Solution-Processed Metal-Oxide pân Charge Generation Junction for

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Solution Processed Metal-Oxide P-N Charge Generation Junction for High Performance Inverted Quantum-dot Light Emitting Diodes Hyo-Min Kim, Jeonggi Kim, Sin-Young Cho, and Jin Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14584 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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ACS Applied Materials & Interfaces

Solution Processed Metal-Oxide P-N Charge Generation Junction for High Performance Inverted Quantum-dot Light Emitting Diodes Hyo-Min Kim, Jeonggi Kim, Sin-Young Cho 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 Junction, Copper Oxide, Inverted QLED, Metal Oxides, Oxygen Vacancy

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ABSTRACT We report the solution processed metal-oxide p-n junction, Li doped CuO (Li:CuO) and Li doped ZnO (Li:ZnO), as a charge generation junction (CGJ) in quantum-dot light-emitting diode (QLED) at reverse bias. Efficient charge generation is demonstrated in a stack of air annealed Li:CuO and Li:ZnO layers in QLEDs. Air annealing of Li:ZnO on Li:CuO turns out to be a key process to decrease the oxygen vacancy (Vo) and to increase the copper(II) oxide (CuO) fraction at the Li:CuO/Li:ZnO interface for efficient charge generation. The green QLEDs (G-QLEDs) incorporating Li:CuO/Li:ZnO CGJ shows the maximum current and power efficiencies of 35.4 cd/A and 33.5 lm/W, respectively.

INTRODUCTION The charge generation layers are typically studied for tandem organic light-emitting diodes (OLED) using small molecule organic layers.1-5 The 4,4,4-tris(3-methylphenylphenylamino) triphenylamine (m-MTDATA, as a p-type)/1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN, as an n-type) or N,N’-diphenyl-N,N’-bis(1-naphthyl)-1,1’-biphenyl-4,4”-diamine (NPD)/HAT-CN layers are widely used as small molecule charge generation in OLEDs.6 The energy level alignment between highest occupied molecule orbital (HOMO) level of hole transport layer (HTL) and lowest unoccupied molecule orbital (LUMO) level of electron transporting layer (ETL) is a key for efficient charge generation. Compared to the organic compounds, metal oxides have the advantages such as good air stability, high carrier mobility and solution processability.7-8 Note that most of metal-oxide precursors are soluble in alkali solvents such as 2-methoxyethanol (2-ME), ethanol (EtOH), isopropyl alcohol (IPA) and methanol (MeOH). In addition, the solution processed metal-oxides can overcome the intermixing issue with polymer layers such as poly(9-vinylcarbazole) (PVK), poly(4-butylphenyl-


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diphenyl-amine) (p-TPD), poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB) which are coated with organic solvents such as toluene, hexane, and dichlorobenzene. Copper and nickel oxides (CuO and NiO) are typical p-type oxide semiconductors which can be used for HIL/HTL.9-13 NiO has a higher band gap of 3.2~3.3 eV compared to that of CuO14-16 and can be an efficient electron blocking layer (EBL) due to the shallow conduction band minimum. Note that CuO has relatively small band gap of 2.2~2.4 eV and both Cu2O and CuO are well known p-type oxides.17-19 S. Höfle, et al. reported the solution processed tandem polymer light emitting diodes (PLEDs) using a Super Yellow as light emitting polymer with solution processed charge generation junctions (CGJs) of zinc oxide (ZnO)/molybdenum trioxide (MoO3), tungsten trioxide (WO3) or vanadium oxide (VOx).20 We reported the tungsten oxide (WOx) doped poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS) / lithium doped zinc oxide (Li:ZnO) used as efficient CGJ in quantum dot light emitting diodes (QLEDs).21 In this work, a Li doped CuO (Li:CuO) / Li:ZnO, p-type oxide/n-type oxide, is used for charge generation. All inorganic QLEDs are of increasing interest for their potential application to stable QLED. In this case the inorganic CGJs should be used for single and tandem QLEDs. The CGJ can be used for cathode and anode buffer layers for electron and hole generations respectively. In addition, the CGJ can be used for the middle charge generation layer (CGL) of double stack QLEDs. The performance of QLED using a top CGJ on the anode is independent of the work function of the anode electrode on the substrate. ITO or other anode electrodes can be used without plasma treatment because the device performance is independent of the work function of the anode. Annealing environment is found to be important to achieve good n-type oxide semiconductor. We compared the annealing effect of Li:ZnO in N2 and in air. The green QLED (G-QLED) with


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Li:CuO/Li:ZnO CGJ with air annealing exhibits the maximum current and power efficiencies of 35.4 cd/A and 33.5 lm/W, respectively. Note that G-QLED with oxide CGJ shows comparable device performance with G-QLED using conventional electron injection and transport layers (34.9 cd/A and 32.4 lm/W). Compared to our previous paper, we achieved a higher G-QLED performance using Li:CuO/air annealed Li:ZnO CGJ. Improved G-QLED performance with metal oxide CGJs in this study is due to the efficient charge generation ability compared to the charge generation in the PEDOT:PSS:WO3/LZO CGJ. It is noted that the Li:CuO/LZO CGJ shows ~2 times higher current density at -3 V than that of PP:WOx/LZO CGJ.

RESULTS AND DISCUSSION Green Quantum-Dots (G-QDs) for the inverted QLEDs. The G-QDs solution with concentration of 10 mg/ml was supplied from Nanosquare. Inc. Korea, showing photoluminescence (PL) peak at 520 nm with full-width at half maximum (FWHM) of 34 nm as shown in Figure S1 (Supporting Information). Note that G-QDs consisting of CdSe core and CdS/ZnS gradient shell (CdSe/CdS/ZnS) are dissolved in toluene. Li Doping Effect in CuO. Li doping effect in CuO has been studied for photochemical cell by C. Chiang et al,23-24 reporting that Li doping in CuO improves both its crystallite size and conductivity. Due to relatively high resistance of CuO, it is essential to reduce its resistance to improve the hole injection property. Among the various dopants such as Li, Ni, Zn, and Mg, Li is selected in this work as a dopant in CuO. We found that the optical band gap of Li:CuO film increases from 2.34 (pristine CuO) to 2.49 eV (40% Li:CuO) as shown in Figure S2 and Table S1. And, we found that the conductivity increases by ~10 times by 20% Li doping in CuO as shown in Figure S3 and Table S2.


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The green phosphorescence OLEDs (Ph.OLEDs) with various HILs such as CuO, Li:CuO and HAT-CN, were fabricated and the device performances are summarized in Supporting information (Figure S4 and Table S3). Device structure is ITO / HILs (20 nm) / NPD (50 nm) / TCTA (5 nm) / TCTA:TPBi: 12% Ir(ppy)3 (15 nm) / TPBi (40 nm) / Liq (1.5 nm) / Al (100 nm). It is found that the OLEDs with Li doped CuO HIL show the similar device performance of the OLED with HAT-CN HIL by vacuum evaporation. Energy Band Alignment for Metal Oxide P-N CGJ in Inverted G-QLED. The energy bands of n- and p-type oxides were measured by ultraviolet photoelectron spectroscopy (UPS) with an ITO reference as shown in Figure 1. The work-functions (WFs) of Li:CuO, Li:ZnO and G-QDs are calculated from the secondary electron cutoff regions and found to be 4.29, 2.95 and 2.90 eV, respectively. Li:CuO is considered to have higher resistivity compared with PEDOT:PSS because its Fermi level exists 0.68 eV above the valence band edge.25-26 Charge Generation at Li:CuO/Li:ZnO Interface. Current density versus voltage characteristics of a CGJ at positive and negative biases are usually measured to check the charge generation and recombination. It is noted that the charge generations at NPD/HAT-CN and mMTDATA/HAT-CN interfaces are well-known phenomena.6 Figure 2 shows energy band diagram and current density versus voltage characteristics of both oxide p-n and NPD/HAT-CN junctions for a comparison. It is found that effective charge generation at the Li:CuO/Li:ZnO interface is confirmed only when air annealed Li:ZnO (not N2 annealed) is adopted on the p-type Li:CuO layer, therefore, we predict that the charge generation at the Li:CuO/Li:ZnO interface is related with O vacancy in metal-oxides. As shown in Figure 2c, the metal oxide p-n diode showing effective charge generation exhibits almost symmetric J-V curve (red symbol) which is similar to a NPD/HAT-


Relative CGE % =

Abs Reverse J = −3 × 100 %. − − − − − − 1 Abs Forward J = +3

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CN junction. Figure 2d shows the normalized current density at 3V versus voltage characteristics of both CGJs to see charge generations at reverse bias, and it is can be seen that the Li:CuO/Li:ZnO junction has better symmetric curve than that of NPD/HAT-CN. The relative charge generation efficiencies (CGEs) of NPD/HAT-CN and Li:CuO/Li:ZnO junctions are calculated to be 67 % and 83 % at -3 V, respectively, which is defined as the following equation (1): The inset in Figure 2d exhibits the current density versus voltage characteristics of Li:CuO/Li:ZnO p-n junction showing good ohmic contact between -0.2 to 0.2 V and it can be an efficient charge generation unit. The Li:CuO and Li:ZnO layers annealed in air exhibit monoclinic crystal and amorphous phases respectively. The Li:CuO on ITO shows the crystalline size of 20 to 80 nm as shown in Figure 3a. A 20 at.% Li:CuO has XRD peaks at 35.601 ° and 38.902 ° with d-spacing of 2.5198 Å and 2.3132 Å, respectively, exhibiting a monoclinic crystal structure. As shown in atomic force microscopy (AFM) and scanning electron microscope (SEM) images (Figure 3b-3h), the Li:CuO on ITO has a root-mean-square (Rq) roughness of 5.48 nm which is higher than that (4.32 nm) of ITO substrate. And, by LZO deposition on Li:CuO, the Rq roughness decreases from 5.48 to 2.44 nm and the details are summarized in Table 1. The relationship between charge generation and O vacancy in metal oxides can be obtained from X-ray photoelectron spectroscopy (XPS) depth profile. Figure 4 shows the XPS depth profile of Li:ZnO layers deposited on the Li:CuO which was annealed in air or in N2. Cu, Ni, Zn and O atoms can be seen at the positions P1, P2, P3 and P4 as shown in Figure 4a and 4b. It is noted that the distances between the top surface and P1 to P4 points are not constant due to the rough surface of Li:CuO as shown in Figure 3c and 3d. The metal-oxygen (M-O), Vo and


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oxygen-hydrogen (O-H) concentrations could be seen at the Li:CuO/N2 annealed Li:ZnO and Li:CuO/air annealed Li:ZnO films from XPS analysis. Note that the Vo induces charge trapping27 and its concentration decreases gradually from the top surface (P1) until P4 through the positions P2 and P3 for the Li:CuO/air annealed Li:ZnO layer compared to those of Li:CuO/N2 annealed Li:ZnO layer. As shown in Figure 4c and 4f, both O-H intensity and Vo concentration at the Li:CuO/air annealed interface are lower than those at the Li:CuO/N2 annealed Li:ZnO. Higher M-O concentration at the Li:CuO/air annealed Li:ZnO can contribute to efficient charge generation. The binding energies for M-O, Vo and O-H at O1s are 529.8, 531.3 and 532.2 eV, respectively, and the details are shown in Figure S5. Additionally, it can be seen that the annealing in air improve the surface roughness of nanocrystalline Li:CuO. The Cu2p3/2 binding at the Li:CuO/Li:ZnO interface is analyzed by XPS depth profile. The copper oxide has two stable states of CuO (Cu2+ + O2- → CuO) and Cu2O (2Cu+ + O2- → Cu2O), and it is known that CuO is a more suitable p-type semiconductor than Cu2O due to its higher conductivity.28 Figure 4d and 4g exhibits the CuxO percentages at the positions from P2 to P4, and it is confirmed that the CuO intensity is higher than Cu2O at the Li:CuO/air annealed Li:ZnO interface. On the contrary, the strong Cu2O intensity is confirmed at the Li:CuO/N2 annealed Li:ZnO interface. It means that the copper oxide formation can be affected by upper-layer formation and the details are summarized in Supplementary Information (Figure S6). The G-QD is deposited on the LZO layer so that its PL intensity depends on the interface states between LZO and G-QD. To confirm the average exciton decay time of G-QDs on various interlayers, we measured time-resolved PL (TRPL) and summarized it in Figure 5 and Table 2. The exciton decay time decreases from 13.2 ns on glass to 9.4 ns on air annealed Li:CuO, to 8.8 ns on air annealed Li:ZnO as shown in Figure 5. However, it is confirmed that the QD layer


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deposition on the N2 annealed LZO recovers the exciton decay time from 8.8 ns to 11.6 ns. The recovered exciton decay time indicates that the exciton quenching site decreases remarkably at the interface between N2 annealed LZO and G-QDs. To find the relationship between charge generation, Cu-O binding and oxygen vacancy at metal oxide p-n junction, two kinds of inverted G-QLEDs with or without air annealed Li:ZnO layer on p-type Li:CuO were fabricated as shown in Figure 6a-6c. Device structure is ITO/Li:CuO (20 nm)/N2 annealed Li:ZnO (50 nm)/G-QDs/TCTA (10 nm)/NPD (20 nm)/HATCN (20 nm)/Al and ITO/Li:CuO (20 nm)/Air annealed Li:ZnO (15 nm)/N2 annealed Li:ZnO (50 nm)/G-QDs/TCTA (10 nm)/NPD (20 nm)/HAT-CN (20 nm)/Al. It is confirmed that the inverted G-QLED introducing air annealed Li:ZnO layer on Li:CuO (red symbol) shows better diode characteristics such as higher on-off current ratio (Ion/Ioff = 106) and better luminance. This is comparable to those of inverted G-QLED without the first air annealed Li:ZnO layer (black symbol). The inverted G-QLED without the air annealed Li:ZnO layer shows extremely low current density ( (ns)

Glass / G-QDs

13.2

Glass / Li-CuO / G-QDs

9.4

Glass / Li-CuO / LZO (air) / G-QDs

8.8

Glass / Li-CuO / LZO (air) / LZO (N2) / G-QDs

11.6


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Table 3. Summary for the device performances of G-QLEDs with metal oxide p-n CGJ using N2 or air annealed Li:ZnO on p-type Li:CuO. The annealing temperature dependence is also shown for the air annealed Li:ZnO on p-type Li:CuO. @ 1,000 cd/m2 VT (V)

VD (V)

CEmax (cd/A)

7.4

-

10.4

3.1

11.2

27.6

N2 annealed LZO on Li:CuO Air annealed LZO on Li:CuO

PEmax (lm/W)

@ 10,000 cd/m2

CE (cd/A)

PE (lm/W)

CE (cd/A)

PE (lm/W)

3.3

-

-

-

-

18.7

20.4

5.7

10.8

2.4

@ 1,000 cd/m2 Annealing temperature (°C)

VT (V)

CEmax (cd/A)

VD (V)

PEmax (lm/W)

CE (cd/A)

@ 10,000 cd/m2

PE (lm/W)

CE (cd/A)

3.1 11.2 27.6 18.7 20.4 5.7 10.8 160 2.6 6.8 25.4 20.2 22.8 10.5 11.7 190 2.6 6.3 27.1 22.2 25.1 12.6 13.6 220 2.7 6.2 27.9 17.2 20.4 10.3 9.5 250 ※ VT and VD were defined as the voltages when luminance is 1.0 cd/m2 and 1,000 cd/m2, respectively.

PE (lm/W) 2.4 3.5 4.4 3.0

Table 4. Summary for the device performance of the QLEDs with metal-oxide CGJ. A comparison with reference device (single ETL) is shown. @ 1,000 cd/m2 Device

VT (V)

VD (V)

CEmax (cd/A)

PEmax (lm/W)

Reference QLED

2.7

5.8

34.9

CGJ QLED

2.8

6.4

35.4

@ 10,000 cd/m2

CE (cd/A)

PE (lm/W)

CE (cd/A)

PE (lm/W)

32.4

30.4

16.4

16.3

6.0

33.5

27.8

13.6

14.0

4.5

2

2

※ VT and VD were defined as the voltages when luminance is 1.0 cd/m and 1,000 cd/m , respectively.


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Solution-Processed Metal-Oxide pân Charge Generation Junction for

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