Layered LEDs show promise

Light-emitting diodes (LEDs) are commonplace, most prominently in lighting and in large-area display units such as television screens. They function by emitting light when an external voltage is applied. Improving their performance and longevity, as well as enabling easy and cheap fabrication, are the main aims of research into these devices. 

Rashid Altamimi and Ahmed Alyamani at KACST — in collaboration with scientists at the University of Cambridge, UK, and the University of Waterloo, Canada — have made high-performance LEDs by adding molecules, called dopants, to a polymer, which fluoresce when irradiated with light.  Devices made by doping polymers are called organic light-emitting diodes (OLEDs). These devices are more efficient, functioning better with a reduced energy input, than similar devices based on conventional electroluminescence — requiring voltage to emit light.

The team tested a variety of molecular dopants, each altering the colour of the light emitted by the OLED to deep blue, green, yellow or red.

They designed their device so that the intensity of its electroluminescence was dependent on magnetic field strength, supporting a mechanism, called ‘triplet fusion’, which is key to their high efficiency.

To improve movement of charges in the OLEDs compared to standard devices, the researchers fabricated multilayer LEDs, incorporating the dopant rubrene, the emitter molecule, in a matrix of a conjugated polymer and a zinc oxide layer. The efficiency of the multilayer LED was 6.3%, an improvement on other reported rubrene-based LEDs.

For real applications, particularly for lighting, LEDs made of solids typically show lower efficiencies than devices made of solutions.  However, some of the solid devices formed in this study achieved efficiencies comparable to solution-based counterparts.

The researchers envisage that the knowledge gained in the design of these triplet fusion OLEDs will help in the fabrication of highly efficient solid-state triplet fusion devices for photovoltaic applications such as roof-mounted solar cells.

1. Di, D. et al. Efficient triplet exciton fusion in molecularly doped polymer light-emitting diodes. Adv. Mater.29, 1605987 (2017). | article