2.3. Device Fabrication

The QLEDs were fabricated by spin coating on patterned ITO glass substrates (sheet resistance ∼18 Ω/sq). The substrates were cleaned in ultrasonic baths of detergent, deionized water, chromatographic grade acetone and 2-propanol for 15 min each, and the cleaned ITO was exposed to a UV-ozone treatment for 15 min. Then, the poly (ethylenedioxythiophene) and polystyrene sulphonate (PEDOT: PSS, filtered with a 0.45 μm PVDF filter) were spin-coated on the substrates at 40 nm thickness at a spin rate of 4000 rpm for 45 s and baked at 140 °C for 15 min in air. Next, they were then transferred into an N₂-filled glove box for further spin coatings of poly [9], 9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine] (TFB), quantum dots and ZnO NPs layers. Afterwards, TFB, the quantum dot solution and the ZnO NPs, filtered with a 0.22 μm PVDF filter before use, were then spin-coated onto the TFB (8 mg/mL, in chlorobenzene) layer at 2000 rpm for 45 s, followed by thermal annealing at 150 °C for 30 min. The quantum dots were spin-coated on the ITO/PEDOT: PSS/TFB layer, and the optimized emission layer thicknesses were ∼30 nm (15 mg/mL, 2500 rpm for 45 s); the ZnO NPs layer was spin-coated on the ITO/PEDOT: PSS/TFB/quantum dots layer and then baked at 60 °C for 30 min. Finally, the multilayered device samples were loaded into a high-vacuum chamber (∼1 × 10⁻⁷ torr) for deposition of an Al cathode (100 nm). To protect the devices from water and oxygen, all the devices were encapsulated with UV-cured epoxy and covered with thin glass slices.