Graphene has been hailed as a potential replacement for indium tin oxide (ITO) in the growing market for transparent conductors, in particular because it might enable the highly sought after flexible portable devices. Technological immaturity has so far hindered the widespread use of graphene as a transparent electrode, however researchers from Philips Research, the University of Cambridge, and Graphenea have engineered organic light emitting diodes (OLEDs) based on graphene which outperform state-of-the-art ITO devices.
Photo: Graphene-based OLED
The high electronic mobility and optical transparency of graphene make the material an excellent candidate for opto-electronic devices, such as solar cells, light emitting diodes (LEDs), and touch screens. Currently ITO is the most widely used material for such applications. However, the rising price and limited geographical availability of Indium, combined with the market trend in the direction of flexible devices, asks for alternative transparent conductors (TCs) for the next generation of devices.
Although graphene offers a remarkably high mobility of charge carriers (for example electrons), the concentration of such carriers in graphene is generally low, leading to an overall unimpressive performance as an electrode. The process of doping the graphene layer with excess carriers is thus a necessary step in the production of graphene electrodes. Furthermore, in the case that the desired application is a transparent electrode, the doping process must preserve the high optical transparency of graphene.
At least as important as the carrier concentration is the efficient exchange of charge carriers between the electrode and the active layer which actually performs the chosen opto-electronic function. The electronic bands of the electrode and active materials bend and adjust to each other, in turn reshaping the opto-electronic performance of the final device. It is thus important to study the band bending as well as the carrier exchange efficiency in actual complete opto-electronic devices.
Typically, metal oxide films are used as intermediate layers between graphene electrodes and an active material to improve the band alignment and increase carrier flow, but also to dope the graphene. Although there have been many reports of the use of such films, recent work published in Scientific Reports is one of the few studies looking into the microscopic details of the charge transfer mechanism. The researchers focused on CVD graphene layers directly transferred onto glass or oxide support. Using a combination of experimental tools (photoemission spectroscopy, sheet resistance measurements, and the characterization of finalized device performance), a network of collaborators from Graphenea in San Sebastian (Spain), Philips Research in Aachen (Germany) and the University of Cambridge found that they can make graphene-based organic LED (OLED) stacks possessing efficiencies exceeding those of standard ITO devices.
The paper, entitled Metal Oxide Induced Charge Transfer Doping and Band Alignment of Graphene Electrodes for Efficient Organic Light Emitting Diodes (open access), shows a study of the commonly used molybdenum trioxide (MoO3) intermediate layer between a graphene electrode and a basic OLED stack. The team's photoemission studies reveal that this combination of materials results in band bending which brings the MoO3 conduction band down towards the Fermi level of graphene, leading to nearly ideal alignment of charge transport levels. As a result, with optimization of MoO3 film thickness, the researchers were able to achieve superior power efficiency compared to a state-of-the-art ITO reference device. It is expected that this result will usher in a new paradigm of band engineering of graphene-based opto-electronic devices, leading to a wider spread of graphene electrodes, particularly in organic opto-electronics.
The work was performed as part of the European project GRAFOL, aimed at roll-to-roll mass production of graphene.