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The development of lasing materials with weak pile‐up effect from triplet excitons, low lasing threshold, and long operational lifetime is much significant for truly continuous wave operation and electrically pumping organic lasers. However, most of organic gain media has been verified to have extreme quenching effect from triplet excitons due to its wide‐band distribution of triplet absorption spectrum. Here, it is found that the phenanthroimidazole derivatives as gain media have congenital advantages on handling triplet's issues. A very low amplified spontaneous emission (ASE) threshold of 0.56 µJ cm−2 and a long operational lifetime of 5.77 × 105 pump pulses in air atmosphere are well obtained. When increasing the pump repetition frequency from 20 Hz to 20 kHz, the operational lifetimes of the thin films have small reduction. Under a high pump repetition frequency of 200 kHz, which is quasi‐continuous wave operational regime, the PI2PhDPO thin film exhibits very low ASE threshold of 0.78 µJ cm−2 due to the tiny overlap between triplet absorption and ASE spectra. [ABSTRACT FROM AUTHOR]

"Hot exciton" molecules that allow the transition of excitons from a high‐lying triplet state (Tn, n≥ 2) to singlet states (Sm, m≥1) by a reverse intersystem crossing (hRISC) process have become an effective strategy to achieve high efficiency fluorescence organic light emitting diodes (OLEDs). However, the understanding of the dynamic behavior of the "hot exciton" process is still very lacking. Herein, the exciton dynamics of an aggregation‐induced emission (AIE) molecule TPA‐An‐mPhCz with the "hot exciton" property are deeply investigated, and the proportion between hRISC and internal inversion (IC) of excitons on Tn is successfully quantified by in situ transient electroluminescent measurements and theoretical calculation. It is found that the IC process increases severe energy loss of Tn excitons. By introducing a triplet–triplet annihilation up‐conversion layer in the emissive layer to efficiently recapture the IC excitons and further doping a blue fluorescent emitter in the TPA‐An‐mPhCz layer to achieve high photoluminescence quantum yield (PLQY), the resulting OLED achieves a maximum external quantum efficiency of 12.5% with negligible efficiency roll‐off. More impressively, the operational lifetime LT75 (lifetime to 75% of the initial luminance) of the device with efficient triplets utilization performs a remarkable 116 h under 1000 cd m−2. This work provides the fundamentals for the design of hot exciton materials with efficient exciton utilization to develop efficient blue fluorescence OLEDs. [ABSTRACT FROM AUTHOR]

Aggregation‐induced emission (AIE) materials are attractive for the fabrication of high‐efficiency organic light‐emitting diodes (OLEDs) owing to the "hot exciton" process by reverse intersystem crossing (hRISC) and high photoluminescence quantum yields (PLQY). However, the internal conversion (IC) from the high‐lying triplet excitation states (Tn, n≥2) to the lower excited triplet state (Tn‐1) is inevitable, resulting in severe exciton losses. Herein, an effective device structure is designed that reuses the lost triplet excitons caused by IC and realizes the breakthrough in the efficiency of blue fluorescence OLEDs based on AIE molecule as an emitter. The maximum external quantum efficiency reached as high as 14.8% and is kept at 14.4% at the luminance of 1000 cd m−2. In the designed device, a triplet–triplet annihilation (TTA) up‐conversion material 1‐[2,5‐dimethyl‐4‐(1‐pyrenyl)phenyl]pyrene (DMPPP) is introduced into the AIE emitter as a triplet sensitizer to receive the lost triplet excitons, and a thin TTA up‐conversion layer 9‐[4‐(10‐phenyl‐9‐anthryl)phenyl]‐9H‐carbazole (CzPA) is introduced in the middle of the doped layer to form the emissive layer (EML). It is found that the hRISC process of iTPB‐2AC greatly enhances the utilization efficiency of TTA intermediate state ([TT]*) excitons on CzPA to iTPB‐2AC so that the utilization of the lost excitons is maximized. This work establishes physical insights into the AIE emission materials and device fabrication of high‐efficiency blue fluorescence OLEDs. [ABSTRACT FROM AUTHOR]

The operational lifetime of blue organic light‐emitting diodes (OLEDs) is still insufficient for practical applications in lighting and display. One type of blue organic emitting materials with hybridized local and charge‐transfer (HLCT) process are beneficial in achieving high‐efficiency OLEDs through "hot exciton" channel by harnessing high‐lying triplet (Tn) excitons. However, the operational lifetime of the resulting blue OLEDs is rarely studied and understood. In this article, the aging properties of blue fluorescent OLEDs based on an HLCT material (2‐(4‐(10‐(3‐(9H‐carbazol‐9‐yl)phenyl)anthracen‐9‐yl)phenyl)‐1‐phenyl‐1H‐phenanthro[9,10‐d]imidazole) (PAC) are systematically investigated by exciton dynamics calculation and transient EL experiments. It is experimentally and theoretically revealed that whether the reverse intersystem crossing (hRISC) process from high‐lying excited triplet to singlet in HLCT materials is completely effective determines the device degradation. A fluorescent emitter is doped into PAC host to accelerate the hRISC process, thus enhancing device operational lifetime to reach T75 = 110 ± 2 h (time to 75% of initial luminance) under 1000 cd m−2. This work provides inspirations to investigate the stability of blue fluorescent OLEDs based on HCLT materials and further enhance the operational lifetime. [ABSTRACT FROM AUTHOR]

Dielectric metasurfaces have significant potential for delivering miniaturized optical systems with versatile functionalities, leading to applications in various fields such as orbital angular momentum generation, imaging, and holography. Among the different materials, crystalline silicon has the advantage of technological maturity and high refractive index, which increases design flexibility and processing latitude. The second, and often overlooked, advantage of silicon is that it affords embedding the metasurface in a protective material such as silica, which is essential for practical applications. The trade‐off against this high refractive index is silicon's absorption at visible wavelength, which requires new design strategies. Here, such a strategy based on metasurfaces supporting air modes is identified that can lead to a transmission efficiency as high as 87% at a wavelength of 532 nm. This exceptional efficiency is obtained by using the high index to confine the electric field in the periphery of the meta‐atoms, thereby reducing absorption losses. As an example, the design of a fully embedded metasurface is described that can generate vortex beams with various orders of orbital angular momentum. It is envisioned that the proposed strategy paves the way for practical applications of high‐efficiency metasurfaces based on crystalline silicon. [ABSTRACT FROM AUTHOR]

Phosphorescent organic light‐emitting diodes (PhOLEDs) using exciplexes as host have achieved unprecedented breakthrough. The energy transfer process between host and guest is the popular way to effectively utilize excitons in this kind of devices. Actually, the intermolecular spin–orbital coupling (ISOC) interaction between the heavy‐atom phosphorescent molecule and exciplex also plays an important role in exciton utilization, thus device performance, but this role is always ignored. Here, the synergistic effects of energy transfer and ISOC interaction on device efficiency are clearly demonstrated in exciplex‐based PhOLEDs by magneto‐electroluminescence (MEL) measurements. It can be seen that the MEL line shape at high field component is significantly wider due to the ISOC effect inducing the accelerated transition process between singlet and triplet states of exciplex host. In this case, the energy transfer process from exciplex host to phosphorescent molecule is more efficient, which is confirmed by the different line widths of MELs. Ultimately, a high‐efficiency exciplex‐based green PhOLED with the maximum external quantum efficiency of 23% is obtained at a low phosphorescent doping concentration of 2 wt%. [ABSTRACT FROM AUTHOR]

White organic light‐emitting diodes (WOLEDs) are one of the most cogent candidates for environment friendly and healthy solid‐state lighting due to many unique merits. Here, a strategic design of exciplex hosts is applied to simultaneously realize the superior color stability, high color rendering index (CRI), high efficiency, and low efficiency roll‐off properties of WOLEDs. The resulting WOLEDs emit the forward‐viewing maximum power efficiency (PE) of 59.9 lm W−1, and the PE still remains 45.3 and 31.9 lm W−1 at 1000 and 5000 cd m−2, respectively, exhibiting extremely low efficiency roll‐off. More remarkably, the devices also show high CRI over 80 and extremely stable spectral emission in the luminance ranges from 1000 to 15 000 cd m−2. The excellent performances indicate the validity of the structure design and provide a new route for the development of high‐performance WOLEDs. [ABSTRACT FROM AUTHOR]

It is well known that one of the major challenges facing white organic light‐emitting diodes (WOLEDs) has been to achieve high efficiency, low efficiency roll‐off, and high color rendering index (CRI) simultaneously. A simple charge‐ and exciton‐confining emitting layer structure formed by employing a blue emitting p/n heterojunction, and combined with red, yellow, and green phosphorescent ultrathin layers, is presented, thus realizing the stable utilization of 100% excitons. The resulting hybrid WOLEDs exhibit the maximum forward‐viewing external quantum efficiency and power efficiency of 23.4% and 50.4 lm W−1, and keep them at 20.3% and 31.2 lm W−1 at 1000 cd m−2, showing high efficiency and low efficiency roll‐off. Furthermore, the device also shows extremely stable electroluminescent spectra in the whole luminance range with a high CRI of 93 and the Commission Internationale de L'Eclairage coordinates of (0.49, 0.43) at 1000 cd m−2, which is perfect warm white for indoor lighting. The achievement of the excellent comprehensive performances is the best result reported to date among the conventional blue fluorescence‐based hybrid WOLEDs possessing such high CRI, which may be a new starting point for further research on high‐performance hybrid WOLEDs. [ABSTRACT FROM AUTHOR]

Triplet‐triplet fusion (TTF) can effectively transfer triplet excitons to singlet energy level and exhibit good stability, making it widely used in blue fluorescence organic light emitting diodes (OLEDs) for practical application. However, the efficiency of the resulting blue fluorescence OLEDs is still fundamentally limited. Herein, this work introduces a hot exciton efficiency enhancement layer (EEL) between TTF emissive layer and electron‐transporting layer for the first time. It can be found that the triplet excitons are greatly utilized by the efficient hot exciton EEL. As a result, a high efficiency blue fluorescence OLED is successfully developed, exhibiting a maximum external quantum efficiency of 15.2% at the luminance of 4068.0 cd m−2, achieving a breakthrough in the efficiency of blue TTF‐OLEDs. Transient electroluminescence (TrEL) measurements and detailed theoretical calculations confirm the key role of the effective hot exciton process from the high‐lying triplet energy level (Tn, n ≥ 2) to S1 via a reverse intersystem crossing (hRISC). Furthermore, the device lifetime is also greatly improved owing to the fast and efficient hRISC in the introduced hot exciton EEL. This work has important guiding significance for constructing high efficiency blue fluorescence OLEDs by introducing hot exciton molecules. [ABSTRACT FROM AUTHOR]

OLEDs with high efficiency and low roll‐off are urgently needed to meet the constantly increasing commercial demands for white organic light‐emitting diodes (WOLEDs) in lighting industry applications. To enhance efficiency and decrease WOLED roll‐off, complicated doping technologies are employed, which result in an additional increase in costs. Herein, a deep‐blue‐light molecule (3TPA‐CN) with aggregation‐induced emission (AIE) is introduced as blue emitter and phosphor host to fabricate WOLEDs. By optimizing the emission zones, high efficiency and low roll‐off hybrid WOLEDs are achieved. The maximum forward‐viewing power and external quantum efficiencies reach 86.7 lm W−1 and 22.3%, and remain 59.2 lm W−1 and 19.6% at a luminance of 1000 cd m−2, fully exhibiting the prospect of AIE molecules as a promising electroluminescence material for the fabrication of high‐performance WOLEDs. [ABSTRACT FROM AUTHOR]