Resonance hosts for high efficiency solution-processed blue and white electrophosphorescent devices

Achieving Balanced Electrical Performance of Host Material through Dual N-P═O Resonance Linkage for Efficient Electroluminescence

Developing high-performance host materials is one of the biggest challenges for blue and white thermally activated delayed-fluorescence (TADF) organic light-emitting diode (OLED) technology due to the rigorous requirements of both efficient carrier flux ability and high triplet energy (E_T) levels in static donor-acceptor molecules. Here, with the aid of a dual-resonance strategy, a host molecule showing dynamic adaption features in the acceptor-resonance-donor-resonance-acceptor (A-r-D-r-A) molecular configuration has been successfully developed through the implantation of two acceptors of diphenylphosphine oxide into electron-donating 5,10-dihydrophenazine with N-P═O resonance linkages. Owing to the dual enantiotropic N⁺═P-O^- resonances, the designed A-r-D-r-A molecule exhibits an extraordinarily balanced charge flux transportation attribute at high E_T (2.96 eV). Excitingly, blue and warm-white TADF OLEDs hosted by the A-r-D-r-A molecule exhibit outstanding external quantum efficiencies of 14.7 and 20.3%, respectively. Our studies not only broaden the scope of resonance molecules but also indicate that a resonance structure is an effective linkage to develop optoelectronic materials with dynamically adaptive properties.

Room-Temperature Phosphorescence of Nicotinic Acid and Isonicotinic Acid: Efficient Intermolecular Hydrogen-Bond Interaction in Molecular Array

Pure organic room-temperature phosphorescence (RTP) has attracted wide interest due to its unique advantages and promising applications. However, it is still challenging to develop efficient RTP through precise molecular design. In this work, RTP is observed from two simple aromatic acids, nicotinic acid (NA) and isonicotinic acid (INA), in the crystal state. Single crystal structure analysis indicates that an intense hydrogen bond between the pyridine nitrogen atom and the carboxyl group results in zigzag and linear molecular packing modes in NA and INA crystal. From theoretical calculations, the hydrogen bond can effectively promote the intersystem crossing process and stabilize triplet exciton. The identical molecular orientations in the molecular array contribute to the larger dipole moment of INA as compared to that of NA, which should be responsible for the red-shifted photoluminescence and RTP of INA. When the hydrogen bond is destructed by grinding or deprotonation, the RTP decreases sharply, further confirming the crucial role of the hydrogen bond on RTP.

Diversity of Luminescent Metal Complexes in OLEDs: Beyond Traditional Precious Metals

Organic light-emitting diodes (OLED) have attracted increasing attention due to their excellent properties, such as self-luminosity, high color gamut and flexibility, and potential applications in display, wearable devices and lighting. The emitters are the most important composition in OLEDs, mainly classified into fluorescent compounds (first generation), metal phosphorescent complexes (second generation), and thermally activated delayed fluorescence (TADF) materials (third generation). In this review, we summarize the advances of novel emitters of organic metal complexes in the last decade, focusing on coinage metals (Cu, Ag, and Au) and non-precious metals (Al, Zn, W, and alkali metal). Also, the design strategy of d¹⁰ and Au(III) complexes was discussed. We aim to provide guidance for exploring efficient metal complexes beyond traditional phosphorescent complexes.

Phosphine Sulfide-Based Bipolar Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes

Three phosphine sulfide-based bipolar host materials, vizCzPhPS, DCzPhPS, and TCzPhPS, were facilely prepared through a one-pot synthesis in excellent yields. The developed hosts exhibit superior thermal stabilities with the decomposition temperatures (T_d) all exceeding 350 °C and the melting temperatures (T_m) over 200 °C. In addition, their triplet energy (E_T) levels are estimated to be higher than 3.0 eV, illustrating that they are applicable to serve as hosts for blue phosphorescent organic light-emitting diodes (PhOLEDs). The maxima luminance, current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of 17,223 cd m^-2, 36.7 cd A^-1, 37.5 lm W^-1, and 17.5% are achieved for the blue PhOLEDs hosted by CzPhPS. The PhOLEDs based on DCzPhPS and TCzPhPS show inferior device performance than that of CzPhPS, which might be ascribed to the deteriorated charge transporting balance as the increased number of the constructed carbazole units in DCzPhPS and TCzPhPS molecules would enhance the hole-transporting ability of the devices to a large extent. Our study demonstrates that the bipolar hosts derived from phosphine sulfide have enormous potential applications in blue PhOLEDs, and the quantity of donors should be well controlled to exploit highly efficient phosphine sulfide-based hosts.