Ultra high luminous efficacy of ZnxCd1-xS white quantum dots-based white light-emitting diodes

White light quantum dots (WQDs) such as CdSe and ZnxCd1-xS are considered source candidates in solid-state lighting (SSL) due to their unique optical properties. However, the low fluorescence quantum yield (QY, < 60 %), low luminous efficacy (similar to 11.9 lm/W), and poor stability (similar to 100 hr) limits their commercial application. In order to solve the above problems, a high QY of ZnxCd1-xS WQDs (x=0.2, 0.5 and 0.8, named as Zn-0.2, Zn-0.5 and Zn-0.8, respectively) was prepared by using unsaturated fatty acid-oleic acid (OA) and non-coordinating solvent-1-octadecene (ODE) at low temperature (180 degrees C). Moreover, the Zn-0.5 WQDs was coated with silica to improve stability, and named as Zn-0.5@SiO2. The results show that the QYs of Zn-0.2, Zn-0.5 and Zn-0.8 WQDs are 82, 86 and 51 %, and the TEM images show that the morphologies of those WQDs are spherical, and the particle size are 2.7, 2.6 and 2.5 nm, respectively. The particle size of Zn-0.5 WQDs in Zn-0.5@SiO2 is 2.6 nm, and the emission wavelength is red-shifted from 391/501 to 418/533 nm due to agglomeration. The CIE coordinates, color rendering index (CRI), correlated color temperature (CCT) and luminous efficacy of Zn-0.5-based white light-emitting diode (WLED) are (0.36, 0.43), 73, 4784 K and 30.6 lm/W, respectively. The CIE coordinates, CRI, CCT and luminous efficacy of Zn-0.5@SiO2-based WLED are (0.42, 0.46), 72, 3732 K and 19.4 lm/W, respectively. After long-term operation for 261 h, the luminous efficacy of Zn-0.5- and Zn-0.5@SiO2-based WLED decreased by 82 and 14 %, respectively. The above results confirm that ZnxCd1-xS WQDs with high QY can be successfully prepared at low temperature by using unsaturated fatty and non-coordinating solvent, and coating the WQDs with silica can increase WLED stability. This result can make WQD a big step towards the application of solid state lighting.