Guijiang Zhou

New Design Tactics in OLEDs Using Functionalized 2-Phenylpyridine-Type Cyclometalates of Iridium(III) and Platinum(II)

As a result of their outstanding attributes, organic light-emitting diodes (OLEDs) and white organic light-emitting diodes (WOLEDs) have been recognized in recent years as the most promising candidates for future flat-panel display technologies and next generation solid-state energy-saving lighting sources. New advancements in the area of high performance triplet emitters become vital for realizing more practical applications. In this regard, several critical issues must be carefully identified and addressed, and these include the ways to enhance device efficiency and suppress efficiency roll-off, to achieve versatile color tuning and simple device manufacture, as well as to obtain high-quality white light from WOLEDs. It has been shown that some functionalized phosphorescent Ir(III) and Pt(II) ppy-type cyclometalated complexes (ppy = 2-phenylpyridine) possess unique features that are suitable for solving these difficult and challenging tasks. In this Focus Review, we will highlight the recent design tactics adopted for these functional metallophosphors and the critical roles they may play in developing more realistic devices.

Recent advances of the emitters for high performance deep-blue organic light-emitting diodes

Blue organic light-emitting diodes (OLEDs) can play a critical role in the field of organic electroluminescence (EL). As the most important applications of OLEDs, both new generation full-color flat-panel displays and future energy-saving solid-state lighting sources require blue color EL to fulfill their functions properly. However, considerable challenges still exist in searching for highly efficient, color stable, and long-lifespan materials and devices that emit blue color, especially in the development of deep-blue emitters, which are indispensable for high-quality displays and lighting sources. Encouragingly, great progress has been made in the area of deep-blue OLEDs in recent years with continuous efforts made by scientists, who are responsible for the significant achievements in the field of OLEDs. Hence, in this review, the crucial tactics employed to obtain high performance deep-blue emitters are presented, including polymers, dendrimers, small organic molecules, delayed fluorescent systems, and phosphorescent emitters. Moreover, the future perspectives and ongoing challenges of this research frontier are also highlighted.

Organometallic acetylides of PtII, AuI and HgII as new generation optical power limiting materials

Within the scope of nonlinear optics, optical power limiting (OPL) materials are commonly regarded as an important class of compounds which can protect the delicate optical sensors or human eyes from sudden exposure to damaging intense laser beams. Recent efforts have been devoted to developing organometallic acetylide complexes, dendrimers and polymers as high performance OPL materials of the next generation which can favorably optimize the optical limiting/transparency trade-off issue. These metallated materials offer a new avenue towards a new family of highly transparent homo- and heterometallic optical limiters with good solution processability which outperform those of current state-of-the-art visible-light-absorbing competitors such as fullerenes, metalloporphyrins and metallophthalocyanines. This critical review aims to provide a detailed account on the recent advances of these novel OPL chromophores. Their OPL activity was shown to depend strongly on the electronic characters of the aryleneethynylene ligand and transition metal moieties as well as the conjugation chain length of the compounds. Strategies including copolymerization with other transition metals, change of structural geometry, use of a dendritic platform and variation of the type and content of transition metal ions would strongly govern their photophysical behavior and improve the resulting OPL responses. Special emphasis is placed on the structure-OPL response relationships of these organometallic acetylide materials. The research endeavors for realizing practical OPL devices based on these materials have also been presented. This article concludes with perspectives on the current status of the field, as well as opportunities that lie just beyond its frontier (106 references).

Simultaneous optimization of charge-carrier balance and luminous efficacy in highly efficient white polymer light-emitting devices.

The use of white organic light-emitting devices (WOLEDs) for solid-state lighting applications is becoming increasingly attractive, [ 1 − 5 ] given that legislation in more countries is banning the use of ineffi cient incandescent lamps. Moreover, since fl uorescent lamps involve the use of mercury and its disposal represents a great challenge, many scientists have been working aggressively to make the replacement of the fl uorescent light sources by WOLEDs a reality. Indeed, the effi ciency of multilayer vacuum-evaporated WOLEDs based on small molecules has been greatly improved in the past several years [ 6 − 10 ] and has already exceeded that of fl uorescent lamps. [ 11 ] In contrast, despite many unique advantages, such as low-cost manufacturing using solution-processing techniques, easy processability over large-areas by spin-coating or ink-jet printing, compatibility with fl exible substrates, a relatively small amount of wasted material, and precise control of the doping level, the application of white polymer light-emitting diodes (WPLEDs) is still severely hindered by the relatively low device effi ciency. [ 3 , 12 − 16 ]

Recent design tactics for high performance white polymer light-emitting diodes

White polymer light-emitting diodes (WPLEDs) represent an intense research subject towards their potential applications in full-color displays, next-generation solid-state lighting sources and back-lighting of liquid-crystal displays due to their merits including low-cost fabrication, flexibility, large area and ease of construction etc. Unfortunately, WPLEDs generally show much poorer EL performance with respect to those made by the vacuum deposition strategy owing to the inherent disadvantages associated with the materials used, device structures and device fabrication processes etc., which has seriously restricted their practical applications. However, the performances of WPLEDs have been improved greatly in recent years, and can even realize some practical devices. In this review, the critical design tactics employed to achieve this goal are presented, which include developing high performance functional light emitters, maintaining a good charge injection/transport balance, introducing new functional layer, surface morphology engineering and employing novel device construction processes etc. In addition, the ongoing challenges and future perspectives of this research frontier are also highlighted.

A non-doped phosphorescent organic light-emitting device with above 31% external quantum efficiency.

The demonstrated square-planar Pt(II)-complex has reduced triplet-triplet quenching and therefore a near unity quantum yield in the neat thin film. A non-doped phosphorescent organic light-emitting diode (PhOLED) based on this emitter achieves (31.1 ± 0.1)% external quantum efficiency without any out-coupling, which shows that a non-doped PhOLED can be comparable in efficiency to the best doped devices with very complicated device structures.

From Mononuclear to Dinuclear Iridium(III) Complex: Effective Tuning of the Optoelectronic Characteristics for Organic Light-Emitting Diodes

Phosphorescent dinuclear iridium(III) complexes that can show high luminescent efficiencies and good electroluminescent abilities are very rare. In this paper, highly phosphorescent 2-phenylpyrimidine-based dinuclear iridium(III) complexes have been synthesized and fully characterized. Significant differences of the photophysical and electrochemical properties between the mono- and dinuclear complexes are observed. The theoretical calculation results show that the dinuclear complexes adopt a unique molecular orbital spatial distribution pattern, which plays the key role of determining their photophysical and electrochemical properties. More importantly, the solution-processed organic light-emitting diode (OLED) based on the new dinuclear iridium(III) complex achieves a peak external quantum efficiency (η(ext)) of 14.4%, which is the highest η(ext) for OLEDs using dinuclear iridium(III) complexes as emitters. Besides, the efficiencies of the OLED based on the dinuclear iridium(III) complex are much higher that those of the OLED based on the corresponding mononuclear iridium(III) complex.

Thiazole-based metallophosphors of iridium with balanced carrier injection/transporting features and their two-colour WOLEDs fabricated by both vacuum deposition and solution processing-vacuum deposition hybrid strategy

New phosphorescent iridium(III) cyclometallated complexes bearing thiazole-based ligands (IrTZ1 and IrTZ2) have been developed. The functionalized organic ligands derived by combining the thiazolyl moiety and triphenylamino group have conferred not only favorable hole-injection/hole-transporting (HI/HT) features but also more balanced charge carrier injection/transporting traits to the as-prepared iridium(III) metallophosphors. Owing to the unique electronic structures afforded by the ligand, the orange organic light-emitting devices (OLEDs) made from IrTZ1 can furnish peak external quantum efficiency (ηext) of 14.82%, luminance efficiency (ηL) of 39.97 cd A−1 and power efficiency (ηp) of 34.95 lm W−1. Inspired by its outstanding electroluminescence (EL) performance, the orange IrTZ1 phosphor complemented with a blue phosphor FIrpic was employed to fabricate highly efficient white organic light-emitting devices (WOLEDs) with a single emission layer. Despite their simple device configuration, the optimized WOLEDs can still maintain decent electroluminescence (EL) ability with ηext of 7.20%, ηL of 18.07 cd A−1 and ηp of 19.57 lm W−1. With the aim to simplify the fabrication process of multi-layered WOLEDs, two-component WOLEDs were obtained through a novel solution processing–vacuum deposition hybrid method with the doped blue fluorescent emission layer deposited by a solution process and the orange phosphorescent emission layer made by vacuum deposition. The WOLEDs prepared using such exploratory approach can show an attractive EL performance with ηext of 9.06%, ηL of 22.72 cd A−1 and ηp of 17.28 lm W−1. All these data have indicated not only the great potential of the orange phosphor in monochromatic and white OLEDs, but also the importance of the hybrid method for simplifying WOLED fabrication.

Versatile phosphorescent color tuning of highly efficient borylated iridium(III) cyclometalates by manipulating the electron-accepting capacity of the dimesitylboron group

Several phosphorescent IrIII ppy-type complexes (ppy = 2-phenylpyridine anion) bearing dimesitylboron (B(Mes)2) units have been designed and some of them have been newly prepared. By changing the substitution positions with different electronic characters that can manipulate the electron-accepting ability of the attached B(Mes)2 moieties, the direction of the metal-to-ligand charge transfer (MLCT) process for these IrIII complexes can be either retained or shifted, which can provide a new strategy toward phosphorescent color tuning. Through computational studies, shifting the substitution position of the B(Mes)2 moiety on the organic ligand, some electronic features, such as the electron injection/electron transporting (EI/ET) properties and charge transport balance, can also be conferred to the phosphorescent IrIII complexes to give excellent electroluminescent (EL) characteristics. Highly efficient red phosphorescent bis(5-(dimesitylboryl)-2-phenylpyridinato)iridium(acetylacetonate) (Ir-B-1) based on the above notion shows a very good compatibility with the choice of host materials which can furnish maximum current efficiency (ηL) of 22.2 cd A−1, external quantum efficiency (ηext) of 14.7% and power efficiency (ηP) of 21.4 lm W−1 for the devices constructed with the conventional host materials. So, these exciting results will not only provide both the systematic guidelines for the phosphorescent color variation on the IrIII complexes with B(Mes)2 units as well as a deeper insight into the conventional color-tuning approach on ppy-type IrIII complexes, but also offer a simple outlet to afford unique electronic features to these phosphorescent emitters to show admirable EL performance.

Phosphorescent Iridium(III) Complexes Bearing Fluorinated Aromatic Sulfonyl Group with Nearly Unity Phosphorescent Quantum Yields and Outstanding Electroluminescent Properties

A series of heteroleptic functional Ir(III) complexes bearing different fluorinated aromatic sulfonyl groups has been synthesized. Their photophysical features, electrochemical behaviors, and electroluminescent (EL) properties have been characterized in detail. These complexes emit intense yellow phosphorescence with exceptionally high quantum yields (ΦP > 0.9) at room temperature, and the emission maxima of these complexes can be finely tuned depending upon the number of the fluorine substituents on the pendant phenyl ring of the sulfonyl group. Furthermore, the electrochemical properties and electron injection/transporting (EI/ET) abilities of these Ir(III) phosphors can also be effectively tuned by the fluorinated aromatic sulfonyl group to furnish some desired characters for enhancing the EL performance. Hence, the maximum luminance efficiency (ηL) of 81.2 cd A(-1), corresponding to power efficiency (ηP) of 64.5 lm W(-1) and external quantum efficiency (ηext) of 19.3%, has been achieved, indicating the great potential of these novel phosphors in the field of organic light-emitting diodes (OLEDs). Furthermore, a clear picture has been drawn for the relationship between their optoelectronic properties and chemical structures. These results should provide important information for developing highly efficient phosphors.