In this paper, we report two new phenanthroline-based compounds, 1,4-bis­(2-phenyl-1,10-phenanthrolin-4-yl)­benzene (p-bPPhenB) and 1,3-bis­(2-phenyl-1,10-phenanthrolin-4-yl)­benzene (m-bPPhenB), for the charge generation unit of tandem organic light-emitting diodes (OLEDs). These two compounds exhibited high electron mobility of (5.8–4.4) × 10–3 cm2/(V s), a very small injection barrier at the p–n junction interface, a high glass transition temperature of 123.9–182.1 °C, and exceptionally good operational stability. Because of such excellent characteristics, a single-stack red phosphorescent OLED (PhOLED) with p-bPPhenB showed a low driving voltage (2.7 V) and significantly improved maximum power efficiency (56.8 lm/W), external quantum efficiency (30.8%), and device lifetime (LT95, 130 h) compared to those of the control device using bathophenanthroline (Bphen) (3.7 V, 39 lm/W, 27.1%, and 13 h). Furthermore, a two-stack (tandem) red PhOLED using p-bPPhenB in the charge generation unit exhibited superior charge generation as well as electron transport properties and excellent device performances (5.0 V, 54.0 lm/W, 56.1%) compared to those of the tandem device using Bphen (6.2 V, 45.2 lm/W, 53.3%).

The subclass of multi resonant thermally activated delayed fluorescent emitters (MR-TADF) containing boron atoms has garnered significant attention in the field of organic light emitting diode (OLED) research. Among boron-based MR-TADF emitters, double boron-embedded MR-TADF (DB-MR-TADF) emitters show excellent electroluminescence performances with high photoluminescence quantum yields, narrow band emission, and beneficially small singlet-triplet energy levels in all the full-color gamut regions. This article reviews recent progress in DB-MR-TADF emitters, with particular attention to molecular design concepts, synthetic routes, optoelectronic properties, and OLED performance, giving future prospects for real-world applications.

In this paper, we report an optical structure design method with the predicted performances of highly efficient three-stacked white organic light emitting diodes (WOLEDs) for solid state lighting applications. The efficiency and color properties of stacked WOLEDs are strongly affected by optical interference inside the thick cavity length; therefore appropriate emissive layer (EML) position is determined by thorough theoretical optical simulations to prevent such optical effect. The theoretically evaluated, three-stacked hybrid WOLED with entirely separated phosphorescent red and green as well as fluorescent blue EML has displayed a CRI and power efficiency of 91 and 33.5 lm/W, respectively. Based on our assumptions, design method, and optical simulation results, the fabricated three-stacked WOLEDs showed a CRI of 93, external quantum efficiency (EQE) of 49.4%, and power efficiencies of 33.4 lm/W. These experimentally measured characteristics are fully correlated with the performances of optically simulated devices. It is important to note that the driving voltage (10.0 V) of optically designed WOLEDs is almost identical to the summation of unit devices (9.9 V) because of good interconnecting units and the same charge balance in the tandem WOLEDs. In addition, the experimentally measured power efficiency of the tandem device is similar to an average value of the unit devices, and most importantly the EQE is nearly equal to the summation of the unit devices with an almost matched white spectrum.

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In this study, we report an Ag-based multilayer structure as the bottom transparent conducting electrode (Ag-TCE) by sandwiching an ultrathin Ag film between commonly used organic hole injection layers for organic light-emitting diode (OLED) applications. This electrode structure fabricated using the same inline fabrication system as organic materials exhibited excellent electrical and optical properties. Ag-TCE showed a high transmittance and low sheet resistance of 79.2% and 17.6 Ω/□, respectively, for the ultrathin Ag layer (9 nm). The phosphorescent yellow OLED fabricated with optimized Ag-TCE showed a driving voltage of 4.0 V, an external quantum efficiency of 23.8%, and a current efficiency of 73.2 cd/A. The electroluminescence performance of Ag-TCE OLED is almost similar to the commonly used ITO electrode (25% and 73.2 cd/A). Based on these results, we demonstrate that this electrode structure is a promising alternative to the ITO electrode.