Sep 2, 2009
Quantum-dot coating whitens LEDs
White light-emitting diodes (WLEDs) featuring a hybrid poly N-vinylcarbazole (PVK) and poly(methyl methacrylate) (PMMA) polymer and colloidal ZnO quantum dots (QDs) have been fabricated by researchers in Korea using a spin-coating technique.
Transmission electron microscopy images show that the ZnO QDs are dominantly distributed at the circumference of the PVK polymer surface. Electroluminescence spectra for the hybrid polymer-QD WLEDs reveal a broad peak around 600 nm. The Commission Internationale de I'Eclairage chromaticity co-ordinates of the WLEDs fabricated utilizing hybrid polymer-QD WLEDs at 10 V are (0.39, 0.41), which is indicative of a white emission.
The electrons and holes are accumulated at the ZnO QDs/PVK heterointerfaces due to the energy-band bending of ZnO QDs, resulting in the formation of the sub-bands between the ZnO QDs/PVK. Because the lifetime of the electrons existing in the ZnO/PVK heterointerface is much larger than that in the ZnO QDs, the light of the LED is emitted in the whole visible range due to continuous recombination between the electrons and the holes during the drop of the electrons from the conduction band to the valence band of the ZnO QDs. This results in the emission of white light because of the combination of the blue, green and red colours that are given out.
The luminescence mechanisms of the WLEDs are described on the basis of the experimental results and the energy band diagram. The device should benefit applications involving next-generation light sources.
The researchers presented their work in Nanotechnology.
About the author
Dong-Ick Son is a PhD graduate student in the Department of Information Display Engineering at Hanyang University. Chan-Ho You is a MS graduate student in the Division of Electronics and Computer Engineering at Hanyang University. Won-Tae Kim is a MS graduate student in the Division of Electronics and Computer Engineering at Hanyang University. Tae-Whan Kim is a professor in the Department of Electronics and Communications Engineering at Hanyang University.