Xiang-Long Li

High-Efficiency WOLEDs with High Color-Rendering Index based on a Chromaticity-Adjustable Yellow Thermally Activated Delayed Fluorescence Emitter.

A chromaticity-adjustable yellow thermally activated delayed fluorescence (TADF) material, PXZDSO2 as a triplet harvester provides a rational device concept, giving two-color and three-color pure organic white organic light-emitting diodes (WOLEDs) with unprecedented color-rendering index of 95 and external quantum efficiency of 19.2%.

Highly efficient single- and multi-emission-layer fluorescent/phosphorescent hybrid white organic light-emitting diodes with ∼20% external quantum efficiency

A blue fluorophore of N,N-diphenyl-4′′-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1′:4′,1′′-terphenyl]-4-amine (BBPI) was utilized as the blue fluorescent emitter and the host of a phosphorescent emitter to fabricate highly efficient fluorescent/phosphorescent (F/P) hybrid white organic light-emitting diodes (WOLEDs) in a single- or multi-emission-layer architecture. For the single-emission-layer WOLEDs consisting of only the blue emitter BBPI and yellow complementary phosphorescent emitter PO-01 in the emission layer, a maximal current efficiency (CE) of 49.1 cd A−1, power efficiency (PE) of 52.0 lm W−1, and external quantum efficiency (EQE) of 16.3% were achieved by modulating the dopant concentration, indicating efficient singlet and triplet exciton separation and utilization ability in the emission layer. An improved EQE of 17.1%, broader spectral coverage, and higher color rendering index (CRI) were achieved by inserting a non-doped blue emission layer of BBPI and a green emission layer of CBP:Ir(PPy)3. A further improved EQE of up to 19.7% could be achieved when an orange-red phosphorescent emitter PQ2Ir was used instead of PO-01, and it is one of the best reported values for the F/P hybrid WOLEDs.

Highly-efficient hybrid white organic light-emitting diodes based on a high radiative exciton ratio deep-blue emitter with improved concentration of phosphorescent dopant

An improved concentration of phosphorescent dopant for highly-efficient hybrid white organic light-emitting diodes based on a high radiative exciton ratio (80%) deep-blue emitter has been developed. The high radiative exciton ratio for the deep-blue emitter was found to be the transfer from the higher triplet (T5) to the lowest singlet state (S1) by a “hot-exciton” process. Notably, when the concentration of Ir(2-phq)3 is up to 0.9 wt%, the OLED still exhibited white emission with a maximum total EQE, CE and PE of 22.3%, 53.7 cd A−1 and 60.2 lm W−1, respectively. The exciton transfer mechanism in a high concentration of phosphorescent dopant was also discussed. The studies provide a way to obtain high performance F/P hybrid WOLEDs with a simple architecture and improved doping concentration.

Deep blue fluorophores incorporating sulfone-locked triphenylamine: the key for highly efficient fluorescence–phosphorescence hybrid white OLEDs with simplified structure

Two novel bipolar isomeric blue fluorophores, PPI-TPA-SO2-1 and PPI-TPA-SO2-2, consisting of electron-withdrawing phenanthro[9,10-d]imidazole and sulfone-locked electron-donating triphenylamine, were designed and synthesized. The sulfone lock induces a more twisted molecular conformation, and thus a higher triplet energy level and better triplet exciton confining ability compared with the analogue TPA-PPI without the sulfone lock. In addition, the introduced sulfone lock also offers the developed materials improved electron affinities and an electron dominant transporting ability. They were utilized as the blue emitter and the host for a yellow phosphorescent emitter to fabricate fluorescence–phosphorescence (F–P) hybrid white organic light-emitting diodes (WOLEDs) in a single-emissive-layer architecture, giving forward-viewing maximum current efficiencies of 44.2 and 47.6 cd A−1, power efficiencies of 49.5 and 53.4 lm W−1, and external quantum efficiencies of 14.4% and 15.6%, respectively, which are much higher than those of the devices based on TPA-PPI (29.5 cd A−1, 33.1 lm W−1, and 9.6%) due to their superior singlet and triplet exciton separation and utilization ability over TPA-PPI. These efficiencies are also the highest values ever reported for the F–P hybrid WOLEDs in a similar architecture, and their power efficiencies are even comparable with most reported highly efficient all phosphorescent WOLEDs without using any out-coupling technology.

Manipulation of exciton distribution for high-performance fluorescent/phosphorescent hybrid white organic light-emitting diodes

White organic light-emitting diodes (WOLEDs) are now approaching the lighting markets and also being aggressively investigated for display applications. In this review, the recent development of fluorescent/phosphorescent hybrid WOLEDs is highlighted. Since the key for the high performance is how to effectively harvest excitons (singlet and triplet), the manipulation of exciton distribution in hybrid WOLEDs is overviewed. In particular, the device structures, design strategies, working mechanisms and electroluminescent processes of the representative high-performance hybrid WOLEDs are systematically reviewed. It begins with the fundamental introduction of the types of hybrid WOLEDs, and next discusses how to manage the excitons for different types of high-performance hybrid WOLEDs. Then, it introduces the doping-free technology to realize hybrid WOLEDs. Lastly, the challenges and opportunities for further enhancement of the device performance are presented.

9,9-Diphenyl-thioxanthene derivatives as host materials for highly efficient blue phosphorescent organic light-emitting diodes

A series of 9,9-diphenyl-9H-thioxanthene derivatives with different valence states of sulfur atoms are reported as host materials in blue phosphorescent organic light-emitting diodes. Their photophysical, electrochemical and thermal properties, as well as device performance were thoroughly investigated to study their structure–property relationships, including the different carbazolyl linkage positions and valence states of sulfur atoms. Extremely low turn-on voltages of around 2.6 V for blue electrophosphorescence, which are already corresponding to the value of the emitted photon energy (hv)/electron charge (e), were achieved by utilizing the developed materials as hosts of the blue phosphor dopant iridium(III)bis(4,6-(difluorophenyl)-pyridinato-N,C2′)picolinate (FIrpic). Notably, a maximal power efficiency of 69.7 lm W−1 and an external quantum efficiency of 29.0% were achieved for an optimal device based on m-DCz-S consisting of the bivalent sulfur atom and meta-combined carbazolyl.

Structure-simplified and highly efficient deep blue organic light-emitting diodes with reduced efficiency roll-off at extremely high luminance

Based on a series of new fluorescent emitters, deep blue non-doped multilayer OLEDs with EQEs exceeding 5.10% and single layer devices excluding any charge carrier transporting materials with an EQE of 4.22% were obtained at an extremely high luminance of 10 000 cd m-2.

Effect of cyano-substitution in distyrylbenzene derivatives on their fluorescence and electroluminescence properties

Efficient organic electroluminescent materials with both high solid-state fluorescence efficiency and high excitons usage efficiency for use in organic light-emitting diodes (OLEDs) are relatively rare. We report two isomeric-phenylethylene compounds with different positions and spatial orientation of the –CN substituent on the vinylene groups, namely α-CN-APV and β-CN-APV. The synthesis, characterization, crystal structure, optical, electrochemical, thermal, and electroluminescence (EL) properties of the two compounds are discussed in detail. The crystal structure of the β-CN-APV shows tight solid-state organization because of two, oppositely pointed, vertically aligned, hydrogen bonds between the two cyano groups and the two vinylene hydrogens (CN⋯H–CC), while the α-CN-APV showed more flexible molecular structure with absence of vinylene hydrogen intermolecular interactions in the crystal. Such tight intermolecular stacking of β-CN-APV ensures the high solid-state fluorescence quantum efficiency. Both the compounds exhibit hybrid local and charge transfer (HLCT) excited states, which facilitate the population of singlet excitons through the reverse intersystem crossing (RISC) process from the high lying triplet states. Experimental and theoretical investigation indicated that the β-CN-APV, when compared to the α-CN-APV, showed higher solid-state fluorescence quantum efficiency and also higher excitons usage efficiency, which eventually provided much higher external quantum efficiency and brightness in the corresponding EL devices.

An ideal universal host for highly efficient full-color, white phosphorescent and TADF OLEDs with a simple and unified structure

The actualization of highly efficient full-color and white organic light-emitting diodes (OLEDs) in a simple and unified device architecture using host materials that are suitable for red, green, and blue dopants has been quite a challenge. Herein, a series of thioxanthene and 9-phenylcarbazole hybrids were designed and synthesized, among which 9,9′-((9H-thioxanthene-9,9-diyl)bis(3,1-phenylene))bis(9H-carbazole) (m-DCz-S) exhibited unique universal host characteristics. Full-color OLEDs were fabricated with phosphorescent and thermally activated delayed fluorescent (TADF) emitters, i.e. FIr6, Ir(ppy)3, PO-01, 2CzPN, ACRDSO2, and PyCN-ACR; among these, the FIr6-based blue device showed the best performance reported to date. In addition, the phosphorescent white OLED with complementary emitters, FIr6: PO-01, has high efficiencies of 61.4 cd A−1, 62.9 lm W−1, and 25.0% and extremely good color stability with a color variation less than (0.0007, 0.0007) in the whole range of luminance. Further improved efficiencies of 74.3 cd A−1, 82.7 lm W−1, and 26.4% have been achieved in the case of the phosphorescent white OLED based on FIrpic: PO-01, which is the most efficient white OLED with this simplified structure reported to date. It is worth noting that all the devices have been fabricated with the same extremely simple architecture by solely changing the emitters doped into the host; thus, the unique universal host material m-DCz-S is advantageous for future low-cost and highly efficient OLED application.

Efficient solution-processed red all-fluorescent organic light-emitting diodes employing thermally activated delayed fluorescence materials as assistant hosts: molecular design strategy and exciton dynamic analysis

The strategy of employing highly soluble, thermally activated, delayed fluorescence (TADF) materials that possess small singlet–triplet splitting energies (ΔEST) for efficient reverse intersystem crossing (RISC) processes results in simplified and low-cost solution-processed all-fluorescence organic light-emitting diodes (OLEDs). However, confined by the energy gap law, it is difficult to strike a balance between a large singlet radiative rate constant (kSr) and very small electron exchange energy (J), which hinders the realization of highly efficient red TADF emission with short excited state lifetimes. Herein, via integrating assistant hosts with TADF character and conventional fluorescent dopants with a large kSr, efficient solution-processed red all-fluorescent OLEDs with external quantum efficiency exceeding the theoretical maximum for traditional fluorescent devices (5%) were prepared. Exciton dynamic studies of the ternary system revealed that the rapid Forster energy transfer channel enabled the significant reduction of singlet and triplet exciton density, which contributed to the suppressed efficiency roll-off of OLEDs operating at high current density. Regarding TADF assistant host molecular design, maintaining a moderate kSr for a large Forster energy transfer radius is as important as keeping a relatively small ΔEST for efficient triplet exciton utilization in this ternary prototype.