Inverted Quantum-Dot Light Emitting Diode Using Solution Processed

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Inverted Quantum-dot Light Emitting Diode using Solution Processed ptype WOx doped PEDOT:PSS and Li doped ZnO Charge Generation Layer Hyo-Min Kim, Jeonggi Kim, Jieun Lee, and Jin Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06505 • Publication Date (Web): 22 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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ACS Applied Materials & Interfaces

Inverted Quantum-dot Light Emitting Diode using Solution Processed p-type WOx doped PEDOT:PSS and Li doped ZnO Charge Generation Layer Hyo-Min Kim, Jeonggi Kim, Jieun Lee and Jin Jang*

Advanced Display Research Center (ADRC), Department of Information Display, Kyung Hee University, Dongdaemoon-ku, Seoul, 130-701, Korea *E-mail: [email protected]

KEYWORDS: charge generation layer, CGL, solution process, quantum-dot, QLED

ACS Paragon Plus Environment

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ABSTRACT Quantum-dots (QDs) are promising material for emissive display with low-cost manufacturing and excellent color purity. In this study, we report colloidal quantum-dot light emitting diodes (QLEDs) with an inverted architecture with solution processed charge generation layer (CGL) of p-type polymer (tungsten oxide doped poly(ethylenedioxythiophene)/polystyrenesulfonate, PEDOT:PSS:WOx) and n-type metal oxide (lithium doped zinc oxide, LZO). The effective charge generation in solution processed PN junction was confirmed by capacitance-voltage (CV) and current density-electric field characteristics. It is also demonstrated that the performances of CGL based QLEDs are very similar when various substrates with different work functions are used.

INTRODUCTION Recently, colloidal quantum-dot light emitting diodes (QLEDs) are of increasing interest for next generation display1-2. Quantum-dot (QD) has such advantages as color tunability, solution processability and high color purity3-5. Size of QDs can be controlled and it decides the emission color of QDs with the same chemical composition6-8. To improve light emission efficiency and absorption of QDs, new synthesis methods such as additional shell on QD core, different chemical composition ratio of core and shell, and gradient chemical composition of shell between core and outer shell have been studied9-13. The improvement in QD synthesis has increased the internal quantum efficiency (IQE) and generated various colors, and thus QD based display could improve its color gamut compared to organic light emitting diode (OLED) displays14-16. Because of wide color gamut, QD film has been adopted for active matrix liquid


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crystal display (AMLCD) to improve color range17-20. However, QDs have some issues for display application such as material instability, device design and QLED manufacturing21-23. Note that the issues on solution permission and inter-mixing during upper layer deposition on QDs of QLED should be solved for high efficiency. To overcome and solve the issues, most of QLEDs have been designed with hybrid structure using both solution and vacuum processes. Note that inverted structured QLEDs can show higher efficiency and longer lifetime24-27. Basically, device design is the most important factor to obtain high efficiency. Reducing the energy barriers for charge injection and transport from both electrodes is necessary to achieve high efficiency QLED. Previous attempts to reduce energy barriers were carried out by choosing suitable materials for energy level alignment28-30. To minimize energy barrier for carrier transport from both electrodes, the substrates such as indium-tin-oxide (ITO), indium zinc-oxide (IZO) for bottom emission are used, and aluminum (Al), molybdenum (Mo), gold (Au), silver (Ag) are adopted for top emission. Because the substrates have different work functions (WFs) such as 4.2 eV (ITO), 4.7 eV (IZO), 4.3 eV (Al), 4.6 eV (Mo), 4.8 eV (Ag) and 5.1 eV (Au), device performance of QLED can be affected by the WFs of the substrates. Note that when charge generation layer (CGL) is used on the substrate, the device performances could be independent on the WF of substrate material. In OLEDs, the methods for enhancing charge injection have been developed at organic/organic or metal/organic junctions31-33. One is to use alkali metal carbonates (e.g. Cs2CO3, CaCO3, Li2CO3 and so on) as dopants in electron transporting layers (ETLs)34-35. In this case, the Fermi level (EF) of n-doped ETL shifts to the conduction band and thus increases free electron concentration at the band edge. Another one is the modification of the WF of the electrode using surface treatments such as UV-ozone, oxygen plasma and so on36-38. Note that


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the WF of bottom electrode of inverted QLED affects the device performance significantly. The previous attempts have been focused on thermal evaporation to form the n- or p-doped charge injection layer. However, the thermal evaporation cannot meet the low-cost large-area process. In this work, we used solution processed CGL for large area displays. Recently, Y. Yang et al, reported highly efficient green QLED exhibiting maximum current efficiency of 63 cd/A and external quantum efficiency of 14.5%. They varied the chemical composition of QDs and adopted it into conventional QLED39. On the other hand, we focused on the application of solution processed CGL in QLED which has advantages of low cost and large area processing. Here, we report the solution processed CGL consisting of p-type conducting polymer and ntype metal oxide for inverted QLEDs. We fabricated inverted red (R-), yellow (Y-), green (G-), light blue (LB-) and deep blue (DB-) QLEDs with CGL. Especially, the inverted G-QLED with CGL showed the maximum current and power efficiencies (CEmax and PEmax) of 27.3 cd A-1 and 19.4 lm W-1, respectively. Also maximum external quantum efficiency (EQE) of the inverted R-, Y-, G-, LB- and DB-QLEDs with CGL were 6.5, 4.6, 8.3, 2.6 and 1.8 %, respectively. The evidences of charge generation could be seen in the current density versus electric field characteristic for CGL device with ITO / CGL / LiF:Al, and in the current density versus electric field plot for the CGL devices with increasing CGL number up to 3. Note that the inverted QLEDs with CGL on various substrates with different WFs show similar and uniform device performances, confirming the charge generation once again.

RESULTS AND DISCUSSION


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Figure 1a and 1b shows respectively the schematic device structure studied in this work and energy level diagram of the inverted G-QLED with CGL: ITO / tungsten oxide (WOx) doped poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic

acid

(PEDOT:PSS,

PP)

(PP:WOx)

(20~23 nm) / Li doped ZnO (LZO) (15 nm) / CdSe/CdS/ZnS QDs / 1,4,7-TriazacyclononaneN,N',N''-triacetate (TCTA) (10 nm) / N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′diamine (NPD) (20 nm) / 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) (20 nm) / Al (100 nm). In this study, we used organic-organic CGL of NPD/HAT-CN, and organicinorganic GGL of p-type PP:WOx/n-type LZO junction, for inverted QLED40. The organic-metal oxide p-n junction acts as an electron injection layer (EIL) by generating electron-hole pairs at the interface and the electrons are injected into QD layer. The generated electrons and holes at the interface are transported to opposite electrodes by electric field. Also, the charge recombination takes place at the interfaces of ITO/PP:WOx and HAT-CN/Al. In this study, we used R-, Y-, G-, LB- and DB-QDs which have photoluminescence (PL) peaks of 629, 571, 520, 474 and 441 nm, respectively. More details about QDs can be seen in Figure S1 and Table S1 in the Supporting Information (SI). To confirm the energy level alignment, we measured ultraviolet photoelectron spectroscopy (UPS) of the layers on ITO, step-by-step. The UPS data with secondary-electron cutoff and zoom-in of valance band edge can be respectively seen in Figure 2a and 2b. Our ITO substrate has WF of 4.2 eV (black square) and vacuum level shifted up by 0.32 eV after p-type PP:WOx deposition on ITO (red triangle) as shown in Figure 2a. And, after LZO deposition on PP:WOx layer, vacuum level shifted down by 1.0 eV compared to WF of ITO (blue circle). To find the highest occupied molecular orbital (HOMO) level for PP:WOx on ITO and the valance band (VB) for LZO on PP:WOx, we calculated valance band shift (∆VB) from EF as shown in red triangle


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and blue circle in Figure 2b, respectively. Note that the optical band-gap of LZO is 3.30 eV. The energy level alignment is summarized in Figure 2c, and the generated electrons and holes transported to opposite directions by electric field through the conduction band (CB) of LZO and HOMO of PP:WOx, respectively. The cross-sectional transmission electron microscopy (TEM) image of the inverted G-QLED with CGL is shown in Figure 3. The thickness of green QD layer was measured to be 15 ~ 20 nm as can be seen in the inset in Figure 3a. The emission layer (EML) could be identified as the existence of sulfur (S), because of ZnS shell in QDs. Also, the interface between LZO and GQDs can be found from the existence of oxygen (O) in LZO. We concluded that the green QD has 3.0 ~ 4.0 MLs, and more detailed energy dispersive x-ray spectroscopy (EDS) data of QLED is shown in Figure 3b. According to the EDS depth profile, the layers in QLED can be clearly seen without damage such as washing-out of layers and permission into under-layer during upper layer deposition. Additionally, it is noted that WOx in PEDOT:PSS can block indium (In) diffusion to CGL as shown in our previous report41-42. To confirm the charge generation at the interface of PP:WOx / LZO, we have fabricated ETL and CGL devices of ITO / LZO (15 nm) / LiF:Al and ITO / PP:WOx (20~23 nm) / LZO (15 nm) / LiF:Al, respectively. Note that PP:WOx / LZO shows efficient charge generation as can be seen in current density versus electric field (Figure S2, SI). At the same electric field, the device with CGL showed higher current density than that of ETL device. Charge generation effect in QLED was studied more by fabricating p-n junction. The current density–voltage (J-V) characteristic of p-n junction is shown at forward and reverse biases (Figure S3, SI) using the device structures of ITO / PP:WOx / LZO / Phenyl-C61-butyric acid methyl ester (PCBM) / 8-hydroxyquinolatolithium (Liq):Al. At forward bias, the recombination


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takes place at the PP:WOx/LZO interface. And, at reverse bias, the charges are generated at the PP:WOx/LZO interface and then contribute to currents. To compare the possibility of charge generation in normal p-n junction, we also fabricated p-n junction without CGL (ITO / PP:WOx / poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB) / PCBM / Liq:Al), and the off-state and on-state currents of the p-n junction without CGL are one order of magnitude lower and one order of magnitude higher than those of p-n junction with CGL, respectively. Note that the charges could not be generated at the interface of TFB/PCBM. Also, in p-n junction with CGL, the J-V characteristic exhibit symmetric behavior at forward and reverse biases31 (Figure S3, SI). To support the charge generation once more, we studied the charge generation effect in QLED with increasing CGL number up to 3 (Figure S4, SI) with Device 1: ITO / LZO / LiF:Al (reference), device 2: ITO / single CGL / LiF:Al, and device 3: ITO / triple CGLs / LiF:Al. The total thickness of devices 1, 2 and 3 were 15, 35, and 105 nm, respectively. The increase in CGL number increases the current density at the same electric field. The device performances of the inverted G-QLED with CGL are shown in Figure 4. Figure 4a shows that the inverted G-QLED with CGL has lower current density at >2 V compared to that of inverted G-QLED with ETL. Note that the lower current density of CGL based QLED is related with energy barrier between ITO and PP:WOx for recombination, and electric field decreases by adding 20 nm thick PP:WOx. However, in low voltage region between -2 V and 2 V in Figure 4b, the G-QLED with CGL shows 2 times higher current density than that of the QLED with ETL. The generated holes and electrons can transport effectively through PP:WOx and LZO, and to EML and cathode, respectively31. In Figure 4c, the inverted G-QLED with CGL shows lower luminance intensity at the same applied voltage compared to that of QLED with a single


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LZO. Note that lower current density of QLED with CGL leads to lower exciton formation rate. The maximum current efficiency of inverted G-QLED with ETL and CGL was 24.7 and 27.3 cd A-1, respectively, and the device performances of the inverted R-, Y-, G-, LB- and DB-QLEDs with ETL or CGL are summarized in Table 1. As can be seen in Figure 4d, the QLED with a CGL showed a similar current efficiency with that of QLED with an ETL. It is noted that the optimization of n-type LZO thickness is very important to evaluate CGL effect. Herein, the optimized LZO thickness was 50 nm for single ETL based QLED, and the thickness was also adopt into the CGL based QLED. Change in capacitance–voltage (C-V) relationship is related with charge generation. If PP:WOx / LZO cannot generate the carriers at its interface during device operating, the capacitance of the device will be similar to that of inverted G-QLED with ETL. The normalized capacitance (C/C0) and luminance of the inverted G-QLEDs with ETL or CGL are shown in the Figure S5. The C/C0 is total charge of the device normalized to that (C0) at driving voltage (VD) of 0 V. It is concluded from capacitance plot that the inverted G-QLED with CGL generates more carriers than the G-QLED with ETL. The performances of the inverted QLEDs with CGL using red, yellow, green, light-blue and deep-blue QDs are shown in Figure 5. Figure 5a and 5b shows the normalized electroluminescence (EL) and EQE for the QLEDs with CGL, respectively. As can be seen in Figure 5 and Table 2, the EL peaks and full-widths at half maximum (FWHMs) of R-, Y-, G-, LB- and DB-QLEDs with CGL are 637, 576, 526, 500 and 444 nm, and 36, 41, 33, 38 and 23 nm, respectively. The narrow FWHMs for the QLEDs with CGL indicate that the excitons are recombined in QDs, without residual emission of the blue EL from TCTA. And, the maximum EQE of R-, Y-, G-, LB- and DB-QLEDs with CGL were found to be 6.5, 4.6, 8.3, 2.6 and 1.8%,


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respectively. The averaged CIE coordinates of the CGL based inverted R-, Y-, G-, LB- and DBQLEDs with operating images is shown in Figure 5c. The current density of the CGL device increased with CGL number at the same electric field (Figure S4). We fabricated the inverted QLEDs with 2 CGLs to verify charge generation effect in device. Notes that the thicknesses of two stack CGLs with PP:WOx / LZO is 70 nm and hole transporting layer (HTL) was fixed at 50 nm. As a result, we could confirm that the device performances are almost independent of CGL number (Figure S6, SI). The device performances of inverted G-QLEDs with CGL number of 1 and 2 are similar; the maximum current and power efficiencies for both devices are very close as shown in Supplementary Figure S6. A big advantage of inverted QLED with CGL is to use the cathode materials with different WFs for inverted QLED. The device performance would be independent of the WFs because of electron-hole generation at the p-n junction. To confirm this assumption, we studied the device performances of inverted G-QLEDs with CGL using the cathode materials with various WFs. The device performances of the inverted G-QLEDs with ETL on ITO, UV/ozone (O3) treated ITO, IZO and on UV/O3 treated IZO substrates can be seen in Figure 6a and 6b. Note that the substrates have different WFs of 4.2 eV (ITO), 4.5 eV (UV/O3 treated ITO), 4.7 eV (IZO) and 5.1 eV (UV/O3 treated IZO). As shown in Figure 6a and 6b, the currents are sensitive to the substrates due to the different energy barriers between WF of substrates and CB of LZO for electron transport. However, we found that the inverted G-QLEDs with CGL have similar device performances for various substrates with different WFs. The inverted G-QLEDs with the CGLs showed uniform diode characteristics compared to that of inverted G-QLEDs with ETL. By introducing p-type organic layer under the n-type oxide layer, the energy barrier for charge transport and recombination remains the same, as a result, the currents of all QLEDs with CGL


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are similar. Although IZO substrate has a little higher sheet resistance (15~17 Ω sq-1) compared to that of ITO (8~9 Ω sq-1), the diode characteristics of the inverted QLEDs with CGL are similar. The inverted G-QLEDs with CGL indicate the similar current efficiency at whole luminance region from 0.1 to 10,000 cd m-2 as shown in Figure 6d, and summarized device performances are shown as Table 3. The independent device performance on the substrates with various WFs opens up many choices for the cathode material for inverted QLED manufacturing. The device performance can be further improved by optimization of PEDOT:PSS:WOx and LZO material properties by surface modification to reduce the series resistance and by using efficient QDs materials. Note that we used commercial QD materials

CONCLUSION In summary, we report a new structure of inverted QLED with solution processed CGL. It is demonstrated that the performance of inverted QLED is almost independent on the transparent conducting oxide (TCO) materials because of charge generation at the interface of PP:WOx/LZO. And we found that charges can be generated at the interface of PP:WOx/LZO which could be confirmed from symmetrical J-V characteristic of p-n junction. The maximum current and power efficiencies of green QLED with CGL exhibited 27.3 cd A-1 and 19.4 lm W-1, respectively. Therefore, the QLED with organic-inorganic p-n CGL can be widely applied to display application.

EXPERIMENTAL Synthesis of Li doped ZnO (LZO). LZO solution was synthesized using sol-gel technique43. We purchased lithium acetate dihydrate (CH3COOLi + 2H2O), zinc acetate dihydrate


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(Zn(C4H6O4)2 + 2H2O) and monoethanolamine (C2H7NO, MEA) from Sigma Aldrich. For 2% LZO solution, a 1.1 g of zinc acetate dehydrate and 2.0 % lithium acetate dihydrate were mixed in a three necked beaker. Then, MEA was added into LZO powders to maintain stability of LZO complex and it was dissolved in 50 ml ethanol. As a last step, the LZO solution was refluxed at 60 °C for 9 hrs until a transparent solution is obtained. Synthesis of tungsten oxide (WOx). WOx solution can be obtained as following sequence. We purchased ammonium metatungstate hydrate ((NH4)6H2W12O40 + xH2O) from Sigma Aldrich. For WOx solution, a 0.06 g of ammonium metatungstate hydrate was dissolved in DI water and hydrogen chloride (HCl) was injected into WOx solution to maintain acidity of WOx solution as pH 2. As a last step, the WOx solution was stirred for 12 hrs at room temperature. Device fabrication of inverted QLED with CGL. We used CdSe/ZnS QDs (core/shell type) for red, yellow and light-blue emissions, and CdSe/CdS/ZnS (core/gradient shell type) QDs for green emission, and ZnCdS/ZnS (core/shell type) QDs for deep-blue emissions which were purchased from Nanosquare Inc, Korea. At the first step, a patterned ITO substrate was cleaned in ultra-sonicator using acetone, methanol and IPA for 15 min, respectively. Then, 20 nm thick PP:WOx was deposited onto ITO substrate with a sheet resistance of 8 ~ 10 Ω sq-1. After annealing PP:WOx layer, the 50 nm thick LZO was deposited onto the PP:WOx for G-QLED. It is noted that LZO thickness could be controlled from 30 to 50 nm for R-, Y-, G-, LB- and DBQLED. The G-QDs with an average diameter of 6 ~ 7 nm in toluene was spin-casted and annealed in a N2 filled glove box. And then, TCTA for electron blocking layer (EBL), NPD and HAT-CN layers were deposited sequentially onto QD layer by thermal evaporation in a vacuum chamber (~4 × 10-7 Torr). Finally, a 100 nm thick Al was thermally evaporated onto the top as anode and the full devices were encapsulated with glass in a N2 filled glove box.


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Characterization and instrumentation. The absorbance and PL of the QD solutions were measured with a Scinco S-4100 UV-visible spectrophotometer and Jasco FP-6500 spectrofluorometer, respectively. The TEM images and EDS data of CGL based inverted green QLEDs were obtained using FEI TitanTM 80-300 operated at an accelerating voltage from 80~300 kV. The UPS results of ITO, ITO/PP:WOx, and ITO/PP:WOx/LZO layers were obtained using Ulvac-PHI (Japan). The current density–voltage (J–V), luminance–voltage (L–V), EL spectra and EQE characteristics were measured using a Konica Minolta CS100A luminance meter and a CS2000A spectrometer coupled with a Keithley 2635A voltage and current source meter.

ASSOCIATED CONTENT Supporting Information Available: Optical characteristics of QD solution, current density plotted as function of electric field for the ETL and CGL devices, evaluation of charge generation in CGL, current density versus electric field characteristic of CGL device with increasing CGL numbers, normalized capacitance and luminance plotted as a function of applied voltage for inverted QLEDs with ETL or CGL, and device performance for the QLEDs with increasing CGL numbers. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Prof. Jin Jang. (E-mail: [email protected]), Tel: +82-2-961-0688, Fax: +82-2-961-9154 Author Contributions


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All authors contributed to this work and wrote the manuscript equally.

ACKNOWLEDGMENT This work was supported by the Human Resources Development program (No. 20134010200490) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

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Hong, S. P.; Park, H.; Oh, J. H.; Yang, H.; Jang, S.; Do, Y. R. Fabrication of Wafer-scale

Free-standing Quantum dot/Polymer Nanohybrid Films for White-Light Emitting Diodes using an Electrospray Method. J. Mater. Chem. C 2014, 2, 10439-10445. (18)

Chen, C.; Lin, C.; Lien, J.; Wang, S.; Chiang, R. Preparation of Quantum dot/Polymer

Light Conversion Films with Alleviated Fӧrster Resonance Energy Transfer Redshift. J. Mater. Chem. C 2015, 3, 196-203. (19)

Liang, R.; Yan, D.; Tian, R.; Yu, X.; Shi, W.; Li, C.; Wei, M.; Evans, D. G.; Duan, X.

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Moon, J. W.; Kim, J. S.; Min, B. G.; Kim, H. M.; Yoo, J. S. Optical Characteristics and

Longevity of Quantum Dot-Coated White LED. Opt. Mater. Express 2014. 4. 2174-2181. (21)

Cho, E.; Jang, H.; Jun, S.; Kang, H. A.; Chung, J. G.; Jang, E. Determination of the

Energy Band Gap Depending on the Oxidized Structures of Quantum Dots. J. Phys. Chem. C 2012, 116, 11792-11796.


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in Dendron Boxes: Superior Chemical, Photochemical and Thermal Stability. J. Am. Chem. Soc 2003, 125, 3901-3909. (23)

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Quantum

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Light-Emitting

Diode

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Low-Work

Function

Organic

Material

polyethylenimine ethoxylated. J. Mater. Chem. C 2014, 2, 510-514. (29)

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Demir, H. V.; Sun, X. W. Solution Processed Tungsten Oxide Interfacial Layer for Efficient Hole-Injection in Quantum Dot Light-Emitting Diodes. Small 2014, 10, 247-252.


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Son, D. I.; Kim, H. H.; Cho, S.; Hwang, D. K.; Seo, J. W.; Choi, W. K. Carrier Transport

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of Indium Tin Oxide Treatment using UV–Ozone and Argon Plasma on the Photovoltaic Parameters of Devices based on Organic Discotic Materials. Polym. Int 2006, 55, 601-607. (37)

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Koch, N. Energy Level Alignment of Electrically doped Hole Transport Layers with Transparent and Conductive Indium Tin Oxide and Polymer Anodes. J. Appl. Phys 2007, 102, 073719, pp.15.


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the Lifetime of a Polymer Light-Emitting Diode by Introducing Solution Processed Tungsten Oxide. J. Mater. Chem. C 2013, 1, 3250-3254. (43)

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Solution-Processed Aluminium–Zinc Oxide as a Cathode Buffer. J. Mater. Chem. C 2013, 1, 1567-1573.


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Page 19 of 27

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Figures

Figure 1. (a) Schematic device structure and (b) energy level diagram of the inverted QLED with CGL. Electrons and holes are generated at the interfaces of PEDOT:PSS (PP):WOx/LZO, and NPD/HAT-CN.


19


Normalized Intensity (a.u.)

(a) 1.00 eV ITO/PP:WOx/LZO

0.32 eV ITO/PP:WOx

4.20 eV

ITO

21

20

19

18

17

16

Binding Energy (eV)

(b) Normalized Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

EF

3.20 eV

ITO/PP:WOx/LZO

0.76 eV ITO/PP:WOx

ITO

4

3

2

1

0

Binding Energy (eV)

(c)

0.32 eV

Evac

1.00 eV

4.20 eV

IP = 5.28 eV

IP = 6.40 eV

EA = 3.10 eV CBM

EF Δ VB = 0.76 eV

HOMO

Eg = 3.30 eV

ΔVB = 3.20 eV VBM

ITO

PP:WOx

2% LZO

Figure 2. Ultraviolet photoelectron spectroscopy (UPS) data with (a) secondary-electron cutoff and (b) zoom-in of valance band edge. (c) Energy level alignment for p-n junction.


20

Page 21 of 27

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Figure 3. Cross-sectional TEM images of the inverted green QLED with CGL. (a) The device cross-section showing PP:WOx, LZO, G-QD, organics and Al anode (inset: zoom-up image showing QDs on LZO layer) and (b) EDS profiles along the depth of the device.


21


(b)

102

10-5 Inverted G-QLED with ETL Inverted G-QLED with CGL

-2

Current Density (mA cm )

101

-2


(a)

100 10-1 10-2 10-3 10-4 10-5 Inverted G-QLED with ETL Inverted G-QLED with CGL

10-6 10-7

10-6

10-7 0

2

4

6

8

10

-2

-1

Voltage (V)

(d)

104

-1

-2

0

1

2

Voltage (V)

Current Efficiency (cd A )

(c) Luminance (cd m )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

103

102

1

10

Inverted G-QLED with ETL Inverted G-QLED with CGL 100

30 Inverted G-QLED with ETL Inverted G-QLED with CGL

25

20

15 10

5

0

2

4

6

8

10

0

Voltage (V)

2000

4000

6000

8000

10000

Luminance (cd m-2)

Figure 4. Device performances for the G-QLED with a CGL consisting of 20 nm p-type PP:WOx and 50 nm n-type LZO. The comparison with G-QLED with an ETL of 50 nm n-type LZO can be seen. (a) Current density – voltage (semi-log scale), (b) zoomed up the current density in between -2 to 2 V (semi-log scale), (c) luminance – voltage (semi-log scale), and (d) current efficiency – luminance (linear scale) characteristics.


22

Page 23 of 27

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Figure 5. (a) Normalized EL spectra and (b) EQE data for the QLEDs with CGL using deepblue, light-blue, green, yellow and red QDs. (c) Averaged CIE coordinates of the CGL based inverted QLEDs with operating images.


23


(a)

(b)

30


101

-1

-2


102

100 10-1 10-2 10-3 10-4 ITO ITO (UV-ozone) IZO IZO (UV-ozone)

10-5 10-6

ITO ITO (UV-ozone) IZO IZO (UV-ozone)

25 20 15 10 5 0

10-7 0

2

4

6

8

0

10

(c)

(d)

6000

8000

10000

30

-1

-2


101 100 10-1 10-2 10-3 10-4 ITO ITO (UV-ozone) IZO IZO (UV-ozone)

10-6

4000

-2

102

10-5

2000

Luminance (cd m )

Voltage (V)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

ITO ITO (UV-ozone) IZO IZO (UV-ozone)

25 20

15

10 5

0

10-7 0

2

4

6

8

10

0

Voltage (V)

2000

4000

6000

8000

10000

-2

Luminance (cd m )

Figure 6. Device performances of QLEDs with (a, b) ETL and (c, d) CGL on various substrates, where UV-ozone treatment was carried out to change the work-function. (a) Current density – voltage (semi-log scale) and (b) current efficiency – luminance (linear scale) characteristics for the QLEDs with ETL. (c) Current density – voltage (semi-log scale) and (d) current efficiency – luminance (linear scale) characteristics for QLEDs with CGL.


24

Page 25 of 27

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Tables

Table 1. Summarized device performances for the inverted QLEDs with CGL or ETL. Devices

ETL

CGL

a) b)

@ 1,000 cd m-2

Colors

VT a) [V]

VD b) [V]

CEmax [cd A-1]

PEmax [lm W-1]

Red

2.4

5.9

4.6

Yellow

2.3

4.8

Green

2.4

Light-Blue

@ 10,000 cd m-2

CE [cd A-1]

PE [lm W-1]

CE [cd A-1]

PE [lm W-1]

4.6

3.7

2.0

2.0

0.7

13.4

10.8

13.4

8.8

9.7

4.4

4.9

24.7

18.4

24.7

15.7

17.0

7.1

2.8

5.2

5.7

3.8

5.7

3.4

4.3

1.9

Deep-Blue

3.3

-

0.5

0.4

-

-

-

-

Red

3.1

8.6

4.8

4.8

3.2

1.1

-

-

Yellow

2.9

7.0

15.4

10.9

14.8

6.7

8.3

2.6

Green

2.6

6.1

27.3

19.4

26.4

13.8

15.1

5.0

Light-Blue

2.9

5.8

5.7

3.4

5.7

3.1

4.0

1.4

Deep-Blue

3.4

-

0.8

0.8

-

-

-

-

-2

Measured voltage when luminance was 1 cd m . Measured voltage when luminance was 1,000 cd m-2.

Table 2. ELpeak, FWHM, maximum EQE and averaged CIE coordinates for the QLEDs with CGL.

Devices

CGL

EL peak [nm]

FWHM [nm]

EQEmax [%]

Red

637

36

Yellow

576

Green

Colors

Averaged CIE coordinates CIEx

CIEy

6.5

0.70

0.30

41

4.6

0.50

0.50

526

33

8.3

0.17

0.75

Light-Blue

500

38

2.6

0.08

0.52

Deep-Blue

444

23

1.8

0.17

0.05


25


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Page 26 of 27

Table 3. Summarized device performances for the inverted green QLEDs with ETL and CGL on various substrates. Work functions for ITO, UV/O3 treated ITO, IZO and UV/O3 treated IZO were 4.2, 4.6, 4.8 and 5.1 eV, respectively. The LZO thickness for both QLEDs was 15 nm. @ 1,000 cd m-2 Structures

G-QLEDs with ETL

G-QLEDs with CGL

a) b)

Substrates

VT a) [V]

VD b) [V]

CEmax [cd A-1]

PEmax [lm W-1]

ITO

2.4

4.3

16.2

ITO (UV/O3)

2.4

3.8

IZO

3.4

IZO (UV/O3)

@ 10,000 cd m-2

CE [cd A-1]

PE [lm W-1]

CE [cd A-1]

PE [lm W-1]

14.3

15.7

11.4

8.8

4.6

12.4

11.4

12.4

10.3

7.7

4.5

6.7

17.0

10.2

15.6

7.3

10.7

3.8

3.1

5.4

16.6

11.6

16.2

9.4

11.2

5.0

ITO

2.5

5.5

23.6

19.0

22.0

12.6

10.6

4.0

ITO (UV/O3)

2.5

4.6

22.2

19.6

21.3

14.6

10.7

4.9

IZO

2.9

6.3

25.8

19.0

22.8

11.3

10.9

3.7

IZO (UV/O3)

2.8

5.0

20.9

16.5

20.1

12.6

9.8

3.9

Measured voltage when luminance was 1 cd m-2. Measured voltage when luminance was 1,000 cd m-2.


26

Page 27 of 27

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ToC figure


27

Inverted Quantum-Dot Light Emitting Diode Using Solution Processed

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