Robust Transparent and Conductive Gas Diffusion Multibarrier Based

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Organic Electronic Devices

Robust Transparent and Conductive Gas Diffusion Multi-Barrier Based on Mg- and Al-Doped ZnO as ITO-Free Electrodes for Organic Electronics Jeong Hyun Kwon, Yongmin Jeon, and Kyung Cheol Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08951 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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ACS Applied Materials & Interfaces

Robust Transparent and Conductive Gas Diffusion Multi-Barrier Based on Mg- and Al-Doped ZnO as ITO-Free Electrodes for Organic Electronics †,‡

†

Jeong Hyun Kwon , Yongmin Jeon , and Kyung Cheol Choi*,

†

†

School of Electrical Engineering, KAIST, Daejeon 34141, Republic of Korea Advanced Nano-Surface Department, Korea Institute of Materials Science, Changwon, Gyeongnam 51508, Republic of Korea ‡

Keywords Doped ZnO, Water vapor transmission rate (WVTR), Trasparent conductive gas diffsuion barriers (TCGDBs), Dielectric/Metal/Dielectric (DMD), ITO-free

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Abstract Thin film encapsulation is strictly required to protect transparent, flexible organic light-emitting diodes (TFOLEDs) based on plastic substrates with poor moisture barrier performances against water vapor and oxygen. However, additional encapsulation process makes OLED fabrication complex and expensive, resulting in lower yield and higher costs for the manufacture of OLEDs. Therefore, to develop simple, transparent conductive gas diffusion barrier (TCGDB) technologies by providing barrier performances to electrodes can be alternatives. Furthermore, TCGDB based on dielectric/metal/dielectric (DMD) structures exhibit not only excellent barrier performances to protect metallic and organic layers against the ambient environment but also mechanical flexibility overcoming the brittleness of oxides. In this work, to improve the moisture-resistant, electrical, and optical properties of ZnO film, periodical dopant layers were inserted during the deposition of ALD ZnO film. These dopant layers make the intrinsic ZnO film more optically and electrically functional. Dopant of MgO with a wide band gap enables blue-shifted optical transmittance, and dopant of Al atoms makes doped ZnO more electrically conductive. In addition, these dopant layers in the ZnO film interrupt the film crystallization, making the film less crystalline with fewer channels and grain boundaries. This effect results in significant improvement of its gas diffusion barrier properties. With a functional and material design that ta kes full advantage of the synergetic combination of highly flexible conductive Ag and a moisture-resist ant MAZO layer, the MAZO/Ag/MAZO (MAM) multilayer with a thickness of approximately 110 nm achieves a sheet resistance of 5.60 Ω/sq, an average transmittance of 89.72 % in the visible range, and a water vapor transmission rate on the order of 10-5 g/m2/day. In addition, OLEDs with the MAM electrode demonstrated great potential of ITO- and encapsulation-free organic electronics. 1. INTRODUCTION Transparent Flexible electrodes (TFEs) have long been in demand for transparent and flexible optoelectronic devices.1–4 Although indium tin oxide (ITO) is the most widely used electrode in organic electronic devices due to its high transparency and electrical conductivity, it has the disadvantages of poor flexibility as well as the scarcity of indium.5,6 To replace previous ITO electrodes, various TFEs with high electrical conductivity and optical transparency have been proposed. However, more recently, beyond the development of TFE as an electrode, new transparent and conductive gas diffusion barrier (TCGDB) films are increasingly proposed for reliable organic electronics. A TCGDB is a gas diffusion barrier film that is conductive and transparent with both gas diffusion barrier and electrode functions. There is an increasing demand for the development of highly reliable and flexible TCGDBs to replace transparent flexible electrodes (TFEs) with poor moisture/oxygen barrier properties (e.g., ITO, graphene, carbon nanotubes, etc.).7,8 For the development of TCGDBs, full-coverage, layered transparent conductive oxides (TCOs) are promising candidates due to their simple and low-temperature process. In particular, atomic layer deposition (ALD) have been used to fabricate impermeable GDBs based on oxide films at low temperatures less than 100 °C.9,10 In other words, ALD enables the low-temperature grown film to provide excellent barrier properties in comparison with other deposition techniques. The plastic substrates, which is used for the realization of flexible organic electronics, do not provide sufficient protection against reactive gases such as water vapor and oxygen, unlike glass substrates.8,11


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Therefore, to satisfy the barrier requirements for a variety of applications, plastics substrates need to be passivated with inorganic single layers or inorganic/organic multilayers.12–14 As mentioned above, for the fabrication of highly impermeable and dense GDBs, ALD have been actively used with various advantages. Previous GDBs, such as Al2O3, MgO, nanolaminates, and so forth, deposited by ALD function as electrically insulating layers.10,14,15 Therefore, the use of these GDBs has been limited to device passivation or gate insulators. In addition, the application of these permeation barriers for the encapsulation of organic electronic devices requires high-cost and complex manufacturing processes with low throughputs. To overcome their limited use, TCGDBs will be in ever-increasing demand and can be used in many ways for various optoelectronic applications, resulting in low-cost fabrication processes without additional passivation layers for plastic substrates.7,16 So far, studies on TCGDBs have little been reported. Chou et al. proposed a TCGDB based on Hf-doped ZnO (HZO) using ALD at 150 °C.16 The mixed HZO films achieved a significantly improved electron concentration and a water vapor transmission rate (WVTR) in comparison to the ZnO films. Andreas et al. also reported robust transparent and conductive tin oxide (SnOx) grown by ALD at 150 °C.7 The SnOx film showed excellent chemical stability without significant change in electrical conductivity under damp heat conditions (85 °C/85% RH). However, although the previously reported TCGDBs grown by ALD exhibit the excellent barrier and electrical properties, their deposition temperatures were 150 °C. In the case of temperature-sensitive flexible substrates, the deposition temperature of 150 ° can give the thermal damage due to poor thermal stability. For example, PET substrates exhibit a glass transition temperature and a crystallizing temperature of 76 °C and 120 °C, respectively, resulting in more thermal damage with increasing process temperature.17 In addition, because most of the organic layers that make up OLEDs have a glass transition temperature of less than 150 °C, applications as a final electrode on OLED devices can be limited.17 Therefore, the thermal budget in the fabrication process is limited by 100 °C for high efficiency and long lifetime of OLED devices. Among various TCOs, doped ZnO films have great potential due to their improved physical and chemical properties in comparison to pure ZnO film. Pure ZnO film shows rapid degradation in electrical conductivity under a harsh environment of 85 °C/85% RH.7 In addition, many channel and grain boundaries have been observed in ZnO thin films due to the crystalline growth. We developed an enhanced ZnO-based film doped with Al and Mg. In general, Al dopant in ZnO film enhances the properties of the original ZnO film. Al-doped ZnO (AZO) has better electrical conductivity and chemical stability than pure ZnO film.18 In addition, Mgdoped ZnO (MZO) films have been reported as transparent conducting layers for various electronic devices. The MZO/Ag/MZO showed blue-shifted optical transmittance due to the wide band gap of Mg (7.8 eV).3 Thus, the doped ZnO films can be suitably tuned depending the kind and content of the dopant materials to achieve various properties. However, TCO single films with poor mechanical flexibility and low electrical conductivity has limitations for use as electrodes in flexible displays.6,7,19 To overcome the limitations of TCOs, dielectric/metal/dielectric (DMD) structures combining the advantages of dielectric and metal have been proposed. The DMD structure is a multi-layered structure that can overcome the opacity of metal and have high conductivity and flexibility by satisfying anti-reflection condition.2 Therefore, we tried to fabricate a TCGDB-based multilayer electrode using a DMD structure. In this study, we demonstrated highly reliable TCGDBs with good barrier properties, flexibility, and high



blue-violet transmittance, fabricated with Mg- and Al-doped ZnO (MAZO)-based multilayer structure. The MAZO/Ag/MAZO (MAM) structure achieved a sheet resistance of less than 6 Ω/sq. and a WVTR value on the order of 10-5 g/m2/day. The ALD ZnO film showed crystalline phase with increasing thickness, which created channels and grain boundaries in the film, resulting in significant permeation paths of water vapor and oxygen. The introduction of a dopant cycle in the ZnO film effectively helps to inhibit the crystalline growth, resulting in improved barrier and mechanical performances. Finally, to prove the feasibility of the proposed electrode, the MAM electrodes were used as anode in phosphorescent organic light-emitting diodes (PhOLEDs). The PhOLEDs based on the synergetic MAM electrodes exhibited good electrical performance and long shelf lifetime reliability. 2. RESULTS AND DISCUSSION

Figure 1. a) Fabrication of TCGDB film with both gas diffusion barrier and TCO functions. b) Conductive and moisture-resistant MAM multilayered structure. The inset displays the schematic of the MAZO thin film structure fabricated by ALD. c) Cross-sectional TEM and EDS images of MAM structure. d) Depth profiles of the MAZO film with the cyclic ratio of MgO:Al2O3:ZnO of 1:2:27. Fabrication of the MAZO-based DMD structure. Passivation layers on both the top and bottom side of the OLED devices are required for conventional flexible OLEDs to prevent water vapor and oxygen penetrating into substrates and devices.9,10 However, these additional passivation processes lead to higher costs and greater complexity. Therefore, there has been a demand for TCGDBs that combine the functions of


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electrodes and passivation (Figure 1a). Functionally designed TCGDB films can be a strong alternative for the realization of reliable, transparent, and flexible displays based on plastic and wearable substrates. A challenge for the development of TCGDB films will make the OLED fabrication process easier and simpler. To develop highly robust and flexible TCGDBs, MAZO thin films with appropriate dopants based on ZnO film were fabricated by ALD to provide better characteristics in comparison to conventional ZnO films, and then the MAM multilayer was fabricated with high impermeability to moisture and oxygen, high transparency, and superior mechanical stability. Al and Mg dopants were added to improve the electrical and optical characteristics in ZnO films, respectively. As shown in Figure 1d, the atomic concentrations of zinc (Zn), aluminum (Al), and magnesium (Mg) of our designed MAZO films were 81.4, 12.4, and 6.2 %, according to X-ray photoelectron (XPS) depth-profiling results. The optimized ALD process for MAZO was set by comparing the electrical conductivity of the MAZO thin film as a function of the ZnO deposition cycle when the cyclic ratio of Al2O3:MgO was fixed as 2:1 (see Figure S1 in the Supporting Information). Using the Matlab program based on the multilayer matrix formula with the optical properties (n, k) of the MAZO and Ag thin in the MAM stacked structure, thickness optimization with the highest luminous transmittance was conducted. Based on the simulation results, the optimized transmittance of the MAM structure could be obtained with a bottom MAZO thickness of about 40 to 60 nm and a top MAZO thickness of 40 to 55 nm. Based on the simulation results, the thickness of each MAZO film in the MAM structure was fixed at 50 nm. The finally fabricated MAM multilayer structure was examined by transmission electron microscopy (XPS) and energy dispersive X-ray spectrometry (EDS), as shown in Figure 1c. For fabrication of the optimized MAM multilayer, after the thickness of the MAZO films was fixed at 50 nm, the thickness of the Ag thin film was optimized by measurement of the optical transmittance and sheet resistance as well as SEM surface imaging according to the Ag thickness. Because thermally evaporated Ag films exhibit island growth in relation to thickness, it is important to confirm the critical thickness at which the Ag film starts to form a continuous conducting layer in optimizing the DMD structure.[20] The DMD structure with a critical thickness of Ag film exhibits the highest optical transmittance. The critical thickness of the Ag film was affected by the deposition rate, seed layer, substrate temperature, and so forth.5,19 Therefore, we deposited the Ag films on MAZO films at a fixed deposition rate of 2.0 Å/s. Figure 2b shows the optical transmittance graphs of MAZO/Ag and MAM structures on PET substrates in relation to Ag thickness. The multilayered structures exhibited the highest luminous transmittance with an Ag thickness of 9 nm, achieving average transmittances of 89.27% and 91.83% in the visible wavelength and blue wavelength regions, respectively. This mean that the Ag film forms a quasi-perfect film at the critical thickness of 9 nm. In addition, SEM surface images of 7-, 8-, and 9-nm-thick Ag films on MAZO films are shown in Figure 2c. We confirmed that the 9-nm-thick Ag film started to show a quasi-perfect film over discontinuous Ag clusters in the SEM images. At the thickness of 9 nm, the MAM structure also exhibited a sharp decrease in sheet resistance and achieved a sheet resistance of 5.60 Ω/sq (Figure 2d). Overall, the Ag film on the MAZO film forms a quasi-perfect continuous surface at the thickness of 9 nm and the MAZO/Ag/MAZO structure was optimized with thicknesses of 50/9/50 nm. Despite the Ag film inserted in the multilayer, the high averaged and blue-wavelength optical transmittance of the MAM structure is attributed to the blue shift of transmittance due to the Al (~6.4 eV) and Mg (~7.8 eV) dopants with high bandgap energy.1,3 Figure S2 of the Supporting Information shows the calculated



transmittances of the ZnO/Ag/ZnO (ZAZ) and MAM structures in the visible wavelength. The optical transmittance of the MAM structure was significantly better than that of ZAZ in the wavelength regions of 400 to 480 nm and 400 to 720 nm, respectively. In particular, the significant transmittance increase of the MAM structure in the blue-wavelength region is attributed to the increase in the optical band gap of MAZO film.1,3 As a result of the insertion of periodic dopant layers, the optical band gap of MAM increased to 3.83 eV in comparison to the ZAZ with the optical band gap of 3.27 eV.

Figure 2. Optical transmittance according to the Ag thickness on a) the MAZO (50 nm)/ Ag (x= 7, 9, 11, 13 nm) structure and b) the MAZO (50 nm)/Ag (x= 7, 9, 11, 13 nm)/MAZO (50 nm) structure. c) SEM images of 7-, 8-, and 9-nm-thick Ag surface morphologies deposited on the MAZO thin film. d) The sheet resistance of


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MAZO/Ag and MAZO/Ag/MAZO structures in relation to Ag thickness. e) Photographs of the patterned MAM structure formed on a glass substrate. Our optimized MAM structure was compared with other electrodes in terms of sheet resistance and transmittance. As shown in Figure 3a, the DMD electrode groups overall exhibited much higher transparency and lower sheet resistance in comparison to the currently reported ITO, graphene, CNT, Ag nanowire, etc. In particular, the sheet resistance of the MAM structure with the average transmittance of approximately 90% without including the PET substrates is as low as 5.60 Ω/sq and the MAM structure exhibited characteristics comparable to or even better than the previously reported high-performance DMD stacks, such as polyethyleneimine (PEI)/Ag/ poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT), Sb-doped SnO2(ATO)/Ag-Ti/ATO, SnOX/Ag/SnOX and so forth.5,7,21–23 Furthermore, we evaluated the Haacke figure of merit (FOM) as an important index for evaluating the performance of the transparent conductive electrodes (TCEs).24 The FOM is a parameter indicating a trade-off between the electrical conductivity and the optical transmittance, which is used as a reference for finding the optimum condition of the electrical and optical properties of a TCE. The FOM is expressed by the following equation (1):

Φ =

(1)

In equation (1), Φ is the FOM, Tavg is the average transmittance of the electrode, and RSH is the sheet resistance of the electrode. Figure 3b and Table 1 show the FOM values of the MAM structure as a function of the Ag thickness and other TCEs calculated by equation (1). A higher FOM means that it is a more competitive electrode as a TCE. This result indicates that ultrathin metal layers sandwiched by dielectric layers are effective to fabricate cost-effective, transparent, and conductive electrodes. The highest FOM of the MAM structure was obtained with an Ag thickness of 9 nm and was 60.4 × 10-3 Ω-1, which was much better than that of ITO and nanostructured materials. Therefore, our MAM structure showed strong potential as a TCE for organic electronics. Flexibility of the electrode, which is an important property for transparent flexible display applications, was evaluated in bending tests. Figures 3c and d show changes in the sheet resistance of MAM structures according to bending strain and the number of bending cycles. Because TCO films are brittle, their electrical conductivities are difficult to maintain with increasing strain. Thus, to overcome the poor flexibility of TCOs, the multilayered structure with an added ductile Ag thin film is used. The Ag film provides an electrical connection between Ag and TCO under high bending strain. In fact, no change in sheet resistance was observed after 1000 bending repetitions at the bending strain of 3%, whereas a 100-nm-thick ITO film started to show a significant increase in sheet resistance at the critical strain of 0.42% due to the cracking of the ITO film.



Figure 3. a) Comparison of sheet resistance and transmittance plot from other reference electrodes: ITO (commercial)7, CNT25, graphene26, Ag NW27, SnOx/Ag/SnOx7, ATO/Ag-Ti/ATO28, and ITO/Ag/ITO29. b) FOM values of the MAM structures as a function of Ag thickness and other TCEs (ITO (commercial)7, CNT25, graphene26, Ag NW27, SnOx/Ag/SnOx7, ATO/Ag-Ti/ATO28, ITO/Ag/ITO29). c) Bending test results of ITO and MAM structure according to the bending strain. d) Normalized sheet resistance of MAM structure according to bending times at tensile strain of 3.1%. Table 1. Sheet resistance, optical transmittance, and FOM of MAZO/Ag/MAZO structures as a function of Ag thickness.


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Figure 4. a) Normalized conductance vs. time of ZnO, AZO, and MAZO under damp heat condition. Inset shows an expanded view of the changes in the normalized conductance in the first 1000 s. (b) Normalized conductance vs. time of bare Ag, Ag/ZnO, and Ag/MAZO. Inset shows an expanded view of the normalized conductance of the Ag film in the first 6000 s. Schematic of c) polycrystalline ZnO film with grain boundaries and channels and d) MAZO film with less-crystallinity interrupted by dopant layers. e) TEM cross-sectional images of 100-nm-thick ZnO and MAZO films.

For device reliability, it is important to consider environmental stability as well as electrical and optical



properties in designing the electrodes. Previous nanostructured materials based on metal and carbon, such as carbon nanotubes, graphene, metal nanowires and so forth, show poor chemical stability under damp heat condition (85 °C/85% RH). Moreover, most oxides are easily corroded due to chemical instability under harsh environmental conditions.7 Among oxides, in particular, the electrical conductivity of ZnO film rapidly deteriorates within a few minutes under damp heat. Therefore, pure ZnO film is unsuitable for application as a TCGDB. To overcome the limitations of pure ZnO film, dopant layers in ZnO film can be a good alternative. Figures 4a and b show real-time resistance changes in various materials over time at 85 °C/85% RH. The measurement results indicate that the doped ZnO films are more corrosion-resistant than pure ZnO films. The degradation of ZnO thin films is known to be due to carrier depletion induced by oxygen chemisorption.7 More specifically, the increase in chemisorbed -OH groups along the grain boundary results in a higher potential-energy barrier, causing degradation of the electrical conductivity of ZnO thin films.30 However, ZnO films doped with both Al and Mg dopant layers show improved environmental stability under damp heat by inhibiting the crystallinity in the ZnO as well as substituting Zn with Al and Mg. Water vapor and oxygen can easily penetrate through the grain boundaries in pure ZnO thin films with open grain boundaries and channels (Figure 4c). However, after the doping of ZnO thin films, the grain boundaries and channels are less open and are partly interconnected between grains by the interruption of crystallinity (Figure 4d). As shown in Figure 4e, the schematics of ZnO and MAZO film structures are in good agreement with cross-sectional images obtained by TEM measurements. Therefore, these effects may explain why environmental species, such as water vapor and oxygen, show reduced penetration of grain boundaries, resulting in decrease in chemisorbed –OH group.


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Figure 5. a) Normalized conductance over time of the Ca films with respective ITO, ZAZ, and MAM structures under storage conditions of 30 °C /90% RH, b) XRD pattern of 50-nm-thick ZnO and MAZO thin films. c) TEM cross-sectional ZnO image (inset: FFT diffraction pattern of the cross-sectional ZnO film). d) TEM cross-sectional MAZO image (inset: FFT diffraction pattern of the cross-sectional MAZO film). We measured the GDB properties of ITO, ZAZ, MAM structures using electrical Ca tests for comparison of their WVTRs. WVTR values, which are regarded as a scale for relative comparison of barrier properties between films, were obtained through Ca testing. The electrical Ca test is a well-known method to obtain WVTR values by measuring the real-time resistance of thick Ca films under the various temperature/humidity conditions. Figure 5a shows the real-time normalized conductance over time of the Ca films with respective ITO, ZAZ, and MAM structures under storage conditions of 30 °C /90% RH. A commonly used 150-nm-thick ITO film achieved a poor WVTR value of 2.41× 10-1 g/m2/day due to defects and grain boundaries induced by crystal growth and sputtering deposition. Likewise, the polycrystalline ZAZ multilayer exhibited poor barrier properties of 8.31 × 10-2 g/m2/day. However, the MAM multilayer achieved an improved WVTR value of 9.2 × 10-5 g/m2/day, which is notably is three orders of magnitude lower than that of the ZAZ multilayer. The significantly lower WVTR values achieved with MAZO films may be attributed to the multi-interfacial system and inhibition of crystalline growth. The crystallinity of a thin film is critical to its barrier and mechanical properties. Crystal growth results in the development of grain boundaries and channels in a film, which facilitate the penetration of water vapor and oxygen with diameters of 0.33 and 0.32 nm, respectively.31 As previously mentioned, the addition of the repeated dopant layers in ZnO films prevents the generation of grain boundaries



and channels by inhibiting crystal growth. Figure 5b shows the X-ray diffraction (XRD) patterns obtained for the 50-nm-thick MAZO and ZnO thin films. The pure ZnO film showed the most striking diffraction peak for the (100) and (002) planes, resulting in high crystallinity. On the other hand, the diffraction peaks in the (100) and (002) orientations were suppressed by the Mg and Al dopant layers. In addition, TEM cross-sectional imaging and FFT analysis of the ZnO and MAZO films were conducted. The results support our explanation, showing clear differences in growth structures (Figures 5c and d). In addition, periodic dopant layers with few cycles were formed with angstrom thickness, with separations between the periodic ZnO films through the formation of interfaces. In addition, the suppressed crystallinity and defect-decoupling system resulting from the insertion of periodic dopant layers in ZnO films improved the flexibility of the MAZO film. The mechanical properties of 50-nm-thick MAZO and ZnO thin films deposited on PET substrates were investigated as shown in Figure S3 (Supporting Information). SEM images in Figure S3 reveal that 50-nm-thick MAZO can withstand tensile strain as large as 3% without developing stress-induced cracks, while 50-nm-thick ZnO starts to exhibit cracks at 2%. The significant crack resistance of MAZO under relatively large strain can be attributed to the suppressed crystallinity and defect-decoupling system, which causes deflection and crack arresting by a microcrack toughening effect.10,32 Therefore, the significant improvement in WVTR and flexibility through these synergistic effects makes MAZO film a good option for application as a TCGDB. However, the electrical conductivity of our ALD MAZO and MAM film quickly degraded at 85°C/85% and exhibited somewhat poor environmental stability in comparison to previously reported studies. Bingel et al. reported that AZO/Ag/AZO fabricated using DC magnetron sputtering showed excellent environmental stability under 85°C/85%.33 As previously reported by other groups, the ALD ZnO film showed poor corrosion resistance due to the poor chemical stability and tend to be abruptly degraded under damp heat conditions.7 In the case of our MAZO/Ag/MAZO stack, although significant stability improvement was observed, various efforts should be made to achieve performance similar to the previously reported level for long-term stability. To enable the DMD stack to have barrier functionality at a WVTR of 10-6 g/m2/day and environmental stability to withstand harsh conditions, a new TCGDB beyond ZnO-based TCOs should be used to block the degradation of the constituent Ag and top dielectric layers. Riedl et al. reported a highly impermeable and robust DMD structure using an ALD tin oxide.7,34 We also confirmed that the sputtered ITO and indium-gallium-zinc-oxide (IGZO) electrode showed good environmental stability due to the stable chemical structure of the tin-based oxide.29 We assumed that tin-based oxides showed excellent environmental stability and conductivity. In addition, tin oxides prepared by ALD showed excellent barrier performance as well as outstanding chemical stability due to the densely packed thin film resulting from the ALD deposition mechanism.7 Therefore, we can fabricate a highly impermeable and conductive GDM using highly impermeable ALD tin-based TCOs as a top dielectric layer and well-known Al2O3/TiO2 nanolaminate, which is reported to have excellent barrier performance and environmental stability, as a bottom dielectric layer.


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Figure 6. a) Schematic of a red-emitting PhOLED device structure. The inset displays the emitting image of a MAM-based OLED. (b, c) Electrical properties of PHOLEDs with MAM and ITO anodes formed on PET substrates: b) current density vs. voltage curve, c) current efficiency-current density curve. d) schematic of glass-encapsulated OLEDs with ITO and MAM anodes deposited on the entire PET substrates for comparison of cell images over time. e) Emitting cell images of glass-encapsulated PhOLEDs with MAM, ZAZ, and ITO anodes after storage for 10 hours under 30 °C/90% RH. f) Operational lifetimes of the glass-encapsulated OLEDs with MAM and ITO electrodes in air at a luminance of 1000 cd/m2. To demonstrate the feasibility of the MAM structure as an electrode, red PhOLEDs were fabricated with a structure of molybdenum trioxide (MoO3, 5 nm)/ 4,4′- bis(N-phenyl-1-naphthylamino)biphenyl (NPB, 68 nm)/ bis(10-hydroxybenzo[h] quinolinato)beryllium complex: tris(1- phenylisoquinoline)iridium (Bebq2: 8% Ir(ppy)3, 70 nm)/ lithium quinolate (Liq, 1 nm)/aluminum (100 nm).10 We also fabricated a PhOLED with an ITO anode as a reference device. Work function matching is very important in fabricating OLEDs to achieve efficient charge injection. Therefore, UV photoelectron spectroscopy (UPS) spectra were measured to investigate the work function of the Ag, Ag/ZnO, and Ag/MAZO (see Figure S4 in the Supporting Information). The work functions in the UPS spectra can be determined by the intercept between the x-axis and the extrapolation of cut-off curves, which is the point where the y-axis equals zero, as shown in Figure S4b. The work function of the AZ and AM structures was 4.13 and 3.97 eV, respectively, and the decrease in the work function is attributed to the shift of the conduction band to a vacuum level by increasing the optical band gap via insertion of the periodic dopant layer of MgO and Al2O3 with a high optical bandgap in the ZnO film. In a recent report, the work function of ITO measured by UPS measurement was 4.2 eV.35 Therefore, in our OLED structure, the MoO3 layer was added as a hole injection layer (HIL) layer for tuning the work function between the anode and polymer layers, resulting in an ohmic contact with the polymer layers.36 Therefore, from the results of measurements, although the ITO and MAM electrodes showed different work functions, the electrical characteristics of the PhOLEDs with ITO and MAM anodes were almost identical (Figures 6b and c).



In addition to the electrical performance, emitting cell images and luminance decay over time were measured to compare reliability effects induced by the difference in the barrier performance on ITO and MAM electrodes. For these measurements, MAM, ZAZ, and ITO anodes were deposited on the entire PET surface, and then the glass-encapsulated PhOLEDs shown in Figure 6d were prepared. Under conditions of 30°C/90% RH, many dark spots in the emitting cell images of the PhOLEDs with ITO and ZAZ anodes were observed after storage for 5 hours (Figure 6e). Generation of these dark spots in the cell images was due to the poor barrier properties of the anodes. On the other hand, the MAM-based PhOLED showed clear cell images without degradation because the barrier performance of the MAM structure was more than 1000 times better than that of the ITO and ZAZ structures, respectively. We also measured the operational lifetimes of the glass-encapsulated OLEDs based on MAM and ITO electrodes in air at room temperature. However, our designed MAM structure with a 9-nm-thick Ag film easily burnt out due to poor stability of the ultrathin Ag under exposure to air, making operational lifetime measurements difficult. Therefore, we used a MAM electrode with a 20-nm-thick Ag film for reliable operational lifetime tests. The luminance decay of the OLEDs was measured over time at an initial luminance of 1000 cd/m2. Figure 6e shows the operational lifetimes of the glass-encapsulated OLEDs with MAM and ITO electrodes in air at an initial luminance of 1000 cd/m2. The half lifetime of the ITO-based OLED was 109 hours, whereas that of the MAM-based OLED was 400 hours. The significant lifetime improvement induced by the use of the MAM is attributed to the clear barrier performance difference between ITO and MAM. As demonstrated by emitting cell images over time and the operational lifetime test, the use of the effective TCGDM in OLEDs effectively protects against the permeation of water vapor and oxygen, resulting in a dramatic lifetime improvement of OLEDs. With a functional design that takes full advantage of the synergetic combination of a moisture-resistant, optically designed MAZO film and a conductive, flexible Ag film, the MAM multilayer structure was demonstrated to be a good alternative for application as a TCGDB for TFOLEDs. 3. CONCLUSION In summary, we demonstrated TCGDBs based on a MAZO film prepared by ALD at low temperatures. Low WVTRs on the order of 10-5 g/m2/day, comparable to values reported for the best electrically insulating GDBs, have been achieved through inhibition of crystal growth. In a stability test under damp heat, the MAZO film showed much better environmental stability than a pure ZnO film, and the Ag film was effectively protected by the MAZO film. In addition, MAM assemblies with an average transmittance of 90.0% and a sheet resistance of 5.6 Ω/sq have been fabricated, resulting in a FOM value of 60.4 × 10-3 Ω-1 at an Ag thickness of 9 nm. When combined with MAZO and Ag films, these synergistic effects enabled the formation of a highly transparent flexible gas diffusion multi-barrier suitable to replace the previous rigid ITO electrodes. Through application of the TCGDM to OLEDs, the MAM-based OLED showed much better operational lifetime and shelf life as well as electrical performance equivalent to that of the commonly used ITO-based OLEDs. However, given the level of corrosion resistance and barrier performance realized in this work, we believe further work on the TCGDM based on highly impermeable and corrosion-resistant TCO films beyond the proposed MAZO film can pave the way for encapsulation-free, transparent, and flexible OLEDs to become next-generation, low-cost, reliable displays with high efficiency and long reliability, and form factor


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advantages. 4. EXPERIMENTAL SECTION Fabrication and Characterization of MAZO and MAM. MAZO films were deposited by ALD with diethylzinc (DEZ), trimethylaluminum (TMA), bis-ethyl-cyclopentadienyl-magnesium (Mg(EtCp)2) and H2O as Zn, Al, Mg, and oxygen precursors, respectively. The MAZO film was deposited in film layers with a thickness of approximately 50 nm under a base pressure of 3.1 × 10-1 Torr and a chamber temperature of 100 °C. The MAZO was periodically doped with an Al and Mg doping layer. The layered ratio of MgO to Al2O3 to ZnO was set as 1:2:27, and the thickness of the MAZO film was adjusted to approximately 50 nm. TMA, DEZ, and H2O sources were cooled to 10 °C, while the Mg(EtCp)2 source was heated to 90 ° to be vaporized. The exposure times of the TMA and DEZ, and Mg(EtCp)2 precursors were 0.25, 0.25, and 0.75 s, respectively, and the H2O exposure times and N2 purging times were 0.25 and 10 s. As a result of the deposition processes, the growth rates of the ZnO, Al2O3, and MgO thin films were 1.20, 0.89, and 1.05 Å, respectively, per cycle in an ALD chamber at 100 °C. After the bottom MAZO film was deposited on a PET substrate, Ag and top MAZO thin films were then sequentially deposited by thermal evaporation and ALD on the PET/MAZO, respectively. Cross-sectional TEM and EDS images were obtained by high-resolution transmission electron microscopy (JEM-ARM200F, JEOL) and EDX (Quantax 400, Bruker), respectively. In addition, atomic concentrations of the MAZO film were determined using XPS (Sigma probe, Thermo VG Scientific) analysis. PhOLED Fabrication and Characterization. Bottom-emitting PhOLEDs were fabricated with the sequential structure of ITO (150 nm) or MAM as an anode, MoO3 as a HIL (5 nm), NPB as a hole-transport layer (68 nm), Bebq as a host, Ir(piq)3 as an emission layer (70 nm, 6%), Liq as an electron injection layer (1 nm), and Al as a cathode (100 nm) on PET substrates using thermal evaporation, and then the fabricated PhOLEDs were encapsulated with glass lids in the glove box. The current density-voltage-luminance characteristics of the OLEDs were measured using a CS-2000 spectroradiometer and a Keithley 2400 source meter. 5. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Electrical conductivity of the MAZO thin film as a function of the ZnO deposition cycle when the cyclic ratio of Al2O3:MgO was fixed as 2:1; Calculated transmittances of the ZnO/Ag/ZnO and MAZO/Ag/MAZO structures in the visible wavelength; Surface SEM images of ZnO and MAZO thin films on PET substrates as a function of bending strain; He(I) UPS spectra of Ag, Ag/ZnO, Ag/MAZO; 6. AUTHOR INFORMATION Corresponding Author K.C. Choi.* Author is with the School of Electrical Engineering, the Korea Advanced Institute of Science and



Technology, Yuseong-gu, Daejeon 34141, Republic of Korea (corresponding author phone: +82-42-350-3482; fax: +82-42-350-8082; e-mail: kyungcc@ kaist.ac.kr). ORCID Kyung Cheol Choi: 0000-0001-6483-9516 Notes The authors declare no competing financial interest. 7. ACKNOWLEDGEMENT This work was supported by the Engineering Research Center of Excellence (ERC) Program (Grant No. NRF2017R1A5A1014708) and the Nano·Material Technology Development Program (Grant No. NRF2016M3A7B4910635) through the National Research Foundation (NRF) funded by Korean Ministry of Science & ICT (MSIT). In addition, this work was supported by the Technology Innovation Program (20000489, Interative fiber based wearable display platforms for clothing displays) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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ToC figure


Robust Transparent and Conductive Gas Diffusion Multibarrier Based

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