Feb 9, 2012
Making efficient OLEDs for flexible solid-state lighting
Graphene could be ideal for replacing transparent anodes made of expensive indium tin oxide (ITO) in applications like organic light-emitting diodes (OLEDs). However, even the best graphene anode OLEDs made today are incapable of emitting light very efficiently because graphene has a low work function and high sheet resistance. Now, researchers in Korea are saying that they may have overcome these problems by fabricating graphene anode OLEDs with luminous efficiencies of 37.2 lmW–1 in fluorescent devices and 102.7 lm W–1 in phosphorescent ones – figures that are significantly greater than those of equivalent OLEDs containing an ITO anode. The result confirms that graphene anodes could be used to make a wide range of high-performance and flexible organic optoelectronics devices.
Graphene – a sheet of carbon just one atom thick – could be an ideal replacement for ITO transparent electrodes in the future. Although ITO has served well until now in OLEDs and liquid-crystal displays, it is becoming more and more expensive to produce. ITO is also brittle and can thus not be used in flexible applications.
However, there is a problem with graphene: the material suffers from the fact that it has a low work function (of around 4.4 eV). This makes it difficult to inject charge carriers (holes) between an anode made of graphene and the overlying organic layers because of the high injection barrier at the interface between the two materials. The result is that graphene anode OLEDs do not conduct electric current as well as their ITO-based counterparts.
And that is not all: graphene sheets also have a resistance that is too high (more than 300 Ω/sq for such applications. To compare, ITO has a resistance of just 10 Ω/sq.
Now, Tae-Woo Lee of the Pohang University of Science and Technology (POSTECH) in Korea and colleagues have developed a way to increase the work function of graphene films to around 6 eV. The researchers achieved this by modifying the surface of the graphene using water-dispersed conducting polymer compositions, including PEDOT:PSS and a perfluorinated ionomer. The technique creates a work-function gradient from the graphene to the overlying organic layer that subsequently allows holes to be easily injected into the organic layer despite the high hole-injection barrier between the anode and the organic layer. Current flow through the device thus increases.
The team also succeeded in lowering the sheet resistance to around 30 Ω/sq by p-doping it with chemicals like HNO3 and AuCl3. This value is low enough for the graphene to be made into an efficient anode for OLED applications and the device operating voltage is almost same as those with an ITO anode, says Lee.
The graphene anodes are flexible too – something that the team confirmed by performing bending tests on the finished OLEDs. "The graphene anode demonstrated excellent bending stability with a bending radius of 0.75 cm and a strain of 1.25%," said Lee. "And we observed that the graphene devices maintained almost the same current density even after being bent and straightened 1000 times."
White OLEDs based on graphene anodes also show good electroluminescence spectra with a Commission Internationale de l'Eclairage (CIE) co-ordinate of 0.32 and 0.42, he told nanotechweb.org. "We believe that this kind of flexible lighting source could thus be effectively utilized for high-end interior/exterior lighting, flexible organic optoelectronics, such as stretchable OLED displays, and solar cells."
The researchers now hope to further improve their graphene anode so that it conducts electricity even better. "We are also trying to apply our strategy to other organic optoelectronics, such as solar cells and transistors," revealed Lee.
The work was detailed in Nature Photonics.
About the author
Belle Dumé is contributing editor at nanotechweb.org