Highly efficient deep blue phosphorescent organic light‐emitting diodes are developed using novel phenylcarbazole‐based phosphine oxide host materials (PPO1 and PPO2). A deep blue phosphorescent dopant, tris((3,5‐difluoro‐4‐cyanophenyl)pyridine) iridium, is doped into PPO1 and PPO2 at a doping concentration of 15% and a high quantum efficiency of 18.4% is obtained with color coordinates of (0.14, 0.15).
Blue-phosphorescent organic light-emitting diodes (PHOLEDs) have been developed for more than 10 years towards use in active-matrix-type organic light-emitting diodes. There has been much improvement in quantum efficiency, lifetime, and color purity although the device performances of the blue PHOLEDs are not yet good enough for practical applications.Most research into blue PHOLEDs was focused on the development of new host and dopant materials.[1-14] The best-known host material in the blue PHOLEDs is N,N-dicarbazolyl-3,5-benzene (mCP).[1] It has good hole-transport properties due to a carbazole unit in the backbone structure and a wide triplet bandgap of 2.90 eV for efficient energy transfer. However, its electron injection and transport properties are poor because of the high energy of the lowest unoccupied molecular orbital (LUMO) of 2.4 eV. Silicone-based wide-triplet-bandgap host materials were also developed [2][3][4][5] and tetraaryl-based silane materials have been used as host materials in blue PHOLEDs. [2,3] However, the energy of the highest occupied molecular orbital (HOMO) of the silane-based host materials is around 7.0 eV, which is not suitable for hole injection. Therefore, it was difficult to balance holes and electrons in the light-emitting layer. To overcome the poor hole injection in the silane-based host materials, silane compounds with a carbazole moiety in the molecular structure were evaluated as triplet host materials in blue PHOLEDs. [4,5] However, the carbazole-based host materials showed strong hole-transport properties and bipolar transport behavior was not observed. In addition, phosphine oxide-type host materials were synthesized, but only sky-blue PHOLEDs were reported due to the low triplet energy. [6,7] Our group also reported phosphine oxide-type host materials with a carbazole moiety in the backbone structure and high efficiency could be obtained. [14] Although several classes of host materials have been synthesized, no host materials could show a theoretical maximum quantum efficiency in the deep-blue PHOLED with Commission International De L'Eclairage (CIE) color coordinate (x þ y) values below 0.30. In this work, we synthesized bipolar-type high-triplet-energy host materials with a carbazole core structure. Phosphine oxide (PPO21) and phosphine sulfide (PPS21) host materials with the carbazole core structure were synthesized and evaluated as host materials in the deep-blue PHOLEDs. A theoretical maximum quantum efficiency over 19% with a deep-blue CIE coordinate of (0.14,0.16) was demonstrated in the deep-blue PHOLEDs using the high-triplet-energy host materials for the first time.The host materials synthesized in this work have a 9-phenylcarbazole core structure with two phosphine oxide or phosphine sulfide units. One diphenylphosphine oxide or sulfide unit was attached to the 3-position of the carbazole unit to control the HOMO level and the charge transport properties. The other diphenylphosphine oxide or sulfide unit was connected to the phenyl group of the 9-phenylcar...
High efficiency blue phosphorescent organic light emitting diodes have been developed by using a simple device structure. A derivative of spirobifluorene based phosphine oxide was used both as a host and an electron transport layer with an exciton blocking function. A maximum quantum efficiency of 19.2% and a current efficiency of 37.2cd∕A were obtained by using a simple device structure without a hole blocking layer.
The ability to image pressure distribution over complex three-dimensional surfaces would significantly augment the potential applications of electronic skin. However, existing methods show poor spatial and temporal fidelity due to their limited pixel density, low sensitivity, or low conformability. Here, we report an ultraflexible and transparent electroluminescent skin that autonomously displays super-resolution images of pressure distribution in real time. The device comprises a transparent pressure-sensing film with a solution-processable cellulose/ nanowire nanohybrid network featuring ultrahigh sensor sensitivity (>5000 kPa −1 ) and a fast response time (<1 ms), and a quantum dot-based electroluminescent film. The two ultrathin films conform to each contact object and transduce spatial pressure into conductivity distribution in a continuous domain, resulting in super-resolution (>1000 dpi) pressure imaging without the need for pixel structures. Our approach provides a new framework for visualizing accurate stimulus distribution with potential applications in skin prosthesis, robotics, and advanced human-machine interfaces.
Transparent organic light emitting diodes were developed by using a thermally evaporable WO3∕Ag∕WO3 (WAW) as a transparent cathode. A thin Ag layer was introduced as an interlayer between the Li doped electron transport layer and the WAW electrode. A high transparency over 80% was obtained and electron injection was greatly improved by using the thin Ag interlayer between the Li doped layer and the WAW electrode. The driving voltage at 1000cd∕m2 was only 4.5V and the sheet resistance of the WAW electrode was as low as 12Ω∕◻.
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