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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]

Aggregation‐induced emission (AIE) and hybridized local and charge‐transfer (HLCT) materials are two kinds of promising electroluminescence systems for the fabrication of high‐efficiency organic light‐emitting diodes (OLEDs) by harnessing "hot excitons" at the high‐lying triplet exciton states (Tn, n ≥ 2). Nonetheless, the efficiency of the resulting OLEDs did not meet expectations due to the possible loss of Tn→Tn−1. Herein, experimental results and theoretical calculations demonstrate the "hot exciton" process between the high‐lying triplet state T3 and the lowest excited singlet state S1 in an AIE material 4⁗‐(diphenylamino)‐2″,5″‐diphenyl‐[1,1″:4′,1″:4″,1′″:4′″,1⁗‐quinquephenyl]‐4‐carbonitrile (TPB‐PAPC) and it is found that the Förster resonance energy transfer (FRET) between two molecules can facilitate the "hot exciton" process and inhibit the T3→T2 loss by doping a blue fluorescent emitter in TPB‐PAPC. Finally, the doped TPB‐PAPC blue OLEDs achieve a maximum external quantum efficiency (EQEmax) of 9.0% with a small efficiency roll‐off. Furthermore, doping the blue fluorescent emitter in a HLCT material 2‐(4‐(10‐(3‐(9H‐carbazol‐9‐yl)phenyl)anthracen‐9‐yl)phenyl)‐1‐phenyl‐1H‐phenanthro[9,10‐d] imidazole (PAC) is used as the emission layer, and the resulting blue OLEDs exhibit an EQEmax of 17.4%, realizing the efficiency breakthrough of blue fluorescence OLEDs. This work establishes a physical insight in the design of high‐performance "hot exciton" molecules and the fabrication of high‐performance blue fluorescence OLEDs. [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]

Aggregation‐induced emission (AIE) materials are highly attractive because of their excellent properties of high efficiency emission in nondoped organic light‐emitting diodes (OLEDs). Therefore, a deep understanding of the working mechanisms, further improving the electroluminescence (EL) efficiency of the resulting AIE‐based OLEDs, is necessary. Herein, the conversion process from higher energy triplet state (T2) to the lowest singlet state (SS1) is found in OLEDs based on a blue AIE material, 4′‐(4‐(diphenylamino)phenyl)‐5′‐phenyl‐[1,1′:2′,1′′‐terphenyl]‐4‐carbonitrile (TPB‐AC), obviously relating to the device efficiency, by magneto‐EL (MEL) measurements. A special line shape with rise at low field and reduction at high field is observed. The phenomenon is further clarified by theoretical calculations, temperature‐dependent MELs, and transient photoluminescence emission properties. On the basis of the T2‐S1 conversion process, the EL performances of the blue OLEDs based on TPB‐AC are further enhanced by introducing a phosphorescence doping emitter in the emitting layer, which effectively regulates the excitons on TPB‐AC molecules. The maximum external quantum efficiency (EQE) reaches 7.93% and the EQE keeps 7.57% at the luminance of 1000 cd m−2. This work establishes a physical insight for designing high‐performance AIE materials and devices in the future. [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]