A Study Of Treating Silver Nanowires Electrode By Hot-Presssing For Application In Organic Light-Emitting Diode

Recently,the flexible organic light-emitting diodes?FOLEDs?have attracted considerable attentions due to their advantages of light weight and high mechanical flexibility in display and lighting applications.One of the key issues of FOLEDs is the transparent conductive electrode?TCE?,which requires several special characteristics,such as being bendable,low-cost,high conductivity and transparency.Up to now,the most widely used transparent electrode material in optoelectronic devices is indium-tin oxide?ITO?.However,some drawbacks restrict the application of ITO in FOLEDs,such as complex time-consuming process,scarcity of indium,fragility upon stretching and bending.In the last decades,several kinds of conductive materials,such as graphene,metallic nanowire,carbon nanotube?CNT?,and conducting polymer have been regarded as the potential replacements for ITO in flexible devices.Among them,silver nanowire?AgNW?has been considered as the most promising candidate for FOLEDs due to their excellent optic-electrical characteristics and outstanding mechanical flexibilities.Although AgNWs have such superior features,currently there are still some issues that inhibit the large-scale application of AgNWs as electrodes in FOLEDs.One is the film preparing temperature and another is the film surface roughness.In this paper,we first investigate a modified two-step polyol reduction method to synthesize AgNWs.According to the synthesis mechanism,we get different kinds of silver nano materials by adjusting the molecular weight of polyvinylpyrrolidone?PVP?,the mole ratio of PVP to silver nitrate?AgNO₃?,the amount of sodium chloride?NaCl?,preparing temperature,stirring speed and dropping speed of AgNO₃.The length and diameter of the AgNWs used for transparent conductive electrode are15?25?m and 50?90 nm,respectively.Then we fabricate conductive film by spin-coating AgNWs on substrates.Although the conductivity and transmittance of the film are good,there are some problems in applying it in the OLEDs.The first is the loose connection between AgNWs which decreases the conductivity,and the second is the accumulation of AgNWs resulting in a height of hundreds nanometers.These protuberances and the coarse surface are the main reasons to induce shorts in OLED devices.We propose a modified hot-pressing method to improve the smoothness by utilizing a mechanical pressure to the conductive film.Taking conductivity,roughness and transparency into consideration,hot-pressing parameters are fixed at 36MPa and 30?with 120 s.AgNW/PEDOT:PSS film on glass substrate prepared by this hot-pressing method has a sheet resistance(R_sh)of 12?/?with a transmittance of 83%at 550 nm,and root mean square roughness?RMS?of 13 nm.Similarly,using the same method to fabricate AgNW/PEDOT:PSS film on polyethylene naphthalate?PEN?substrate can also reach a R_sh of 19?/?with a transmittance of 70%at 550 nm and smooth surface.The sheet resistance of the AgNW/PEDOT:PSS film on PEN substrate almost keeps the same even after 800 bending cycles.This mechanical flexibility is better than the ITO electrode on PEN.Finally we use this hot-pressed AgNW/PEDOT:PSS film as electrode in the fabrication of fluorescence OLED,phosphorescence OLED and flexibleOLED.Comparingthedeviceswithunpressed AgNW/PEDOT:PSS film as anodes,the OLEDs with hot-pressed AgNW/PEDOT:PSS film as anodes show lower leakages and better performance in luminance and current efficiency.The test fluorescence and phosphorescence devices are hole-abundant so the OLEDs with hot-pressed AgNW/PEDOT:PSS film as anodes have a higher current efficiency than OLEDs with ITO as anode.These results indicate that hot-pressing method is a simple but effective way to reduce the roughness and improve the conductivity of AgNW film.Meanwhile they prove AgNW is a promising material to replace ITO for fabricating flexible OLED.