex-forming couple 4, 4′, 4″ -tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB), the efficient charge injection and transporting, barrier-free hole-electron recombination for the formation of the interfacial exciplex, and elimination of charge traps of phosphors in the emissive layer are realized simultaneously, resulting in a turn-on voltage of 2.36 V, a record high PE of 97.2 lm W−1, as well as extremely low driving voltage of 2.60 V at 100 cd m−2, 3.03 V at 1000 cd m−2 and 4.08 V at 10000 cd m−2. This report is the first time that the PE performance of s-PhOLED approaches 100 lm W−1 high level, even superior to the corresponding state-of-the-art performance of the same color vacuum-deposited PhOLED (v-PhOLED) counterpart. We anticipate this report opens a new avenue for achieving power efficient monochromatic and white s-PhOLEDs with simple structures.

Starburst 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine-based deep-blue fluorescent emitters with tunable oligophenyl length for solution-processed undoped organic light-emitting diodes

On the basis of a well-known hole transporting material, namely 4,4′,4′′-tris(carbazol-9-yl)-triphenylamine (TCTA), a series of star-shaped deep-blue fluorescent emitters (2P-TCTA, 3P-TCTA, 4P-TCTA and 5P-TCTA) have been successfully developed via a simple extension of the oligophenyl chain between two N atoms. When the number of phenyl rings increases, it is found that both the absorption and emission for these TCTA-based starbursts are red-shifted and finally become saturated for 5P-TCTA consisting of a pentaphenyl bridge. Interestingly, on going from 2P-TCTA to 5P-TCTA, the film photoluminescence quantum yield is gradually enhanced from 11.4% to 35.5%. The same trend is also observed for their corresponding solution-processed undoped OLEDs. As a consequence, 5P-TCTA shows the best device performance, revealing a maximum luminescence of 7300 cd m−2, and a peak luminous efficiency of 2.48 cd A−1 (2.15 lm W−1; 2.30%) together with CIE coordinates of (0.15, 0.09).

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

Dendron engineering in self-host blue Ir dendrimers is reported to develop power-efficient nondoped electrophosphorescent devices for the first time, which can be operated at low voltage close to the theoretical limit (Eg/e: corresponding to the optical bandgap divided by the electron charge). With increasing dendrons HOMO energy levels from B-POCz to B-CzCz and B-CzTA, effective hole injection is favored to promote exciton formation, resulting in a significant reduction of driving voltage and improvement of power efficiency. Consequently, the nondoped device of B-CzTA achieves extremely low driving voltages of 2.7/3.4/4.4 V and record high power efficiencies of 30.3/24.4/16.3 lm W-1 at 1, 100 and 1000 cd m-2, respectively. We believe that this work will pave the way to the design of novel power-efficient self-host blue phosphorescent dendrimers used for energy-saving displays and solid-state lightings.

Facile synthesis of self-host functional iridium dendrimers up to the fourth generation with N-phenylcarbazole-based polyether dendrons for non-doped phosphorescent organic light-emitting diodes

A facile synthesis has been demonstrated for the first time to construct self-host functional Ir-cored dendrimers up to the fourth generation on the basis of a newly developed polyether dendron, where the N-phenylcarbazole (NPC) moiety is used as the basic building block instead of benzene to improve charge transport whilst keeping the ease of preparation. With the growing generation number, the dendrimer size can be well tuned in a wide range of 4–10 nm. The obtained fourth generation dendrimer 45NPC-G4 is the largest Ir complex ever reported so far, having a diameter up to 10 nm and a molecular weight as high as 15.9 kDa. Most interestingly, the performance of non-doped phosphorescent organic light-emitting diodes (PhOLEDs) is found to be greatly dependent on the molecular size. For example, 9NPC-G2 (R ≈ 30 A) reveals the best luminous efficiency as high as 50.5 cd A−1 (56.6 lm W−1, 14.8%), whereas the efficiency of 45NPC-G4 (R ≈ 50 A) sharply drops to 10.5 cd A−1 (5.6 lm W−1, 3.4%). The results suggest that an appropriate size of 6 ± 2 nm is desirable to balance the dilemma between luminescence quenching and charge transport, and thereby realize highly efficient non-doped PhOLEDs.

Interfacial triplet confinement for achieving efficient solution-processed deep-blue and white electrophosphorescent devices with underestimated poly(N-vinylcarbazole) as the host

Highly efficient deep-blue and white PhOLEDs with FIr6 as a blue emitter and PVK as a host are developed by using a high triplet level interfacial layer to confine triplet excitons within the emissive layer. Incorporation of an interfacial layer with a higher triplet level such as TPCz can effectively cut off the potential loss pathways of the triplet excitons within the PVK:FIr6 emissive layer. The resultant PVK:FIr6-based deep-blue and white solution-processed PhOLEDs exhibit an unprecedented forward-viewing EQE of 16.1% and a total EQE of 28.0% (38.4 lm W−1) at a practical luminance of 1000 cd m−2, respectively.

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

The single molecular tuning of charge balance has been demonstrated here by integrating a p-type dendron, an n-type dendron and a blue emissive Ir core into one dendritic platform. Compared with the commonly used physical blending, not only can the charge balance be well tailored, but also the intrinsic phase separation can be successfully eliminated in such developed single molecular systems (B-TCz2TPO1 and B-TCz1TPO2). As a consequence, their corresponding nondoped solution-processed PhOLEDs achieve more than doubled external quantum efficiencies accompanied by a negligible efficiency roll-off.

Influence of Al3+ and P5+ ion contents on the valence state of Yb3+ ions and the dispersion effect of Al3+ and P5+ ions on Yb3+ ions in silica glass

Three series of Yb3+-doped silica glasses containing different amounts of Al2O3 and P2O5 were prepared successfully by using a sol–gel method. Absorption, excitation and fluorescence spectra of Yb2+ ions in these silica glasses as well as X-ray photoelectron spectra (XPS) of Yb4d were recorded and analysed systematically. It is found out that the addition of Al3+ or P5+ ions has great influence on the redox state of ytterbium ions. With increasing Al3+ ion contents in these silica glasses, more trivalent Yb3+ ions are reduced to divalent Yb2+. In contrast, the increase of P5+ ion contents greatly promotes divalent Yb2+ ions to be oxidized to trivalent Yb3+. The possible redox mechanisms have been explored and discussed in detail. The influence of Al3+ and P5+ ion contents on the near-infrared luminescence intensity of Yb3+ ions and cooperative luminescence of Yb3+ ion pairs was also discussed. Both the near-infrared luminescence intensity of Yb3+ ions and cooperative luminescence of Yb3+ ion pairs decrease gradually with increasing Al3+ and P5+ ion contents. The decrease of cooperative luminescence of Yb3+ ion pairs indicates a good dispersion effect of Al3+ and P5+ ions on Yb3+ ions in Yb3+-doped silica glass. The results are useful for optimization of fabrication of the high quality Yb3+-doped silica fiber by the composite design of Yb–Al–P co-doped silica glass.

An oligocarbazole-encapsulated heteroleptic red iridium complex for solution-processed nondoped phosphorescent organic light-emitting diodes with over 10% external quantum efficiency

A novel self-host red Ir dendrimer D-(PPQ)2Ir(acac) has been developed for solution-processed nondoped phosphorescent organic light-emitting diodes (PhOLEDs) by fully encapsulating the heteroleptic complex (PPQ)2Ir(acac) with oligocarbazole at both the C∧N and O∧O ligands. Due to the shielding effect of the dendritic wedge, the intermolecular interactions and luminescence quenching are found to be significantly reduced from (PPQ)2Ir(acac) to D-(PPQ)2Ir(acac). Correspondingly, the maximum external quantum efficiency (EQE) of hybrid-solution-processed electrophosphorescent devices is increased from 0.5% to 9.9%. Moreover, D-(PPQ)2Ir(acac) shows a good alcohol resistance in the presence of the large-size carbazole dendrons. Such a feature does favor the successful fabrication of all-solution-processed devices via an orthogonal solvent processing, revealing an optimized EQE as high as 11.1% (8.7 cd A−1, 6.0 lm W−1) with CIE coordinates of (0.67, 0.33). The results indicate that highly efficient solution-processed red-emitting nondoped PhOLEDs with over 10% EQE can also be realized based on a self-host phosphorescent dendrimer system.

Solution processable red iridium dendrimers containing oligocarbazole dendrons for efficient nondoped and doped phosphorescent OLEDs

Solution processable red Ir dendrimers named R-D1, R-D2 and R-D3, which contain a quinoline-based homoleptic complex as the core and oligocarbazole as the dendron, have been facilely and successfully designed and synthesized via a post-dendronization procedure. With the increasing dendron generation from R-D1 to R-D3, the intermolecular interactions and luminescence quenching in solid states are found to be effectively prevented because of encapsulation from the outer dendrons. As a result, the third-generation dendrimer R-D3 achieves the best nondoped device performance, revealing a promising EQE of 10.5% (9.2 cd A−1, 7.0 lm W−1) with CIE coordinates of (0.67, 0.33). Furthermore, the doped devices of R-D3 show a wide doping concentration window in the range of 5–30 wt%, and a maximum EQE as high as 18.3% (25.7 cd A−1, 33.0 lm W−1) is realized at about a 10 wt% doping content. The results can compete well with vacuum-deposited small molecular red phosphors, representing important progress on solution processable phosphorescent dendrimers with red emission.