Chih-Jung Chen

Cesium Azide—An Efficient Material for Green Light-Emitting Diodes With Giant Quantum Dots

A novel efficient and air-stable electron injection layer (EIL) of cesium azide (CsN3) was compared with conventional ones including CsF, Cs2CO3, LiF and without EIL in type-II quantum dot light-emitting diodes (QLEDs) with both organic electron and hole transport layers. Via directly decomposing to pristine cesium (Cs), the low-temperature evaporated CsN3 provided a better interfacial energy level alignment without damaging the underneath organic layer. Consequently, the current efficiencies of 7.45 cd/A was achieved in the CsN3-based green QLEDs consisting of giant CdSe@ZnS/ZnS quantum dots at 544 nm, which was 310% (at 10 mA/cm2) improvement over the LiF-based QLEDs. Moreover, the light turn-on voltage in CsN3-devices significantly decreased ~5.5 V in comparison with LiF-devices.A novel efficient and air-stable electron injection layer (EIL) of cesium azide (CsN3) was compared with conventional ones including CsF, Cs2CO3, LiF and without EIL in type-II quantum dot light-emitting diodes (QLEDs) with both organic electron and hole transport layers. Via directly decomposing to pristine cesium (Cs), the low-temperature evaporated CsN3 provided a better interfacial energy level alignment without damaging the underneath organic layer. Consequently, the current efficiencies of 7.45 cd/A was achieved in the CsN3-based green QLEDs consisting of giant CdSe@ZnS/ZnS quantum dots at 544 nm, which was 310% (at 10 mA/cm2) improvement over the LiF-based QLEDs. Moreover, the light turn-on voltage in CsN3-devices significantly decreased ~5.5 V in comparison with LiF-devices.

Enhanced thermal stability of green-emission quantum-dot light-emitting diodes via composition-gradient thick-shell quantum dots

We investigated the thermal properties of quantum-dot light-emitting diodes (QLEDs) using composition-gradient thick-shell CdSe@ZnS/ZnS QDs. Thick-shell QDs with low defective structures effectively prevented electron–hole pairs from nonradiative Auger recombination. More specifically, defects were prevented from thermal-stress-induced expansion at elevated temperatures and high driving currents. Consequently, 97% of EL remained after the device was thermally stressed at temperatures higher than 110 °C, indicating that the nanostructure design of QDs is an important factor for high-performance QLEDs.

High color-rendering warm-white lamps using quantum-dot color conversion films

Colloidal quantum dots are promising next-generation phosphors to enhance the color rendition of light-emitting diodes (LEDs) while minimizing the brightness droop. In order to exploit the beneficial tunability of quantum dots for highly efficient devices, optimization and determination of the performance limit are of crucial importance. In this work, a facile preparation process of red-emission quantum dot films and simulation algorithm for fitting this film with two commercial LED flat lamps to the optimized performance are developed. Based on the algorithm, one lamp improves from cold-white light (8669 K) with poor color rendition (Ra = 72) and luminous efficacy (85 lm/W) to warm-white light (2867 K) with Ra = 90.8 and R9 = 74.9, and the other reaches Ra = 93 ∼ 95. Impressively, the brightness droop is only about 15 ∼ 20% and the luminous efficacy of 68 lm/W is achieved. Furthermore, our device shows reliability over 1000 hours with only PET (polyethylene-terephthalate) films as the barrier, indicating that this auxiliary red-emission film can be easily applied to improve the color rendition of most commercial LED flat lamps.

Reducing roll-off effect of efficient green quantum-dot light-emitting diodes via composition-gradient thick-shell quantum dots

In this study, we investigated the stable and efficient quantum-dot light emitting diodes (QLEDs) under high driving current by using composition-gradient thick-shell CdSe@ZnS/ZnS QDs. Thick-shell QDs with low defective structure can effectively avoid the electron-hole pairs from nonradiative Auger recombination and prevent thermal-stress-induced expansion at high driving current. Consequently, roll-off effect in electroluminescent feature can be reduced almost two times. Moreover, the maximum current efficiency of thick-shell-device is 10.3 cd/A, which much higher than 1.57 cd/A in thin-shell-device.