Degradation Dynamics of Quantum Dots in White LED Applications

Abstract

Quantum dots (QDs) are being investigated in a hybrid white light LED structure which inculcates phosphor in the package with blue LED chip as light source. Upon prolong operation, degradation of CdS QDs is observed which can limit its practical applications. The degradation includes intensity reduction as well as emitted wavelength shift of the white light. Three stages of degradation are observed, namely an enhancement state where light intensity is observed to increase, followed by a rapid degradation stage where light intensity decreases rapidly, and finally a slower degradation state where light intensity degradation rate slows down and continue till end of test.In this work, the degradation of CdS QDs is analyzed using various material analysis tools. Density functional theory (DFT) calculation is performed to confirm the spectroscopy results. It is found that the time evolving degradation of QDs begins from the oxidation of sulfur vacancy of CdS QDs by nearby oxygen atoms due to the imperfection of the protective coating around the QDs in combination of the presence of blue light, and this oxidation renders a transformation of CdS into CdO at the initial state. The final stage is the formation of CdSO4 via some intermediate processes.