Improved hole injection for blue phosphorescent organic light-emitting diodes using solution deposited tin oxide nano-particles decorated ITO anodes

Seung Il Lee,Woo Young Kim,Kunyu Liang,Ayse Turak,Lazar Kojvic,Geum Jae Yun,Gregory Hanta,Jin Wook Kim,Lok Shu Hui

doi:10.1038/s41598-019-39451-4

Abstract

Blue phosphorescent organic light-emitting diodes (PHOLEDs) were fabricated with tin oxide (SnOx) nano-particles (NPs) deposited at the ITO anode to improve their electrical and optical performances. SnOx NPs helped ITO to increase the work function enhancing hole injection capability. Charge balance of the device was achieved using p- and n-type mixed host materials in emissive layer and the devices’ luminance and maximum external quantum efficiency (EQE) increased about nearly 30%. Tuning the work function using solution processed NPs allows rapid optimization of device efficiency.

Highlights

The evolution of organic light-emitting diodes (OLED) into flat panel display and solid state planar lightning applications has been the result of much development in luminous efficiency, low power consumption and life-time
Common materials with large triplet energies used for blue PhOLEDs include, blue dopant material bis[3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)]-iridium (FIrpic) (T1 = 2.7 eV), host for blue phosphorescent dyes, 1,3-Bis(N-carbazolyl)benzene with a large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap as well as high triplet energy (=2.9 eV) and hole transport layer (HTL) materials 1-Bis[4-[N,N-di(4-tolyl)amino]phenyl]-cyclohexane (TAPC) T1 = 2.9 eV and a very deep HOMO level (5.9 eV)[6]
By deliberately introducing Sn4+ components with submonolayer coverage of SnOx NPs, we propose an alternative method of controlling the surface electronic structure of indium tin oxide (ITO) for better matching with the deep HOMO of novel HTLs

Summary

Introduction

The evolution of organic light-emitting diodes (OLED) into flat panel display and solid state planar lightning applications has been the result of much development in luminous efficiency, low power consumption and life-time. Among the most significant was the development of doped guest-host matrix systems that utilizing both singlet and triplet excitons for emissions[1,2,3,4] Such phosphorescent OLEDs (PHOLEDs) have significantly higher luminous efficiencies than their fluorescent counterparts, due to the potential for 100% internal quantum efficiency[2]. Common materials with large triplet energies used for blue PhOLEDs include, blue dopant material bis[3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)]-iridium (FIrpic) (T1 = 2.7 eV), host for blue phosphorescent dyes, 1,3-Bis(N-carbazolyl)benzene (mCP) with a large HOMO-LUMO energy gap as well as high triplet energy (=2.9 eV) and HTL materials 1-Bis[4-[N,N-di(4-tolyl)amino]phenyl]-cyclohexane (TAPC) T1 = 2.9 eV and a very deep HOMO level (5.9 eV)[6]. As a result of the large hole injection barriers at the ITO/HTL interface, improvement of carrier injection in blue PHOLEDs is expected to have a more significant effect on the device performance than for green or red PHOLEDs and OLED structures. The possibility of tailoring the work function to match the energy level of the active organic layer is of great interest in the fabrication of organic devices to form barrier-free Ohmic contacts

Methods

Results

Conclusion