Metal-Oxide Stacked Electron Transport Layer for Highly Efficient

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Metal-Oxide Stacked Electron Transport Layer for Highly Efficient Inverted Quantum-Dot Light Emitting Diodes Hyo-Min Kim, Di Geng, Jeonggi Kim, Eunsa Hwang, and Jin Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10314 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 8, 2016

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ACS Applied Materials & Interfaces

Metal-Oxide Stacked Electron Transport Layer for Highly Efficient Inverted Quantum-Dot Light Emitting Diodes Hyo-Min Kim, Di Geng, Jeonggi Kim, Eunsa Hwang 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: high efficiency, LZO, metal-oxide, quantum-dot, QLED, stack structure

ACS Paragon Plus Environment

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ABSTRACT We report highly efficient inverted QLED using Al doped ZnO (AZO)/Li doped ZnO (LZO) stack electron transport layer (ETL). An introduction of LZO layer on AZO improved the current and power efficiencies of the green (G-) QLED from 10.5 to 34.0 cd A-1 and from 5.4 to 29.6 lm W-1, respectively. The red (R-), G- and blue (B-) QLEDs fabricated in this work exhibited the maximum external quantum efficiencies (EQEs) of 8.4, 12.5 and 4.3 %, respectively. It is found from time-resolved photoluminescence (PL) and transient electroluminescence (EL) decay that exciton loss at the interface between ETL and emission layer can be significantly reduced by introducing LZO.

INTRODUCTION Colloidal quantum-dots (QDs) are of increasing interest for their applications for biological imaging1-3, photo-detectors4,5, solar cells6,7, and light-emitting diodes (LEDs)8,9. Quantum-dot light emitting diodes (QLEDs) have been extensively studied for low power consumption solidstate lightings and flat-panel displays, since QDs have unique size dependent properties, such as color tenability and good color purity of narrow full-width at half maximum (FWHM) less than 40 nm10-13. Therefore, QDs based LEDs can offer significant advantages in display applications with low-cost manufacturing process. To improve the efficiency of QLED, the materials and device architecture should be optimized. QLEDs have the issues such as QD material stability, QD particle aggregation and uniform layer formation because of their small size (3 ~ 10 nm). Therefore, the under-layer of QDs should have smooth surface in order to have uniform QD printing14,15. Recently, organic/inorganic hybrid QLEDs have been shown to be highly efficient as compared to organic/organic or inorganic/inorganic QLEDs16,17.


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Typical QLEDs consist of a QD layer sandwiched between an organic hole transporting layer and an electron transporting layer, but the organic under-layer can be damaged by QD solvent. Organic materials can be dissolved in solvents such as toluene, chlorobenzene, hexane, etc., and these solvents can induce inter-mixing QDs with the under-layer18,19. To overcome material intermixing, an inverted QLED with metal-oxide as an electron injection or/and transporting layer (EIL or/and ETL) has been designed, with zinc oxide (ZnO), titanium oxide (TiOx) and aluminum doped ZnO (AZO) 16,20. The inverted QLEDs exhibit better performance with lower turn-on (VT), and driving voltages (VD), higher maximum luminance and higher external quantum efficiency (EQE) compared to regular structured QLEDs having a hole injection layer (HIL) on indium tin oxide (ITO) anode20. The use of metal oxides as EIL or HIL has been reported to be more stable against oxygen and moisture exposures compared to organic material counterparts21. An inverted QLED is promising device structure for display application when oxide thin-film transistor (TFT) backplane is used because the cathode of QLED is connected to the drain of nchannel oxide TFT. The oxide TFTs can be used for large area and high resolution display because of their high mobility and low cost manufacturing. To make efficient QLEDs, it is important to find appropriate materials which have wellmatched energy levels with QDs; the energy level alignments between ETL, QDs and HTL. The energy barrier from ETL or HTL to QDs can affect the device performances such as threshold voltage, driving voltage and current efficiency (CE), and thus it is important to reduce the energy barrier. The most important condition for highly efficient device is charge balance in active layer. For efficient charge injection to EML and charge balance in EML, an inverted QLED using polyethylenimine ethoxylated (PEIE) on ITO is used to reduce its work function22,23. And, to have


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efficient electron injection to QDs, an inverted QLEDs using TiO2/ZnO nanoparticles (NPs) or ZnO NPs/Cs2CO3 are reported16,24. Inverted green QLED (G-QLED) with 4,4′-bis(carbazole-9-yl)biphenyl (CBP) having lower HOMO level as HTL exhibited the maximum CE and external quantum efficiency (EQE) of 19.2 cd A-1 and 5.8 %, respectively21. Recently, highly efficient G-QLED with conventional structure with large-sized (12.7 nm) green QDs having ZnS outer shell shows the high CE of 46.4 cd A-1 and EQE of 12.6 %25. The solution processed Li doped ZnO (LZO) TFTs is found to have lower carrier concentration than that of undoped ZnO26. It also reported that 2% Li doped ZnO based TFT has a higher fieldeffect mobility than that of 0, 5 or 10% Li doped ZnO TFT. In addition, LZO TFTs using spray deposition shows the electron mobility over 50 cm2 V-1s-1 27. The role of AZO/LZO stack used in this work is electron injection/transport to the QD. The charge generation layer (CGL) consisting of WOx doped PEDOT:PSS (p-type) and Li doped ZnO (n-type) is used for electron/hole generations in our previous paper which is very different from current work28. The generated holes are injected into the substrate side and the generated electrons transport to the QD layer. The advantage of CGL is to use the substrates with wide variation of work functions28. In our SID 2014 paper a stack EIL/ETL was used29, but the use of LZO material as an ETL, its process conditions and properties could be found only in this work29. A key material for stack ETL is LZO and thus the optimization of its properties were carried out in this paper and thus higher efficiency could be achieved. In this study, we have studied the inverted red (R-), green (G-) and blue (B-) QLEDs with stacked metal oxide as ETL, 2% LZO layer (2nd ETL) on a 30% Al doped ZnO (1st ETL). The device performance of inverted green QLED, such as CE and power efficiency (PE), was improved


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by ~3 times compared to those of control device (AZO based QLED). And, we have fabricated the G-QLED with a single LZO ETL, exhibiting the maximum CE of 16.2 cd/A and maximum PE of 14.3 lm/W. The electrical property of LZO was studied by the analysis of electron-only devices (EODs). Atomic force microscopy (AFM), ultraviolet photon spectroscopy (UPS), transient electroluminescence (EL), time resolved photoluminescence (TRPL) and X-ray Photoelectron spectroscopy (XPS) were measured to characterize LZO and AZO layers.

RESULTS AND DISCUSSION Figure 1 shows the UPS spectra of AZO/LZO stack ETL on ITO substrate and energy band diagram of the inverted G-QLED. Figure 1a exhibits the secondary-electron cutoff region of the stacked ETL and Figure 1b, 1c and 1d shows Fermi-edge regions of ITO, AZO and LZO, respectively. Note that black, red and blue symbols show the UPS data of ITO, ITO/AZO and ITO/AZO/LZO, respectively, and the work function (WF) of ITO substrate is 4.2 eV. In the secondary electron cutoff regions between red and blue, we confirmed that the WFs for AZO and LZO are 3.52 and 3.45 eV, respectively. And, Figure 1b, 1c and 1d shows a zoom-in of the valance band (VB) edge, indicating the VB shift (ΔVB) relative to the Fermi level of ITO, showing that ΔVB’s of AZO and LZO are 3.13 and 3.08 eV, respectively. Note that the optical band gaps (Eg) for the AZO and LZO were found to be 3.4 and 3.15 eV, respectively. According to the results of ΔVB and Eg, the conduction band minimum (CBM) and valance band maximum (VBM) levels are 3.25 and 6.65 eV for AZO, and 3.38 and 6.53 eV for LZO, respectively. Therefore, LZO has a small energy difference of 0.07 eV between CB and WF. Note that our G-QDs has 3.84 eV for CBM and 6.14 eV for VBM (Figure S1, supporting information), therefore, the energy level diagram is shown in Figure 1e. It is noted that the device structure of the inverted G-QLED is ITO


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/ AZO (50 nm) / LZO (15 nm) / CdSe/CdS/ZnS QDs / Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) (10 nm) / N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPD) (20 nm) / dipyrazino[2,3-f:2',3'-h]quinozaline-2,3,6,7,10,11-hexacarbonitrile (HAT-CN) (20 nm) / Al (100 nm). The device was designed to have efficient electron blocking to confine electrons in QD layer because of shallow lowest unoccupied molecular orbital (LUMO) level of TCTA (2.5 eV) (See Figure 1e). The cross-sectional TEM images of the device are shown in Figure 2a, indicating a clear line between LZO and QDs layers. And we found that the green QDs have 3.5 ~ 4.0 multi-layers (MLs) as shown in Figure 2b. Additionally, the R-, G- and B-QDs solutions have PL peaks at 629, 520 and 441 nm, respectively (Figure S2). And Figure 2c, 2d, 2e and 2f shows the AFM images for AZO, AZO/LZO, AZO/G-QD, AZO/LZO/G-QD layers respectively. A 50 nm thick AZO was deposited on glass and it was annealed at 225 °C for 10 min in air (Fig. 2c). And then, LZO layer was deposited on AZO and annealed at 160 °C for 10 min in a N2 glove box (Fig. 2d). Note that the root-mean-square (RMS) roughness (Rq), peak-to-valley roughness (Rpv) and average roughness (Ra) are improved upon LZO deposition on AZO. Green QDs (G-QDs) were deposited on AZO and on AZO/LZO using ITO substrate and the thickness of each layer was the same as that of QLED. It is noted that the Rpv, Rq and Ra of AZO/GQDs are 12.61, 1.64 and 12.45 nm, respectively. But, the Rpv, Rq and Ra for LZO/AZO/G-QDs decrease to 11.97, 1.59 and 11.79 nm, respectively, by LZO coating. Among the roughness values, Rsk is a factor to define the surface skewness of mean value30. The skewness describes the shape and asymmetry of the amplitude distribution function (ADF) and can be calculated by the following equation with L and r represents the total number of heights and height values in the AFM topographic map, respectively;


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L

1 Rsk = r 3 ( x)dx 3 ∫ LRq 0

.

(1)

It is noted that a symmetric height distribution of thin-film surface can have Rsk value close to 0. Also, the negative and positive Rsk values are due to the surface morphology such as holes and peaks30. The Rsk of QDs on AZO / LZO decrease from 0.07 to 0.02 by LZO deposition, indicating that LZO coating improves the roughness balance between spikes and valleys of QD layer (Table 1). Figure 2g shows that there is no LZO crystalline peak corresponding to ~34°, confirming amorphous structure of LZO31. The performances of the optimized QLEDs are shown in Figure S3. Here, we defined VT and VD as the voltage giving the luminance of 0.1 cd m-2 and of 100 cd m-2, respectively. Device performances with various LZO thicknesses are shown in Figure S3a and S3b, exhibiting VT, VD, maximum current and power efficiencies as a function of LZO thickness. It is noted that VT and VD decrease by inserting LZO on AZO, and they are almost constant with increasing LZO thickness over ~ 15 nm. However, the device performance, such as CE and PE, degrades significantly with increasing LZO thickness from 15 to 55 nm. The device performances as a function of LZO annealing temperature with fixed LZO thickness of ~ 25 nm are shown in Figure S3c and S3d. As the annealing temperature increases from 160 to 250 °C, VT and VD does not change, however, the maximum CE and PE degrade significantly. When LZO annealing temperature is 160 °C, the maximum current and power efficiencies are 28.3 cd A-1 and 22.1 lm W-1, respectively. And, those degrade to 12.4 cd A-1 and 7.1 lm W-1, respectively, when LZO annealing temperature increases to 250 °C. Additionally, the current density increases steadily from 10-5 to 10-1 mA cm-2 at 2 V when LZO annealing temperature increases from 160 °C to 250 °C (Figure S4, Table S1). We have also fabricated the devices at different annealing conditions; in N2 or in air, at the


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fixed annealing temperature of 160 °C and found that the performance of QLED depends strongly on annealing environment (Figure S5). The increased conductivity of LZO with increasing its thickness affects to charge balance in QD EML and thus degrade QLED performances (Figure S6, Table S2). More details are shown in Table S2 achieved from Figure S6 using the device structure of ITO / Al / ETLs / LiF:Al. The optimized thickness, annealing temperature and annealing condition of LZO layer are ~15 nm, 160 °C and in N2, respectively. Structural and electrical properties of solution processed oxide semiconductor used for oxide TFT depend on its thickness. For example, it is found that conductivity of solution processed oxide semiconductor increases with increasing thickness and thus the transfer curve of solution processed oxide TFT shifts to negative gate voltage32,33. It is noted that the defect density increases with increasing film thickness and thus the conductivity of LZO film increases with increasing thickness. Annealing temperature for the solution processed oxide is also a key process parameter for solution processed oxide semiconductor. It is found that the conductivity of solution processed ITO and ZnO nanoparticles increase with annealing temperature34,35. There is no particular optimum annealing temperature which can apply for all solution processed oxide semiconductors but it depends on the material. In this work the optimum temperature was found to be 160 °C. Annealing atmosphere is an important parameter for the structure and electrical properties of oxide semiconductor because oxygen can be diffused into oxide semiconductor if it is annealed in air. It is found that air annealed ZnO TFT exhibits lower electron mobility than N2 annealed ZnO TFT at >240 °C. In contrast, under 240 °C annealing, air annealed ZnO TFT shows higher electron mobility than N2 annealed device36. Therefore, there is no general rule for the annealing atmosphere of solution processed oxide semiconductors. The annealing in air leads to higher conductivity for LZO layer compared to the film annealed in N2 and affects to charge balance in


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QD EML. Therefore, higher QLED efficiency could be achieved by using LZO annealed in N2. To check the increased conductivity of AZO/LZO layer (Table S2), we have measured the capacitances of AZO and AZO/LZO thin-films as function of frequency. The deposition conditions of AZO and LZO films were the same as for QLED fabrication, and the capacitance data are shown in Figure S7. The capacitances of both films do not change as a variation of frequency between 100 Hz to 10 kHz, however, the AZO/LZO layer has 3.5 times higher capacitance than that of AZO film at 1 kHz, which is due to much higher conductivity of LZO compared with that of AZO. The device performances are shown in Figure 3. Figure 3a exhibits current density and luminance characteristics of the inverted QLEDs with and without LZO layer as function of voltage. Note that open and close symbols exhibit current density and luminance characteristics, respectively. The luminance of AZO/LZO based QLED is much higher than that of AZO based QLED at the same applied voltage. The reference device (QLED with AZO ETL) exhibited the maximum current and power efficiencies of 10.5 cd A-1 and 5.4 lm W-1, respectively. And, the current efficiencies at 1,000 cd m-2 and 10,000 cd m-2 were 10.4 cd A-1 and 8.9 cd A-1, respectively. And Figure 3b and 3c shows current and power efficiencies versus luminance, respectively, showing that the maximum current and power efficiencies of stacked ETL QLED are 34.0 cd A-1 and 29.6 lm W-1, respectively. The significant roll-off for the current efficiency for the QLED with AZO/LZO may be related with charge balance in QD layer which is due to the fast electron injection and transport at high electric field from ITO to QDs. As shown in Figure 3a, the AZO/LZO based QLED shows rapidly increasing current density compared to that of AZO based QLED. Note that the AZO/LZO based QLED has a relatively ideal diode property than AZO based QLED. The lower current density of AZO/LZO based QLED below ~ 3 V is related with an interface of ETL/QDs. The PL intensity result of Figure 4a indicates


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that the insertion of LZO layer on AZO reduces the defects at the interface of ETL/QD, and thus increases the PL intensity by 200 %. It means that AZO based QLED have higher leakage current than AZO/LZO based QLED. On the other hand, in the operating voltage region of 3 to 7 V, the current density of AZO/LZO based QLED increases rapidly compared to that of AZO based QLED because of better diode performance. It is noted that the AZO based QLED shows a relatively higher current density below ~ 3 V compared to that of AZO/LZO based QLED due to a lot of defects at the AZO surface. Exciton decay time is the most important factor to evaluate exciton loss at the interface of ETL/EML or EML/HTL. We have measured the PL intensity of G-QDs on AZO and on AZO/LZO; sample#1 (black): glass / G-QDs, sample#2 (red): glass / AZO / G-QDs and sample#3 (blue): glass / AZO / LZO / G-QDs (Figure 4a). By 50 nm AZO deposition on glass (red symbol), PL intensity of G-QDs drops by 44%. However, the PL intensity of G-QDs increases by ~200 % by coating LZO on AZO layer (Sample #2), which is due to the reduction of exciton loss at the LZO/G-QDs interface. Note that the excitons at QDs can be recombined at the ETL/GDs and QDs/HTL interfaces. We have measured TRPL and transient EL to study the effect of LZO deposition on exciton recombination (Figure 4b, 4c, 4d and Table 2). In Figure 4b, the black and red lines exhibit the normalized PL intensity for AZO/G-QDs/TCTA and AZO/LZO/G-QDs/TCTA, respectively, as function of time. The excitation wavelength, power and pulse duration for TRPL measurements were 355 nm, 1 mW and ~150 fs, respectively. The LZO coating recovers exciton decay time from 4.5 ns to 5.0 ns. PL intensity depends strong on exciton quenching sites at interface37. Liu. S et al. reported that OH species such as nickel oxy-hydroxide (NiOOH) are well-known luminance quenchers. To have the relationship between OH bonds at the metal oxide surface and PL


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quenching, we measured XPS of O1s for the AZO and LZO surfaces and the results are shown in Figure S8. The O-H concentrations of AZO and LZO surfaces were 24.9 and 13.9 %, respectively, therefore we concluded that the exciton quenching sites by O-H bonds at the interface of ETL/GQDs can be reduced by LZO deposition on AZO layer. To support the increase of exciton decay time of G-QDs, we measured transient EL for both QLEDs. We fixed current density as ~3.4 mA cm-2 and obtained the luminance decay time from a bi-exponential plot. Note that the pulse width and frequency for transient EL measurement were 30 μs and 10 kHz, respectively. The luminance decay times (@ L0=50%) for AZO ETL based and AZO/LZO ETL based QLEDs are 90 and 250 ns, respectively (Figure 4c and 4d). An interesting point in the transient EL is the difference in luminance overshoot between the two devices and it is related to the radiative and non-radiative recombination of injected charges38. The longer decay and strong overshoot in transient EL of the QLED with AZO/LZO is due to the less non-radiative recombination in the QLED. Lee. J et al39 report the luminance overshoot phenomenon at transient EL data when charges are accumulated at interface of ETL/EML or HTL/EML. And they explained the luminance overshoot as the existence of residual trapped charges upon the voltage pulse turn-off. From Figure 4(c), it appears that relatively large amount of electrons are accumulated at the interface of LZO/G-QDs in AZO/LZO based G-QLED at same current level compared to AZO based G-QLED. To study the longer decay time with transient EL, we have simulated RC delay by SPICE simulator using the measured maximum resistance and capacitance of AZO- and AZO/LZO- based QLEDs as function of voltage using equivalent circuit (Figure S9a, S9b and S9c). The pulse width of 30 μs and frequency of 10 kHz were used in the simulation according to the measurement conditions. The AZO/LZO- based QLED has a slightly longer delay time of 24.3 ns than that (21.4 ns) of AZO based QLED. The simulated RC delay time of


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AZO/LZO based QLED is much smaller compared to the measured luminance decay time (250 ns) of AZO/LZO based QLED (Figure S9d, S9e and S9f). Therefore, the elongated luminance decay time for AZO/LZO based QLED is mainly due to the exciton recombination rather than RC delay. The longer decay time is due to much less nonradiative centers at the QD/LZO interface. In case of R-QLED, the maximum current and power efficiency are improved from 3.3 to 6.1 cd A-1 and 4.2 to 7.1 lm W-1, respectively, by adding LZO. And, the B-QLED with stacked ETL shows the maximum current and power efficiencies of 0.9 cd A-1 and 0.6 lm W-1, respectively. The stacked ETL based R- and B-QLEDs have the NPD layer with 20 nm which is same as that in GQLED. The device performances for R-, G- and B-QLEDs with stacked ETL are summarized in Table 3. Additionally, note that there is LZO defect emission at 510 nm when the NPD thickness is 20 nm (Figure S10). The energy level and band gap of QDs can affect charge balance in QDs. The different VBs of R- and B-QDs (5.8 eV for R-QDs and 6.3 eV for B-QDs) compared to GQDs (6.1 eV) lead to change the energy barrier for hole transport, therefore, the thickness of charge transporting layers should be controlled to have charge balance in QDs. We fabricated electron- and hole-only devices (EOD and HOD); EOD: ITO / AZO (50 nm) / LZO (15 nm) / LiF:Al (100 nm), and HOD: ITO / PEDOT:PSS (40 nm) / TCTA (10 nm) / NPD (20 nm) / HAT-CN (20 nm) / Al (100 nm), respectively. For HOD, we deposited 40 nm PEDOT:PSS as HIL and the thickness of the other layer is the same as that of QLED. Compared to the current flow between EOD and HOD, we could confirm that hole current is faster than electron current at below 2 V (Figure S11). Thus, holes can accumulate in the LZO layer due to the energy barrier of 0.39 eV between VB of LZO and VB of G-QDs. It was confirmed the existence of residual 520 nm EL peak during our optimization process of R-QLED which can be due to the LZO defect luminescence40-42.


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The device performances for R-, G- and B-QLEDs with stacked ETL are shown in Figure 5. Figure 5a, 5b and 5c shows the normalized EL spectra, EQE versus current density and CIE coordinates of the inverted R-, G- and B-QLEDs, respectively. The EL peaks are at 643, 523 and 444 nm at the maximum EQEs with the FWHMs of 32, 34 and 24 nm, respectively. The optimized R-, G- and B-QLEDs exhibit the maximum EQEs of 8.4, 12.5 and 4.3 %, respectively (Table 4). The color gamut with R-, G- and B-QLEDs is 119 % of NTSC (the National Television System Committee 1931). And Figure 5d and 5e shows the histograms of current and power efficiencies of 30 devices from 7 batches, yielding an average current and power efficiencies of 33.1 cd A-1 and 29.1 lm W-1, respectively. The good reproducibility of the devices demonstrates the feasibility of LZO as 2nd ETL in the inverted QLEDs. Additionally, we have compared AZO/LZO based G-QLED with G-QLED with a single LZO to verify the advantage of stack ETL. The device performances of single LZO and AZO/LZO based G-QLEDs are shown in Figure S12, indicating that the stack ETL G-QLED shows much higher device performance than that of single AZO or single LZO based G-QLEDs. The G-QLEDs with stack ETL and single LZO ETL showed the maximum CE’s of 34.0 and 16.2 cd/A, respectively. The lower current density of AZO/LZO based QLED compared to single LZO based QLED is due to the higher resistance of 50 nm thick AZO layer43, however, it leads to positive effect for charge balance in QD EML. The advantages of LZO layer for QLED can be summarized as followings: i) the LZO on AZO layer increases the conductivity; ii) the surface roughness of the under-layer of QD is improved by introducing LZO; iii) higher electron concentration of AZO/LZO induces efficient charge transport to the QD EML; iv) the reduction in exciton loss and quenching sites at the interface with QD layer increase current efficiency; v) LZO increases exciton and EL decay times in QLED. The effect of


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reduction of exciton quenching sites appears to be the most important and the improvement of roughness balance between spikes and valleys under QD layer is also related with the reduction of the quenching sites. Therefore, by introducing LZO, current and power efficiencies could be improved significantly when a stack ETL is used.

CONCLUSION We report highly efficient inverted QLED using sol-gel processed metal oxide stack layers as ETLs. An introduction of LZO layer on AZO can improve the current efficiency of G-QLED by ~3 times compared to the control device (AZO based inverted G-QLED). The maximum current and power efficiencies of the inverted G-QLED exhibit 34.0 cd A-1 and 29.6 lm W-1, and those are 6.1 cd A-1 and 7.1 lm W-1 for R-QLED, 0.9 cd A-1 and 0.6 lm W-1 for B-QLED, respectively. The R-, G- and B-QLEDs fabricated in this work exhibit the maximum EQEs of 8.4, 12.5 and 4.3 % with CIE coordinates (0.70, 0.30), (0.16, 0.75) and (0.17, 0.05), respectively. It is found from TRPL and transient EL decay results that loss of excitons at the interface of ETL/EML with QDs can be reduced remarkably by introducing LZO.

EXPERIMENTAL Materials synthesis: The details on the material preparation of 30% AZO used as a 1st ETL appear in our previous article43. Introducing LZO showed better device performance as compared to pristine ZnO; For 2% LZO sol-gel solution, zinc acetate dihydrate (Zn(C4H6O4)2 + 2H2O) 1.10 g and 2.0% of lithium acetate dihydrate (CH3COOLi + 2H2O) were mixed in a three necked beaker. Monoethanolamine (MEA) was mixed with LZO powders in order to have a stable LZO complex


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and then it was dissolved in 50 mL ethanol and then refluxed at 60 °C for 12 hrs at a stirring speed of 400 rpm until a transparent solution was obtained28. QLED fabrication: We used CdSe/ZnS QDs (core/shell type) for red, CdSe/CdS/ZnS QDs (core/gradient shell type) for green and ZnCdS/ZnS QDs (core/shell type) for blue emissions, supported from Nanosquare Inc., Korea. The green QDs have conduction band (CB) and VB edge energy levels of 3.9 and 6.2 eV, respectively. And, ~50 nm thick 30% AZO was used as the 1st ETL achieved by spin-coating at 2000 rpm onto ITO having a sheet resistance of 8~10 Ω sq-1 and then annealed at 225 °C for 10 min in ambient air. After annealing the AZO layer, the LZO was spin-coated at 2,000 rpm onto the AZO layer as the 2nd ETL, and then annealed at 160 °C for 10 min in a N2 filled glove box. The thickness of the LZO layer was fixed as ~ 15 nm. For a G-QLED, CdSe/CdS/ZnS QDs with an average diameter of 6~7 nm in toluene (concentration 10 mg ml-1) was spin-coated at 3000 rpm and then annealed at 190 °C for 10 min in a glove box. After QDs spin-coating, a TCTA as both HTL and electron blocking layer (EBL), NPD as a HTL and HATCN as a HIL were deposited in a high vacuum chamber (~5 × 10-7 Torr). Then, a 100 nm Al was evaporated onto the top as the anode. Finally, the devices were encapsulated with glass in a N2 filled glove box. Characterization and instrumentation: An Agilent 4156C semiconductor parameter analyzer was used to monitor electrical characteristics of the QLEDs. The absorbance and photoluminescence (PL) of the QDs were measured with a Scinco S-4100 UV-visible spectrophotometer and Jasco FP-6500 spectrofluorometer, respectively. The time resolved PL (TRPL) and transient EL of the thin-films and QLEDs were measured with PicoHarp300 and Tektronix TDS 3054B, respectively. The transmission electron microscopy (TEM) images of red, green and blue QDs and G-QLEDs were obtained using FEI TitanTM 80-300 operated at an


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accelerating voltage from 80~300 kV. The AFM images of thin-films of LZO and AZO were obtained using XE-100 (Park Systems, Korea). And, the XRD data of LZO thin-film was obtained using ATX-G (Rigaku, Japan). The UPS results of ITO, ITO/AZO, and ITO/AZO/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. XPS results of AZO and LZO films were obtained using PHI 5000 VersaProbe (ULVAC PHI, Japan).

ASSOCIATED CONTENT Supporting Information Available: UPS results of G-QD, top-view TEM image and optical characteristic of R-, G-, and B-QDs, optimization of device performance with variation of LZO thickness and annealing temperature, device characteristics of inverted G-QLEDs with LZO annealed at various temperature, device characteristics of inverted G-QLEDs with LZO annealed in air or N2 condition, current versus electric field characteristics with LZO thickness variation at N2 annealing, capacitance versus frequency characteristic for AZO- and AZO/LZO films, voltage simulation results for internal RC delay of AZO- and AZO/LZO-based G-QLEDs, normalized EL spectra for R-QLEDs with and without LZO layer, current versus voltage characteristic of EOD and HOD, XPS data of O1s region at surface of LZO and AZO layers. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author


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

Figure 1. He (I) UPS spectra of AZO/LZO stack ETL on ITO; a) Secondary-electron cutoff regions. b), c) and d) Fermi-edge regions of ITO substrate, AZO and LZO, respectively. The work functions of ITO, AZO and LZO are calculated as 4.2, 3.52 and 3.45 eV, respectively, at secondary-electron cutoff region. e) Energy band diagram of inverted green QLED.


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Figure 2. a) Cross-sectional TEM image of the inverted G-QLED with LZO as 2nd ETL. b) Zoomup image showing G-QDs on LZO layer. AFM images of the stacked ETL with and without GQD layer; c) AZO, d) AZO / LZO, e) AZO / G-QD, and f) AZO / LZO / G-QD. g) XRD intensity for 30 nm LZO layer. The thicknesses of AZO and LZO layers for AFM measurement were fixed at 50 and 15 nm, respectively. The RMS roughness of c), d), e) and f) are 0.93 (AZO), 0.31(AZO/LZO), 1.64 (AZO/G-QD) and 1.59 nm (AZO/LZO/G-QD), respectively.


24

8000

100 10-1

6000

10-2 10-3

4000

10-4 10-5

2000

-2

10

10-6 10-7

0

2

4

6

0 10

8

Voltage (V) 40

-1

Current Efficiency (cd A )

(b)

10000

AZO ETL QLED AZO/LZO ETLs QLED

1

Luminance (cd m )

102

-2

(a)

AZO ETL QLED AZO / LZO ETLs QLED

35 30 25 20 15 10 5 0

0

2000

4000

6000

8000

10000

-2

40

-1

(c)

Luminance (cd m )

Power Efficiency (lm W )

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


Current Density (mA cm )

Page 25 of 30

AZO ETL QLED AZO / LZO ETLs QLED

35 30 25 20 15 10 5 0

0

2000

4000

6000

8000

10000

-2

Luminance (cd m )

Figure 3. Device performances of inverted G-QLED fabricated with 15 nm thick LZO on AZO. Device structure is ITO/AZO/(with or without LZO)/G-QD/TCTA/NPB/HAT-CN/Al. a) Current density (open symbol) and luminance (close symbol) versus voltage, b) current efficiency and c) power efficiency versus luminance characteristics.


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Figure 4. a) PL intensity for glass/QD, glass/AZO/QD and glass/AZO/LZO/QD, b) time-resolved PL for glass/AZO/QD/TCTA and glass/AZO/LZO/QD/TCTA, c) transient EL for the G-QLED with AZO and G-QLED with AZO/LZO at the current density of 3.4 mA cm-2 and d) zoomed-up transient EL result at off-state.


26

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Figure 5. a) Normalized EL spectra, b) EQEs and c) CIE coordinates for the fabricated R-, G- and B-QLEDs with LZO as 2nd ETL. The device structure of QLEDs with stack ETL; ITO / AZO / LZO / QDs (R, G- and B-) / TCTA / NPB / HAT-CN / Al. Histogram of maximum d) current and e) power efficiencies measured from 30 devices. The device structure of G-QLEDs with stack ETL; ITO / AZO / LZO / G-QDs / TCTA / NPB / HAT-CN / Al. The average current and power efficiencies of G-QLED with stack ETL are 33.1 cd A-1 and 29.1 lm W-1, respectively, and black line is Gaussian fitting.


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Table 1. AFM surface roughness for ETL and ETL/QD layers. Peak to valley (Rpv), root mean square (Rq), skewness of the line (Rsk) and average (Ra) of roughness are shown. Substrate

Layer

Rpv [nm]

Rq [nm]

Ra [nm]

Rsk

AZO

11.47

0.96

0.70

-1.55

AZO / LZO

4.26

0.31

0.23

-1.14

AZO / G-QDs

12.61

1.64

12.45

+0.07

AZO / LZO / G-QDs

11.97

1.59

11.79

+0.02

On Glass

On ITO

Table 2. The exciton and luminescence decay times obtained from TRPL and transient EL characteristics respectively. The pulse width and frequency for transient EL measurement were 30 μs and 10 kHz, respectively. Analysis

Structure

Exciton decay time (@ L50%)

Glass / AZO / G-QDs / TCTA

4.5 ns

Glass / AZO / LZO / G-QDs / TCTA

5.0 ns

TRPL Luminance decay time (@ L50%) G-QLED with AZO

90 ns

G-QLED with AZO / LZO

250 ns

Transient EL

Table 3. Summarized device performance of the inverted R-, G- and B-QLEDs with 2% LZO as 2nd ETL. @ 1,000 cd m-2 Thickness [nm] Device 1st ETL

B-QLED a) b)

~ 50

VD b) [V]

CEmax [cd A-1]

PEmax [lm W-1]

2.0

3.5

6.1

2.5

3.9 -

CE [cd A-1]

PE [lm W-1]

CE [cd A-1]

PE [lm W-1]

7.1

4.8

3.1

2.9

1.1

34.0

29.6

30.1

19.0

15.8

6.9

0.9

0.6

-

-

-

-

2nd ETL

R-QLED G-QLED

VT a) [V]

~ 15

4.0

@ 10,000 cd m-2

m-2.

Measured voltage when luminance was 0.1 cd Measured voltage when luminance was 100 cd m-2.


28

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Table 4. Summarized FWHM, ELpeak, EQEmax and CIE coordinate data of the fabricated R-, Gand B-QLEDs with 2% LZO as 2nd ETL. Thickness [nm] Devices 30% AZO

B-QLED

~ 50

ELpeak [nm]

EQEmax [%]

CIE coordinates (CIEx, CIEy)

34

643

8.4

(0.70, 0.30)

32

523

12.5

(0.16, 0.75)

24

444

4.3

(0.17, 0.05)

2% LZO

R-QLED G-QLED

FWHM [nm]

~ 15


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Table of Contents Graphic


30

Metal-Oxide Stacked Electron Transport Layer for Highly Efficient

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