Approaching ultimate flexible organic light-emitting diodes using a graphene anode

Tae-Hee Han,Sung-Joo Kwon,Sang-Hoon Bae,Jong-Hyun Ahn,Tae-Woo Lee,Himchan Cho,Min-Ho Park,Hong-Kyu Seo

doi:10.1038/am.2016.108

Abstract

Ultimate flexible organic light-emitting diodes (OLEDs) should have an ultra-high device efficiency, a low-efficiency roll-off at a high luminance and excellent flexibility. Here, we realized flexible tandem OLEDs using a graphene anode with a very high electroluminescent efficiency of ~205.9 cd A−1, 45.2% (~396.4 cd A−1, 87.3% with a hemispherical lens) and a very low efficiency roll-off at a high luminance of ~6.6% at 10 000 cd m−2 (~3.8% with a hemispherical lens) by stacking two organic electroluminescence (EL) units. For the first time, we used an easily controlled and low-temperature processable charge generation layer with lithium nitride (Li3N). This simultaneously provided efficient stacking of EL units and enhanced compatibility of the flexible device on a thin plastic substrate. The flexible tandem OLEDs with a graphene anode also showed great flexibility against bending up to a bending strain of 6.7%. These results represent a significant advancement towards the production of next-generation flexible displays and solid-state lighting that use a graphene anode. Bendable displays with nearly unmatched strain resistance and device efficiency can be realized using graphene-based transparent electrodes. Organic light-emitting diodes (OLEDs) are integral to today's touch-screen technology, but future applications are limited by a reliance on brittle, indium tin oxide electrodes. Researchers led by Tae-Woo Lee from Pohang University of Science and Technology in South Korea have made an alternative OLED prototype that assembles multi-electroluminescent units of organic materials onto high-quality, graphene electrodes transferred to thin plastic substrates. The team improved on previous graphene OLED designs by inserting a charge-generation layer - an organic material mixed with lithium ions that can be evaporated at low temperatures - between separate electroluminescent stacks. This setup provides ample charge carriers and mechanical strength to keep the OLED efficiency high, even after 1,000 bending cycles. We realized highly flexible tandem OLEDs using a graphene anode with very high electroluminescent efficiency ~205.9 cd A−1, 45.2% (~396.4 cd A−1, 87.3% with a hemispherical lens) and very low efficiency roll-off at high luminance ~6.6% at 10 000 cd m−2 (~3.8% with a hemispherical lens) by stacking two organic electroluminescence units using a easily controllable and low-temperature processable charge generation layer on thin flexible plastic substrates. The flexible OLEDs showed great flexibility against bending stress up to bending strain of 6.7%.

Highlights

Organic light-emitting diodes (OLEDs) have shown a strong potential for use in next-generation flexible displays and solid-state lighting.[1,2,3,4,5,6,7,8] indium–tin oxide (ITO), the most widely used electrode in OLEDs, has a very poor tolerance to external mechanical stress.[9]its cost has increased gradually owing to a scarcity of indium.[9]
We developed and used a low-temperature-processable charge generation layer (CGL) between two EL units that is formed by a stable co-deposition of the organic electron transporting layer (ETL) and an n-type metal dopant in a single organic chamber
The optical transparency decreased by ~ 2.3% per layer of stacked single layer graphene (SLG); this gradual decrease ensures that stacking of SLGs for the multilayered graphene anode does not disrupt the optical properties of the SLGs

Summary

Introduction

Organic light-emitting diodes (OLEDs) have shown a strong potential for use in next-generation flexible displays and solid-state lighting.[1,2,3,4,5,6,7,8] indium–tin oxide (ITO), the most widely used electrode in OLEDs, has a very poor tolerance to external mechanical stress.[9]its cost has increased gradually owing to a scarcity of indium.[9]. Organic light-emitting diodes (OLEDs) have shown a strong potential for use in next-generation flexible displays and solid-state lighting.[1,2,3,4,5,6,7,8] indium–tin oxide (ITO), the most widely used electrode in OLEDs, has a very poor tolerance to external mechanical stress.[9]. To enable the fabrication of flexible organic optoelectronic devices, the brittle ITO electrode should be replaced with a flexible transparent conducting electrode. Much effort has been made to replace the ITO electrode with transparent conducting materials such as conducting polymers, carbon nanotubes and metal nanowires.[10,11,12] Among these materials, graphene, which is a twodimensional sheet of sp2-bonded carbon atoms, provides several advantages for flexible transparent electrodes, such as high optical transparency, low sheet resistance, excellent flexibility, smooth surface ( preventing electrical shortage in devices), and no optical haze.[13,14,15,16,17,18,19] The chemical stability of graphene is an additional advantage for its practical use in organic opto-electronic devices.[6,16,17] several researchers have considered the possibility of using graphene electrodes in flexible organic opto-electronics.[6,20,21,22,23,24,25,26,27,28,29,30,31] There has been significant progress in the use of flexible OLEDs with graphene electrodes because of the existence of methods to modify the high sheet resistance (4300 Ω per square) and low work function (WF) (~4.4 eV) of pristine graphene.[6,20,21,22,23,24,25,26,27,32,33]

Methods

Results

Conclusion