Influence of Encapsulation on the Efficiency and Positive Aging Behavior in Blue Quantum Dot Light-Emitting Devices

Encapsulatingblue quantum dot light-emitting devices (QLEDs) usingan ultraviolet curable resin is known to lead to a significant increasein their efficiency. Some of this efficiency increase occurs immediately,whereas some of it proceeds over a period of time, typically overseveral tens of hours following the encapsulation, a behavior commonlyreferred to as positive aging. The root causes of this positive aging,especially in blue QLEDs, remain not well understood. Here, it isrevealed that contrary to the expectation, the significant improvementin device efficiency during positive aging arises primarily from animprovement in electron injection across the QD/ZnMgO interface andnot due to the inhibition of interface exciton quenching as is widelybelieved. The underlying changes are investigated by XPS measurements.Results show that the enhancement in device performance arises primarilyfrom the reduction in O-related defects in both the QDs and ZnMgOat the QD/ZnMgO interface. After 51.5 h, the blue QLEDs reach theoptimal performance, exhibiting an EQE(max) of 12.58%, whichis more than sevenfold higher than that in the control device withoutencapsulation. This work provides design principles for realizinghigh efficiency in blue QLEDs with oxide electron-transporting layers(ETLs) and provides a new understanding of the mechanisms underlyingpositive aging in these devices and thus offers a new starting pointfor both fundamental investigations and practical applications.