Qing Zhang

White organic light-emitting diodes (WOLEDs) are highly attractive in the fields of solid-state lighting. The biggest challenge that is facing at present is how to maximize the exciton utilization to further enhance the efficiency, while taking into account the stability. Here, highly efficient all-fluorescence and fluorescence/phosphorescence (F/P) hybrid WOLEDs with low efficiency roll-off by designing exciplex-sandwich emissive architecture and precisely manipulating the exciton allocation are demonstrated. The resulting complementary-color hybrid WOLEDs realize the maximum external quantum efficiency of 28.3% and power efficiency of 102.9 lm W⁻¹, and remain 26.9% and 73.5 lm W⁻¹ at 500 cd m⁻² and yet as high as 25.8% and 63.5 lm W⁻¹ at 1000 cd m⁻², respectively, revealing very low roll-off. By using the efficient blue exciplex combined with red and green phosphorescent emitters, the three-color WOLEDs yield a high color rendering index of 86, an external quantum efficiency of 29.4%, and a power efficiency of 75.5 lm W⁻¹. It is anticipated that the exciplex engineering will open an efficient avenue to precisely allocate excitons, and finally producing high-performance WOLEDs for next-generation solid-state lighting technology.

Highly efficient all-fluorescence and fluorescence/phosphorescence hybrid white organic light-emitting diodes (WOLEDs) are demonstrated by using the blue thermally activated delayed fluorescence exciplex emission. The hybrid WOLED realizes a maximum external quantum efficiency of 28.3% and a power efficiency of 102.9 lm W⁻¹, and remain 25.8% and 63.5 lm W⁻¹ at 1000 cd m⁻², respectively, revealing very low efficiency roll-off.

Although pyrimidine-based emitters are one of the most promising classes of compounds, a structure–property relationship for an effective molecular design strategy has not yet been elucidated. In this study, the authors develop three types of blue pyrimidine emitters for high-performance thermally activated delayed fluorescence (TADF) organic light-emitting devices (OLEDs). 2,4,6-triphenylpyrimidine is used as an acceptor, and tri- or penta-substituted acridine as a donor unit for the blue TADF emitters. The authors investigate the effects of the nitrogen position on the photophysical properties and OLED performances, and develop a blue device with a high external quantum efficiency close to 25%. The device performances are among the best so far for blue TADF OLEDs.

A small structural differencein the nitrogen positions of pyrimidine-based thermally activated delayed fluorescence emitters significantly influences the photophysical events and organic light-emitting device performances. A blue device exhibits a high external quantum efficiency close to 25%.

Solution-processed CsPbBr₃ quantum-dot light-emitting diodes with a 50-fold external quantum efficiency improvement (up to 6.27%) are achieved through balancing surface passivation and carrier injection via ligand density control (treating with hexane/ethyl acetate mixed solvent), which induces the coexistence of high levels of ink stability, photoluminescence quantum yields, thin-film uniformity, and carrier-injection efficiency.

Phosphorescent organic light-emitting diodes (OLEDs) are leading candidates for next-generation displays and solid-state lighting technologies. Much of the academic and commercial pursuits in phosphorescent OLEDs have been dominated by Ir(III) complexes. Over the past decade recent developments have enabled square planar Pt(II) and Pd(II) complexes to meet or exceed the performance of Ir complexes in many aspects. In particular, the development of N-heterocyclic carbene-based emitters and tetradentate cyclometalated Pt and Pd complexes have significantly improved the emission efficiency and reduced their radiative lifetimes making them competitive with the best reported Ir complexes. Furthermore, their unique and diverse molecular design possibilities have enabled exciting photophysical attributes including narrower emission spectra, excimer -based white emission, and thermally activated delayed fluorescence. These developments have enabled the fabrication of efficient and “pure” blue OLEDs, single-doped white devices with EQEs of over 25% and high CRI, and device operational lifetimes which show early promise that square planar metal complexes can be stable enough for commercialization. These accomplishments have brought Pt complexes to the forefront of academic research. The molecular design strategies, photophysical characteristics, and device performance resulting from the major advancements in emissive Pt and Pd square planar complexes are discussed.

Recent developments in square planar Pt(II) and Pd(II) complexes for OLED applications have enabled dramatic improvements in electroluminescent performance, particularly through the application of N-heterocyclic carbene or tetradentate ligands, which now meet or exceed the performance of Ir complexes in many aspects. The progress in square planar metal complexes toward solving the challenges of developing efficient and stable blue emitters and their use for stable and efficient single-dopant white OLEDs are discussed.

Concentration quenching of thermally activated delayed fluorescence is found to be dominated by electron-exchange interactions, as described by the Dexter energy-transfer model. Owing to the short-range nature of the electron-exchange interactions, even a small modulation in the molecular geometric structure drastically affects the concentration-quenching, leading to enhanced solid-state photoluminescence and electroluminescence quantum efficiencies.

Efficient and tunable thermally activated delayed fluorescence emitters are presented by C.-C. Wu, K.-T. Wong, and co-workers on page 7560. A toolbox composed of acridine donor units and cyano-substituted acceptor units allows for systematically probing acceptor strengths as well as tunable acceptor confirmations like twist angles. Highly efficient blue-green to yellow OLEDs with up to 31.3% external quantum efficiency are achieved.

Typically, luminescent π-conjugated 2,2′:6′,2′′-terpyridine (tpy) derivatives are versatile components for tridentate metal ligands, supramolecular materials, two-photon absorption bioimaging probes, fluorescent ion sensors, and organic light-emitting devices. However, a limited number of luminescent tpy materials, other than metal complexes, have been reported. This study introduces a series of π-conjugated tpy derivatives that exhibit strong thermally activated delayed fluorescence (TADF). We have observed that a blue tpy emitter outperforms conventional fluorescent emitters. Additionally, a green tpy emitter has exhibited a performance that is almost comparable to that of its green phosphorescent counterparts, realizing an external quantum efficiency close to 25 % and a power efficiency exceeding 80 lm W⁻¹ with an exceptionally low efficiency roll-off. This study demonstrates the first example of highly luminescent tpy-based TADF emitters.

Terpyridine-based TADF emitters: A series of terpyridine/acridine conjugates has been prepared, the members of which exhibit strong blue-to-green emission and thermally activated delayed fluorescence (TADF) character in the solid state. An organic light-emitting device with a green emissive terpyridine/acridine conjugate shows an external quantum efficiency of close to 25 %, a power efficiency of over 80 lm W⁻¹, and an exceptionally low efficiency roll-off (see graphic).