Investigation of double emissive layer structures on phosphorescent blue organic light-emitting diodes

doi:10.1016/j.synthmet.2009.03.026

Synthetic Metals

Volume 159, Issue 14, July 2009, Pages 1460-1463

https://doi.org/10.1016/j.synthmet.2009.03.026 Get rights and content

Abstract

The performances of blue phosphorescent organic light-emitting diodes (PHOLEDs) at high current densities have been investigated with double emissive layer structures (D-EMLs). The D-EMLs are comprised of two emissive layers with a hole-transport-type host of N,N′-dicarbazolyl-3,5-benzene (mCP) and an electro transport-type ultrawide band-gap host of m-bis-(triphenylsilyl)benzene (UGH3) both doped with a blue electro-phosphorescent dopant of iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C^2′) picolinate (FIrpic). The expansion of hole/electron recombination zone in D-EMLs has been successfully achieved by controlling of each EML properties, therefore external quantum efficiency, especially at high current density region was significantly enhanced. Moreover, the blue PHOLED with D-EMLs showed substantially reduced roll-off with the external quantum efficiency of 10.0% at 5000 cd/m².

Introduction

Organic light-emitting devices (OLEDs) have been received considerable attention for both display and lighting applications due to their high light-emitting performances [1], [2], [3]. Especially, electro-phosphorescent organic light-emitting diodes (PHOLEDs) using green and red phosphorescent dyes have reached almost 100% internal quantum efficiency by efficient utilizing both singlet and triplet excitons [3]. However, there are still several challenges for highly efficient blue PHOLEDs because they require wider band-gap materials for both host and dopant than those for green or red PHOLEDs. There have been many approaches for developing efficient blue PHOLEDs [4], [5], [6], [7], [8], [9], [10]. An endothermic host–guest energy transfer by using (4,4′-N,N′-dicarbazole) biphenyl (CBP) and iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C^2′) picolinate (FIrpic) system was reported by Adachi et al. [4]. Incorporation of high triplet energy host – N,N′-dicarbazolyl-3,5-benzene (mCP) or 4,4′-bis(9-carbazolyl)-2,2′-dimethyl-biphenyl (CDBP) enabled effective exothermic energy transfer, and resulted in further enhancement of blue OLEDs performance [5], [6]. A stepwise doping structure of blue PHOLEDs has been also reported [7]. On the other side, direct charge trapping on the blue dopants by using ultrawide band-gap hosts was also demonstrated [8], [9], [10].

Previously, we reported a peak external quantum efficiency (EQE) over 20% by employing the blue light emitting FIrpic doped in ultrawide band-gap host UGH3 with the adequate interlayers [10]. Despite this encouraging report, the efficiency of the blue PHOLEDs at a high current density region is quite limited due to severe roll-off (the decrease of efficiency with increasing current density) [11] in electro-phosphorescent devices. In the previous work, the EQE of FIrpic-based device showed significant reduction of 9.5% at 10 mA/cm² which is almost half the value of the maximum efficiency [10]. This roll-off of the quantum efficiency was understood by the triplet–triplet annihilation (TTA) or triplet–polaron annihilation (TPA) in the emissive layer with heavy doping ratio [12], [13].

In this paper, blue PHOLEDs performances at a high current density have been investigated with double emissive layer structures (D-EMLs) based on a hole-transport-type host and an electron-transport-type ultrawide band-gap host. With the D-EMLs structure, we have successfully achieved the distribution of exciton generation region in the emissive layer by controlling of each EML thickness, therefore obtained highly enhanced blue PHOLEDs performance as well as reduced roll-off at high current density region.

Section snippets

Experimental

A series of organic light-emitting devices in the current study were made using the configuration (Device A): indium tin oxide (ITO)/NPB (40 nm)/EML-1 (x nm)/EML-2 (y nm)/Bphen (50 nm)/LiF (1 nm)/Al (120 nm); x (EML-1) + y (EML-2) = 30 as shown in Fig. 1. For the device with D-EMLs, the x-nm-thick mCP layer doped with 7% FIrpic for the EML-1 and the y-nm-thick UGH3 layer doped with 10% FIrpic for the EML-2, respectively, were used as emissive layers. 4,4′-bis[N-(1-nathyl)-N-phenyl-amino]biphenyl (NPB),

Results and discussion

Four types of blue PHOLEDs were fabricated based on the Device A structure. Device A-1 and A-4 containing single emitting layer (S-EML) of UGH3 and mCP host for control devices and Device A-2 and A-3 containing a double emitting layer (D-EML) of UGH3 and mCP host with different thicknesses were prepared (Fig. 1). The reported triplet energy gap of UGH3 and mCP hosts were 2.9 and 3.5 eV, respectively. Therefore, triplet energy gaps of both hosts are higher than that of FIrpic dopant (2.62 eV), in

Conclusion

We have demonstrated that blue phosphorescent OLEDs performance at high current region was significantly improved by using D-EMLs with the hole-transport-type host (mCP) and the electro transport-type host (UGH3). Moreover, this D-EMLs exhibited highly enhanced performance and reduced roll-off at high brightness (EQE = 10.0% at 5000 cd/m²) compared to conventional blue PHOLEDs with a single EML. We attributed this enhancement to the distribution of exciton generation zone along with confinement of

Acknowledgment

We gratefully acknowledged Prof. Jun Yeob Lee (Department of Polymer Science and Engineering of Dankook University) for fruitful discussion, and Ms. K.-I. Song and Ms. S.J. Lee (ETRI) for assistance of EL device experiments. This work was supported by the future technology development program of MOCIE/ITEP [2006-10028439, OLED Lighting].

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Investigation of double emissive layer structures on phosphorescent blue organic light-emitting diodes

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgment

Org. Electron.

Nature

Appl. Phys. Lett.

J. Appl. Phys.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.