Jul 15, 2009
Nickel electrodes challenge ITO in OLED test
The successful entry of organic light-emitting diodes (OLEDs) into the general lighting market requires not just high-performance devices, but also a significant reduction in cost.
One of the limiting factors for the industrial implementation of organic devices is the use of indium tin oxide (ITO) as a bottom electrode, which apart from being an expensive and scarce material, also involves high temperature processing. In addition, polymer degradation occurs due to the diffusion of both indium and oxygen into the emissive layer under device operation. Finally, the use of a thick ITO layer, which is a high index material, introduces optical modes that lower the out-coupling efficiency of the device. As much as 50–80% of the light emission can be lost to waveguiding modes in the glass, ITO and organic layers.
In a recent paper, published in Nanotechnology, researchers have investigated the possibility of using a thin nickel layer instead of ITO as anodes for bottom-emitting organic light-emitting diodes. Identical single-layer devices based on a blue-emitting polymer were fabricated as bottom-emitting diodes, and equipped with either an ITO or nickel hole injecting electrode. The sputtering fabrication process gives very smooth, continuous and conductive nickel films, even for a film thickness as low as 4 nm.
The team, which is based at ICFO, Spain, has demonstrated that similar efficiencies were reached for devices with either ITO or nickel films less than 10 nm. The thin nickel films show higher roughness, higher square resistance and a lower transmittance of about 2.5 times compared with ITO. These characteristics lead to a less efficient hole injection in the nickel-based device, which results in lower current densities at a given voltage and a slightly higher threshold voltage than the ITO-based device. Nevertheless, similar luminance levels were obtained for ITO-based and nickel-based devices. Most importantly, no optical modes could be observed within the nickel layer because the film is too thin for a light trapping mechanism.
To conclude, thin nickel films could provide a route to ITO-free devices, as a semitransparent anode material for bottom-light-emitting diodes, with several advantages including simple deposition, no need for post-deposition treatment and lower cost.
About the author
The work was performed at the Institute of Photonics, ICFO in Barcelona, Spain, and is the result of a collaboration between the Optoelectronics group and the Nanophotonic devices group. Danny Krautz is a PhD student in the Nanophotonic devices group and D S Ghosh is a PhD student in the Optoelectronic group. Prof. V Pruneri is head of the Optoelectronics group where the thin metal electrodes are being investigated and developed for device applications. Dr S Cheylan is a research fellow in the Nanophotonic devices group and her current research involves the fabrication and study of polymer light-emitting diodes and photovoltaic cells.