Highly Efficient Deep Blue Cd-Free Quantum Dot Light-Emitting Diodes by a p-Type Doped Emissive Layer

Environmentally friendly ZnSe/ZnS core/shell quantum dots (QDs) as an alternative blue emission material to Cd-based QDs have shown great potential for use in next-generation displays. However, it remains still challenging to realize a high-efficiency quantum dot light-emitting diode (QLED) based on ZnSe/ZnS QDs due to their insufficient electrical characteristics, such as excessively high electron mobility (compared to the hole mobility) and the deep-lying valence band. In this work, the effects of QDs doped with hole transport materials (hybrid QDs) on the electrical characteristics of a QLED are investigated. These hybrid QDs show a p-type doping effect, which leads to a change in the density of the carriers. Specifically, the hybrid QDs can balance electrons and holes by suppressing the overflow of electrons and improving injection of holes, respectively. These electrical characteristics help to improve device performance. In detail, an external quantum efficiency (EQE) of 6.88% is achieved with the hybrid QDs. This is increased by 180% compared to a device with pure ZnSe/ZnS QDs (EQE of 2.46%). This record is the highest among deep-blue Cd-free QLED devices. These findings provide the importance of p-type doping effect in QD layers and guidance for the study of the electrical properties of QDs.