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>
	<li>Shirasaki, Y., Supran, G. J., Bawendi, M. G., Bulovi&#263;, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Emergence+of+colloidal+quantum-dot+light-emitting+technologies.">Emergence of colloidal quantum-dot light-emitting technologies.</a> Nat. Photonics. 7 (1), 13-23 (2012).</li><li>Chen, O., Wei, H., Maurice, A., Bawendi, M., Reiss, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Pure+colors+from+core-shell+quantum+dots.">Pure colors from core-shell quantum dots.</a> MRS Bull. 38 (09), 696-702 (2013).</li><li>Dai, X., Deng, Y., Peng, X., Jin, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantum-Dot+Light-Emitting+Diodes+for+Large-Area+Displays:+Towards+the+Dawn+of+Commercialization.">Quantum-Dot Light-Emitting Diodes for Large-Area Displays: Towards the Dawn of Commercialization.</a> Adv. Mater. 29 (14), (2017).</li><li>Wang, L., 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-performance+azure+blue+quantum+dot+light-emitting+diodes+via+doping+PVK+in+emitting+layer.">High-performance azure blue quantum dot light-emitting diodes via doping PVK in emitting layer.</a> Org. Electron. 37, 280-286 (2016).</li><li>Colvin, V., Schlamp, M., Alivisatos, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Light-emitting+diodes+made+from+cadmium+selenide+nanocrystals+and+a+semiconducting+polymer.">Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer.</a> Nature. 370 (6488), 354-357 (1994).</li><li>Tan, Z., 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=Colloidal+nanocrystal-based+light-emitting+diodes+fabricated+on+plastic+toward+flexible+quantum+dot+optoelectronics.">Colloidal nanocrystal-based light-emitting diodes fabricated on plastic toward flexible quantum dot optoelectronics.</a> J. Appl. Phys. 105 (03), 034312 (2009).</li><li>Tan, Z., 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=Bright+and+color-saturated+emission+from+blue+light-emitting+diodes+based+on+solution-processed+colloidal+nanocrystal+quantum+dots.">Bright and color-saturated emission from blue light-emitting diodes based on solution-processed colloidal nanocrystal quantum dots.</a> Nano Lett. 7 (12), 3803-3807 (2007).</li><li>Lee, C. -. L., Nam, S. -. W., Kim, V., Kim, J. -. J., Kim, K. -. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electroluminescence+from+monolayer+of+quantum+dots+formed+by+multiple+dip-coating+processes.">Electroluminescence from monolayer of quantum dots formed by multiple dip-coating processes.</a> physica status solidi (b). 246, 803-807 (2009).</li><li>Lee, T. -. C., 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=Rational+Design+of+Charge-Neutral,+Near-Infrared-Emitting+Osmium(II)+Complexes+and+OLED+Fabrication.">Rational Design of Charge-Neutral, Near-Infrared-Emitting Osmium(II) Complexes and OLED Fabrication.</a> Advanced Functional Materials. 19, 2639-2647 (2009).</li><li>Qian, L., Zheng, Y., Xue, J., Holloway, P. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stable+and+efficient+quantum-dot+light-emitting+diodes+based+on+solution-processed+multilayer+structures.">Stable and efficient quantum-dot light-emitting diodes based on solution-processed multilayer structures.</a> Nat. Photonics. 5 (9), 543-548 (2011).</li><li>Kwak, J., 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=Bright+and+efficient+full-color+colloidal+quantum+dot+light-emitting+diodes+using+an+inverted+device+structure.">Bright and efficient full-color colloidal quantum dot light-emitting diodes using an inverted device structure.</a> Nano Lett. 12 (5), 2362-2366 (2012).</li><li>Dai, X., 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=Solution-processed,+high-performance+light-emitting+diodes+based+on+quantum+dots.">Solution-processed, high-performance light-emitting diodes based on quantum dots.</a> Nature. 515 (7525), 96-99 (2014).</li><li>Lee, K. -. H., Lee, J. -. H., Song, W. -. S., Ko, H., Lee, C., Lee, J. -. H., Yang, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Highly+efficient,+color-pure,+color-stable,+blue+quantum+dots+light-emitting+devices.">Highly efficient, color-pure, color-stable, blue quantum dots light-emitting devices.</a> ACS Nano. 7 (8), 7295-7302 (2013).</li><li>Shen, H., 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-efficient+deep-blue+light-emitting+diodes+by+using+high+quality+ZnxCd1-xS/ZnS+core/shell+quantum+dots.">High-efficient deep-blue light-emitting diodes by using high quality ZnxCd1-xS/ZnS core/shell quantum dots.</a> Adv. Funct. Mater. 24 (16), 2367-2373 (2014).</li><li>Wang, A., 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=Bright,+efficient,+and+color-stable+violet+ZnSe-based+quantum+dot+light-emitting+diodes.">Bright, efficient, and color-stable violet ZnSe-based quantum dot light-emitting diodes.</a> Nanoscale. 7 (7), 2951-2959 (2015).</li><li>Shen, H., 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,+low+turn-on+voltage+blue-violet+quantum-dot-based+light-emitting+diodes.">High-efficiency, low turn-on voltage blue-violet quantum-dot-based light-emitting diodes.</a> Nano Lett. 15 (2), 1211-1216 (2015).</li><li>Fokina, A., 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=The+role+of+emission+layer+morphology+on+the+enhanced+performance+of+light-emitting+diodes+based+on+quantum+dot-semiconducting+polymer+hybrids.">The role of emission layer morphology on the enhanced performance of light-emitting diodes based on quantum dot-semiconducting polymer hybrids.</a> Adv. 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. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Efficient+polymer+light-emitting+diode+with+air-stable+aluminum+cathode.">Efficient polymer light-emitting diode with air-stable aluminum cathode.</a> J. Appl. Phys. 119 (9), 095503 (2016).</li><li>Yu, L., 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=Optimization+of+the+energy+level+alignment+between+the+photoactive+layer+and+the+cathode+contact+utilizing+solution-processed+hafnium+acetylacetonate+as+buffer+layer+for+efficient+polymer+solar+cells.">Optimization of the energy level alignment between the photoactive layer and the cathode contact utilizing solution-processed hafnium acetylacetonate as buffer layer for efficient polymer solar cells.</a> Acs Appl. Mater. Interfaces. 8 (1), 432-441 (2016).</li><li>Li, F., Tang, H., Anderegg, J., Shinar, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fabrication+and+electroluminescence+of+double-layered+organic+light-emitting+diodes+with+the+Al2O3/Al+cathode.">Fabrication and electroluminescence of double-layered organic light-emitting diodes with the Al2O3/Al cathode.</a> J. Shinar, Appl. Phys. Lett. 70 (10), 1233-1235 (1997).</li><li>Bai, Z., 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=Hydroxyl-Terminated+CuInS2+Based+Quantum+Dots:+Toward+Efficient+and+Bright+Light+Emitting+Diodes.">Hydroxyl-Terminated CuInS2 Based Quantum Dots: Toward Efficient and Bright Light Emitting Diodes.</a> Chemistry of Materials. 28, 1085-1091 (2016).</li><li>Wang, Z., 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=Efficient+and+Stable+Pure+Green+All-Inorganic+Perovskite+CsPbBr3+Light-Emitting+Diodes+with+a+Solution-Processed+NiOx+Interlayer.">Efficient and Stable Pure Green All-Inorganic Perovskite CsPbBr3 Light-Emitting Diodes with a Solution-Processed NiOx Interlayer.</a> The Journal of Physical Chemistry C. , (2017).</li></ol>

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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Spatial Separation of Molecular Conformers and Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

We present a technique that allows the spatial separation of different conformers or clusters present in a molecular beam. An electrostatic deflector is used to separate species by their mass-to-dipole moment ratio, leading to the production of gas-phase ensembles of a single conformer or cluster stoichiometry.

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold ...

Casting Protocols for the Production of Open Cell Aluminum Foams by the Replication Technique and the Effect on Porosity

Metal foams are interesting materials from both a fundamental understanding and practical applications point of view. Uses have been proposed, and in many cases validated experimentally, for light weight or impact energy absorbing structures, as high surface area heat exchangers or electrodes, as implants to the body, and many more. Although great progress has been made in understanding their structure-properties relationships, the large number of different processing techniques, each producing material with different characteristics and structure, means that understanding of the individual effects of all aspects of structure is not complete. The replication process, where molten metal is infiltrated between grains of a removable preform material, allows a markedly high degree of control and has been used to good effect to elucidate some of these relationships. Nevertheless, the process has many steps that are dependent on individual &#8220;know-how&#8221;, and this paper aims to provide a detailed description of all stages of one embodiment of this processing method, using materials and equipment that would be relatively easy to set up in a research environment. The goal of this protocol and its variants is to produce metal foams in an effective and simple way, giving the possibility to tailor the outcome of the samples by modifying certain steps within the process. By following this, open cell aluminum foams with pore sizes of 1&#8211;2.36 mm diameter and 61% to 77% porosity can be obtained.

Replication is one of the processing techniques used for the production of porous metal sponges. In this paper one implementation of the method for the production of open celled porous aluminum is shown in detail.

Metal foams are interesting materials from both a fundamental understanding and practical applications point of view. Uses have been proposed, and in many ...

Fabrication of White Light-emitting Electrochemical Cells with Stable Emission from Exciplexes

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active layer which consists of blue-fluorescent poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and &#960;-conjugated triphenylamine molecules. This white light emission originates from exciplexes formed between PFD and amines in electronically excited states. A device containing PFD, 4,4&#39;,4&#39;&#39;-tris[2-naphthyl(phenyl)amino]triphenylamine (2-TNATA), Poly(ethylene oxide) and K2CF3SO3 showed white light emission with Commission internationale de l&#39;&#233;clairage (CIE) coordinates of (0.33, 0.43) and a Color Rendering Index (CRI) of Ra = 73 at an applied voltage of 3.5 V. Constant voltage measurements showed that the CIE coordinates of (0.27, 0.37), Ra of 67, and the emission color observed immediately after application of a voltage of 5 V were nearly unchanged and stable after 300 sec.

The authors present a method for fabricating stable white-light-emitting electrochemical cells utilizing emission from exciplexes formed between a blue-emitting fluorene polymer and aromatic amines.

The authors present an approach for fabricating stable white light emission from polymer light-emitting electrochemical cells (PLECs) having an active ...

Blue-hazard-free Candlelight OLED

A candlelight-style organic light emitting diode (OLED) is a human-friendly type of lighting because it is blue-hazard-free and has a low correlated color temperature (CCT) illumination. The low CCT lighting is deprived of high-energy blue radiation, and it can be used for a longer duration before causing retinal damage. This work presents the comprehensive protocols for the fabrication of blue-hazard-free candlelight OLEDs. The emission spectrum of the OLED was characterized by the maximum exposure time limit of the retina and the melatonin suppression sensitivity. The devices can be fabricated using dry and wet processes. The dry-processed OLED resulted in a CCT of 1,940 K and exhibited a maximum retinal exposure limit of 1,287 s at a brightness of 500 lx. It showed 2.61% melatonin suppression sensitivity relative to 480 nm blue light. The wet-processed OLED, where the spin coating is used to deposit hole injection, hole transport, and emissive layers, making fabrication fast and economical, produced a CCT of 1,922 K and showed a maximum retinal exposure limit of 7,092 at a brightness of 500 lx. The achieved relative melatonin suppression sensitivity of 1.05% is 86% and 96% less than that of the light emitting diode (LED) and compact fluorescent lamp (CFL), respectively. Wet-processed blue-hazard-free candlelight OLED exhibited a power efficiency of 30 lm/W, which is 2 times that of the incandescent bulb and 300 times that of the candle.

We present a protocol for the fabrication of a blue-hazard-free candlelight organic light emitting diode (OLED) for eye protection and melatonin secretion.

A candlelight-style organic light emitting diode (OLED) is a human-friendly type of lighting because it is blue-hazard-free and has a low correlated color ...

Reaction Kinetics and Combustion Dynamics of I4O9 and Aluminum Mixtures

Tetraiodine nonoxide (I4O9) has been synthesized using a dry approach that combines elemental oxygen and iodine without the introduction of hydrated species. The synthesis approach inhibits the topochemical effect promoting rapid hydration when exposed to the relative humidity of ambient air. This stable, amorphous, nano-particle material was analyzed using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) and showed an exothermic energy release at low temperature (i.e., 180 &#176;C) for the transformation of I4O9 into I2O5. This additional exothermic energy release contributes to an increase in overall reactivity of I4O9 when dry mixed with nano-aluminum (Al) powder, resulting in a minimum of 150% increase in flame speed compared to Al + I2O5. This study shows that as an oxidizer, I4O9 has more reactive potential than other forms of iodine(V) oxide when combined with Al, especially if I4O9 can be passivated to inhibit absorption of water from its surrounding environment.

Reaction Kinetics and Combustion Dynamics of I4O9 and Aluminum Mixtures

A protocol for measuring flame speeds of a reactive mixture composed of tetraiodine nonoxide (I4O9) and aluminum (Al) is presented. A method for resolving reaction kinetics using differential scanning calorimetry (DSC) is also presented. It was found that I4O9 is 150% more reactive than other iodine(V) oxides.

Tetraiodine nonoxide (I4O9) has been synthesized using a dry approach that combines elemental oxygen and iodine without the introduction of hydrated ...

Experimental Procedure for Warm Spinning of Cast Aluminum Components

High performance, cast aluminum automotive wheels are increasingly being incrementally formed via flow forming/metal spinning at elevated temperatures to improve material properties. With a wide array of processing parameters which can affect both the shape attained and resulting material properties, this type of processing is notoriously difficult to commission. A simplified, light-duty version of the process has been designed and implemented for full-size automotive wheels. The apparatus is intended to assist in understanding the deformation mechanisms and the material response to this type of processing. An experimental protocol has been developed to prepare for, and subsequently perform forming trials and is described for as-cast A356 wheel blanks. The thermal profile attained, along with instrumentation details are provided. Similitude with full-scale forming operations which impart significantly more deformation at faster rates is discussed.

An experimental protocol for instrumented warm rotary forming of cast aluminum alloys employing a bespoke industrially scaled apparatus is presented. Experimental considerations including thermal and mechanical effects are discussed, as well as similitude with full-scale processing of automotive wheels.

High performance, cast aluminum automotive wheels are increasingly being incrementally formed via flow forming/metal spinning at elevated temperatures to ...

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, &#960;-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (&#181;-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, &#181;-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, ...

Resonance Fluorescence of an InGaAs Quantum Dot in a Planar Cavity Using Orthogonal Excitation and Detection

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). Resonant excitation without fluorescence detection &#8211; for example, a differential transmission measurement &#8211; can determine some properties of the emitting system, but does not allow applications or measurements based on the emitted photons. For example, the measurement of photon correlations, observation of the Mollow triplet, and realization of single photon sources all require collection of the fluorescence. Incoherent excitation with fluorescence detection &#8211; for example, above band-gap excitation &#8211; can be used to create single photon sources, but the disturbance of the environment due to the excitation reduces the indistinguishability of the photons. Single photon sources based on QDs will have to be resonantly excited to have high photon indistinguishability, and simultaneous collection of the photons will be necessary to make use of them. We demonstrate a method to resonantly excite a single QD embedded in a planar cavity by coupling the excitation beam into this cavity from the cleaved face of the sample while collecting the fluorescence along the sample&#39;s surface normal direction. By carefully matching the excitation beam to the waveguide mode of the cavity, the excitation light can couple into the cavity and interact with the QD. The scattered photons can couple to the Fabry-Perot mode of the cavity and escape in the surface normal direction. This method allows complete freedom in the detection polarization, but the excitation polarization is restricted by the propagation direction of the excitation beam. The fluorescence from the wetting layer provides a guide to align the collection path with respect to the excitation beam. The orthogonality of the excitation and detection modes enables resonant excitation of a single QD with negligible laser scattering background.

Resonant excitation of a single self-assembled quantum dot can be achieved using an excitation mode orthogonal to the fluorescence collection mode. We demonstrate a method using the waveguide and Fabry-Perot modes of a planar microcavity surrounding the quantum dots. The method allows complete freedom in the detection polarization.

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). ...

Production and Characterization of Vacuum Deposited Organic Light Emitting Diodes

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex donor-acceptor emitters is presented. With a step-by-step procedure, readers will be able to repeat and produce OLED devices based on simple organic emitters. A patterning procedure allowing the creation of personalized indium tin oxide (ITO) shape is shown. This is followed by the evaporation of all layers, encapsulation and characterization of each individual device. The end goal is to present a procedure that will give the opportunity to repeat the information presented in cited publication but also using different compounds and structures in order to prepare efficient OLEDs.

A protocol for the production of simple structured organic light-emitting diodes (OLEDs) is presented.

A method for producing simple and efficient thermally-activated delayed fluorescence organic light-emitting diodes (OLEDs) based on guest-host or exciplex ...

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Attainment of high-quality Schottky contacts is imperative for achieving efficient gate modulation in heterostructure field effect transistors (HFETs). We present the fabrication methodology and characteristics of Schottky diodes on Zn-polar BeMgZnO/ZnO heterostructures with high-density two dimensional electron gas (2DEG), grown by plasma-assisted molecular beam epitaxy on GaN templates.

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device ...