Ultraflexible and High-Performance Multilayer Transparent Electrode

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Ultra-flexible and High-performance Multilayer Transparent Electrode based on ZnO/Ag/CuSCN Yixiong Ji, Jun Yang, Wei Luo, Linlong Tang, Xiangxing Bai, Chongqian Leng, Chaoyan Ma, Xingzhan Wei, Jing Wang, Jun Shen, Shirong Lu, Kuan Sun, and Haofei Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15902 • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 18, 2018

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ACS Applied Materials & Interfaces

Ultra-flexible and High-performance Multilayer Transparent Electrode based on ZnO/Ag/CuSCN Yixiong Ji†#, Jun Yang†#, Wei Luo†, Linlong Tang†, Xiangxing Bai†, Chongqian Leng†, Chaoyan Ma†, Xingzhan Wei†, Jing Wang†, Jun Shen†, Shirong Lu†*, Kuan Sun‡* and Haofei Shi†* #

These two authors contribute equally to this work.

†

Chongqing Institute of Green and Intelligent Technology, CAS, Fang Zheng Road 266,

Chongqing, 400714, P. R. China. ‡

Key Laboratory of Low-grade Energy Utilization Technologies and Systems of the

Ministry of Education of China; School of Power Engineering, Chongqing University, Shazheng Road 174, Chongqing, 400044, P. R. China. *

E-mail: [email protected], [email protected], [email protected]

KEYWORDS:

flexible

multilayer

transparent

electrode,

CuSCN,

organic

light-emitting diodes, light outcoupling, high performance

ABSTRACT: Driven by huge demand for flexible opto-electronic devices, high-performance flexible transparent electrodes are continuously sought. In this work, a flexible multilayer transparent electrode with structure of ZnO/Ag/CuSCN (ZAC) is

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engineered, featuring inorganic solution-processed cuprous thiocyanate (CuSCN) as a hole-transport antireflection coating. The ZAC electrode exhibits an average transmittance of 94% (discounting the substrate) in the visible range, a sheet resistance (Rsh) of 9.7 Ω/sq, a high mechanical flexibility without Rsh variation after bending 10,000 times, a long-term stability of 400 days in ambient environment and scalable fabrication process. Moreover, spontaneously formed nano bulges are integrated into ZAC electrode and light outcoupling is significantly improved. As a result, when applied into Super Yellow (SY) based flexible OLED, the ZAC electrode provides a high current efficiency of 23.4 cd/A and excellent device flexibility. These results suggest multilayer thin films with ingenious materials design and engineering can serve as a promising flexible transparent electrode for opto-electronic applications.

INTRODUCTION: The fabrication of transparent conductive films on mechanically flexible substrates is crucial for the realization of foldable optoelectronic devices, including organic light emitting diodes (OLEDs), organic photovoltaic cells (OPV) and touch panels1. In the past 20 years, exciting development on optoelectronic devices was made, nevertheless the development of flexible electronic devices still lags behind due to the absence of ideal flexible transparent electrodes, which require high electrical conductivity, optical transparency, mechanical flexibility and long-term stability. For flexible electronics, the desire for mechanically flexibility cannot be easily satisfied by the most commonly used indium tin oxide (ITO) electrode because of its brittle nature2. To fill the gap, a variety of flexible transparent conductive materials have been developed, such as conducting polymers3-6, metallic nanowires7-9, carbon materials10-11,


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ultra-thin metal films12-13 and some hybrids such as graphene/conducting polymer14, polymer-metal15 etc. However, all of these technologies suffered from some critical problems, including poor stability and noticeable color of conducting polymers16-17, high surface roughness and high corrosion rates of metallic nanowires caused by their stacked mesh structures and large surface area18, the high sheet resistance (Rsh) of carbon materials, and the obsessional trade-off between transmittance and conductivity of ultra-thin metal films.

Recently, multilayer transparent electrodes (MTEs) that combine the advantages of several materials are emerging as a new type of flexible transparent electrodes to overcome the limitations of electrodes associated with single material19. Much attention has been paid on sandwiching an ultra-thin noble metal film between two antireflection layers, where the lower one also works as a wetting layer. Such a multilayer structure offers comprehensive properties including good stability, high conductivity, high optical transmittance, continuous smooth surface etc20-23. Since the materials choice and fabrication technique for the middle noble metal layer is quite limited and has been well studied6, 24, the key to enhance the opto-electrical properties of such MTEs shifts to material engineering of antireflection layers. Currently, transition metal oxides, such as WO322, NiO23 and MoO321, 25 are commonly employed in MTEs as antireflection layers, but high vacuum or high-temperature calcination above 300 oC are sometimes required to achieve high quality films. Such harsh film-formation processes limited

thier applications. To solve this issue,

solution-processed materials at low temperature, such as ZnO26, NiO27, MoO328 and



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PEDOT:PSS29 have been studied, delivering promising results in the development of flexible

solar

cells

and

light

emitting

diodes.8

Among

them,

poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the most widely used material in conventional organic optoelectronics. But,it suffers from some issues such as electrical and physical inhomogeneity, and long-standing acidic nature of PSS.15, 30.

In this work, we present a high-performance MTE with an architecture of ZnO/Ag/CuSCN (ZAC), featuring a solution-processed p-type cuprous thiocyanate (CuSCN) as antireflection layer and a spontaneously formed photonic structure. The ZAC MTE exhibits an average transmittance of 94% in the visible range and a low Rsh of 9.7 Ω/sq. In addition, the electrode exhibits excellent flexibility and stability so that its Rsh keeps constant during bending for 10,000 cycles at a radius of 5 mm or exposing to air with humidity up to 68% for more than 400 days. Moreover, nano bulges, which are created on PET substrate during the sputtering process and then imbedded into the ZAC electrode, can effectively reduce the energy trapped in waveguide modes and plasmonic modes and thus enhance light outcoupling with minimized mode losses. When applied into Supper Yellow (SY) based flexible OLEDs, the ZAC electrode affords a record current efficiency of 23.4 cd/A, as well as an excellent device flexibility, e.g. 90% of original current efficiency/luminance is maintained after bending at 5 mm radius for 1000 times. The results suggest ZAC MTE, which benefits from the synergetic effect of the novel antireflection material and nano photonic structures, will become an attractive transparent electrode for flexible opto-electronics.


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RESULTS AND DISCUSSION

Structure and fabrication of ZAC electrode. A schematic diagram of the ZAC MTE is illustrated in Fig. 1a. To obtain highly conductive and transparent ultra-thin metal film with reasonable stability for MTE, zinc oxide (ZnO) is chosen as a dual-functional wetting and antireflection layer, while silver as conducting material. In view of many merits of inorganic p-type CuSCN (vide infra), it is chosen as the top antireflection material as well as hole transport material. The complete fabrication process of the proposed ZAC MTE is illustrated in a flow chart (Fig. S1). A 9 nm-thick continuous silver layer can be formed on 20 nm-thick ZnO layer via magnetron sputtering. Interestingly, during this sputtering process, randomly-distributed nano bulges are formed spontaneously on the surface of PET substrate (Fig. S1) and then embedded into the ZAC electrode in the following fabrication process. The CuSCN layer is deposited via spin-coating at room temperature.

The crystal structure of CuSCN and its physical form in solid and solution are shown in Fig. 1b; a few relevant physical properties are summarised in Table S1. CuSCN is a p-type semiconductor with electrical conductivity in the range of 10-2 ~ 10-3 S/cm, a decent hole mobility of 0.01–0.1 cm2/Vs and a deep homo level of 5.5 eV, which are beneficial for hole injection/collection of electrodes. Furthermore, CuSCN possesses a wide bandgap of 3.6 eV and a large dielectric constant of 5.1 (±1.0) 36, which promises high optical transparency in visible range and efficient light antireflection. In addition, CuSCN is soluble in solvents such as ammonia, ethyl ether, dipropyl sulfide, diethyl



sulfide, etc., high-quality thin films of CuSCN can be easily obtained by various printing techniques, allowing for manufacturing large-area MTEs at high throughput and low cost. Finally, the inorganic nature of CuSCN provides physical protection for the underneath Ag against oxidation as well as mechanical robustness, thus can potentially improve the lifetime for the ZAC electrode and the corresponding devices. Because of the above-mentioned merits, CuSCN plays an indispensable role in the ZAC electrode.

PET substrate is composed of semicrystalline and amorphous domains. The selectively etching of the amorphous domains by plasma causes the formation of nanostructure on the surface of PET34-35. Randomly-distributed nano bulges on PET substrate then embedded into the ZAC electrode have great potential for light extraction by reducing the total light reflection and the mode-trapped energy. It worth noting that these nano bulges play a key role in their performances on light extraction, whose size is controllable by tuning the Ar plasma power during sputtering process, i.e. higher sputtering power leads to larger size, as evidenced by atomic force microscopy (AFM) (Fig. S3). To balance the light extraction and the surface roughness of ZAC, 70 W is chosen to sputter deposit the ZnO wetting layer and to create nano bulges on PET substrate simultaneously (Fig. S4). To further clarify that the formation of nano bulges is due to plasma corrosion of the organic PET, glass substrate as a control sample is subjected to the same process, and no obvious change on surface morphology is observed (Fig. S5).


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Figure 1(c) illustrates the surface morphology of each layer of ZAC electrode after being deposited on PET or glass substrate, acquired by field-emission scanning electron microscopy (FE-SEM) and AFM. The nano bulges created on PET substrate have an average size of around 100 nm with a density of 11 bugles/µm2. They are still observable after deposition of ZnO and Ag layers, but smoothened by the following spin-coated CuSCN film. To investigate whether the Ag film is successive, the surface morphology of PET/ZA samples with different thickness of Ag film was measured by SEM and the results are illustrated in Fig. S12. When the thickness of Ag film of PET/ZA is 7nm, it shows poor continuity at the bulge area (Fig. S12(a)). However, it is obviously that the Ag film with thickness of 9nm is continuous with compact crystals (Fig. S12(b)). As a result, the ZAC on PET exhibited a root-mean-square surface roughness (Rrms) as low as 6 nm, which implies good contact with the active layer and reduction of short circuit channels in the devices. The XRD data prove the formation of CuSCN crystals with high crystallinity (Fig. S2), which promises high hole mobility and low density of recommbination defects. In contrast, every layer of the ZAC electrode prepared on glass substrate is quite smooth, for that the inorganic glass is able to tolerant the plasma sputtering process and so no nano bulges appears on the glass surface. Notably, promoted by the strong interfacial interaction with oxides, conductive Ag metal presents good wetting ability on ZnO layer and thus forms compact and uniform conductive layer with a Rrms as low as 2 nm, much lower than the one without the wetting layer (Fig. S9).



Figure 1. (a) Schematics of the flexible ZAC electrode consisting of an ultra-thin Ag film sandwiched in between the ZnO and CuSCN layers on PET substrate, (b) CuSCN crystal structure, photos of CuSCN powder and diethyl sulfide solution of CuSCN, (c) Surface morphology images of the ZnO, ZnO/Ag and ZnO/Ag/CuSCN layers on glass or PET substrates, taken by scanning electron microscopy (SEM) with scale bars of 400 nm and atomic force microscopy (AFM).

Optical and electrical properties. In this ZAC electrode, CuSCN antireflection layer plays a key role in reducing the light reflection at the interface of the electrode,


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through increasing the destructive interference (Fig. S6). The average total reflectance of the ZAC electrode with a 40 nm CuSCN layer as antireflection layer goes down to 9.3% over the visible range, decreased by almost 58% in comparison with that of the electrode without CuSCN (Fig. 2a). The total reflectance of ZAC electrodes and their substrates are shown in Fig. 2(a). Interestingly, before deposition of ZAC, the reflectance of glass is lower than PET. While after depositing the ZAC on these substrates, the nano bulges formed in PET/ZAC, which provided an average total reflectance of only 7.7%, lower than Glass/ZAC (9.3%). We ascribe it to the positive contribution from the embedded nano bulges or the synergetic antireflection effect caused by the ZAC layers and nano bulges.

The influence of the antireflection layer thickness on the light transmission of ZAC electrode is studied theoretically and experimentally. Based on the finite element method using Comsol Multiphysics, the thicknesses for CuSCN and ZnO are optimized to be around 40 nm and 20 nm, respectively (Fig. S7). To verify the calculation results, we measure the transmittance of electrodes with various thicknesses of ZnO and CuSCN, and obtain consistent results (Fig. 2b and Fig. S8). Consequently, the optimized ZAC provides a transmittance >96% at 550 nm discounting the substrate (Fig. 2b).

The nano bulges in the ZAC electrode can enhance the overall light transmittance. ZAC with nano bulges exhibits a higher average transmittance of 86.3% than the counterpart without nano bulges (84.6%) fabricated with the same process during the



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visible spectrum (Fig. 2c). The improvement of transmittance (1.7%) coincides with the value of reduced internal reflection (1.6%, Fig. 2a), which together suggests the nano bulge-induced transmittance improvement is mainly due to the reduction of the internal reflection. If discounting the substrate, the average transmittance of ZAC electrode with embedded nano bulges goes up to an average 94% in the visible range (inset of Fig. 2c). Moreover, the introduction of nano-bulge structures does not exert obvious influences on the sheet resistant of electrode, because these structures do not degrade the integrity and compactness of the Ag layer (Fig. 1c). With optimized size and distribution of the nano bulges on PET by controlling the magnetron sputtering process, in the end the ZAC MTE on flexible PET substrate shows better optoelectronic properties than the counterpart on the rigid glass.

Typically, figure-of-merit (FoM) is used to evaluate the overall performances of transparent conductive electrodes37. The FoM (F) values are obtained by fitting Equation (1) with the experimental data through plotting the optical transmittance (T) at 550 nm versus the Rsh. ଵ଼଼.ହ ିଶ ) ೞ೓ ி

ܶ = (1 + ோ

(1)

Typically, a FoM value greater than 35 is considered to be viable for commercial application38. Figure 2d compares ZAC MTE with previously reported MTEs. This novel ZAC electrode on PET substrate, with a sheet resistance (Rsh) of 9.7 Ω/sq and a transmittance of 96.3% at 550 nm (excludingthe substrate), exhibits a FoM value near 1000, which is far more higher than the commercial ITO electrode. The high FoM of


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ZAC MTE shows the tremendous potential in the transparent electrode application.

Figure

2.

(a)

Total

reflectance

of

the

glass/ZnO/Ag

(black

square),

glass/ZnO/Ag/CuSCN (red circle), PET/ZnO/Ag/CuSCN (blue triangle), Glass (pink triangle), PET (green rhomboid), (b) Optical transmittance of ZAC on glass substrate with different thickness of CuSCN, (c) Optical transmittance of PET/ZAC (blue triangle, with nano bulges embedded), Glass/ZAC (pink triangle, without nano bulges), PET (red circle), Glass (black square), inset is the conceptual diagram for the enhanced transmission caused by nano bulges embedded in ZAC electrode, (d) Characteristic curve of optical transmittance at the wavelength of 550 nm (including substrate contribution) versus the corresponding Rsh for this ZAC electrode on PET



substrate, and also other representative transparent electrodes. The lines represent fits of Equation (1) to the clusters of data points, which determines their figure of merit (FoM). The thicknesses of ZnO, Ag and CuSCN films in the Fig. (a, c and d) are 20nm, 9nm, 40nm separately.

Mechanical properties and stability. The mechanical flexibility and durability under bending stress, as the key parameters of flexible transparent electrodes, are also studied, and encouraging results are received. As shown in Fig. 3a, the resistance varieties of the electrodes under bending are measured as a function of the bending cycles39. When the ZAC electrode is bent with a radius around 5 mm for 10,000 times, no obvious increase in resistance is observed. As comparison, the resistance of the ITO electrode after bending with 100 cycles at even larger bending radius (10 mm) increases by nearly three orders. Bending at 5 mm radius is fatal to the brittle ITO, resulting in sharp resistance increase after one or two cycles. Cracks, which can be seen on PET/ITO film (Fig. 3b) after bending for 100 times, cut off the transporting path of charge carriers. While the PET/ZAC electrode is so flexible that no microscopic crack is found even under 10,000 bending cycles (Fig. 3b).

Benefited from its inorganic material property, CuSCN works very well as capping layer with great stability. CuSCN is insensitive to water, and almost insoluble to common organic solvents, which means it can protect conductive film very well from erosion, and keep the optical and electrical properties of transparent electrodes for a


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long time. To examine the stability of ZAC electrode, the electrodes are stored and tested without encapsulation in an ambient environment at room temperature with humidity of 63 ± 5%. After more than 400days , the Rsh of ZAC on both glass and PET substrates remains outstanding (13.5Ω/sq and 12.3Ω/sq respectively) (Fig. 3c).

Encouragingly, this flexible ZAC electrode could be readily expanded to a conductive sheet with a large area up to 112 cm2 (8 cm × 14 cm), which still maintains comparable performances with the small-size counterpart (Fig. 3d), and demonstrates that the architecture and manufacture procedure for ZAC electrode are scalable, providing a good chance to realize large-area flexible electronic devices.

Figure 3. (a) Characteristic curves of Rsh versus the bending cycles for PET/ZAC with



5 mm radius (red circle), and for PET/ITO with 5 mm (blue triangle) or 10 mm (black square) radii. Bending radii are tested according to the inset image. (b) SEM images of ITO and ZAC electrodes after bending for 100 times. (c) Rsh Variations of the glass/ZAC (black square) and PET/ZAC (red circle) electrodes in 63 ± 5% humidity without encapsulation for 400 days. (d) Diagram of flexible large-area ZAC (112 cm2) on PET with good transparency and conductivity. Cigit logo was used with permission.

Application to development of high-performance flexible OLEDs. To demonstrate the application potential of this high-performance ZAC, OLED devices employing it as transparent electrode are fabricated on either glass or PET substrates with the cell architecture: ZAC/SY/LiF/Al (Fig. 4a). Besides, two glass/ITO based control devices, i.e. ITO/PEDOT:PSS or CuSCN/SY/LiF/Al, are fabricated for comparison. To review objectively the properties of the ZAC electrode, SY is chosen as emitter for its universal popularity. According to the schematic energy structure diagram shown in Fig. 4b, CuSCN with a deeper HOMO level (-5.5 eV)17 than that of PEDOT:PSS (-5.1 eV)40 implies a better energy match and better hole injection capability, according to integer charge transfer (ICT) model41.

The current density−voltage−luminance (J−V−L) characteristic curves of OLEDs with different transparent electrodes are shown in Fig. 4c. All the devices exhibit competitive performances and turn on at ~2.3 V, which is smaller than that of band gap of SY, indicating good energy level match between different functional layers of


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OLEDs. Benefitted from the embedded nano bulges, the PET/ZAC flexible transparent electrode based OLED provides much better photoelectrical properties as compared to the glass/ZAC electrode without nanostructures, which are even better than ITO based devices. The PET/ZAC based OLED provides a champion current efficiency of 23.4 cd/A at a current density of 65 mA/cm2, much higher than that of the devices based on glass/ZAC (15.2 cd/A), glass/ITO/CuSCN (16.3 cd/A) and glass/ITO/PEDOT:PSS (15.1 cd/A) electrodes (Fig. 4d and Table S2), profiting from the enhanced luminance. This improvement is mainly ascribed to the remarkable outcoupling of internal light from SY emitting layer into air, induced by the nano bulges embedded in ZAC electrode.

The enhancement of light out-coupling of ZAC electrode induced by the embedded nano bulges is investigated in detail. The light emission behaviors of the OLED are analyzed by the well-established rigorous dipole model through treating the emissive dipoles in the active layer as forced damped harmonic oscillators. From such a model, the power dissipation spectra generated by the dipoles in the active layer can be calculated. The case of ZAC electrode without nano bulges corresponds to the left spectra (Fig. 5a), which can be divided into four regions, including the air mode region (k < nair/ne), the substrate mode region (nair/ne < k < nsub/ne), the waveguide mode region (nsub/ne < k < 1), and the plasmonic mode region (k>1)42. It is clear that, in this situation, the waveguide modes and plasmonic modes dominated the power dissipation spectra, indicating that most of the light generated by the active layer is dissipated in these two modes, so the outcoupling efficiency is relatively low. By contrast, the ZAC electrode



with embedded nano bulges produces totally different results (Fig. 5b). In this situation, the power dissipation intensity of waveguide modes and plasmonic modes decreases considerably, while the dissipation intensity of air modes increases. Such a result is expected because nano bulges in the ZAC electrode can act as light scatters to decouple waveguide and plasmonic modes into air modes. Therefore, benefited from the nano bulges embedded, the light outcoupling efficiency of the flexible ZAC on PET has been significantly enhanced, in good accordance with the experimental results.

Besides the excellent photoelectric performances, this OLED with PET/ZAC as flexible transparent electrode obtains excellent mechanical flexibility of device as well. The current efficiency/luminance of OLED on ZAC is able to keep 90% of the original value after 1000 bending cycles at a radius of 5 mm (Fig. S12). In great contrast, the ITO based device degraded quickly under the same bending conditions, probably due to the easy fragmentation and the sharply-reduced electrical conductivity of ITO electrode under bending.


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Figure 4. (a) Device architecture of OLEDs with ZAC or ITO/PEDOT:PSS as transparent electrodes and Super Yellow (SY) as emissive layer. (b) Schematic energy level diagram for the component materials in OLEDs. (c) Characteristic curves of current efficiency/luminance versus voltage (J–V–L) and (d) current efficiency versus current for glass/ITO/PEDOT:PSS (black square), glass/ITO/CuSCN (red circle), PET/ZAC (blue triangle) and glass/ZAC (purple inverted triangle) based devices.



Figure 5. Calculated power dissipation spectra weighted with the emitter spectrum in arbitrary units (mapped as a color defined in the color bars.) versus normalized in-plane wave vector of OLEDs based on ZAC electrodes (a) without nano bulges and (b) with nano bulges.

CONCLUSIONS

In summary, multilayer transparent electrode with inorganic solution-processed p-type CuSCN as a hole-transport antireflection coating (ZAC) is developed to afford an average transmittance of 94% in the visible range and a Rsh of 9.7 Ω/sq. The flexibility and stability of the electrode allows it to tolerate 10,000 cycles of bending at 5 mm radius or after exposure to air with high humidity up to 68% for more than 400 days. Benefited from its scalable fabrication process, the electrode size can be expanded to a large area of 112 cm2 without obvious degradation of photo-electronic properties. When applied into OLED with SY as emitter, the flexible ZAC electrode with embedded nano bulges provided a current efficiency as high as 23.4 cd/A, much


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higher than the counterpart without nanostructures or with ITO electrode. Calculation simulation reveals that the enhanced light outcoupling arises from the nano bulges, which can act as light scatters to decouple waveguide and plasmonic modes. Moreover, the ZAC electrode provides OLED devices with excellent flexibility, e.g. it maintains 90% of the original current efficiency/luminance after bending 1000 cycles at a small bend radius of 5 mm. These results suggest the ZAC multilayer electrode featured with p-type solution-processed CuSCN for light antireflection and embedded nano bulges for light out-coupling is a promising flexible transparent electrode for the next-generation opto-electronics.

METHODS

Material preparation and film characterization. PEDOT:PSS 4083 is purchased from Heraeus. CuSCN powder is purchased from Sigma–Aldrich and dissolved in diethyl sulfide (boiling point: 92 °C) at room temperature. The emitting material SY dissolves in toluene (5mg/ml). XRD measurements are performed at room temperature with a PANalytical X’Pert3 powder diffract meter operating in Bragg–Brentano scanning mode, with an angular resolution of 0.01° and Cu–K radiation (0.154056 nm wavelength). Film thicknesses are measured by using an Alpha-step IQ system. Surface morphology of each film is characterized by AFM and SEM. AFM surface topography images are tested by and acquired by using a dimension EDGE in contact tapping mode. SEM images are measured by using JSM-7800F.

Electrode fabrication and characterization. The structure of the fabricated ZAC



electrode is ZnO/Ag/CuSCN. ZAC electrodes are deposited on glass or PET substrates by magnetron sputtering of ZnO and Ag, and subsequent solution process of CuSCN in diethyl sulfide at room temperature. The bottom ZnO film is deposited by sputtering a ZnO target and the thickness is controlled by sputtering time. The chamber is evacuated to a base pressure of 1.7 × 10-5 Pa before sputtering, and working pressure is kept at 2 Pa by introducing Ar gas (99.9999%) at a flow rate of 20 sccm. The radio frequency (RF) power for ZnO sputtering process varies from 60 W to 260 W. Ag layer is deposited on the bottom ZnO via sputtering Ag target at a power of 40 W. The deposition is performed at 0.5 Pa by introducing a gas mixture of Ar (flow rate of 45 sccm) and O2 (flow rate of 2 sccm). The CuSCN film with desired thickness is prepared by spin coating the CuSCN solution (20 mg/ml in diethyl sulfide) on Ag layer at 3500 rpm for 50 s and baked at 100 °C for 20 min in a nitrogen environment. The Rsh of electrode is obtained by four-point probe technology and the transmittance is acquired by a UV-vis spectro-photometer (Lambda 35, Perkin Elmer). Reflectance are measured by an integrating sphere with power source (QE-LXE 75 W xenon lamp, Enli Technology Co., Ltd).

OLED device fabrication and measurement. With glass or PET/ZAC electrodes in hands, the glass/ITO/CuSCN or PEDOT-PSS electrodes are also prepared for comparison. The ITO-coated glass substrates are first ultrasonically cleaned with detergent, deionized water, acetone, and ethyl alcohol in sequence, then treated for 15 min with ozone generated by a UV-Ozone cleaner. CuSCN solution (20 mg/ml in diethyl sulfide) or PEDOT:PSS 4083 is spin-coated on the ITO substrates at 3500 rpm


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for 50s and baked at 100 °C for 20 min. After the transparent electrodes are prepared, SY in toluene (5mg/ml) is deposited on them by spin-coating at 1500 rpm for 45s and baked at 80 °C for 30 min. Finally, 1 nm LiF and 100 nm Al are sequentially thermally evaporated under a high vacuum through a shadow mask with an active area of 0.11 cm2. Current density-voltage-luminance (J-V-L) characteristic curves are tested by a source meter (Keithley 2400) and a chroma meter (CS-100A, Konica Minolta). The emitting spectrum is obtained by an ocean spectrometer (USB4000). All measurements are conducted in ambient atmosphere.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.xxxxxxx.

Physical and chemical properties of CuSCN; Performance parameters of OLED devices; Fabrication flow chart of the ZAC electrode with nano bulges; Relationship between the plasma power and the physical dimension of the nano bulges; Optical properties of ZAC electrode related to zinc oxide and cuprous thiocyanate layers; Sheet resistance of ZAC electrode related to ultra-thin silver film; Performances of ZAC electrode based flexible OLEDs；Fresnel coefficients’ modifying in transfer matrix to caculate the outcoupling of bulges embedded OLED.



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AUTHOR INFORMATION

Corresponding Author *

E-mail: [email protected], [email protected], [email protected]

Author contributions

Y. Ji and J. Yang initiate the project and perform the experiments. W. Luo assist the design and fabrication of OLED. L. Tang, X. Bai, and J. Wang finish the calculation simulation. C. Leng and C. Ma assist the material synthesis and characterization. X. Wei and J. Shen analyzes and interprets the experimental data. S. Lu, K. Sun and H. Shi supervise the experimental work and write the paper.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

This paper is financially supported by the Project of National Natural Science Foundation of China, Fundamental & Advanced Research Project of Chongqing and Chongqing University Research Funding (21642013, 61504015, 61504148, cstc2015jcyjB0628,

cstc2013jcyjC00001,

cstc2016jcyjA0315

106112017CDJQJ148805).

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Table of Contents Graphic


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Ultraflexible and High-Performance Multilayer Transparent Electrode

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