Highly efficient inverted phosphorescent organic light-emitting devices with ZnO nanoparticles electron injection layer

Highlights

• We developed highly efficient inverted phosphorescent OLEDs with ZnO nanoparticles.

• ZnO nanoparticles were used as an electron injection layer.

• ZnO NPs efficiently inject electrons from cathode to the electron transport layer.

• The maximum external quantum efficiency of the inverted device was 15.8%.

Abstract

We have developed highly efficient inverted phosphorescent organic light-emitting devices (OLEDs) using ZnO nanoparticles (NPs). The wurtzite structured ZnO NPs having an average particle size of about 5 nm were synthesized by solution-precipitation method. The inverted phosphorescent OLEDs with a tris(2-phenylpyridine) iridium(III) emitter have fabricated using ZnO NPs and 2,2’,2”-(1,3,5-benzinetriyl)-tris-(1-phenyl)− 1-H-benzimidazole (TPBi) electron transport layer. ZnO NPs effectively inject electrons from cathode to the electron transport layer. The maximum external quantum efficiency and current efficiency of the phosphorescent inverted OLEDs with ZnO NPs were 15.8% and 55.1 cd/A, respectively.

Introduction

Organic light emitting-devices (OLEDs) have been widely used during past few decades for displays and lighting applications because they can provide superior characteristics such as wide viewing angle, fast response, and excellent mechanical flexibility [1], [2], [3], [4]. Although the efficiency and lifetime of OLEDs have been substantially improved by the highly efficient phosphorescent organic emitters and the sophisticated device structures, the low electron mobility and low environmental stability of organic materials are still major concerns in enhancing the device efficiency and stability for commercial applications [5], [6], [7], [8], [9], [10], [11], [12].

Recently, zinc oxide (ZnO) nanoparticles (NPs) have been widely studied for using as electron injection/transport layers of OLEDs because of their strong potentials in improving the device efficiency and stability due to their high electron mobility [13], [14], low conduction band minimum energy, and high chemical stability [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Since the ZnO NP layers are coated by solution processes such as spin coating and inkjet printing, the solvents in ZnO solutions may damage underlying organic layers in the conventional structures so that the inverted structures have been typically used for the ZnO NP layers [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. The inverted OLED structure can take several advantages such as low operating stress and image sticking, and low luminance drop compared with the conventional structure [28], [29]. Several authors have reported the inverted OLEDs with ZnO NP electron injection/transport layers based on the polymer emitting layers [18], [19], [20], [21], [22], [23]. For example, Y. Murat et al. demonstrated the inverted polymer device exhibiting the maximum current efficiency of 13.1 cd/A using Super Yellow as an emitting material [18]. S. Höfle et al. reported the maximum current efficiency of 22 cd/A using the same polymer emitting material [19]. In addition, B. R. Lee et al. reported the highly efficient inverted polymer device of the maximum external quantum efficiency of 17.8% using poly(9,9’-dioctylfluorene-cobenzo-thiadiazole) emitting layer [20]. Since the work function of ZnO is not sufficient to inject electrons from ZnO to the polymer emission layer, they inserted the interlayers such as polyethyleneimine (PEI), polyethylenimine ethoxylated, 2-methoxyethanol and ethanolamine between ZnO and emission layer [18], [19], [20]. These interlayers improve the efficiency of device by enhancing the electron injection into the emission layer and confining electrons and holes inside the emission layer.

In this paper, we report the inverted green phosphorescent OLEDs with ZnO NP electron injection layers. There are very few reports on the inverted phosphorescent OLEDs with ZnO NP layers although the phosphorescent emitters can provide high efficiency by harvesting singlet and triplet excitons generated inside the emission layer [23], [24], [25], [26]. H. Lee et al. demonstrated the iridium(III)bis(4,6-di-fluorophenyl)-pyridinato-N,C^2’]picolinate doped blue phosphorescent OLEDs with an inverted structure of ZnO NP layer. The maximum external quantum efficiency of the device was 8.2% [25]. Recently, J. Zhang et al. reported the inverted green phosphorescent OLEDs with a current efficiency of 60.87 cd/A [24]. They used ZnO/PEI for enhancing the electron injection into the emission layer. In this paper, we fabricated the inverted phosphorescent OLEDs using ZnO NP electron injection layer and 2,2’,2”-(1,3,5-benzinetriyl)-tris-(1-phenyl)− 1-H-benzimidazole (TPBi) electron transport layer. ZnO NPs enhance the electron injection from cathode to the TPBi electron transport layer, resulting in highly efficient inverted OLEDs exhibiting the maximum external quantum efficiency of 15.8%.