The balanced injection of holes and the electrodes into an emissive quantum-dot layer for evasion of radiative recombination is critical to the color purity, stability, and efficience of the quantum-dot-based light-emitting diodes. We proposed a partially-oxidized aluminum cathode to enhance the electrode injection under exciton confinement for high-performance blue quantum-dot light-emitting diodes. The main advantage of this technique is that no additional electron-transport layer is emoted in the device, which guarantees color purity and simplifies the fabrication processes.
Demonstrating the procedures will be done by Wenda Sun, a graduate student in my lab. To begin the procedure, cut a 12-centimeter-by-12-centimeter piece of ITO-coated glass with a resistivity of about 10 ohms per square into 15-millimeter-wide strips. Clean the ITO glass strips with a dust-free cloth and pure ethanol.
Allow the ITO glass to air dry. Check the conductive side of the ITO glass for defects with a digital multimeter. Use standard adhesive tape to cover a two-millimeter-wide section lengthwise along the center of the ITO glass strip.
Then, place the ITO glass in a shallow acid-resistant container, such as a petri dish. Pour zinc powder over the ITO glass to a thickness of about 0.5 millimeters. Pour 36%by weight hydrochloric acid over the glass until the glass and the zinc powder are completely submerged.
Allow etching to proceed for 15 seconds. Then, pour off the hydrochloric-acid solution. Immediately rinse the etched ITO glass thoroughly with tap water.
Dry the glass with a dust-free cloth, and remove the adhesive tape. Soak the etched ITO glass in acetone for 15 minutes. Use a cotton ball to remove any adhesive residue from the ITO glass.
Then, cut the ITO glass into 15-millimeter-by-15-millimeter pieces with a glass cutter. Sonicate the ITO-glass pieces in dilute detergent solution, tap water, deionized water, acetone, and isopropyl alcohol, in sequence, for 15 minutes each. Afterwards, dry the ITO-glass pieces under a stream of nitrogen gas.
Dry the pieces further in an oven at 150 degrees Celsius for five minutes in air. Store the dry pieces in air when finished. Prior to the fabrication, in a nitrogen-filled moisture-free glovebox add one milliliter of orthodichlorobenzene to 25 milligrams of poly-TPD.
Stir the mixture at room temperature overnight. To begin the layer fabrication, remove a one-to-six PEDOT:PSS solution from the refrigerator in which it was stored. Stir the PEDOT:PSS solution at room temperature for 20 minutes.
While the PEDOT:PSS solution stirs, treat a clean dry ITO-glass substrate in an ultraviolet ozone chamber for 15 minutes. Once the stirring and ultraviolet-ozone treatment have both finished, draw about two milliliters of the evenly-dispersed PEDOT:PSS solution into a 10-milliliter syringe. Equip the syringe with a 0.45-micrometer polyether cellphone filter.
Fix the treated ITO-glass substrate at the center of a spin coaterchuck. Apply two drops of filtered PEDOT:PSS solution to the ITO glass. Spin-coat the ITO glass at 3, 000 RPM for 30 seconds.
Then, anneal the PEDOT:PSS-coated ITO glass at 150 degrees Celsius for 15 minutes to obtain a 30-nanometer-thick PEDOT:PSS film. Transfer the annealed PEDOT:PSS-coated substrate to a spin-coater in a nitrogen-filled glovebox. Apply 35 microliters of the 25-milligram-over-milliliter poly-TPD solution to the substrate.
Spin-coat the substrate at 3, 000 RPM for 30 seconds, and bake the substrate at 150 degrees Celsius for 30 minutes to obtain a 40-nanometer-thick poly-TPD film. Next, in the glovebox, dissolve 4.29 milligrams of zinc-cadmium-sulfide coated by zinc-sulfide quantum dots, capped with short-chain one-octane thiol in 300 microliters of toluene. Fix the poly-TPD-coated substrate at the center of the spin coaterchuck.
Apply 35 microliters of the quantum-dot solution to the substrate. Spin-coat the substrate at 3, 000 RPM for 30 seconds to obtain a 40-nanometer-thick film of zinc-cadmium-sulfide coated by zinc-sulfide quantum dots. Store the substrate in the nitrogen-filled moisture-free glovebox.
Prior to the aluminum-metal deposition, use a narrow scraper, such as a graver, to scrape away the PEDOT:PSS poly-TPD and zinc-cadmium-sulfide coated by zinc-sulfide quantum-dot layers from the two-millimeter-wide ITO-coated strip on each prepared substrate. Cover the substrates with patterned metal masks, and transfer the masked substrates into a thermal evaporation chamber. Once the chamber is at or below 10 to the minus four pascals deposit 10 nanometers of aluminum at one angstrom per second to form the aluminum electrodes.
To begin the aluminum-electrode autoxidation procedure, place the devices in a vacuum oven connected to a container of an anhydrous gas mixture of 80%nitrogen and 20%oxygen. Bump down the oven to about 267 pascals. Then, pressurize the oven to three times 10 to the four pascals with the anhydrous nitrogen-gas mixture.
Oxidize the devices at room temperature for the desired length of time to obtain the blue QD-LEDs. Store the devices in a nitrogen-filled moisture-free glovebox. The x-ray photo electron spectrum of an autoxidized aluminum cathode could be fitted with three peaks corresponding to metallic aluminum ions, the gamma phase of Aluminum-3 oxide, and amorphous alumina, respectively.
Time-resolved photoluminescence spectroscopy showed that quantum dots by aluminum by aluminum-oxide samples had longer lifetimes than quantum dots by aluminum samples, which was attributed to an improved electron injection and the suppression of non-radiative recombination in the quantum-dots by aluminum by aluminum-oxide samples. A QD-LED device with a pure aluminum cathode had low luminance, which was attributed to inefficient charge injection. Introducing a 15-nanometer aluminum Q3 layer to facilitate electronic transmission improved the luminance to 1300 candelas per square meter.
Oxidation treatment dramatically improved the device performance, with the aluminum by aluminum-oxide device achieving a maximum luminance of 13, 002 candelas per square meter and a maximum current efficiency of 1.15 candelas per ampere, after 12 hours of oxidation. The aluminum by aluminum-oxide device could suppress excitron quenching at high current densities because of its enhanced electron injection and hole-blocking abilities. The emission peak of the aluminum-by-aluminum-oxide device occurred at 457.3 nanometers, with a full-width half max of 21.4 nanometers, indicating pure blue light without parasitic electroluminescent emission.
The emission remains stable under various applied biases. We first getting the idea for this method is inspired by our perusal of the research that the exposure of the aluminum cathodes to air can improve the performance of the blue QD devices. A coated luminescence of blue zinc-cadmium-sulfide coat by zinc-sulfide QD-LEDs has been achieved using a partially-oxidized cathode.
Once mastered, this technique can be done in 12 hours if it is performed properly. While attempting this procedure, remember to strictly consider the oxidation time. Following this procedure, other methods, like introducing an additional electron-transport layer can be performed to further improve the device performance.
After its development, this technique paved the way for researchers in the field of blue QD-LED to explore the oxidized aluminum cathode for improving device performance. After watching this video, you should have a good understanding of how to fabricate high-performance QD-LEDs by an easily-controlled oxidation procedure.
