Improving Charge Injection via a Blade-Coating Molybdenum Oxide Layer: Toward High-Performance Large-Area Quantum-Dot Light-Emitting Diodes.

A solution-processed molybdenum oxide (MoO x) as the hole injection layer (HIL) by doctor-blade coating was developed to improve the efficiency and lifetime of red-emitting quantum-dot light-emitting diodes (QD-LEDs). It has been demonstrated that by adding isopropyl alcohol into the MoO x precursor during the doctor-blade coating process, the morphology, composition, and the surface electronic structure of the MoO x HIL could be tailored. A high-quality MoO x film with optimized charge injection was obtained, based on which all-solution-processed highly efficient red-emitting QD-LEDs were realized by using a low-cost doctor-blade coating technique under ambient conditions. The red QD-LEDs exhibited the maximum current efficiency and external quantum efficiency of 16 cd/A and 15.1%, respectively. Moreover, the lifetime of red devices initializing at 100 cd/m2 was 3236 h under ambient conditions, which is about twice as long as those with a conventional poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) HIL. Large-area QD-LEDs with 4 in. emitting areas were fabricated with blade coating as well, which exhibit a high efficiency of 12.1 cd/A for red emissions. Our work paves a new way to the realization of efficient large-area QD-LEDs, and the processing and findings from this work can be expanded into next-generation lighting and flat-panel displays.

Efficient All-Solution Processed Quantum Dot Light Emitting Diodes Based on Inkjet Printing Technique.

Quantum dot light emitting diodes (QLEDs) are increasingly attractive owing to their compatibility with the inkjet printing process and potential application in low-cost large-area full-color pixelated display. The strategy for controlling the morphology of the quantum dot layer is definitely critical for realizing all-solution processed QLEDs with high performance, which certainly requires in-depth thinking regarding the design of ink composition and their optimization in the printing process. Herein, by carefully controlling the quantum dot ink composition and physicochemical properties, we demonstrate that the viscosity, contact angle, and the three-phase contact line moving would affect the final morphology of the quantum dot film formed by inkjet printing. We achieved coffee ring-free and low-roughness quantum dot film, and all-solution processed QLEDs with normal structure were fabricated for the first time. The devices have a low turn-on voltage of 2.0 V, a luminance of 12100 cd/m2 at the voltage of 12 V, and a maximum current efficiency of 4.44 cd/A at the luminance of 1974 cd/m2, which is the best result to date for inkjet-printed red QLEDs. The results will pave the way for future application of inkjet printing in solution processed pixelated QLED display.