Optoelectronic materials: Optimum solution

Organic light-emitting diodes (OLEDs) are seen as a promising replacement for the liquid-crystal displays (LCDs) used in many flat-screen televisions because they are cheaper to mass-produce. Zhikuan Chen at the A*STAR Institute of Materials Research and Engineering and co-workers have now shown how meticulous engineering of fluorescent molecules can dramatically increase OLED efficiency.

Conventional light-emitting diodes are made of inorganic crystals such as gallium arsenide. OLEDS, on the other hand, utilize carbon-based materials that can be made flexible. Today, OLED technology is commonly used to make large-area outdoor displays and wearable displays. However, further improvements in operation efficiency are required if OLEDs are to become truly competitive against the alternative options.

Chen and his co-workers have now reported blue light-emitting devices that reach an external quantum efficiency (EQE) of as high as 9.4%—almost double the classical upper limit of 5% for fluorescence-based OLEDs. “This improvement is important because higher efficiency means a lower driving voltage and thus lower power consumption and increased device lifetime,” explains Chen.

EQE is an important measure of LED operation as it determines what fraction of the charge carriers injected into the device are converted into photons that can be emitted. EQE takes into account the chance that the two types of charge carriers—negatively charged electrons and positively charged holes—recombine with each other, as well as the intrinsic probability that this results in the creation of a photon and the chance that this photon will escape from the device. Chen and his team have now used computer models to optimize these various processes.

Simulations enabled the researchers to find a structure for their active molecule—an oligofluorene—that best balanced charge carrier injection into the material and charge transport through it to enhance device emission efficiency. Further improvements were made by selecting the best emitting-layer thickness and by doping the emitter in an appropriate organic host material to minimize efficiency loss.

The OLEDs emitted blue light centered at a wavelength of 450 nanometers. Chen and his co-workers found that the high EQE was possible because the fraction of charge carriers that recombine without emitting light was negligible. Importantly, the light output was stable during operation, making it more amenable to use in practical situations. “Soon we hope to develop these materials further for lighting and displays applications,” says Chen.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering

References
Zhen, C. G. et al. Achieving highly efficient fluorescent blue organic light-emitting diodes through optimizing molecular structures and device configuration. Advanced Functional Materials 21, 699–707 (2011).