Abstract
We demonstrate the effect of air exposure on optical and electrical properties of ZnMgO nanoparticles (NPs) typically exploited as an electron transport layer in Cd-based quantum-dot light-emitting diodes (QLEDs). We analyze the roles of air components in modifying the electrical properties of ZnMgO NPs, which reveals that H2O enables the reduction of hole leakage while O2 alters the character of charge transport due to its ability to trap electrons. As a result, the charge balance in the QDs layer is improved, which is confirmed by voltage-dependent measurements of photoluminescence quantum yield. The maximum external quantum efficiency is improved over 2-fold and reaches the value of 9.5% at a luminance of 104 cd/m2. In addition, we investigate the problem of electron leakage into the hole transport layer and show that trap-mediated electron transport in the ZnMgO layer caused by adsorbed O2 ensures a higher leakage threshold. This work also provides an insight into the possible disadvantages of device contact with air as well as problems and challenges that might occur during open-air fabrication of QLEDs.
Highlights
It has been proposed that contamination with water, which is created by chemical reaction between the resin and ZnMgO electron transport layer (ETL), is a key factor influencing device lifetime.[9−11] Some authors have pointed to spontaneous interfacial reaction of ZnMgO with Al as a major cause of the aging process,[12] while others proposed vacancy reduction in ZnMgO as a possible reason.[13]
We show that the second approach can be done by modifying electrical transport in ZnMgO NPs by air exposition
We incorporated CdSe@ ZnS/ZnS QDs emitting at 515 nm (FWHM ∼ 21 nm, photoluminescence quantum yield (PLQY) ∼ 55%) with a thickness of a single monolayer into quantum-dot light-emitting diodes (QLEDs), which assures optimal device performance in terms of brightness and turn-on voltage (Figure S1a,b)
Summary
Colloidal quantum-dot light-emitting diodes (QLEDs) have attracted considerable attention in the optoelectronic industry due to their remarkable performance, which makes them promising candidates for displays and lightning technology with a particular emphasis on color purity, brightness, and emission tunability.[1,2] state-of-the-art devices have reached excellent performance in terms of both efficiency and stability,[3,4] some papers reported abnormal behavior during device operation and storage including positive aging,[5] instabilities of current and luminance during device testing,[6] and hole transport layer (HTL) degradation.[7]. A lot of attention has been paid to encapsulation as a potential source of unwanted chemical moieties such as water or organic acids.[8] For instance, it has been proposed that contamination with water, which is created by chemical reaction between the resin and ZnMgO electron transport layer (ETL), is a key factor influencing device lifetime.[9−11] Some authors have pointed to spontaneous interfacial reaction of ZnMgO with Al as a major cause of the aging process,[12] while others proposed vacancy reduction in ZnMgO as a possible reason.[13] Recently, a more sophisticated phenomenon such as resistive switching, i.e., electric-field-induced oxygen vacancies migration, which is an intrinsic property of ZnMgO, has been investigated as potentially detrimental to operational stability.[14]. From the point of view of scalable QLED manufacturing, open-air fabrication of QLED is highly desired. For this reason, environmental testing is an important part of research on optoelectronic devices. We perform stability tests and discuss the limitations of open-air fabrication of QLEDs
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