Research Article www.acsami.org
Fully Transparent Quantum Dot Light-Emitting Diode with a Laminated Top Graphene Anode Li Yao,† Xin Fang,† Wei Gu,‡ Wenhao Zhai,† Yi Wan,† Xixi Xie,† Wanjin Xu,† Xiaodong Pi,‡ Guangzhao Ran,*,† and Guogang Qin*,† †
State Key Laboratory for Microscopic Physics, School of Physics, Peking University, Beijing 100871, China State Key Laboratory of Silicon Materials and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China
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S Supporting Information *
ABSTRACT: A new method to employ graphene as top electrode was introduced, and based on that, fully transparent quantum dot light-emitting diodes (T-QLEDs) were successfully fabricated through a lamination process. We adopted the widely used wet transfer method to transfer bilayer graphene (BG) on polydimethylsiloxane/polyethylene terephthalate (PDMS/PET) substrate. The sheet resistance of graphene reduced to ∼540 Ω/□ through transferring BG for 3 times on the PDMS/PET. The T-QLED has an inverted device structure of glass/indium tin oxide (ITO)/ZnO nanoparticles/(CdSSe/ZnS quantum dots (QDs))/1,1-bis[(di-4tolylamino)phenyl] cyclohexane (TAPC)/MoO3/graphene/ PDMS/PET. The graphene anode on PDMS/PET substrate can be directly laminated on the MoO3/TAPC/(CdSSe/ZnS QDs)/ZnO nanoparticles/ITO/glass, which relied on the van der Waals interaction between the graphene/PDMS and the MoO3. The transmittance of the T-QLED is 79.4% at its main electroluminescence peak wavelength of 622 nm. KEYWORDS: graphene, transparent top electrode, lamination, quantum dots, light emitting diodes
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INTRODUCTION Colloidal quantum dots light-emitting diodes (QLEDs), which possess the advantages of relatively high luminescence efficiency and size-tunable color, have received increasing attention in next generation solid-state lighting applications.1,2 Fully transparent and flexible light emitting diodes have great potential in future smart display and light source application,3−6 such as cell phones and car windshields. The demand for the transparent electrodes material is constantly rising, particularly for transparent top-electrode. As a transparent top conductor, indium tin oxide (ITO) is often used, but the plasma sputtering process may causes critical damage to the underlying organic layers.7 So many studies of transparent top electrode material have been carried out in carbon nanotubes,8,9 metal nanowires,10 or graphene.11 Among these options, graphene show more superiority in the combination of many advantageous features, for example, low sheet resistance, high transparency and outstanding mechanical characteristics.12−14 Graphene has been widely studied as bottom electrodes in organic light emitting diodes and solar cells,15−18 but only a few research articles reported graphene used as top electrodes.19−21 The fabrication of graphene top electrode is still a challenging task. As far as we know, only three reports6,20,22 studied graphene as top electrode in light-emitting diodes, which could be summarized to two strategies. The wet strategy required © 2017 American Chemical Society
immersing the remaining part of the lighting device into water and isopropyl alcohol solution to wet transfer graphene top electrode, which restricted the choice of active components to water insensitive materials.20 The dry strategy was to dry transfer graphene top electrode with the help of bonding layer on the remaining part of the device. However, it is more suitable for multilayer graphene grown on Ni substrate.6,13 For the single-layer or bilayer graphene grown on Cu foil, which enjoys the advantages of large area, high uniformity, and outstanding quality and is preferred for electrode application,23 using the second strategy is difficult to transfer complete graphene, and thus requiring the insertion of silver nanowires between layers of graphene to reduce the sheet resistance of graphene electrode.22 Herein, we developed a new strategy to fabricate graphene top electrode and hence built fully transparent quantum dot light-emitting diode. In our work, we wet transfer graphene from Cu foil onto polydimethylsiloxane/polyethylene terephthalate (PDMS/PET), and then laminated these wet-transferred graphene/PDMS/PET onto the remaining part of the lighting diode as electrodes. Our strategy not only avoids the Received: February 11, 2017 Accepted: June 20, 2017 Published: June 20, 2017 24005
DOI: 10.1021/acsami.7b02026 ACS Appl. Mater. Interfaces 2017, 9, 24005−24010
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic fabrication process of a T-QLED with a laminated graphene anode. (b) The energy diagram of a T-QLED with a graphene anode and a control QLED with an Al anode. (c) Photos for the T-QLED without bias (left one) and with an operating voltage of 6 V (right one), respectively. rinsed with alcohol and DI water. Then use graphene/PDMS/PET to transfer desired number of graphene layers successively. Finally, a 60 nm Au ohmic contact electrode was deposited on the corner of graphene/PDMS/PET by thermal evaporation with a mask. In this work, the CdSSe/ZnS quantum dots (CdSSe/ZnS QDs, quantum yield ≥ 40%) were purchased from Najing Tec. Company. The colloidal ZnO nanoparticles were synthesized through a solutionprecipitation procedure.1 The device structure of the T-QLED was glass/ITO/ZnO nanoparticles/(CdSSe/ZnS QDs)/1,1-bis[(di-4tolylamino)phenyl] cyclohexane (TAPC)/MoO3/graphene/PDMS/ PET. In a nitrogen-filled glovebox, on the ITO substrate, we first spincoated the ZnO nanoparticles (40 mg/mL in ethanol) electron transport layer, then the CdSSe/ZnS QDs (10 mg/mL in toluene) emitting layer. Further preparation details of ZnO nanoparticles layer were provided elsewhere.26 The CdSSe/ZnS QDs were spin-coated for 30 s at a speed of 1200 rpm and then baked for 20 min under 110 °C. Afterward, the samples were transferred to a high vacuum thermal evaporation system, and then a TAPC hole transport layer and a MoO3 hole injection layer were sequentially deposited on the samples. Then the samples were transferred into the nitrogen-filled glovebox again. Finally, the graphene/PDMS/PET was laminated on the top MoO3 layer of the samples without pressure and heat. Control QLEDs, possessing the structure of glass/ITO/ZnO nanoparticles/ (CdSSe/ZnS QDs)/TAPC/MoO3/Al, were also fabricated. The light-
immersion of device into water, but more importantly could stack many complete single or bilayer graphene together to significantly reduce the sheet resistance of graphene electrodes without the help of other conducting material, which is successfully combine the respective advantages of the dry and wet strategies.
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EXPERIMENTAL SECTION
The mixture of PDMS prepolymer (Sylgard 184) and a curing agent in 7:1 volume ratio was spin-coated on PET substrate at 2000 rmp for 30 s, then baked for 10 min under 100 °C to get the PDMS/PET substrate. Graphene films were synthesized on 25 μm Cu foils in a chemical vapor deposition (CVD) system.24 Poly(methyl methacrylate) (PMMA) was spin-coated on the graphene/Cu at 4000 rmp for 30 s, and then dried in the air.25 The Cu foil was removed from PMMA/graphene by etching in an aqueous solution of FeCl3. Then PMMA/graphene rinsed in deionized (DI) water for many times. Just before the first wet transfer of graphene, we used the O2 plasma to treat the PDMS/PET substrate for 40 s to tune the surface hydrophilicity of PDMS. After placing the PMMA/graphene stack on the PDMS/PET substrate, the PMMA/graphene/PDMS/PET dried in the air for about 6 h. To remove the PMMA, we immersed the PMMA/graphene/PDMS/PET in acetone for about 1 h and then 24006
DOI: 10.1021/acsami.7b02026 ACS Appl. Mater. Interfaces 2017, 9, 24005−24010
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Raman spectrum of a pristine graphene transferred on a 300 nm-SiO2/Si substrate. (b) Transmittance spectra of PDMS/PET, graphene/PDMS/PET, glass/ITO, and T-QLED with a laminated graphene anode. Here, graphene used is 6 layers.
Figure 3. EL characteristics of a T-QLED with a laminated graphene anode and a control QLED with an Al anode: (a) the current density versus voltage curves, (b) the luminescence versus voltage curves, (c) the current efficiency versus voltage and the external quantum efficiency (EQE) versus voltage curves, and (d) the power efficiency versus voltage curves. For the T-QLED, the luminescence was measured not only from the ITO side (bottom emitting), but also from the graphene side (top emitting). For the control QLED, the luminescence was only measured from the ITO side. emitting areas of the T-QLEDs and the control QLEDs were ∼20 and ∼5 mm2, respectively. The QLEDs were all without encapsulation, and their electroluminescence (EL) measurements were conducted immediately after their fabrication, under air ambient and room temperature. The EL measurements of the QLEDs and the Raman characterization of bilayer graphene were the same as those, which were described particularly in our previous work.24 The photoluminescence (PL) characterization of the spin-coated CdSSe/ZnS QDs film was also conducted on the Raman system and the wavelength of excitation laser was 325 nm. To measure sheet resistances (Rs) of graphene, Hall HL5500 is utilized. A UV−vis−NIR spectrophotometer (Shimadzu UV-3100) is utilized to do the transparency measurement. We used a surface profiler (Veeco, Dektak 150 Stylus) to measure the thicknesses of ZnO nanoparticles, TAPC, and MoO3 layers and used an atomic
force microscope (AFM, Asylum Research) to measure the thickness of CdSSe/ZnS QDs layer.
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RESULTS AND DISCUSSION Figure 1a presents the fabrication process of the T-QLED. There are four main steps in the process: (1) Obtain the floating graphene with the supporting of PMMA (a−c); (2) obtain PDMS/PET substrates through spin-coating of PDMS on PET (I); (3) transfer graphene on the PDMS/PET substrate for three times (II−IV and IV′); and (4) fabricate a T-QLED by laminating the graphene/PDMS/PET anode on the remaining part of the diode (V). The two components can directly contact with each other tightly without heating and pressure. This lamination method with the help of PDMS had 24007
DOI: 10.1021/acsami.7b02026 ACS Appl. Mater. Interfaces 2017, 9, 24005−24010
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) EL spectra of a T-QLED with a laminated graphene anode under different forward voltages. The inset shows a photo for the T-QLED biased at 8 V. (b) PL spectrum of the CdSSe/ZnS QDs, an EL spectrum of a T-QLED, and an EL spectrum of a control QLED.
top side (graphene side) and from the bottom side (ITO side) of the T-QLED. For the control QLED, the Al anode (∼80 nm) also played the role of reflector, so the luminance, which was measured from the ITO side of the control QLED, included the light output reflected back from opaque Al anode. As shown in Figure 3a and b, under the voltage range of 5− 10 V, the current density and luminescence of the T-QLED are both more than 1 order of magnitude lower than those of the control QLED. That may be the result of the higher sheet resistance of graphene (∼540 Ω/□) compared with that of Al (
