Electroluminescence From Colloidal ZnS QDs

With size-tunable luminescent properties, colloidal quantum dots (QDs) have been applied in display and lighting devices. CdSe QDs were adopted in display devices, thanks to good monochromaticity, but not in lighting devices. ZnS QDs shows broadband emissions from the transitions between the abundant of intrinsic defects. Due to variable spectra and high luminescent efficiency, they are expected to realize high-efficiency white light emission with varying color temperature. In this dissertation, we focus on the natue of photoluminescence (PL) and electroluminescences (EL) from the defects of ZnS QDs. The main experiments and discussions are as follows:1) The new synthesis progress named Diffusion of Solvents in Overweight State, and chemical precipitation were co-adopted to prepare ZnS QDs. The series of QDs were obtained by changing the preparation conditions of overweight state, Zn/S molar ratios and temperature. The nature and mechanism of PL from the QDs are discussed. QDs and QDs-polymer composite films were fabricated by spin-coating technology and characterized by SEM images and PL spectra.2) Series of ITO/ZnS QDs/Al single-layer and multilayer EL devices were fabricated. Behaviors of the EL devices were studied systematically and their working mechanisms were discussed with the help of PL analysis. EL emission peak were tuned from yellow-blue (560 nm) to blue (470 nm) with the increasing voltage, which determined by selective excitation processes (SEPs). And extended-defect-bands were proposed to explain the contrary shift between PL and EL spectrum for different Zn/S ratios, in which shallow and deep defect levels were selected to be excited by PL and EL. Particle size-dependent wideband EL of ZnS QDs suggested a possibility of adjustment of color temperature for lighting devices. Its origin may be not only related to quantum size effect but also the emission proportion from different defects. Further detailed discussion on regulating color temperature by means of QDs-polymer composite film showed that ZnS QDs/MEH-PPV and QDs/ PVK:NPB composite film device were beneficial to achieve good performance.3) Another composite film device was fabricated with ZnO QDs/SiO₂ composite which were synthesized in different concentration of SiO₂-ethanol solution. SiO₂ not only balanced the carriers of device but controlled the size and the defects of ZnO QDs which co-operated to shift the EL spectra.