In the growing transparent conductor market, the potential of graphene as an alternative to indium tin oxide (ITO) is gaining significance, especially in the development of highly portable, flexible devices.
However, graphene’s application as a transparent electrode has been greatly affected due to technological immaturity. Now, scientists at Philips Research, Graphenea, and the University of Cambridge have fabricated graphene-based organic light emitting diodes (OLEDs) (Figure 1) that deliver superior performance compared to advanced ITO devices.
Figure 1. Graphene-based OLED
Graphene’s Potential in Opto-Electronic Applications
Graphene exhibits high optical transparency and electronic mobility, making it a potential material for opto-electronic applications such as touch screens, LEDs, and solar cells. At present, such devices are fabricated predominantly from ITO.
However, demand for alternative transparent conductors (TCs) to fabricate next-generation devices is increasing due to the ever-increasing price and limited geographical availability of indium as well as the market trend towards flexible devices
Graphene has high charge carrier mobility, but typically has low carrier concentrations. Its overall performance as an electrode therefore needs to be improved, by doping to increase the number of available charge carriers. Care must be taken to avoid damaging the high optical transparency of graphene during the doping process, as this is an important quality for a transparent electrode.
Efficient exchange of charge carriers between the active layer and the electrode is equally important in order to perform the desired opto-electronic function. The electronic bands of the active materials and the electrode bend and modulate with one another, thereby fine-tuning the opto-electronic performance of the final device. Hence, it is essential to investigate the carrier exchange efficiency and band bending in the complete optoelectronic device.
Metal oxide films are generally applied as intermediate layers between graphene electrodes and the active material in optoelectronic devices like OLEDs, in order to improve carrier flow and band alignment, and for graphene doping. A number of reports on the use of metal oxide films have been published. However, a recent study reported in Scientific Reports is one of the few research works focusing on the microscopic features of the charge transfer mechanism.
In this study, titled “Metal Oxide Induced Charge Transfer Doping and Band Alignment of Graphene Electrodes for Efficient Organic Light Emitting Diodes (open access),” the research team explored CVD graphene layers that are directly deposited onto glass or oxide support.
The researchers from Graphenea, the University of Cambridge, and Philips Research demonstrated the fabrication of graphene-based OLED stacks that have efficiencies better than standard ITO devices.
In this work, a molybdenum trioxide (MoO3) intermediate layer is placed between a basic OLED stack and a graphene electrode. From the photoemission studies, the researchers revealed that this combination of materials leads to band bending.
As a result, the MoO3 conduction band is brought down towards the Fermi level of graphene, thus resulting in nearly ideal alignment of charge transport levels. This, in turn, enabled the researchers to achieve better power efficiency than a high-tech ITO reference device through the optimization of MoO3 film thickness.
The study results open the door for a new paradigm of band engineering of graphene-based opto-electronic devices. This, in turn, will pave the way for a widespread adoption of graphene electrodes, especially in organic opto-electronics. The study was carried out under GRAFOL, which is a European project aiming at roll-to-roll volume production of graphene.
This information has been sourced, reviewed and adapted from materials provided by Graphenea.
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