Name: enhanced electron injection and exciton confinement for pure blue quantum-dot light-emitting diodes by introducing partially oxidized aluminum cathode
Uploaded: 2018-05-31T15:00:00
Description: Watch this Scientific Journal Video about Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode at JoVE

This work was supported by the NSFC (51573042), The National Key Basic Research Program of China (973 project, 2015CB932201), Fundamental Research Funds for the Central Universities, China (JB2015RCJ02, 2016YQ06, 2016MS50, 2016XS47).

1. Pattern Etching of Indium Tin Oxide (ITO) Glass
<ol>
	<li>Cut large pieces of ITO glass (12 cm &#215; 12 cm) into 15 mm wide strips. Clean the ITO glass surface using a dust-free cloth with alcohol.</li>
	<li>Check the conductive side of the ITO glass with a digital multimeter. Cover the active area of the ITO glass with adhesive tape, so that the active area is 2 mm wide in the middle.</li>
	<li>Pour the zinc powder on the ITO glass (to a thickness of about 0.5 mm).</li>
	<li>Pour the hydrochloric acid solution (36...

<ol>
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Mater. Interfaces. 3 (18), 1600279 (2016).</li><li>Yang, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-efficiency+light-emitting+devices+based+on+quantum+dots+with+tailored+nanostructures.">High-efficiency light-emitting devices based on quantum dots with tailored nanostructures.</a> Nat. Photonics. 9, 259-266 (2015).</li><li>Cheng, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pure+Blue+and+Highly+Luminescent+Quantum-Dot+Light-Emitting+Diodes+with+Enhanced+Electron+Injection+and+Exciton+Confinement+via+Partially+Oxidized+Aluminum+Cathode.">Pure Blue and Highly Luminescent Quantum-Dot Light-Emitting Diodes with Enhanced Electron Injection and Exciton Confinement via Partially Oxidized Aluminum Cathode.</a> Adv. Opt. Mater. 5 (11), 1700035 (2017).</li><li>Rotole, J. A., Sherwood, P. M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gamma-Alumina+(&#947;-Al2O3)+by+XPS.">Gamma-Alumina (&#947;-Al2O3) by XPS.</a> Surf. Sci. Spectra. 5 (1), 18-24 (1998).</li><li>Liu, J., Yang, W., Li, Y., Fan, L., Li, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrochemical+studies+of+the+effects+of+the+size,+ligand+and+composition+on+the+band+structures+of+CdSe,+CdTe+and+their+alloy+nanocrystals.">Electrochemical studies of the effects of the size, ligand and composition on the band structures of CdSe, CdTe and their alloy nanocrystals.</a> Phys. Chem. Chem. Phys. 16 (10), 4778-4788 (2014).</li><li>Abbaszadeh, D., Wetzelaer, G. A. H., Doumon, N. Y., Blom, P. W. 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The device architecture of the blue QD-LED consists of an ITO transparent anode, a PEDOT:PSS HIL (30 nm), a Poly-TPD HTL (40 nm), a ZnCdS/ZnS QDs EML (40 nm), and an Al:Al2O3 cathode (100 nm). Due to the porous character of the Al cathode, we obtained an oxidized Al cathode by exposing it to oxygen. Figure 2e and Figure 2f display the energy level alignment diagrams of QDs layer with Al and Al:Al2O3. When the QDs conta...

Light-emitting diodes (LEDs) based on colloidal semiconductor quantum dots have attracted great interest due to their unique advantages, including solution processability, tunable emission wavelength, excellent color purity, flexible fabrication, and low processing cost1,2,3,4. Since the first demonstrations of QDs-based LEDs in 1994, tremendous efforts have been devoted to engineering the materials and device structures5,6,7. A typical QD-LED device is designed to have a three-layered sandwich architecture which consists of a hole transport layer (HTL), an emissive layer, and an electron transport layer (ETL). The choice of a suitable charge transport layer is critical to balance the injected holes and electrons into the emissive layer for radiative recombination. Currently, vacuum-deposited small molecules are widely used as ETL, for instance, bathocuproine (BCP), tris(8-Hydroxyquinolinate) (Alq3), and 3-(biphenyl-4-yl)-5-(4-tertbutylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ)8. However, the unbalanced carrier injection often causes the recombination region shift to ETL, making unwanted parasitic electroluminescence (EL) emission and deteriorating the device performance9.
To enhance the device efficiency and environmental stability, solution-processed ZnO nanoparticles were introduced as an electron transport layer instead of the vacuum-deposited small-molecule materials. Highly bright RGB QD-LEDs were demonstrated for conventional device architecture, showing luminance up to 31,000, 68,000, and 4,200 cd m-2 for emission of orange-red, green, and blue, respectively10. For an inverted device architecture, high performance RGB QD-LEDs with low turn on voltage were successfully demonstrated with brightness and external quantum efficiencies (EQE) of 23,040 cd m-2 and 7.3% for red, 218,800 cd m-2 and 5.8% for green, and 2,250 cd m-2 and 1.7% for blue, respectively11. To balance the injected charges and preserve the QDs emissive layer, an insulating poly(methylmethacrylate) (PMMA) thin film was inserted between the QDs and ZnO ETL. The optimized deep-red QD-LEDs exhibited high external quantum efficiencies up to 20.5% and a low turn-on voltage of only 1.7 V12.
Besides, optimizing the optoelectronic properties and nanostructures of QDs also plays a crucial role in boosting the device performance. For instance, highly fluorescent blue QDs with photoluminescence quantum yield (PLQE) up to 98% were synthesized through optimizing the ZnS shelling time13. Similarly, high-quality, violet-blue QDs with near 100% PLQE were synthesized by precisely controlling the reaction temperature. The violet-blue QDs-LED devices showed remarkable luminance and EQE up to 4,200 cd m-2 and 3.8%, respectively14. This synthesis method is also applicable to violet ZnSe/ZnS core/shell QDs, the QD-LEDs exhibited high luminance (2,632 cd m-2) and efficiency (EQE=7.83%) by using Cd-free QDs15. Since blue quantum dots with high PLQE have been demonstrated, high charge injection efficiency in the QDs layer plays another crucial role in fabricating high performance QD-LEDs. By substituting long chain oleic acid ligands to shorten 1-octanethiol ligands, the electron mobility of QDs film was increased two-fold, and a high EQE value over 10% was obtained16. The surface ligand exchange can also improve the morphology of QDs film and suppress the photoluminescence quenching among QDs. For instance, QDs-LED showed improved device performance by using chemically grafted QDs-semiconducting polymer hybrids17. Besides, high-performance QDs were prepared through reasonable optimization of the graded composition and thickness of the QDs shell, due to the enhanced charge injection, transport, and recombination18.
In this work, we introduced a partial autoxidized aluminum (Al) cathode to improve the performance of ZnCdS/ZnS graded core/shell-based blue QD-LEDs19. The change of the potential energy barrier of the Al cathode was confirmed by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Furthermore, the fast charge carrier dynamics at the QDs/Al and QDs/Al:Al2O3 interface were analyzed by time-resolved photoluminescence (TRPL) measurements. In order to further validate the influence of partially oxidized Al on device performance, QD-LEDs with different cathodes (Al only, Al:Al2O3, Al2O3/Al, Al2O3/Al:Al2O3, and Alq3/Al) were fabricated. As a result, high performance pure blue QD-LEDs were demonstrated by employing Al:Al2O3 cathodes, with a maximum luminance of 13,002 cd m-2 and a peak current efficiency of 1.15 cd A-1. Furthermore, there was no additional organic ETL involved in the device architecture, which can avoid unwanted parasitic EL to guarantee the color purity under different working voltages.

<table>
	<tbody>
		<tr>
			<td>Indium Tin Oxide (ITO)-coated glass 
			substrate</td>
			<td>CSG Holding Co., Ltd.</td>
			<td></td>
			<td>Resistivity&#8776;10 &#937;/sq</td>
		</tr>
		<tr>
			<td>Zinc powder</td>
			<td>Sigma-Aldrich</td>
			<td>96454</td>
			<td>Molecular Weight 65.38</td>
		</tr>
		<tr>
			<td>Isopropyl alcohol</td>
			<td>Beijing Chemical Reagent</td>
			<td>67-63-0</td>
			<td>Analytically pure</td>
		</tr>
		<tr>
			<td>Toluene</td>
			<td>Innochem</td>
			<td>I01367</td>
			<td>Analytically pure</td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>Innochem</td>
			<td>I01366</td>
			<td>Analytically pure</td>
		</tr>
		<tr>
			<td>Hydrochloric acid</td>
			<td>acros</td>
			<td>124210025</td>
			<td>1 N standard solution</td>
		</tr>
		<tr>
			<td>O-dichlorobenzene</td>
			<td>acros</td>
			<td>396961000</td>
			<td>98+%, Extra Dry</td>
		</tr>
		<tr>
			<td>Poly(3,4-ethylenedioxythiophene) doped polystyrene sulfonate (PEDOT:PSS)</td>
			<td>H. C.Stark</td>
			<td>Clevious P VP Al 4083</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly(N,N&#8242;-bis(4-butylphenyl)-N,N&#8242;-bis(phenyl)-benzidine) (Poly-TPD)</td>
			<td>Luminescence Technology</td>
			<td>LT-N149</td>
			<td></td>
		</tr>
		<tr>
			<td>Aluminum tris(8-Hydroxyquinolinate) (Alq3)</td>
			<td>Luminescence Technology</td>
			<td>LT-E401</td>
			<td></td>
		</tr>
		<tr>
			<td>UV-O cleaner</td>
			<td>Jelight Company</td>
			<td>92618</td>
			<td></td>
		</tr>
		<tr>
			<td>Filter</td>
			<td>Jinteng</td>
			<td>JTSF0303/0304</td>
			<td>Polyether sulfone (0.45 &#956;m)</td>
		</tr>
		<tr>
			<td>Ultrasonic cleaner</td>
			<td>HECHUANG ULTRASONIC</td>
			<td>KH-500DE</td>
			<td></td>
		</tr>
		<tr>
			<td>Digital multimeter</td>
			<td>UNI-T</td>
			<td>UT39A</td>
			<td></td>
		</tr>
		<tr>
			<td>Spin coater</td>
			<td>IMECAS</td>
			<td>KW-4A</td>
			<td></td>
		</tr>
		<tr>
			<td>Digital hotplate</td>
			<td>Stuart</td>
			<td>SD160</td>
			<td></td>
		</tr>
	</tbody>
</table>

enhanced electron injection and exciton confinement for pure blue quantum-dot light-emitting diodes by introducing partially oxidized aluminum cathode

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation displays and solid-state lighting technologies. The brightness and efficiency of blue QDs-based light-emitting diodes (LEDs) remain inferior to their red and green counterparts, due to the inherently unfavorable energy levels of different colors of light. To solve these problems, a device structure should be designed to balance the injection holes and electrons into the emissive QD layer. Herein, through a simple autoxidation strategy, pure blue QD-LEDs which are highly bright and efficient are demonstrated, with a structure of ITO/PEDOT:PSS/Poly-TPD/QDs/Al:Al2O3. The autoxidized Al:Al2O3 cathode can effectively balance the injected charges and enhance radiative recombination without introducing an additional electron transport layer (ETL). As a result, high color-saturated blue QD-LEDs are achieved with a maximum luminance over 13,000 cd m-2, and a maximum current efficiency of 1.15 cd A-1. The easily controlled autoxidation procedure paves the way for achieving high-performance blue QD-LEDs.

UV-Vis absorption and photoluminescence (PL) spectra were used to record the optical properties of ZnCdS/ZnS graded core/shell-based blue QDs. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images were collected for the morphologies of QDs (Figure 1). X-ray photoelectron spectroscopy (XPS), electrochemical study, and ultraviolet photoelectron spectroscopy (UPS) were employed to detect the structural properties and energy levels ...

Watch this Scientific Journal Video about Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode at JoVE

Enhanced Electron Injection and Exciton Confinement for Pure Blue Quantum-Dot Light-Emitting Diodes by Introducing Partially Oxidized Aluminum Cathode

A protocol is presented for fabricating high-performance, pure blue ZnCdS/ZnS-based quantum dots light-emitting diodes by employing an autoxidized aluminum cathode.

Stable and efficient red (R), green (G), and blue (B) light sources based on solution-processed quantum dots (QDs) play important roles in next-generation ...

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.

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