Graphene-based Transparent Electrodes for Highly Efficient Flexible OLEDs

Ultra-efficient and flexible organic-light emitting diodes (OLEDs) have been developed by adopting an ideal electrode structure composed of graphene and layers of titanium dioxide and conducting polymer.

Article  |  Fall 2016

Graphene holds great promise as a transparent electrode in wearable/flexible optoelectronic devices due to its distinctive capability to deform significantly. However, such advantages in form factors carry only limited importance if devices made thereof are inefficient. In fact, emerging display applications, such as those in wearable IT devices, require even greater efficiency than conventional devices because they have to rely on batteries with fairly limited capacity due to their strict constraints in weight, size, and/or deformability.

A key challenge is to develop a structure or methodology to unlock their full optical potential yet to retain graphene’s merits in form factors. The present work is about our quest to provide an effective solution to this vital task in an example of organic light-emitting diodes (OLEDs) – light sources regarded as an ideal match to graphene in that they are among the best platforms for deformable light sources and, to this end, have been in need of affordable and/or flexible transparent electrodes.

As a solution, a joint research team, led by Prof. Seunghyup Yoo from the School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST) and Prof. Tae-Woo Lee from the Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), proposed a new device architecture that can maximize the efficiency of graphene-based OLEDs. The key design of the proposed OLEDs involves a synergetic interplay of high-index TiO₂ layers and low-index hole injection layers (HILs) sandwiching graphene electrodes. A combinatory use of both TiO₂ layers and low-index HILs results in a situation where cavity resonance enhancement is maximized yet loss to surface plasmon polariton (SPP) is mitigated due to the low refractive index of conducting polymers used as HILs, pumping up the efficiency of OLEDs made thereof to their limit. Under this approach, graphene-based OLEDs exhibit ultrahigh external quantum efficiency of 40.8% and power efficiency of 160.3 lm/W, which are unprecedented in OLEDs using graphene as a transparent electrode. Furthermore, these devices remain intact and operate well, even after 1,000 bending cycles at a radius of curvature as small as 2.3 mm. This is a remarkable result for OLEDs containing oxide layers such as TiO₂ because oxides are typically brittle and prone to bending-induced fractures, even at a relatively low strain. The research team, which further includes Professors Sung-Yool Choi (Electrical Engineering) and Taek-Soo Kim (Mechanical Engineering) of KAIST and their students, found that TiO₂ has a crack-deflection toughening mechanism that tends to prevent bending-induced cracks from easily forming.

Prof. Yoo said, “What’s unique and advanced about this technology, compared with previous graphene-based OLEDs, is the synergy of high- and low-index layers that enables optical management of both resonance effect and SPP loss, leading to significant enhancement in efficiency, all with little compromise in flexibility.” He added, “Our work was the achievement of collaborative research, transcending the boundaries of different fields, through which we have often found meaningful breakthroughs.

The research results were published in Nature Communications (7, 11791 (2016); DOI: 10.1038/NCOMMS11791) under the title “Synergetic Electrode Architecture for Efficient Graphene-based Flexible Organic Light-emitting Diodes”.

This picture shows an OLED with the composite structure of a TiO2/graphene/conducting polymer electrode in operation. The OLED exhibits 40.8% of ultrahigh external quantum efficiency (EQE) and 160.3 lm/W of power efficiency. The device prepared on a plastic substrate shown on the right remains intact and operates well even after 1,000 bending cycles at a radius of curvature as small as 2.3 mm.

This picture shows a new architecture to develop highly flexible OLEDs with excellent efficiency by using graphene as a transparent electrode (TE).