High-Efficiency Blue Phosphorescent Organic Light-Emitting Devices with Low Efficiency Roll-Off at Ultrahigh Luminance by the Reduction of Triplet-Polaron Quenching

Highly Efficient Ultra-Thin EML Blue PHOLEDs with an External Light-Extraction Diffuser

In this study, various diffusers are applied to highly efficient ultra-thin emission layer (EML) structure-based blue phosphorescent organic light-emitting diodes (PHOLEDs) to improve the electroluminescence (EL) characteristics and viewing angle. To achieve highly efficient blue PHOLEDs, the EL characteristics of ultra-thin EML PHOLEDs with the various diffusers having different structures of pattern-shape (hemisphere/sphere), size (4~75 μm), distribution (surface/embedded), and packing (close-packed/random) were systematically analyzed. The diffusers showed different enhancements in the overall EL characteristics of efficiencies, viewing angle, and others. The EL characteristics showed apparent dependency on their structure. The external quantum efficiency (EQE) was enhanced mainly by following the orders of pattern, size, and shape. Following the pattern size, the EQE enhancement gradually increased; the largest-sized diffuser with a 75 μm closed-packed hemisphere (diffuser-1) showed a 1.47-fold EQE improvement, which was the highest. Meanwhile, the diffuser with a ~7 μm random embedded sphere with a low density (diffuser 5) showed the lowest 1.02-fold-improved EQE. The reference device with ultra-thin EML structure-based blue PHOLEDs showed a maximum EQE of 16.6%, and the device with diffuser 1 achieved a maximum EQE of 24.3% with a 5.1% wider viewing angle compared to the reference device without a diffuser. For the in-depth analysis, the viewing angle profile of the ultra-thin EML PHOLED device and fluorescent green OLEDs were compared. As a result, the efficiency enhancement characteristics of the diffusers show a difference in the viewing angle profile. Finally, the application of the diffuser successfully demonstrated that the EL efficiency and viewing angle could be selectively improved. Additionally, we found that it was possible to realize a wide viewing angle and achieve considerable EQE enhancement by further investigations using high-density and large-sized embedded structures of light-extraction film.

Flexible organic optoelectronic devices on paper

Paper substrate has many advantages, such as low cost, bendable, foldable, printable, and environmentally friendly recycling. Nowadays, paper has been further extended as a flexible platform to deliver electronic information with the integration of organic optoelectronic devices, such as organic thin-film transistor, organic solar cell, organic electrochromic device, and organic light-emitting device. It has great potential to become the new generation of flexible substrate. Given rough surface and porous of paper, many efforts have been underway in recent years to enable the compatibility between optoelectronics and paper substrate. In this review, we present the development history of paper and its physicochemical properties, and summarize the current development of paper-based organic optoelectronic devices. We also discuss the challenges that need to be addressed before practical uses of paper-based organic optoelectronic devices.

Highly Efficient Simple-Structure Sky-Blue Organic Light-Emitting Diode Using a Bicarbazole/Cyanopyridine Bipolar Host

Up to now, the most efficient blue phosphorescent organic light-emitting diode (PhOLED) was achieved with a maximum external quantum efficiency (η_ext) of 34.1% by using an exciplex cohost. It still remains a challenge to obtain such high efficiencies using a single-host matrix. In this work, a highly efficient sky-blue PhOLED is successfully fabricated using a newly developed bipolar host material, namely 5-(2-(9H-[3,9'-bicarbazol]-9-yl)phenyl)nicotinonitrile (o-PyCNBCz), which realizes a η_ext of 29.4% at a practical luminance of 100 cd m^-2 and a maximum η_ext of 34.6% (at 23 cd m^-2). The present device is characterized by simple configuration with a single host and single emitting layer. o-PyCNBCz also reveals high efficiency of 28.2% (94.8 cd A^-1) when used as the host for green PhOLED. Under identical conditions, o-PyCNBCz always outperforms than its isomer 3-PyCNBCz (5-(9-phenyl-9H-[3,9'-bicarbazol]-6-yl)nicotinonitrile) in terms of more balanced charge transportation, higher photoluminescent quantum yields of over 90%, and higher horizontal orientation ratio of the emitting dipole for the host-dopant films, which finally lead to its superior performance in PhOLEDs. It is observed that all these merits of o-PyCNBCz benefit from its ortho-linking style of carbazole (p-type unit) and cyanopyridine (n-type unit) on the phenylene bridge and the resultant molecular conformation.

Tuning the NLO response of bis-cyclometalated iridium(iii) complexes by modifying ligands: experimental and structural DFT analysis

DFT calculations have been carried out to investigate two synthesized iridium(iii) complexes with substituted Phbd (1-phenyl-2-(pyridin-2-yl)-1H-benzo[d]imidazole) and Crbd (9-(4-(2-(pyridin-2-yl)-1H-benzo[d]imidazol-1-yl)phenyl)-9H-carbazole) as ancillary ligands.