A Study Of Solution Processed Transparent And Conductive Films For Application In Organic Light-Emitting Diode

Recently the flexible organic light emitting diodes?OLEDs?have attracted tremendous attention owing to their lightness,thinness and the high mechanical flexibility,and are considered as the next–generation display.The electrodes for flexible OLEDs should possess some characteristics,such as bendable,transparent,highly conductive and low-cost.Indium tin oxide?ITO?has been widely used as transparent electrode for organic optoelectronic devices due to its fairy good conductivity and transparency.However,a few drawbacks of ITO includes high price due to indium scarcity,complicated processing requirements,prone to cracking upon bending,rendering it unsuitable for application that requires stretchability or flexibility.Therefore,graphene,poly?3,4-ethylenedioxythiophene?:poly?styrenesulfonate??PEDOT:PSS?and silver nanowires?AgNWs?films have been studied here,and considered as the potential candidates for ITO to prepare flexible OLED devices.Firstly,a highly conductive and transparent graphene multilayer films were produced by a layer-by-layer?LbL?assembling method.Such technique was realized by alternate deposition of cationic surfactant N,N,N-trimethyl-1-dodecanaminium bromide?CTAB?and anionic surfactant sodium dodecyl benzene sulfonate?SDBS?modified graphene oxide?GO?.After 1000°C annealing in N₂ atmosphere for 2 hours,the graphene film yielded a conductivity of 900 S/cm.On the other hand,the thickness,transmittance,and sheet resistance of the film can be controlled by changing deposition cycles.The improved conductivity is attributed to the better graphitization and nitrogen-doping introduced by CTAB.The OLED using such multilayer graphene film fabricated by LbL method as anode obtains higher current density and luminance at low voltage region compared to that with ITO anode.Moreover,the current efficiency of the graphene-based device is comparable to that of ITO-based device.Then,a highly conductive,transparent and uniform PEDOT:PSS film has been developed by dipping treatment with hydriodic acid?HI?solution.The film after dipping into HI solution can obtain a higher conductivity and uniformity compared to the film treated by dropping method or with other acids.The HI-treated PEDOT:PSS film reaches a sheet resistance of 68?per square and a transmittance of 87%at 550 nm.The conductivity enhancement for the HI-treated film is ascribed to the permeation of proton and iodine anion of HI into PEDOT:PSS film,resulting in the separation of PSS and PEDOT chains.The phase separation of PSS and PEDOT chains provides more conductive pathways for carriers to improve conductivity of the film.Using the optimized HI-treated PEDOT:PSS film as anode,we have fabricated ITO free OLEDs.The device can obtain the luminance of 1000 cd/m² at 6.4V,and this voltage is0.4V lower than that of the device with ITO anode.Later,the PEDOT:PSS and GO hybrid film was studied.In order to improve the conductivity of the hybrid film,the dimethyl sulfoxide?DMSO?was added in the mixture,or the film was treated with dipping method in HI solution.PEDOT:PSS hybrid film mixed with 0.02 wt%GO and 6 wt%DMSO exhibits a lowest sheet resistance of 85 ohm per square and a transmittance of 87%at 550 nm.The HI-treated PEDOT:PSS with0.01 wt%GO hybrid film exhibits a sheet resistance of 92 ohm per square and a transmittance of 91%at 550 nm wavelength.The enhancement of conductivity is mainly caused by the weakening of Coulombic attraction between PEDOT and PSS chains due to the HI and DMSO.The hydrophilic PSS domains are rinsed in the presence of aqueous solution,and the hydrophobic PEDOT domains can provide more conductive pathways for carriers to improve the conductivity of the film.Meanwhile,the GO sheets together with PEDOT chains extend the conductive network and further improve the conductivity of the film.After mixing the GO sheets,the air stability of the PEDOT:PSS hybrid film is also dramatically improved.Using PEDOT:PSS hybrid films mixed with different ratios of GO:SDBS as anodes,we fabricated OLED devices.The device using PEDOT:PSS hybrid anode doped with 0.02 wt%GO:SDBS reaches a luminance of 1000 cd/m² only at 4.6 V,and this voltage is much lower than those of other devices.The device with PEDOT:PSS doped with 0.02 wt%GO:SDBS hybrid anode has the best charges transport capability,and the current efficiency reaches 2.7 cd A^-1 at 100 mA cm^-2,which is 70%higher than the best value of the device with ITO anode.Lastly,the AgNWs with the high length-diameter aspect ratio can be obtained by optimizing the molecular weight and the mixing ratios of the polyvinyl pyrrolidone?PVP?as stabilizer and template in solution.The film with the highest uniformity,conductivity and transmittance can be obtained by using the PVP with 360000 molecular weight,and the molar ratio of PVP to silver nitrate is 4:1.The AgNWs and PEDOT:PSS hybrid film can keep highly uniform after hot-pressing treatment.The conductivity and transmittance of the hybrid film does not change significantly with or without hot-pressing treatment.The OLED using hot-pressed silver nanowire and PEDOT:PSS hybrid film as anode shows lower leakage current compared to the device without hot-pressing treatment.Moreover,thicker PEDOT:PSS film can further reduce the leakage current,but the current density of the device is also lower.To sum it up,the graphene,PEDOT:PSS,AgNWs and their hybrid films can obtain high conductivity,transmittance and uniformity,and are promising electrodes for low cost flexible OLED.