Highly efficient solution-processed green and red electrophosphorescent devices enabled by small-molecule bipolar host material

Abstract

A solution-processable host molecule TPO comprised of hole-transporting triphenylamine and electron-transporting oxadiazole has been synthesized. Through meta-linkage between the donor and the acceptor, complete charge localization of the HOMO and LUMO and proper triplet energy are imparted. The new compound shows good thermal stability with a high glass-transition temperature of 131 C. Smooth and homogeneous film can be obtained by spin-coating from a TPO/iridium complex blend as probed by atomic force microscopy. The solution-processed red phosphorescent organic lightemitting device (PhOLED) achieves a maximum current efficiency of 13.3 cd A À1 with Commission Internationale de l'Eclairage coordinates of (0.64, 0.36); while the green device reaches a maximum current efficiency of 40.8 cd A À1 , and the value is still as high as 39.6 cd A À1 at a practical luminance of 1000 cd m À2 . The low roll-off can be attributed to the bipolar nature of the host material TPO. An optimized device further elevates the efficiency to 56.8 cd A À1 , which is among the highest ever reported for small-molecule based green PhOLEDs fabricated by a wet process.

High Efficiency in a Solution‐Processed Thermally Activated Delayed‐Fluorescence Device Using a Delayed‐Fluorescence Emitting Material with Improved Solubility

“…In most cases, a small‐molecule host and dopant materials for vacuum‐deposited phosphorescent organic light‐emitting diodes were modified or used without modification for application in solution‐processed devices, which improved the quantum efficiency of the solution‐processed phosphorescent OLEDs. High quantum efficiency close to 20% was reported in red, green and blue phosphorescent OLEDs …”

“…There have been many papers reporting improved device performances in solution‐processed OLEDs and most research has been directed to enhancing the quantum efficiency using soluble triplet host and phosphorescent emitting materials . In most cases, a small‐molecule host and dopant materials for vacuum‐deposited phosphorescent organic light‐emitting diodes were modified or used without modification for application in solution‐processed devices, which improved the quantum efficiency of the solution‐processed phosphorescent OLEDs.…”

Solution‐Processible Carbazole Dendrimers as Host Materials for Highly Efficient Phosphorescent Organic Light‐Emitting Diodes

“…Consequently the PVK‐based devices usually have relatively high turn‐on voltages and driving voltages, which are not favorable for high power efficiencies of the devices 18. Most recently, there have been several reports exploring small molecular materials as triplet hosts to fabricate PHOLEDs by spin coating 19, 20. Although acceptable performance was obtained, the device stability is still a concern because of the comparatively lower glass transition temperatures and inferior film forming ability of the solution techniques, especially when large area application is taken into account.…”

High quantum efficiency in solution processed blue phosphorescent organic light-emitting diodes based on an asymmetric benzothienopyridine host

“…Other than these, a triphenylsilyl modified carbazole [3], a triazole modified tetraphenylsilane [9], a benzofuropyridine modified carbazole [10], and diphenylphosphine oxide based fluorene [11] were effective as the host material for blue PHOLEDs and mixed hosts of small molecules were efficient in red and green PHOLEDs [12][13][14][15][16]. High current quantum efficiency of above 15% was reported in the red, green and blue PHOLEDs [17][18][19][20][21][22]. Although blue PHOLEDs with high quantum efficiency above 15% have been reported, the search for a solution processed host materials with stable morphology is still under pursue.…”