Energy-efficient lighting: less is more

Solution-processed transparent blue organic light-emitting diodes with CVD-grown graphene as the electrodes

According to a report by the International Energy Agency, lighting accounts for approximately 19% of total global power consumption. Pursuing energy-efficient lighting is the trend in the future. Organic light-emitting diodes (OLEDs) are a type of LED in which a thin film of organic compounds is used to emit light when electricity is applied by adjacent electrodes, enabling OLEDs to produce paper-thin bendable and stretchable displays and lighting. Moreover, they can be transparent and emit light from both sides of the device, greatly expanding the design possibilities and creating a completely new lighting experience. For example, transparent displays allow users to view what is shown on a glass video screen while still being able to see through it. This capability is ideal for a broad range of applications, including transportation, museums and corporate settings.

   Compared with other displays, OLEDs are more efficient because they are generated by organic material instead of a backlight that is always “on.” When they are “off,” they consume no power. Moreover, OLEDs can be configured as larger-area, more diffuse light sources, which may be more practical for general ambient lighting because the soft light can be viewed directly, with less need for shades, diffusers, or parabolic shells.

Research on OLEDs is not new; some innovative products have recently been released to become a part of everyday use. However, progress in the investigation of transparent OLEDs has been relatively slow due to the lack of suitable transparent conductive electrodes (TCEs). The most commonly used TCEs are metal oxide materials such as tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO) and aluminum zinc oxide (AZO). However, there are several shortcomings that limit their usage in future electronics. One major issue is the scarcity of materials, which leads to higher costs. In addition, they are brittle and thus unsuitable for applications in flexible and stretchable electronic devices. Some candidates for TCEs include carbon nanotubes (CNTs), nanostructured metals and graphene.

The research team led by Professor Chih-I Wu in the Department of Electrical Engineering at National Taiwan University has invented a novel polymer-free method that can routinely transfer large-area graphene to any substrate and preserve the optimal properties of as-grown samples compared with those of graphene transferred with conventional polymer-assisted methods.

This transfer process is employed to enable layer-by-layer transfers of multiple stacked graphene layers with n-type doping. These n-type doped graphene films are incorporated on solution-processed organic layers as a transparent cathode, and fully solution-processed blue-light transparent polymer OLEDs are thus achieved. The photographs show the device (a) before and (b) during operation with an active size of 2 × 3 mm2. The inset shows a photograph of a working blue-light polymer OLED in front of a mirror, showing emission from both surfaces. Figures (c) and (d) show the current density–voltage–luminance (J–V–L) curve of the transparent polymer OLEDs and the luminance efficiency versus current density characteristics. The inset figure in (d) shows electroluminescence spectra of the blue-light transparent polymer OLEDs from the graphene cathode side.

Although the efficiency and brightness of this transparent device are insufficient due to the poor contact between the organic layer and the graphene film cathode, an approach to realize fully solution-processed transparent OLEDs without vacuum deposition was developed and demonstrated. With this transfer method, graphene electrodes can be used in a wide variety of organic optoelectronics with more efficient doping and simple transfer techniques.

Jung-Hung Chang, Wei-Hsiang Lin, Po-Chuan Wang, Jieh-I Taur, Ting-An Ku, and Chih-I Wu* (2015). Solution-processed transparent blue organic light-emitting diodes with graphene as the top cathode. Scientific Reports 5, 9693. Published online 20 April 2015. DOI:10.1038/srep09693

Professor Chih-I Wu
Department of Electrical Engineering