Silver Nanowires Modified with PEDOT: PSS and Graphene for Organic Light-Emitting Diodes Anode

Silver nanowires (AgNWs) networks are promising candidates for the replacement of indium tin oxide (ITO). However, the surface roughness of the AgNWs network is still too high for its application in optoelectronic devices. In this work, we have reduced the surface roughness of the AgNWs networks to 6.4 nm, compared to 33.9 nm of the as-deposited AgNWs network through the hot-pressing process, treatment with poly (3,4ethylenedioxythiophene)–poly (styrenesulfanate), and covered with graphene films. Using this method, we are able to produce AgNWs/PEDOT: PSS/SLG composite films with the transmittance and sheet resistance of 88.29% and 30 Ω/□, respectively. The OLEDs based on the AgNWs/PEDOT: PSS/SLG anodes are comparable to those based on ITO anodes.

and current transport paths. It is well known that PEDOT: PSS with good photoelectric property and high work function (5.1 eV-5.2 eV) is usually used as a hole transporting layer in organic optoelectronic devices 26 . We used PEDOT: PSS to reduce the roughness of the composite films, and the surface roughness of the composite film was decreased to 6.4 nm, as shown in Fig. 4(c). Figure 4(d-f) show the 3D display corresponding to Fig. 4(a-c), and it can be more intuitive to see the change of the roughness. Song et al. reported that they improved the characteristics of the AgNWs network through a process of irradiating it with intense pulsed light (IPL) 27 . The surface roughness of the AgNWs/PEDOT: PSS composite film treated with IPL can reach 5.9 nm. However, it is similar with the AgNWs/PEDOT: PSS composite film treated with hot-press process.
Hydrophilic control of the AgNWs films. It is difficult to cover the PEDOT: PSS aqueous dispersion on the AgNWs surface after hot press with good uniformity due to the poor hydrophilicity of the AgNWs network. A sol-gel TiO 2 solutions were spin-coated on the AgNWs network to improve the surface wettability of the AgNWs films. Figure 5(a) shows the contact angles of the AgNWs networks after the sol-gel TiO 2 solutions treatment with different concentrations. With the increasing concentration of the sol-gel TiO 2 solutions, the contact angle was decreased from 89.25° to 53.41°, which suggests an improved hydrophily of the AgNWs network for the further deposition of PEDOT: PSS on the surface.
Characterization of the AgNWs/PEDOT: PSS/SLG composite film. To further reduce the sheet resistance of the AgNWs/PEDOT: PSS films and improve the carrier mobility of the composite films, we use graphene films transferred to the surface of the composite film. Figure 5(b) shows the Raman spectrum of the graphene film to verify the number of the graphene layers. The sharp peaks at 1590 cm −1 and 2680 cm −1 are corresponding to the G band and 2D band of graphene, respectively 28 . From the TEM of the graphene film, we can see that it has only a single set of diffraction pattern for a regular hexagon. The intensity ratio of 2D band to G band is 1.9, and the weak peak (D band) at 1352 cm −1 shows a high crystalline quality of the single-layer    It is obviously that the emitting light colors tend to be consistent and the influence of the different transmittances of the diverse anodes can be ignored.

Conclusions
In conclusion, we have developed the AgNWs/PEDOT: PSS (~40 Ω/□ , T 550nm~9 1.04%) and AgNWs/PEDOT: PSS/SLG (~30 Ω/□ , T 550nm~8 8.29%) transparent conductive films with high transmittance, good conductivity and lower surface roughness. By a simple hot-pressing process, the roughness of the AgNWs network decreased from 33.9 nm to 9.5 nm. Further covered with PEDOT: PSS, which is readily used for the hole transport layer or hole injection layer in OLEDs, reduced the surface roughness to 6.4 nm. We have also demonstrated that the AgNWs network can be used as the transparent and conductive anodes in OLEDs. The performance of the device based on the AgNWs/PEDOT: PSS/SLG films is higher than the device based on the AgNWs/PEDOT: PSS films due to the contribution of graphene. These results suggest that the AgNWs/PEDOT: PSS/SLG film is a promising candidate as the transparent conductive electrode in optoelectronic devices. Hot-press process. The sample was placed on the bearing platform of the imprinting equipment (NC-AX1401, Nano Carve), which was covered with a PI film (25 μ m). The pressure of 0.3-1 MPa was applied between the top substrate and the PI film in the imprinting chamber with different heating temperatures to achieve non-contact hot-pressing.
Hydrophilic treatment. The TiO 2 solution was prepared by an ordinary method 30 . TiO 2 sol-gel solution was spin-coated on the AgNWs network for 30 s at 6000 rpm to make it hydrophilic and annealed at 150 °C for 10 min. Then PEDOT: PSS (Clevios PH 1000, Heraeus) was spin-coated on the AgNWs network.
Graphene growth and transfer. The SLG films were synthesized by CVD on a 25 μ m polishing copper foil (99.999%, Alpha). Before growth, the foils were annealed at 1000 °C in H 2 atmosphere for 5 min. Then, the source gas CH 4 was infused with a flow rate of 0.5-3 sccm while keeping the same temperature for 3-10 min. Finally, the copper foils were rapidly cooled to room temperature. The graphene films were transferred to the surface of the AgNWs/PEDOT: PSS film to form the AgNWs/PEDOT: PSS/SLG composite film by a typical method 26 . To characterize the composite films, field-emission scanning electron microscope (SEM, FEI Quanta 600), transmission electron microscope, and atomic force microscope (NT-MDT) were used to examine the surface morphology. Raman measurements were performed using Thermo Scientific DXR Raman microscope spectrometer with a laser wavelength of 532 nm at room temperature. For sheet resistance measurements, we used a semiconductor analyzer (Agilent, B1500A) combined with a four-probe station (CASCADE, alessi REL-4800). The optical transmittance in the wavelength range of 300-1100 nm was obtained by a PV Measurements QEX10. The current-voltage (I-V) characteristics of the fabricated OLEDs were measured with an experimental set-up including a Keithley 2400 source meter. A spectroradiometer (PR750) was also employed to measure the electroluminescence spectrum of the 3 × 3 mm 2 emitting area of the devices. The reference OLEDs with the same layer structures, except the AgNWs/PEDOT: PSS/SLG composite film was replaced by a conventional ITO layer (15 Ω/ sq), were also fabricated for comparison.