Quantum dot white light emitting diodes with high scotopic/photopic ratios

Alloy core/shell Cd_xZn_1-xS/ZnS quantum dots (QDs) are emerging as robust candidates for light-emitting diodes (LEDs), however the emission range of the current Cd_xZn_1-xS/ZnS is quite limited, ranging from 390 to 470 nm. It still remains a challenging task to construct white LEDs based on current Cd_xZn_1-xS/ZnS system. Here, a versatile ZnSe with a moderate band gap is introduced onto the Cd_0.1Zn_0.9S core. The ZnSe shell, on one hand, can passivate the core surface which leads to bright emissions. On the other hand, it is essential in extending the emission to red region so that the emission wavelengths of Cd_0.1Zn_0.9S/ZnS and Cd_0.1Zn_0.9S/ZnSe QDs can cover the whole visible region, which is very important for white LED applications. Two- and four-hump QD-based LEDs are computationally and experimentally investigated. Results show that four-hump quantum dot light-emitting diodes (QLED) have better performances than the two-hump one, in the luminous and the vision properties. The fabricated white LEDs (WLEDs) based on Cd_0.1Zn_0.9S/ZnS and Cd_0.1Zn_0.9S/ZnSe QDs exhibits a scotopic/photopic ratio (S/P) ratio as high as 2.52, which exceeds the current limit of 2.50 by common lighting technologies, a color rendering index of 90.3, a luminous efficacy of optical radiation of 460.78 lumen per unit optical power, and a correlated color temperature of 5454 K. These results suggest that Cd_xZn_1-xS/ZnS and Cd_xZn_1-xS/ZnSe quantum dots serving as emitters hold great promise for the next-generation white light source with better S/P ratio.