Light-emitting diodes (LEDs) constructed using quantum dots as active elements offer a promising alternative to existing LED technologies because of their superior color tunability and narrow-band light emission. However, the efficiency of quantum dot LEDs (QD-LEDs) has previously been lacking. Researchers from Samsung Electronics in Korea1 have now demonstrated QD-LEDs with enhanced efficiency based on a crosslinked QD structure.

Several factors affect the light emission efficiency of QD-LEDs, including the match between the energy levels of the components, which affects the efficiency of electron and hole injection into the device, and the electrical resistance between the QDs, which when low ensures a homogenous current distribution in the active QD layer. “There is still room for improvement and it is important to increase their efficiency and reliability in order to meet the criteria for their commercialization in the display market,” says Byoung Lyong Choi from the research team.

The high-performance devices developed by the Samsung group are based on a mixture of organic and inorganic materials, chosen so as to obtain the best match between electron levels and thereby enhance charge-carrier injection. The QDs were then crosslinked by an organic molecule, which lowers the energy states and further improves hole injection into the QD layer, and finally annealed to improve the electrical conductance between the dots.

Fig. 1: Photograph of a 4-inch QD-LED display device with 320x240 pixel resolution.

The efficiency of red QD-LEDs was several times higher than that for QD-LEDs without cross-linking, and was sufficient to construct a prototype monochrome display (Fig. 1). Although this demonstration is remarkable, further advances are necessary before these QD-LEDs can be considered suitable for commercialization. “The present devices only operate at one wavelength, and it is very important to develop a full-color display,” says Choi. This could be achieved by patterning individual pixels having multiple QDs emitting light at different wavelengths.

The efficiency of the QD-LEDs must also be improved, since organic LEDs are still 10 times more efficient than the new devices developed by Choi's team. Nevertheless, Choi is confident that this gap can be closed soon. “It is only a matter of time before QD-LEDs are commercialized, for example in lighting applications and possibly solar cells as well.”