Enhancement of tandem organic light-emitting diode performance by inserting an ultra-thin Ag layer in charge generation layer

Abstract

White organic light-emitting diodes (WOLEDs) have attracted both scientific and industrial interest in the solidstate lighting and display applications due to their exceptional merits,such as high luminances,low power consumptions, high efficiencies,fast response times,wide-viewing angles,flexibilities and simple fabrications.The power efficiency of WOLED has been step-by-step improved in the last 20 years,however,the lifetime of WOLED is still unsatisfactory, which greatly restricts the further development of WOLED.In general,the tandem structure can be used to obtain high-efficiency and long-lifetime WOLED.One of the most important features of this kind of structure is that the different-colors emitting units can be connected by the charge generation layer.Therefore,the key to achieving a highperformance tandem device is how to design the charge generation layer.In this paper,we first develop a tandem green OLED by using an effective charge generation layer with an ultra-thin Ag layer between 4,7-diphenyl-1,10-phenanthroline:CsCO3 and hexaazatriphenylenehexacabonitrile,achieving high luminance,low voltage,high efficiency and long lifetime.The green tandem device with ultra-thin Ag layer (device C) obtains a highest luminance of 290000 cd/m2,which is 1.4 and 1.9 times higher than those of the tandem devices without ultra-thin Ag (device B) and singleunit device (device A),respectively.The driving voltage of device C is 7.2 V at 1000 cd/m2,1.4 V lower than that of device B.Besides,the maximum current efficiency of device C is 60.4 cd/A,which is 2.4% and 220% higher than those of device B (59 cd/A) and device A (18.7 cd/A),respectively.The power efficiency of device C is 26 lm/W,which is 21% higher than that of device B (21.5 lm/W).Moreover,the lifetime (T80) of device C reaches 250 h at an initial luminance of 10000 cd/m2,which is nearly 100 times higher than that of device B (2.7 h).Finally,we fabricate a white tandem device with the optimized charge generation layer,achieving a current efficiency and power efficiency of 75.9 cd/A and 36.1 lm/W at 1000 cd/m2,respectively.In addition,the lifetime (T80) is 77 h at an initial luminance of 10000 cd/m2.All the excellent performances are ascribed to the introduction of the ultra-thin Ag layer into the charge generation layer, which can effectively block the charge generation layer from diffusing.This exciting discovery can provide an effective way to design efficient and stable WOLED,which is beneficial to the solid-state lighting and display markets.