A Flexible and Thin Graphene/Silver Nanowires ... - ACS Publications

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A Flexible and Thin Graphene/Silver Nanowires/Polymer Hybrid Transparent Electrode for Optoelectronic Devices Hua Dong,†,⊥ Zhaoxin Wu,*,† Yaqiu Jiang,† Weihua Liu,‡ Xin Li,‡ Bo Jiao,†,§ Waseem Abbas,† and Xun Hou† †

Key Laboratory of Photonics Technology for Information, Key Laboratory for Physical Electronics and Devices of the Ministry of Education, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China ‡ Departmen of Microelectronics, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China § Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ⊥ Department of Biomedical Engineering, School of Life Science and Technology, Xi’an Jiaotong University, Xi’an 710049, P. R. China S Supporting Information *

ABSTRACT: A typical thin and fully flexible hybrid electrode was developed by integrating the encapsulation of silver nanowires (AgNWs) network between a monolayer graphene and polymer film as a sandwich structure. Compared with the reported flexible electrodes based on PET or PEN substrate, this unique electrode exhibits the superior optoelectronic characteristics (sheet resistance of 8.06 Ω/□ at 88.3% light transmittance). Meanwhile, the specific up-to-bottom fabrication process could achieve the superflat surface (RMS = 2.58 nm), superthin thickness (∼8 μm thickness), high mechanical robustness, and lightweight. In addition, the strong corrosion resistance and stability for the hybrid electrode were proved. With these advantages, we employ this electrode to fabricate the simple flexible organic light-emitting device (OLED) and perovskite solar cell device (PSC), which exhibit the considerable performance (best PCE of OLED = 2.11 cd/A2; best PCE of PSC = 10.419%). All the characteristics of the unique hybrid electrode demonstrate its potential as a high-performance transparent electrode candidate for flexible optoelectronics. KEYWORDS: flexible transparent electrode, metal nanowires, graphene, sandwich, up-to-bottom

1. INTRODUCTION Transparent electrodes have recently attracted considerable attention due to the key components for optoelectronic devices. As a traditional electrode, indium tin oxide (ITO) hinders the applications in flexible optical and electrical devices due to the deficiency of the indium source and brittle nature of the ITO.1,2 To overcome these limitations, a number of alternative transparent electrodes materials have been investigated such as conductive polymer, carbon nanotubes (CNTs), graphenes, random metal nanowires, and electrospun metallic nanofibers.3−9 Among the reported candidates, silver nanowires (AgNW) exhibit the superior electric conductive and optical transmittance.10−17 However, oxidation and corrosion problems of © 2016 American Chemical Society

bare AgNWs always exist in previous research which caused by exposure to atmosphere and chemical attack in harsh environments.9,18 Subsequently, hybrid structures were developed to solve the weakness. Employing the graphene (GN) to combine with silver nanowires was one of the most effective strategies: With the advantages of high chemical and thermal stability, high mechanical property, high optical transparency, and easy fabrication, graphene could provide an effectively protective screen for the metal matrix. In addition, the 2D GN Received: July 23, 2016 Accepted: October 28, 2016 Published: October 28, 2016 31212

DOI: 10.1021/acsami.6b09056 ACS Appl. Mater. Interfaces 2016, 8, 31212−31221

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ACS Applied Materials & Interfaces

2. EXPERIMENTAL DETAILS OF HYBRID TRANSPARENT ELECTRODE

structure also improved the extra electron pathway for the hybrid film.19−21 Although the optical−electrical and stability properties of the reported AgNW−GN hybrid electrodes were comparable to conventional ITO electrode, there remain various challenges: high surface roughness, semiflexibility, and high thickness. As for the reported AgNW−GN hybrid electrodes, the common operation procedure was transferring the CVD-grown GN layer onto the AgNW, and the hybrid conductive matrix was localized on the flexible plastic substrate.22,23 This traditional method of transferring the GN layer onto the rough AgNW surface probably lead to the ripples, defects, and cracks of the GN layer, which could destroy the conductive path and reduce the charges carriers of the electrode.22,23 In addition, the surface roughness of majority of reported GN−AgNW electrodes was less than desirable even after the treatment,24,25 and this highly coarse surface of the electrode should be avoided cause the uniformity and the smooth of the electrode could significantly influenced the performance of the applied optoelectronic devices. Beyond that, other properties related to the commercial and industrial application were equally important, that is, superflexible, ultrathin, and lightweight. It was noticed that the current researches on AgNW−GN hybrid electrode were almost based on plastic substrates such as PET (poly(ethylene terephthalate)) or PEN (poly(ethylene naphthalate)), which were both semiflexible and thick.26−28 The superflexibility and lightweight property of the novel transparent electrode were also crucial to develop the low-cost and large-size commercial application of the flexible optoelectronic devices. In this article, we developed a unique sandwich construction to prepare a high-performance and fully flexible graphene− AgNWs−polymer transparent electrode. AgNWs were completely encapsulated between the monolayer graphene and mixed polymer matrix (polycarbonate:Pc and additives) via a specific up-to-bottom fabrication process, and this unique hybrid electrode exhibits advantages in terms of various aspects. First, as the main framework of the electrode, the polymer could promise the compact combination of graphene and AgNWs as well as the firm junctions between AgNWs. Meanwhile, the entire hybrid film exhibits the superthin, lightweight, and fully flexible features due to the development of the mixed polymer matrix, which was ignored in previous studies. It was also found that the buried AgNW could be protected from the oxidation and corrosion by the graphene and the polymer layer. This type of electrode exhibits the remarkable advantages on many properties in terms of high electrical conduction (sheet resistance of 8.06 Ω/□ at 88.3% light transmittance), low surface roughness (RMS ∼ 2.5 nm), high mechanical robustness, excellent chemical corrosion, superthin (thickness ∼ 8 nm), and lightweight (∼1/11 weight of PET with same area). Moreover, the uniform and large-area (10 cm × 10 cm) hybrid electrode can be obtained by this convenient fabrication process. Finally, the application of the unique graphene−AgNWs− polymer hybrid transparent electrode for flexible devices was also investigated by preparing the organic light-emitting devices (OLEDs) and perovsikite solar cells (PSC) on this electrode, and the performances were considerable. It was prospected that out study has the potential to boost the development of the novel transparent electrodes and flexible optoelectronic devices.

2.1. Preparation of Silver Nanowires. The air-assisted polyol method was used to synthesize the AgNWs.29,30 In this synthesis, 10 mL of ethylene glycol (EG) solution (0.6 M) of polyvinylpyrrolidone (PVP), 1 mL of AgNO3 solution (0.93 M in EG), and 0.2 mL of EG solution of NaCl (0.032 M) were injected into a three-neck round flask, which was equipped with a thermocontroller and a magnetic bar. The mixture was heated to the boiling point of EG (198 °C) in a flask and should be maintained for another 20 min; then it was cooled naturally to room temperature. Air was pumped continuously into the mixed solution during the whole treatment. Then, we took the centrifugation method to separate the AgNWs. The SEM images, TEM images, absorption spectrum, and XRD of AgNWs are shown in Figures S1, S2, and S3, respectively. 2.2. Preparation of Monolayer Graphene. For monolayer graphene growth,31,32 before the growth of graphene, the Cu foil (Alfa Asear, 99.99%) was placed in a CVD system with a quartz tube and annealed at at 980 °C, 80 mbar, and 10 sccm H2 /150 sccm Ar for 30 min for preparing a high-density step structure on Cu, which contributed to the higher decomposition rate of methane (CH4) during graphene growth. After that, the system was slowly cooled to room temperature under H2. Then the temperature was increased to 1000 °C under the conditions of 1 mbar, 1 sccm H2, and 15 sccm CH4 for the growth of graphene. After 15 min of growth, the system was cooled to room temperature under H2. After CVD growth, PMMA (950 PMMA A 6) was spin-coated on graphene/Cu for the first time and standing for several seconds; then the spin-coat process was repeated again. The redissolution of the PMMA tends to mechanically relax the underlying graphene, leading to a better contact with the substrate.33 Meanwhile, a more sufficient coverage with PMMA on the graphene could be achieved. After being annealed at 80 °C for 10 min, subsequently the composite was immersed into 0.1 M ammonium persulfate solution to etch away the Cu foil. PMMA/graphene was washed with deionized water for three times, and then it was transferred onto the PTFE (polytetrafluoroethylene) structure. Here we kept the PMMA/graphene/PTFE quiescence in acetone for 48 h to remove the PMMA. After that, the isolated graphene/PTFE was kept in alcohol for 5 min to complete the final clean process. At the end, the graphene/PTFE substrate was obtained. 2.3. Preparation of the Hybrid Electrode. Graphene/AgNW/ Polymer Hybrid Electrode. PTFE was chosen as the transfer to promise the smooth surface of the hybrid electrode. AgNWs solution with different concentrations (1.25, 2.50, 3.75, 5.00, and 6.25 mg/mL shown in Figure S4) were spin-coated at 1000 rpm for 60 s onto the cleaned graphene/PTFE substrates to form a randomly dispersed AgNWs network. Then the air-dried AgNWs film was subjected to a pressure of 10 MPa for 10 s to ensure the compact connection between AgNWs and AgNW mesh and graphene. After that, the PTFE/graphene/AgNWs structure was annealed at 100 °C for 15 min. Subsequently, mixed polymer in chloroform solution was prepared as follows: (weight ratio: polycarbonate:polystyrene:ethylene glycol monobutyl ether acetate:oleamide = 95:2:2:1). Here the polycarbonate (PC) was used as the polymer matrix, polystyrene (PS) was used as the softer, ethylene glycol monobutyl ether acetate was used as the adhesive, and oleamide served as the mold release agent. All these materials were purchased from Sigma-Aldrich, and the chemical structures are shown in Figure S5. The mixed solution was stirred for 5 h; then it was spin-coated at 1000 rpm for 30 s onto the surface of the PTFE/graphene/AgNWs structure, which was also followed by thermal annealing at 100 °C for 10 min. Then the graphene/ AgNWs/polymer hybrid film formed on the surface of the PTFE substrate. For the final peeling process of the hybrid electrode, a specific “lowtemperature exfoliation technology” was utilized. Here we placed the whole structure in top of the liquid nitrogen with a proper distance (no direction contact). Under this low temperature atmosphere (nearfreezing), the hybrid electrode could be intactly stripped from the PTFE substrate due to the different surface tension. The conductive 31213


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Figure 1. (a−g) Schematic diagram of the hybrid graphene−AgNW−polymer (MG-A-P) electrode. (h) An optical image of MG-A-P hybrid electrode and (i) curved exhibition of MG-A-P hybrid electrode. film could be peeled from the PTFE substrate with almost no technical damages. AgNW/Polymer Hybrid Electrode. Here for the AgNW/polymer hybrid electrode, at first step, bare PTFE without graphene was employed as the initial transfer substrate. The subsequent process was the same with the graphene/AgNW/polymer hybrid electrode. Graphene/Polymer Hybrid Electrode. Here for the graphene/ polymer hybrid electrode, the process of spin-coating AgNWs and being hot-treated is omissible, and the other process was the same with the graphene/AgNW/polymer hybrid electrode. AgNW/PET Hybrid Electrode. AgNWs solution with concentration of 3.75 mg/mL was spin-coated at 1000 rpm for 60 s onto the cleaned PET substrates to form a randomly dispersed AgNWs network. Then the air-dried AgNWs film was subjected to a pressure of 10 MPa for 10 s to ensure the compact connection between AgNWs. ITO/PET Electrode. The ITO/PET electrode was purchased from Sigma-Aldrich with the properties (sheet resistance of 25 Ω/□ at ∼84% light transmittance). 2.4. Measurements. The morphology of the AgNWs was investigated by a scanning electron microscope (SEM) (Quanta 250, FEI) and a transmission electron microscope (TEM) (2100, JEOL). Atomic force microscopy (AFM) (NT-MDT solver P47H-PRO, Russia) was used to characterize the surface roughness of the films. The surfaces of various films were examined by scanning electron microscopy (SEM) (FEI Quanta 250) and optical microscopy (Nikon eclipse lv100). Optical transmission spectra of the different films were recorded by using a UV−vis−NIR spectrometer (Hitachi U-3010, Japan). Sheet resistance measurements were taken using the fourprobe tester (M3, Suzhou Jingge Electronic Co. Ltd., China). Crystalline structures were performed by X-ray diffraction (XRD) (D/MAX-2400, Rigaku, Japan) with Cu Kα radiation. PSC device characteristics were evaluated in ambient under AAA solar simulator (XES-301S, SAN-EI Electric Co. Ltd.), AM 1.5G illumination, with an intensity of 100 mW/cm2 (1 sun, calibrated by a NREL-traceable KG5 filtered silicon reference cell). The current density−voltage (J−V) curves of OLED and PSC were measured by a Keithley digital source meter (Model 2602). Incident photon-to-current conversion efficiency (IPCE) spectra of PSC were obtained by the solar cell quantum efficiency measurement system (SolarCellScan 100, Zolix Instruments Co. Ltd.).

3. RESULTS AND DISCUSSION OF HYBRID TRANSPARENT ELECTRODE The schematic diagram of the hybrid graphene−AgNW− polymer (MG-A-P) electrode preparation process is shown in Figure 1 (steps a−g). The CVD-grown monolayer graphene (MG) was transferred onto the PTFE as the top structure of the hybrid electrode, and then different concentrations of AgNWs in isopropyl alcohol (IPA) were spin-coated onto the MG layer. A hot-press process was adopted to ensure the compact combination of AgNWs and MG as well as form the firm junctions between AgNWs (shown in Figure S6).34 Subsequently, a mixed polymer in chloroform solution was spin-coated over the AgNWs-MG surface. Here a 30 s standing for the polymer was needed to ensure that the conductive matrix was entire buried, and then the hybrid was annealed to prepare an integral structure. It was noticed that here we introduced an specific exfoliated method called the “lowtemperature exfoliation method” as described in the Experimental Details section, which could promise the integrity and the uniformity of the hybrid electrode. PTFE was chosen as the transfer substrate for its lower surface energy than the other conventional substrate such as SiO2, which could promise the easily stripping of the hybrid electrode.34 Finally, the highquality, flexible, and conductive hybrid MG-A-P transparent electrode was obtained. 3.1. Characterization of the Monolayer Graphene (MG). The quality and completeness of graphene layers significantly influenced the performance of the hybrid electrode.27 The uniform monolayer graphene (MG) on the Cu foil was first obtained via a CVD method, and then it was transferred onto a PTFE substrate (process shown in Figure 2). Figure 3a shows the SEM of the initial MG film on the surface of the Cu foil. We can see that the Cu surface was covered by the continuous graphene film with few wrinkles (white lines), which was due to the typical growth process of the graphene (described in the Supporting Information).31 There was some “polishing marks” (dark scratches) on the Cu surface which were the fabrication and treatment results of the fabricating high purity copper.35 Here the large-area morphology of the graphene deposited on the Cu foil was also observed, and the 31214


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electrode, Figure 3c exhibits the optical microscope (OM) image of the unfolded graphene film on PTFE substrate transferred from the Cu foil, and Figure 3d shows the SEM image of the unfolded graphene film on PTFE. It can be observed that under a large-scale resolution the well-transfer graphene film was obtained with little broken after a successful PMMA-assisted transfer process. Raman spectra in Figure 3e show the synthesized graphene is high quality without any obvious defects. A more than twice intensity ratio of 2D peak to G peak can be observed, and the full widths at half-maximum (fwhm) of the 2D peak are ∼30 cm−1, suggesting that the asprepared graphene was monolayer graphene (MG).31 3.2. Properties of the Hybrid Electrode. By reason for the typical encapsulation technique of the hybrid electrode, that is, burying the conductive mesh into the flat polymer matrix, both the sandwich structure (with graphene) and the doublelayer structure (without graphene) exhibit the excellent optical and electrical characteristics. To investigate the properties of the unique hybrid electrode in our study, the conductive resistance and light transmittance of two types of electrodes (graphene−AgNW−polymer (MG-A-P); AgNW−polymer (AP)) were measured. Here the concentration of AgNW increased from 1.25 to 6.25 mg/mL (1.25, 2.50, 3.75, 5.00, and 6.25 mg/mL) to search the optimal balance between the optical and the conductive properties. For comparison, we also

Figure 2. Schematic diagram of the preparation of monolayer graphene.

corresponding SEM image is shown in Figure S7, indicating the uniformity of the CVD-grown graphene. The transfer approach was important to obtain a uniform and smooth graphene film. Two types of graphene (folded and unfolded) were transferred onto substrate for observing and applying, respectively. Since the morphology of single graphene layer can be hardly observed due to the ultrathin and high optical transmittance property, a folded sample was prepared to study its morphology. Figure 3b shows the high-power SEM image of folded exfoliation graphene on Si, and the wrinkles were the folded areas of the graphene. As for the graphene layer employed by the hybrid

Figure 3. (a) SEM image of the monolayer graphene on the Cu surface. (b) SEM image of the exfoliated folded monolayer graphene. (c) Optical image of the monolayer graphene transferred on PTFE. (d) SEM image of the monolayer graphene transferred on PTFE. (e) Raman spectra of the transferred graphene. 31215


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ACS Applied Materials & Interfaces measured the sheet resistance of the graphene/polymer films. The resistances of MG-P film, MG-A-P film, and A-P film are shown in Figure 4.

mL. When the graphene was introduced, the corresponding parameter of the MG-A-P changes from 93.1% to 80.3%. It was found that the optical transmittance of the graphene modified hybrid film has a slight reduction compared to the film without graphene at the same AgNWs concentration. This tiny decline was probably attributed to the existence of the monolayer graphene, which obstructed the light to some extent. However, considering the introduction of the MG has the advantage of increasing the conductivity, the comprehensive optical− electrical properties of the hybrid electrodes should be evaluated via a justicial approach. To determine the optimum performance of the various hybrid electrodes, the value of figure of merit (FOM) was calculated.36 FOM was based on sheet resistance (Rs) and optical transmittance (T) at a wavelength of 550 nm, and it was defined as FOM = T10/Rs. Large FOM was corresponding to the high performance of electrode. Figure 5c shows the optical transmittance and FOM values of the hybrid electrodes with and without graphene, respectively. It was found that the FOM of the MG-A-P electrode exhibits a constantly larger value of FOM than that of the A-P electrode, and the largest FOM of MG-A-P electrode was 35.7 × 10−3/Ω with the concentration of AgNW at 3.75 mg/mL. Here the corresponding optical and electronic property of the electrode were Rs = 8.06 Ω/□ and 88.3% optical transmittance at 550 nm, respectively. The outstanding performance was comparable to the conventional ITO and other hybrid transparent electrodes based on metal nanowires (parameters of different electrodes shown in Table S1).25,28,34,37−41 Moreover, the flat and high optical transmittance of this hybrid electrode at the near-infrared region was better than that of the ITO/PET (show in Figure S8), revealing the potential to applied in photoelectric detectors and solar cells for utilizing the near-infrared region more effectively. The transmittance spectra of MG-P film were measured and are shown in Figure S9 (97.15% at 550 nm), and the FOM was calculated as approximately 5.6 × 10−3/Ω. The FOM of MG-P film was rather lower than that of the best MG-A-P and A-P film, which demonstrated that the comprehensive optical− electrical properties of pure MG-P film have an unsatisfactory performance as the conductive film. Differing from the reported bottom-up fabrication of the transparent electrode, the unique up-bottom process in our study could achieve a flat and smooth surface of the hybrid electrode. The morphologies of the various hybrid electrodes were investigated, and the AFM images are shown in Figure 6. Figures 6a−d show the AFM images of A-P hybrid films. We can see that the distribution of the uniform AgNWs became gradually dense with the increased concentrations of the

Figure 4. Sheet resistance of the MG-P, MG-A-P, and A-P hybrid films with different concentrations of AgNWs.

It was found that with the increase of the concentrations of AgNWs both types of the electrodes have the reduced trend of the resistance, which can be attributed to the dense conductive mesh. As for the electrode with or without graphene at the same concentration of AgNWs, we can see that the MG-A-P electrode has the further decreased resistance. This result was presumably due to the existence of the graphene, which could effectively improve the conductive property of the electrode as the conduction channel. It was noticed that when the concentration of the AgNWs was over 5 mg/mL, the conductive property just improved slightly. Considering the excessive AgNWs would influence the light transmittance of the electrode, electrodes with four densities were studied in the following studies (1.25, 2.50, 3.75, and 5.00 mg/mL). However, the average sheet resistance of MG-P films is just 1600−1700 Ω/□, which indicated the low conductive property of the pristine graphene. The important property of transparent electrode was optical transmittance over a broad range of wavelengths. The transmittance spectra of two types of hybrid films (MG-A-P and A-P) were measured and are shown in Figures 5a and 5b. Research shows that when the deposition density of the AgNWs increased, the optical transmittance for both the A-P hybrid films and MG-A-P hybrid films decreased with a similar trend. At the wavelength of 550 nm, the optical transmittance of the A-P films decreases from 95.3% to 82.2% with the concentration of the AgNW increasing from 1.25 to 5.00 mg/

Figure 5. (a) Optical transmittance spectra of AgNWs−polymer (A-P) film and (b) graphene−AgNWs−polymer (MG-A-P) film with various concentrations of AgNWs. (c) FOM values of A-P and MG-A-P and the corresponding transmittance at 550 nm. 31216


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Figure 6. AFM images of the surface of A-P films (a−d) and MG-A-P films (e−h) with different concentrations of AgNWs. Here the images on the sane line were measured under the same concentrations of AgNWs, and the RMS was also indicated in the images.

Figure 7. (a) SEM surface image of MG-A-P film (3.75 mg/mL). (b) SEM cross-section image of MG-A-P film and enlarged surface. (c) OM crosssection image of MG-A-P film. (d) Raman spectra (graphene peak) of the pure monolayer graphene and MG-A-P film.

shown in Figure 7a (a magnified image is shown in Figure S10a). This low roughness was attributed to the specific construction; that is, both the MG layer and AgNWs were buried in the polymer matrix, resulting in a compact and integrate structure. Relative to the semiflexible conventional ITO-PET(∼100 μm thickness shown in Figure S11), MG-A-P electrode demonstrated a superflexible feature with natural bend (shown in Figure S12) due to the development of the mixed polymer matrix. Such a fully flexible and smooth electrode was the vital requirements of the high-performance optoelectronic devices. Moreover, the thickness of the entire electrode can be

AgNWs. It was worth noting that the surface roughness (RMS) of the films was observed at a rather low level (from 2.12 to 2.98 nm), which was due to the unique transfer-fabricating process. When the monolayer graphene was introduced (shown in Figure 6e−h), fluctuations of the RMS (right column to the left column) were inconspicuous. In view of the thickness of a monolayer graphene film was less than 1 nm (diameter of a carbon atom), the roughness analysis indicated that the graphene layer combined with the other components of the hybrid electrode uniformly and completely. Here the RMS of the MG-A-P hybrid electrode with optimal performance (3.75 mg/mL) was around 2.58 nm, and the surface SEM image is 31217


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Figure 8. (a) Variation in sheet resistance versus bending of different electrodes. (b) Variation in sheet resistance versus pasting of different electrodes. (c) Variation in sheet resistance of different electrodes expose to atmosphere for 30 days. (d) Variation in sheet resistance of different electrodes under the attack of aqueous Na2S solution.

controlled by modified the spin-coating conditions of the polymer. Figures 7b and 7c show the cross-section SEM and OM images of the MG-A-P electrode (AgNWs at 3.75 mg/ mL); it was found that the thickness was around 8 μm (a magnified SEM cross-section image of MG-A-P film was shown in Figure S10b), and a magnified SEM image (Figure 7b) exhibits a flat surface of the electrode with AgNWs embed in the polymer film. The production in our study was much more flexible and thinner compared to the other flexible electrodes based on PET or PEN substrate (shown in Table S1), and these excellent properties were credited to thickness-adjusted polymer matrix. The Raman spectra of the MG-A-P electrode were measured and are shown in Figure 7d. It was noted that the characteristic peaks (high ratio of 2D peak to G peak) of the monolayer graphene can still be observed. This result demonstrated that the creative up-to-bottom transfer process contributed to the complete and high-uniform graphene layer in hybrid electrode. To further demonstrate the uniformity of the hybrid electrode, a large-scale (10 cm × 10 cm) MG-A-P electrode was prepared (shown in Figure S13), and then sheet resistance of multiple areas was measured and is shown in Table S2. In addition, considering that the thickness of the MG-A-P electrode was superthin, we further compared the weight between the MG-P-A electrode and a ITO/PET (∼100 μm shown in Figure S11) electrode with the same area, and the result indicated a remarkable lightweight characteristic (weight ratio: 1:11) of the MG-P-A electrode (shown in Figure S14). Lightweight, large-scale, and superthin characteristics all have the critical significance of developing the commercial flexible optoelectronic devices.

A typical sandwich structure which buries the AgNW and graphene in the polymer matrix has its unique characteristics compared to other transparent conductive films in the property of mechanical robustness against bending and adhesion.42,43 Here the ITO-PET and AgNW-PET film (AgNWs at 3.75 mg/ mL) were both prepared as the reference, and the flexibility of the three types of films was evaluated by the bending test with a radius of curvature of 2.0 mm. The sheet resistance evolutions of three films with increasing bending cycles are shown in Figure 8a (ΔR = R − R0). It was found that the ITO film rapidly cracked, and the resistance remarkably increased only after five cycles of bending, which exhibit the fragile feature. The bare AgNW-PET film had a considerable performance, and its resistance increased 2-fold after 250 cycles of bending test. For comparison, the resistance of the MG-A-P film increased only 8% after 250 cycles of tensile folding, and no visible cracking or tearing on the surface was observed. This robust mechanical characteristic was owing to the fact that conductive mesh (AgNWs and graphene) was firmly anchored by the polymer matrix, superior to that of other transparent electrodes such as CNT and graphene films.44,45 The conventional AgNWs-based hybrid electrode has the drawback that the metal mesh can be easily erased from the substrate due to the weak adhesive forces. By contrast, the MG-A-P electrode via the integrate encapsulation could suffer the strict adhesive test. Here we exploited 3M tape (Scotch Magic Tape) to evaluate different electrodes (MG-A-P, AgNW-PET, and ITO-PET) under a pressure of approximately 2 MPa for 30 s. The result in Figure 8b shows that after 10-fold pasting tests ΔR/R of AgNW-PET dramatically increased due to the poor adhesion between AgNW and PET. By contrast, ΔR/R of G-A-P film was 31218


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Figure 9. Structure diagram of OLED (a) and perovskite solar cell devices (b) based on MG-A-P electrode. (c) J−V curve of PSC devices and the sample of the PSC. (d) J−V curve (e) brightness and (f) power efficiency of OLED.

4. FABRICATION AND THE CHARACTERISTICS OF THE OPTOELECTRONIC DEVICES BASED ON THE MG-A-P ELECTRODES To illustrate the practicability of the MG-A-Pfilm as a highperformance flexible transparent electrode in optoelectronic devices, we applied the hybrid film to the simple organic lightemitting device (OLED) and perovskite solar cell (PSC). The structure diagram of the two types of devices is shown in Figure 9a,b. The fabrication process of the optoelectronic devices is shown as follows: OLED devices: 5 nm MoO3 was deposited on the hybrid electrode by evaporation; then the 50 nm thick NPB and 50 nm thick Alq3 were deposited subsequently. After that, the cathode was prepared by thermal evaporation of a thin LiF layer (0.5 nm) and then an Al layer (120 nm). Perovskite devices: 10 nm MoO3 was deposited on the hybrid electrode. Then 100 nm PbI2 was deposited on the MoO3 by evaporation; subsequently, 1 M CH3NH3I in IPA was spin-coated at 2000 rpm onto the PbI2 layer and annealed at 90 °C for 1 h. Then 30 nm PC61BM, 5 nm BCP, and 120 nm Ag were deposited subsequently. For the PSC, the active area of the cells was defined by a metal mask with square aperture of the area of 0.070 65 cm2. The premasked active area of the solar cells was approximately 0.1256 cm2 (circle) nominally defined by the overlap area of the Ag and flexible conductive electrodes. Devices were masked for all the current−voltage measurements. For the OLED device, the light-emitting area was 0.16 cm2 (square) nominally defined by the overlap area of the Ag and flexible conductive electrodes. Each batch of devices containing 15 samples were prepared, and the various parameters are shown in Tables S3 and S4. For the optimal OLED device, a considerable performance can be obtained and is shown in Figure 9d−f (3.38 V of turn-on voltage, 4297 cd/m2 brightness, and 2.11 cd/A power efficiency). For the best PSC device, the scan rate is 0.05 V/s for both reverse and forward. The current−voltage character-

approximately invariable when suffered 100-fold tests, which was comparable to that of ITO-PET. The chemical inertness46 of graphene has been demonstrated in recent studies, and this corresponding corrosion-inhibiting ability could provide the passivated interface on transparent electrodes. However, it was difficult to achieve a complete and compact graphene coverage on the metal mesh.22,25 In our study, the specific treatment and encapsulation process eliminated the weakness: the initial hot-press process first achieved the close connection between AgNW and graphene, and subsequently the hybrid conductive layers were entirely wrapped by the polymer matrix. Thus, the dual-protection effects for AgNW by both polymer and graphene were developed, facilitating the multiaspect stability of the MG-A-P electrode. Here we investigated the change of resistance ΔR/R in both MG-A-P film and AgNWs-PET film exposed in atmosphere for 30 days, and the results are shown in Figure 8c. It can be seen that a considerable stability of MG-A-P was observed compared to that of the AgNW-PET, and bare AgNW without protective layer was obviously corroded after exposure in atmosphere (the AFM image of corrosive morphology was inserted in Figure 8c). The comparison demonstrated the superior antioxidation property of the MG-A-P film. Besides this, the unique sandwich construction of the MG-AP film can sufficiently avoid the conductive metal mesh from chemical erosion by corrosive liquids. Here sulfuration test of MG-A-P film and AgNW-PET film by utilizing Na2S solution was investigated, and the result is shown in Figure 8d. Obvious change of resistance ΔR/R was observed in AgNW-PET cause at ∼10 s, and the AFM image of corrosive morphology was inserted in Figure 8d. On the contrary, MG-A-P film exhibit a strong resistibility against sulfuration due to the encapsulation of inert graphene and polymer layer. 31219



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istics under 100 mW/cm2 of AM 1.5 illumination are shown in Figure 9c. The highest performance of the device D is shown in Figure 9c, exhibiting a performance of PCE = 10.419% under reverse scanning and PCE = 9.269% under forward scanning. The hysteresis between forward and reverse J−V scan is the manifestation of a slow response time of the cell to a change in load.47 The integrated current density derived from the IPCE spectra (Figure S15) was in close agreement with the value measured under simulated sunlight shown in Figure 9c. By fitting the Nyquist plots using the equivalent circuit in the inset of Figure S16, the Rct for PSC device is 434.3 Ω, and the series resistance Rs is 23.6 Ω. The considerable resistance proves the excellent interface contact and charge injection process in the flexible PCS device. To further investigate the optoelectronic characteristics of the MG-A-P hybrid film, a series of OLED and PSC devices were also fabricated on commercially ITO-PET films (sheet resistance of 25 Ω/□ at ∼84% light transmittance) under the same conditions using as the references (device based on ITOPET fabrication process was the same as device based on MGA-P). The performance of two types of best devices is shown in Figure S17. We can see that the performance of the optimal OLED device on ITO-PET exhibits as (3.97 V of turn-on voltage, 3833 cd/m2 brightness, and 2.02 cd/A power efficiency), and power efficiency of optimal PSC device on ITO-PET exhibits as PCE = 10.231%. By comparison, the optoelectronic devices based on MG-A-P film present the more considerable performance than ITO-PET-based devices, approximately due to the higher conductive property and light transmittance. It was prospected that more superior performance of flexible devices can be achieved by further reducing the surface roughness and optimizing the encapsulation way of the hybrid electrode.

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Corresponding Author

*E-mail [email protected] (Z.W.). Author Contributions

H.D. and Z.W. equally contributed to the work. H.D. and Z.W. conceived and designed the study. H.D., Z.W., Y.J., W.L., and X.L. performed the experiments. H.D. and Z.W. wrote the paper. B.J., W.A., and X.H. reviewed and edited the manuscript. All authors read and approved the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Basic Research Program of China (2013CB328705), National Natural Science Foundation of China (Grant No. 11574248,61275034), Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20130201110065), International Cooperation by Shaanxi (Grant No. 2015KW-008), China Postdoctoral Science Foundation (Grant No. 2016M590947), and the Fundamental Research Funds for the Central Universities (Grant No. xjj2016031). The SEM and TEM work was done at International Center for Dielectric Research (ICDR), Xi’an Jiaotong University, Xi’an, China.

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REFERENCES

(1) Jung, Y. C.; Shimamoto, D.; Muramatsu, H.; Kim, Y. A.; Hayashi, T.; Terrones, M.; Endo, M. Robust, Conducting, and Transparent Polymer Composites Using Surface-Modified and Individualized Double-Walled Carbon Nanotubes. Adv. Mater. 2008, 20, 4509−4512. (2) Gu, H.; Swager, T. M. Fabrication of Free-standing, Conductive, and Transparent Carbon Nanotube Films. Adv. Mater. 2008, 20, 4433−4437. (3) Lee, J. Y.; Connor, S. T.; Cui, Y.; Peumans, P. Solution-Processed Metal Nanowire Mesh Transparent Electrodes. Nano Lett. 2008, 8, 689−692. (4) Geng, H. Z.; Kim, K. K.; So, K. P.; Lee, Y. S.; Chang, Y.; Lee, Y. H. Effect of Acid Treatment on Carbon Nanotube-Based Flexible Transparent Conducting Films. J. Am. Chem. Soc. 2007, 129, 7758− 7759. (5) Hsu, P. C.; Kong, D. S.; Wang, S.; Wang, H. T.; Welch, A. J.; Wu, H.; Cui, Y. Electrolessly Deposited Electrospun Metal Nanowire Transparent Electrodes. J. Am. Chem. Soc. 2014, 136, 10593−10596. (6) Wu, H.; Kong, D. S.; Ruan, Z. C.; Hsu, P. C.; Wang, S.; Yu, Z. F.; Carney, T. J.; Hu, L. B.; Fan, S. H.; Cui, Y. A Transparent Electrode Based on a Metal Nanotrough Network. Nat. Nanotechnol. 2013, 8, 421−425. (7) Zhang, D. Q.; Wang, R. R.; Wen, M. C.; Weng, D.; Cui, X.; Sun, J.; Li, H. X.; Lu, Y. F. Synthesis of Ultralong Copper Nanowires for High-Performance Transparent Electrodes. J. Am. Chem. Soc. 2012, 134, 14283−14286. (8) De, S.; Lyons, P. E.; Sorel, S.; Doherty, E. M.; King, P. J.; Blau, W. J.; Nirmalraj, P. N.; Boland, J. J.; Scardaci, V.; Joimel, J.; Coleman, J. N. Transparent, Flexible, and Highly Conductive Thin Films Based on Polymer - Nanotube Composites. ACS Nano 2009, 3, 714−720. (9) De, S.; Higgins, T. M.; Lyons, P. E.; Doherty, E. M.; Nirmalraj, P. N.; Blau, W. J.; Boland, J. J.; Coleman, J. N. Silver Nanowire Networks as Flexible, Transparent, Conducting Films: Extremely High DC to Optical Conductivity Ratios. ACS Nano 2009, 3, 1767−1774. (10) Azulai, D.; Belenkova, T.; Gilon, H.; Barkay, Z.; Markovich, G. Transparent Metal Nanowire Thin Films Prepared in Mesostructured Templates. Nano Lett. 2009, 9, 4246−4249. (11) Kim, T.; Kim, Y. W.; Lee, H. S.; Kim, H.; Yang, W. S.; Suh, K. S. Uniformly Interconnected Silver-Nanowire Networks for Transparent Film Heaters. Adv. Funct. Mater. 2013, 23, 1250−1255.

5. CONCLUSIONS In conclusion, a high-quality and fully flexible transparent conductive graphene/AgNWs/polymer film was developed via an up-to-bottom method. The monolayer graphene and the polymer matrix were introduced as the inert cover to avoid the metal mesh from oxidation and corrosion. Meanwhile, the unique sandwich construction achieved the compact combination of the various layers. Besides the excellent optical-electrical perporties (8.06 Ω/□ at 88.3% light transmittance), this typical hybrid film exhibits the remarkable mechanical and anticorrosion robustness. In particular, the superthin and lightweight features of the hybrid electrode were observed which attracted little attention in reported researches. We also fabricated the simple OLED and solar cells device based on this hybrid electrode, and the performance is considerable. It was expect that our study is beneficial for boosting the commercial and industrial application of transparent electrode in emerging flexible optoelectronic devices.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09056. Materials, molecular structures, characteristic of films, and device performance (PDF) 31220


Research Article

ACS Applied Materials & Interfaces (12) Hauger, T. C.; Al-Rafia, S. M. I.; Buriak, J. M. Rolling Silver Nanowire Electrodes: Simultaneously Addressing Adhesion, Roughness, and Conductivity. ACS Appl. Mater. Interfaces 2013, 5, 12663− 12671. (13) Gaynor, W.; Lee, J. Y.; Peumans, P. Fully Solution-Processed Inverted Polymer Solar Cells with Laminated Nanowire Electrodes. ACS Nano 2010, 4, 30−34. (14) Dong, H.; Wu, Z. X.; Lu, F.; Gao, Y. C.; El-Shafei, A.; Jiao, B.; Ning, S. Y.; Hou, X. Optics-Electrics Highways: Plasmonic Silver Nanowires@TiO2 Core-Shell Nanocomposites for Enhanced DyeSensitized Solar Cells Performance. Nano Energy 2014, 10, 181−191. (15) Song, M.; You, D. S.; Lim, K.; Park, S.; Jung, S.; Kim, C. S.; Kim, D. H.; Kim, D. G.; Kim, J. K.; Park, J.; Kang, Y. C.; Heo, J.; Jin, S. H.; Park, J. H.; Kang, J. W. Highly Efficient and Bendable Organic Solar Cells with Solution-Processed Silver Nanowire Electrodes. Adv. Funct. Mater. 2013, 23, 4177−4184. (16) Song, M.; Park, J. H.; Kim, C. S.; Kim, D. H.; Kang, Y. C.; Jin, S. H.; Jin, W. Y.; Kang, J. W. Highly Flexible and Transparent Conducting Silver Nanowire/Zno Composite Film for Organic Solar Cells. Nano Res. 2014, 7, 1370−1379. (17) Song, M.; Kim, J. K.; Yang, S. Y.; Kang, J. W. Solution-Processed Silver Nanowires as a Transparent Conducting Electrode for Air-Stable Inverted Organic Solar Cells. Thin Solid Films 2014, 573, 14−17. (18) Chen, R. Y.; Das, S. R.; Jeong, C.; Khan, M. R.; Janes, D. B.; Alam, M. A. Co-Percolating Graphene-Wrapped Silver Nanowire Network for High Performance, Highly Stable, Transparent Conducting Electrodes. Adv. Funct. Mater. 2013, 23, 5150−5158. (19) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (20) Kuzmenko, A. B.; van Heumen, E.; Carbone, F.; van der Marel, D. Universal Optical Conductance of Graphite. Phys. Rev. Lett. 2008, 100, 117401−117401. (21) Pang, S. P.; Hernandez, Y.; Feng, X. L.; Mullen, K. Graphene as Transparent Electrode Material for Organic Electronics. Adv. Mater. 2011, 23, 2779−2795. (22) Kholmanov, I. N.; Magnuson, C. W.; Aliev, A. E.; Li, H. F.; Zhang, B.; Suk, J. W.; Zhang, L. L.; Peng, E.; Mousavi, S. H.; Khanikaev, A. B.; Piner, R.; Shvets, G.; Ruoff, R. S. Improved Electrical Conductivity of Graphene Films Integrated with Metal Nanowires. Nano Lett. 2012, 12, 5679−5683. (23) Liu, Y.; Chang, Q. H.; Huang, L. Transparent, Flexible Conducting Graphene Hybrid Films with a Subpercolating Network of Silver Nanowires. J. Mater. Chem. C 2013, 1, 2970−2974. (24) Toimil-Molares, M. E.; Balogh, A. G.; Cornelius, T. W.; Neumann, R.; Trautmann, C. Fragmentation of Nanowires Driven by Rayleigh Instability. Appl. Phys. Lett. 2004, 85, 5337−5339. (25) Hsiao, S. T.; Tien, H. W.; Liao, W. H.; Wang, Y. S.; Li, S. M.; MMa, C. C.; Yu, Y. H.; Chuang, W. P. A Highly Electrically Conductive Graphene-Silver Nanowire Hybrid Nanomaterial for Transparent Conductive Films. J. Mater. Chem. C 2014, 2, 7284−7291. (26) Peng, P.; Hu, A. M.; Huang, H.; Gerlich, A. P.; Zhao, B. X.; Zhou, Y. N. Room-Temperature Pressureless Bonding with Silver Nanowire Paste: Towards Organic Electronic and Heat-Sensitive Functional Devices Packaging. J. Mater. Chem. 2012, 22, 12997− 13001. (27) Deng, B.; Hsu, P. C.; Chen, G. C.; Chandrashekar, B. N.; Liao, L.; Ayitimuda, Z.; Wu, J. X.; Guo, Y. F.; Lin, L.; Zhou, Y.; Aisijiang, M.; Xie, Q.; Cui, Y.; Liu, Z. F.; Peng, H. L. Roll-to-Roll Encapsulation of Metal Nanowires between Graphene and Plastic Substrate for HighPerformance Flexible Transparent Electrodes. Nano Lett. 2015, 15, 4206−4213. (28) Zhang, Q.; Di, Y. F.; Huard, C. M.; Guo, L. J.; Wei, J.; Guo, J. B. Highly Stable and Stretchable Graphene-Polymer Processed Silver Nanowires Hybrid Electrodes for Flexible Displays. J. Mater. Chem. C 2015, 3, 1528−1536. (29) Sun, Y. G.; Xia, Y. N. Large-Scale Synthesis of Uniform Silver Nanowires through a Soft, Self-Seeding, Polyol Process. Adv. Mater. 2002, 14, 833−837.

(30) Tang, X. L.; Tsuji, M.; Jiang, P.; Nishio, M.; Jang, S. M.; Yoon, S. H. Rapid and High-Yield Synthesis of Silver Nanowires Using AirAssisted Polyol Method with Chloride Ions. Colloids Surf., A 2009, 338, 33−39. (31) Liu, W.; Li, H.; Xu, C.; Khatami, Y.; Banerjee, K. Synthesis of High-Quality Monolayer and Bilayer Graphene On Copper Using Chemical Vapor Deposition. Carbon 2011, 49, 4122−4130. (32) Yan, Z.; Lin, J.; Peng, Z. W.; Sun, Z. Z.; Zhu, Y.; Li, L.; Xiang, C. S.; Samuel, E. L.; Kittrell, C.; Tour, J. M. Toward the Synthesis of Wafer-Scale Single-Crystal Graphene on Copper Foils (vol 6, pg 9110, 2012). ACS Nano 2013, 7, 2872−2872. (33) Li, X. S.; Zhu, Y. W.; Cai, W. W.; Borysiak, M.; Han, B. Y.; Chen, D.; Piner, R. D.; Colombo, L.; Ruoff, R. S. Transfer of LargeArea Graphene Films for High-Performance Transparent Conductive Electrodes. Nano Lett. 2009, 9, 4359−4363. (34) Jiang, Y. Q.; Xi, J.; Wu, Z. X.; Dong, H.; Zhao, Z. X.; Jiao, B.; Hou, X. Highly Transparent, Conductive, Flexible Resin Films Embedded with Silver Nanowires. Langmuir 2015, 31, 4950−4957. (35) Ok, K. H.; Kim, J.; Park, S. R.; Kim, Y.; Lee, C. J.; Hong, S. J.; Kwak, M. G.; Kim, N.; Han, C. J.; Kim, J. W. Ultra-Thin and Smooth Transparent Electrode for Flexible and Leakage-Free Organic LightEmitting Diodes. Sci. Rep. 2015, 5, 9464−9464. (36) Jeong, J. A.; Kim, H. K. Ag Nanowire Percolating Network Embedded in Indium Tin Oxide Nanoparticles for Printable Transparent Conducting Electrodes. Appl. Phys. Lett. 2014, 104, 071906−071906. (37) Yang, S. B.; Choi, H.; Lee, D. S.; Choi, C. G.; Choi, S. Y.; Kim, I. D. Improved Optical Sintering Efficiency at the Contacts of Silver Nanowires Encapsulated by a Graphene Layer. Small 2015, 11, 1293− 1300. (38) Choi, D. Y.; Kang, H. W.; Sung, H. J.; Kim, S. S. Annealing-Free, Flexible Silver Nanowire-Polymer Composite Electrodes via a Continuous Two-Step Spray-Coating Method. Nanoscale 2013, 5, 977−983. (39) Zeng, X. Y.; Zhang, Q. K.; Yu, R. M.; Lu, C. Z. A New Transparent Conductor: Silver Nanowire Film Buried at the Surface of a Transparent Polymer. Adv. Mater. 2010, 22, 4484−4488. (40) Tokuno, T.; Nogi, M.; Karakawa, M.; Jiu, J. T.; Nge, T. T.; Aso, Y.; Suganuma, K. Fabrication of Silver Nanowire Transparent Electrodes at Room Temperature. Nano Res. 2011, 4, 1215−1222. (41) Jeong, C. W.; Nair, P.; Khan, M.; Lundstrom, M.; Alam, M. A. Prospects for Nanowire-Doped Polycrystalline Graphene Films for Ultratransparent, Highly Conductive Electrodes. Nano Lett. 2011, 11, 5020−5025. (42) Xu, H. M.; Wang, H. C.; Wu, C. P.; Lin, N.; Soomro, A. M.; Guo, H. Z.; Liu, C.; Yang, X. D.; Wu, Y. P.; Cai, D. J.; Kang, J. Y. Direct Synthesis of Graphene 3d-Coated Cu Nanosilks Network for Antioxidant Transparent Conducting Electrode. Nanoscale 2015, 7, 10613−10621. (43) Song, J. Z.; Li, J. H.; Xu, J. Y.; Zeng, H. B. Superstable Transparent Conductive Cu@Cu4Ni Nanowire Elastomer Composites against Oxidation, Bending, Stretching, and Twisting for Flexible and Stretchable Optoelectronics. Nano Lett. 2014, 14, 6298−6305. (44) Tung, V. C.; Chen, L. M.; Allen, M. J.; Wassei, J. K.; Nelson, K.; Kaner, R. B.; Yang, Y. Low-Temperature Solution Processing of Graphene-Carbon Nanotube Hybrid Materials for High-Performance Transparent Conductors. Nano Lett. 2009, 9, 1949−1955. (45) Connolly, T.; Smith, R. C.; Hernandez, Y.; Gun’ko, Y.; Coleman, J. N.; Carey, J. D. Carbon-Nanotube-Polymer Nanocomposites for Field-Emission Cathodes. Small 2009, 5, 826−831. (46) Novoselov, K. S.; Fal’ko, V. I.; Colombo, L.; Gellert, P. R.; Schwab, M. G.; Kim, K. A Roadmap for Graphene. Nature 2012, 490, 192−200. (47) Dong, H.; Wu, Z. X.; Xia, B.; Xi, J.; Yuan, F.; Ning, S. Y.; Xiao, L. X.; Hou, X. Modified Deposition Process of Electron Transport Layer for Efficient Inverted Planar Perovskite Solar Cells. Chem. Commun. 2015, 51, 8986−8989.

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