Electron transport phenomena at the interface of Al electrode and heavily doped degenerate ZnO nanoparticles in quantum dot light emitting diode

Enhanced Luminance of CdSe/ZnS Quantum Dots Light-Emitting Diodes Using ZnO-Oleic Acid/ZnO Quantum Dots Double Electron Transport Layer

The widely used ZnO quantum dots (QDs) as an electron transport layer (ETL) in quantum dot light-emitting diodes (QLEDs) have one drawback. That the balancing of electrons and holes has not been effectively exploited due to the low hole blocking potential difference between the valence band (VB) (6.38 eV) of ZnO ETL and (6.3 eV) of CdSe/ZnS QDs. In this study, ZnO QDs chemically reacted with capping ligands of oleic acid (OA) to decrease the work function of 3.15 eV for ZnO QDs to 2.72~3.08 eV for the ZnO-OA QDs due to the charge transfer from ZnO to OA ligands and improve the efficiency for hole blocking as the VB was increased up to 7.22~7.23 eV. Compared to the QLEDs with a single ZnO QDs ETL, the ZnO-OA/ZnO QDs double ETLs optimize the energy level alignment between ZnO QDs and CdSe/ZnS QDs but also make the surface roughness of ZnO QDs smoother. The optimized glass/ITO/PEDOT:PSS/PVK//CdSe/ZnS//ZnO-OA/ZnO/Ag QLEDs enhances the maximum luminance by 5~9% and current efficiency by 16~35% over the QLEDs with a single ZnO QDs ETL, which can be explained in terms of trap-charge limited current (TCLC) and the Fowler-Nordheim (F-N) tunneling conduction mechanism.

Optimization of the electron transport in quantum dot light-emitting diodes by codoping ZnO with gallium (Ga) and magnesium (Mg)

In our study, to optimize the electron–hole balance through controlling the electron transport layer (ETL) in the QD-LEDs, four materials (ZnO, ZnGaO, ZnMgO, and ZnGaMgO NPs) were synthesized and applied to the QD-LEDs as ETLs. By doping ZnO NPs with Ga, the electrons easily inject due to the increased Fermi level of ZnO NPs, and as Mg is further doped, the valence band maximum (VBM) of ZnO NPs deepens and blocks the holes more efficiently. Also, at the interface of QD/ETLs, Mg reduces non-radiative recombination by reducing oxygen vacancy defects on the surface of ZnO NPs. As a result, the maximum luminance (L_max) and maximum luminance efficiency (LE_max) of QD-LEDs based on ZnGaMgO NPs reached 43 440 cd m⁻² and 15.4 cd A⁻¹. These results increased by 34%, 10% and 27% for the L_max and 450%, 88%, and 208% for the LE_max when compared with ZnO, ZnGaO, and ZnMgO NPs as ETLs.

Optimized QD-LEDs are fabricated using Ga–Mg-codoped ZnO NPs as ETL, which reached the LE_max and PE_max at 15.4 cd A⁻¹ and 10.3 lm W⁻¹.