Achieving High-Performance Pure-Red Electrophosphorescent Iridium(III) Complexes Based on Optimizing Ancillary Ligands

Two new iridium(III) complexes were synthesized by introducing two trifluoromethyl groups into an ancillary ligand to develop pure-red emitters for organic light-emitting diodes (OLEDs). The electron-donating ability of the ancillary ligands is suppressed, owing to the electron-withdrawing nature of trifluoromethyl groups, which can reduce the HOMO energy levels compared with those of compounds without trifluoromethyl groups. However, the introduction of trifluoromethyl groups into the ancillary ligand has little impact on the LUMO energy levels. Therefore, a well-tuned, pure-red, excited-state energy was achieved by regulating the relative energy level between the HOMO and LUMO. OLEDs with these complexes as emitters showed high external quantum efficiencies (EQEs) of 26 % and realized high EQEs of about 25 % and fairly low driving voltages of 3.3-3.6 V for practical luminance of 1000 cd m^-2 , as well as excellent Commission Internationale de L'Eclairage (CIE) coordinates of (0.66, 0.33) and (0.67, 0.33); thus, this demonstrates the successful molecular design strategy by modifying the electron-donating ability of ancillary ligand.

Synthesis and optoelectronic properties of spirofluorenexanthene-based carbazole host materials

The structure–property relationship of carbazole substituted SFX derivatives at different positions for RGB PhOLEDs.

Thermally Activated Delayed Fluorescence Materials: Towards Realization of High Efficiency through Strategic Small Molecular Design

Thermally activated delayed fluorescence (TADF) is one of the most intriguing and promising discoveries towards realization of highly-efficient organic light emitting diodes (OLED) utilizing small molecules as emitters. It has the capability of manifesting all excitons generated during the electroluminescent processes, consequently achieving 100 % of internal quantum efficiency. Since the report of the first efficient OLED based on a TADF small molecule in 2012 by Adachi et al., the quest for optimal TADF materials for OLED application has never stopped. Various TADF molecules bearing different design concepts and strategies have been designed and produced, with the aim to boost the overall performances of corresponding OLEDs. In this minireview, the general principles of TADF molecular design based on three basic categories of TADF species: twisted intramolecular charge transfer (TICT), through-space charge transfer (TSCT) and multi-resonance induced TADF (MR-TADF) are discussed in detail. Several key aspects with respect to each category, as well as some effective methods to enhance the efficiency of TADF materials and corresponding OLEDs from the molecular engineering perspectives, are summarized and discussed to exhibit a general landscape of TADF molecular design to a wide variety of scientific researchers within this particular disciplinary area.

Low energy consumption phosphorescent organic light-emitting diodes using phenyl anthracenone derivatives as the host featuring bipolar and thermally activated delayed fluorescence

A bipolar host material 10-(4-(5,5-dimethylbenzofuro[3,2-c]acridin-13(5H)-yl)phenyl)-10-phenylanthracen-9(10H)-one (DphAn-5BzAc) with TADF properties, has been synthesized.

Diarylboron-Based Asymmetric Red-Emitting Ir(III) Complex for Solution-Processed Phosphorescent Organic Light-Emitting Diode with External Quantum Efficiency above 28

Organic light-emitting diodes (OLEDs) are one of the most promising technologies for future displays and lighting. Compared with the blue and green OLEDs that have achieved very high efficiencies by using phosphorescent Ir(III) complexes, the red OLEDs still show relatively low efficiencies because of the lack of high-performance red-emitting Ir(III) complexes. Here, three highly efficient asymmetric red-emitting Ir(III) complexes with two different cyclometalating ligands made by incorporating only one electron-deficient triarylboron group into the nitrogen heterocyclic ring are reported. These complexes show enhanced photoluminescence quantum yields up to 0.96 and improved electron transporting capacity. In addition, the asymmetric structure can help to improve the solubility of Ir(III) complexes, which is crucial for fabricating OLEDs using the solution method. The photoluminescent and oxidation-reduction properties of these Ir(III) complexes are investigated both experimentally and theoretically. Most importantly, a solution-processed red OLED achieves extremely high external quantum efficiency, current efficiency, and power efficiency with values of 28.5%, 54.4 cd A-1, and 50.1 lm W-1, respectively, with very low efficiency roll-off. Additionally, the related device has a significantly extended operating lifetime compared with the reference device. These results demonstrate that the asymmetric diarylboron-based Ir(III) complexes have great potential for fabricating high-performance red OLEDs.

Dispiro and Propellane: Novel Molecular Platforms for Highly Efficient Organic Light-Emitting Diodes

The incorporation of spatially oriented aromatic motifs in rigid molecular platforms is of great interest for the design of organic electronic materials. These structures can create unusual packing patterns and charge transport properties in the solid state which are not possible for simple planar structures. Herein, we showed that the novel dispiro and propellane motifs were successfully used as robust molecular platforms for the construction of host materials (TPA, Cz, SF, and SO). The propellane derivative with three functional groups arranged in the staggered conformation was studied for the first time as the host for organic light-emitting diodes (OLEDs). The green and red phosphorescent OLEDs hosted by these dispiro and propellane derivatives exhibited excellent electroluminescence performance. Particularly, the red OLED hosted by the propellane-type SF achieved maximum efficiencies of 47.3 cd A^-1, 40.2 lm W^-1, and 26.6% and 97.6 cd A^-1, 77.8 lm W^-1, and 27.0% for the green OLED without any light out-coupling enhancement. These results suggest that the dispiro and propellane molecular platforms have great potential in the construction of OLED materials.

Versatile functionalization of trifluoromethyl based deep blue thermally activated delayed fluorescence materials for organic light emitting diodes

Four deep blue TADF compounds with trifluoromethyl-substituted phenyl/N-heterocyclic rings were investigated both as hosts and guests in OLEDs with excellent performance.

First N-Borylated Emitters Displaying Highly Efficient Thermally Activated Delayed Fluorescence and High-Performance OLEDs

Despite the fast boom of thermally activated delayed fluorescence (TADF) emitters bearing borane-based acceptor, so far, no TADF emitter with a direct B-N linkage between N-donor and boryl acceptor has been reported. The latter should simplify the molecular architecture and hence facilitate the synthetic design and versatility. We report here the preparation and characterization of a new series of N-borylated compounds with functional acridine donor unit; namely: ACBM, PACBM, and SACBM. Spectroscopic studies were performed to explore their photophysical properties that exhibited prominent solvatochromism and thermally activated delayed fluorescence. The time-dependent DFT calculation indicated the involvement of substantial intramolecular charge transfer character for which HOMO and LUMO are spatially separated. For compound SACBM, fabrication of green emitting OLED gave CIE chromaticity of (0.22, 0.59) and maximum external quantum efficiency, luminance efficiency and power efficiency of 19.1%, 60.9 cd/A, and 43.6 lm/W, respectively, demonstrating for the first time the highly efficient OLEDs using N-borylated TADF emitters.

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes

The design of thermally activated delayed fluorescence (TADF) materials both as emitters and as hosts is an exploding area of research. The replacement of phosphorescent metal complexes with inexpensive organic compounds in electroluminescent (EL) devices that demonstrate comparable performance metrics is paradigm shifting, as these new materials offer the possibility of developing low-cost lighting and displays. Here, a comprehensive review of TADF materials is presented, with a focus on linking their optoelectronic behavior with the performance of the organic light-emitting diode (OLED) and related EL devices. TADF emitters are cross-compared within specific color ranges, with a focus on blue, green-yellow, orange-red, and white OLEDs. Organic small-molecule, dendrimer, polymer, and exciplex emitters are all discussed within this review, as is their use as host materials. Correlations are provided between the structure of the TADF materials and their optoelectronic properties. The success of TADF materials has ushered in the next generation of OLEDs.

Stable green phosphorescence organic light-emitting diodes with low efficiency roll-off using a novel bipolar thermally activated delayed fluorescence material as host

A novel bipolar hosting material, 11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (DPDDC), was designed, synthesized, and characterized for green phosphorescent organic light-emitting diodes (PhOLEDs). The DPDDC exhibits excellent hole and electron transport properties, superior thermal stability, a high glass-transition temperature and a small singlet-triplet energy gap for efficient reverse intersystem crossing from triplet to singlet, reducing the triplet density of the host for PhOLEDs. The electrophosphorescence properties of the devices using DPDDC as the host and three green phosphorescent iridium(iii) complexes, bis(2-(4-tolyl)pyridinato-N,C2')iridium(iii) acetylacetonate, bis(2-phenylpyridine)iridium(iii) acetylacetonate, and bis(4-methyl-2,5-diphenylpyridine)iridium(iii) acetylacetonate [(mdppy)2Iracac] as the emitter were investigated. The green PhOLED with 5 wt% (mdppy)2Iracac presents an excellent performance, including a high power efficiency of 92.3 lm W-1, high external quantum efficiency of 23.6%, current efficiency roll-off as low as 5.5% at 5000 cd m-2 and a twentyfold lifetime improvement (time to 90% of the 5000 cd m-2 initial luminance) over the reference electrophosphorescent device.

Recent advances in organic thermally activated delayed fluorescence materials

Thermally activated delayed fluorescence: harvesting dark triplet excitons to generate bright emissive singlet excitons.

A versatile ferrocene-containing material as a p-type charge generation layer for high-performance full color tandem OLEDs

A ferrocene derivative, DPAF, is developed as a p-type charge generation layer and gives excellent device performance for RGB tandem OLEDs.