Enhanced Lifetime and Efficiency of Red Quantum Dot Light-Emitting Diodes with Y-Doped ZnO Sol-Gel Electron-Transport Layers by Reducing Excess Electron Injection

Charge balance between electrons and holes, which are injected into the quantum dot (QD) emission layer (EML), is critical for realizing stable and efficient QD light-emitting diodes (QLEDs). ZnO has been widely used as an electron-transport layer (ETL) because of its superior performance compared to other metal oxides. However, nearly barrier-free electron injection into the QD EML leads to spontaneous charge transfer and excess electron injection, resulting in reduced device performance. Here, to adjust electron-hole balance and thereby improve the lifetime and efficiency of QLEDs, we introduce an yttrium (Y)-doped ZnO (YZO) ETL into QLEDs. The sol-gel processed YZO film, with a mobility that could be simply tuned by varying the Y concentration, provides enhanced charge-transport balance by suppressing excess electron flow to the QD EML. Furthermore, YZO helps suppress QD charging and smooth the surface morphology. Applying the YZO ETL into the inverted-structure QLED enables us to achieve color-saturated red emission, an improved external quantum efficiency of up to 8.6%, and an eight times longer lifetime compared to the device with undoped ZnO. This result provides a simple method for enhancing the performance of QLEDs by easily controlling metal doping concentration in the sol-gel processed ZnO ETL.