Fabrication of inverted inorganic-organic quantum-dot light-emitting diodes with solution-processed n-type oxide electron injection layers and QD-polymer blend light-emitting layers

We fabricated inorganic-organic hybrid quantum-dot light-emitting-diodes (QD-LEDs) consisting of several types of solution-processed n-type oxide electron injection layers (EILs)/quantum-dot (QD) and poly (9-vinylcarbazole) (PVK) blend light emitting layer (EMLs)/4,4-bis(carbazole-9yl)bihpheyl (CBP)/a-NPD/1,4,5,8,9, 11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) hole injection layer/Al structures. We compared the electrical properties of hybrid QD-LEDs with solution-processed n-type oxide electron injection layers consisting of Al-doped ZnO nano-particles (AZO-NP), polyethyleneimine (PEI), titanium oxide nanosheet (TiO-NS) on PEI (PEI/TiO-NS), and AZO-NP/TiO-NS multilayers. The combination of the PEI dipole layer and ultra-thin TiO-NS nanosheet (similar to 1 nm) layers reduced the potential barrier at ITO/TiO-NS interface. However, a considerable barrier height of >0.3 eV exists at the TiO-NS/QD interface. The use of small-work function AZO-NP (3.9 eV) effectively improves external quantum efficiency (EQE) compared with relatively large work-function AZO-NP (4.3 eV) and TiO-NS (4.1 eV). The capacitance-voltage curves and the current density-voltage-luminance curves strongly depend on the thickness of the QD:PVK blend (2:1 in weight) layer, and we obtained the optimized thickness for EML as ca. 30 nm. With the improved charge balance and morphology, an EQE of above 3.0% is obtained for green light-emitting QD-LED and an EQE of 0.86% for blue light-emitting QD-LED.