Metal oxide nanoparticles as phosphors for white light emitting diodes
Résumé
Zinc oxide is a direct and wide band gap semiconducting material (3.37 eV at room temperature), which has usually been used for the optoelectronic applications in the UV spectral range [1, 2]. A good crystalline quality of ZnO is mandatory to obtain high luminescent efficiency, but the recombination on the inevitable intrinsic defects in ZnO crystal lattice is competing with the excitonic emission, lowering the efficiency of ZnO-based UV devices. Scientists have dedicated a considerable effort to reduce the presence of these undesirable defects, but at present they are attracting a growing interest, as they are at the origin of a broad range light emission from blue to IR. This is especially true in the case of the nanoparticles, where the presence of the surface defects strongly contributes to the white light emission. Complete understanding of the causes of this visible emission is still lacking, but several reviews have summed up the state-of-the art knowledge [3, 4]. Another major result is that the photoluminescent quantum efficiency (PL QE) is highly dependent on the quantum dot size: the smaller the size, the larger the PL QE. Thus, very high QY have only been reached for very small (~ some nm in diameter) ZnO nanoparticles (NPs). When incorporated into appropriate host polymer matrices, ZnO NPs can be applied as phosphors in white light emitting diodes (WLEDs). In addition, rare earth-free (RE) materials applied to WLEDs are attractive and necessary from the viewpoint of the uneven distribution of RE in the modern world, as it is China who covers 95 % of their global demand.
We demonstrate broadband white light emission from rare earth (RE)-free ZnO NPs and layers (pure or doped with metallic ions) and synthesized with easy to scale up chemical routes [5]. The RE-free ZnO-based NPs exhibit white light emission, maintaining a high value of PL QE. It should also be noted that that the peak wavelength of the broad light emission (and thus the emission color) can be tuned by or the amount or valence of the incorporated metallic ion. We demonstrate the use of these phosphors in a novel organic-inorganic WLED in combination with a solid state UV light-emitting source.
References:
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5.Y. Zhu, A. Apostoluk, P. Gautier, A. Valette, T. Cornier, J.-M. Bluet, K. Masenelli-Varlot, S. Danièle, B. Masenelli, Scientific Reports 6, pp. 23557 (1-11), 2016.