High-quality Blue-violet Core/shell Quantum Dots And Their Application In Light Emitting Diodes

Quantum dots(QDs) have very unique and attractive characteristics, such as high photoluminescence(PL) quantum yield, size tunability and good chemical stability and have been widely applied in biological labels, photovoltaic cells, light-emitting diodes(LEDs) and other fields. With the development of synthesis technology for QDs, we have been able to produce relatively high quality of quantum dots, and the properties of QD-LEDs by the use of high quality quantum dots have been greatly improved. But the PL quantum yield of violet-blue emitting QDs is still lower, and the performance of QD-LEDs compared with red and green leds for poor performance. Therefore, it is particularly important to prepare high-quality violet-blue emitting QDs and improve the performance of violet-blue emitting QD-LEDs. In this dissertation, we successfully prepared high-quality violet-blue emitting ZnxCd1-xS/ZnS core/shell quantum dots and Cd-free ZnSe/ZnS core/shell quantum dots and also prepared better performance of violet-blue emitting QD-LEDs by employing these QDs. The main research results of this dissertation are following:(1) In the organic phosphine free system, high-quality violet-blue emitting ZnxCd1-xS/ZnS core/shell quantum dots(QDs) are synthesized by a new method, called “nucleation at low temperature/shell growth at high temperature”. The resulting nearly monodisperse core/shell QDs have high PL quantum yield(near to 100%), high color purity(FWHM) <25 nm), good color tunability in the violet-blue optical window from 400 to 470 nm, and good chemical/photochemical stability. Highly efficient deep-blue quantum dot-based light-emitting diodes(QD-LEDs) are demonstrated by employing the ZnxCd1-xS/ZnS core/shell QDs as emitters. The bright and efficient QD-LEDs show a maximum luminance up to 4100 cd/m2 and peak external quantum efficiency(EQE) of 3.8%, corresponding to 1.13 cd A-1 in luminous efficiency. Such high value of the peak EQE can be comparable with OLED technology. These results signify a remarkable progress, not only in the synthesis of high-quality QDs but also in QD-LEDs that offer a practicle platform for the realization of QD-based violet-blue display and lighting.(2) we successfully prepared high-quality violet-blue emitting Cd-free ZnSe/ZnS core/shell by a novel “low temperature injection and high temperature growth” method In the organic phosphine free system. The resulting nearly monodisperse ZnSe/ZnS core/shell QDs exhibit excellent characteristics, such as a high color saturation(typical spectral full width at half-maximum between 12 and 20 nm), good emission tunability in the violet-blue range of wavelengths from 400 to 455 nm, a high absolute PL quantum yield(up to 83%), and superior chemical and photochemical stability. By employing ZnSe/ZnS core/shell quantum dots(QDs) as emitters with a fully solution processable method, bright, efficient, and color-stable violet Cd-free quantum dot-based light-emitting diodes(QD-LEDs) with maximum luminance up to 2632 cd/m2 and a peak EQE of 7.83% have been demonstrated successfully. Considering the factors of the photopic luminosity function, the brightness and efficiency results of such violet QD-LEDs not only represent a 12-fold increase in device efficiency and an extraordinary 100 times increase in luminance compared with previous Cd-free QD-LEDs but also can be much superior to the best performance(1.7%) of their Cd-based violet counter-parts. These results demonstrate significant progress in short-wavelength QD-LEDs and shed light on the acceleration of commercial application of environmentally-friendly violet QD-based displays and lighting.