Flexible Single-crystal Graphene/Copper Nanowire Composite Transparent Conductive Films:Preparation,Properties And Applications

With the development of smart wearable technology,flexible electronic devices have attracted great attention.Nowadays,the most widely used and well-developed indium-tin-oxide（ITO）transparent electrode has been not compatible with future flexible electronic technologies due to its inherent brittleness.With the rapid increase of the demand for ITO electrodes in the traditional transparent electronic device market,the gradual depletion of indium resources have resulted in the increase of ITO cost.In addition,the toxicity of ITO electrodes can also be harmful to the environment and human health.In recent years,graphene as an emerging two-dimensional crystal composed of a single layer of carbon atoms,is of excellent flexibility（Young’s modulus 1 TPa,tensile strength 130 GPa）and high carrier mobility(200000cm² V^-11 s^-1),high thermal conductivity(5300 W m^-1K^-1),high specific surface area(2630 m² g^-1),high transmittance（97.7%）,as well as excellent chemical stability.Thus it has been considered as one of the ideal candidates for next-generation flexible transparent electrodes.Exploring graphene-based transparent electrodes and their construction methods is crucial for the development of new generation flexible electronic devices.To date,chemical vapor deposition（CVD）method has been mainly used to prepare large-area graphene thin films.Polycrystalline graphene has a large number of grain boundary defects,which result in low conductivity and poor mechanical properties of the film,etc.,thus such graphene films have not yet met the requirements of high-performance flexible optoelectronic devices.To address above issue,this paper focuses on the preparation and transfer of single-crystal graphene films,the construction of graphene/copper nanowire composite structures,as well as their performances and application of those electrodes.It mainly includes the following aspects:1.Single-crystal graphene films prepared by CVD.The strong electron scattering induced by the grain-boundary defects of polycrystalline graphene films Leads to the increase of film resistance.Avoiding grain boundary generation is facile to reduce the decrease of the graphene conductivity.Herein,the single-layer single-crystal graphene thin films have been prepared via chemical vapor deposition using copper foil as the growth substrate.The morphology,crystal quality,and layer number of graphene were characterized.The non-destructive transfer method of the graphene film was studied to achieve a large-area,single-crystal graphene film.The relationship between the transmittance and the conductivity of the transparent conductive graphene film was measured and analyzed,and an optimal strategy for preparing a high-transmittance and high-conductivity graphene film was provided.Finally,the monolayer graphene film exhibited the sheet resistances ranged from 200 to 250Ωsq^-11 and the transmittance of c.a.97%.2.Preparation and properties of flexible single-crystal graphene/copper nanowire composite transparent conductive films.Although the single-layer graphene films exhibits good transmission,the square resistance of the graphene film still does not meet the requirements of high-performance optoelectronic devices(<20Ωsq^-1).The stacking of graphene multilayers will increase the times of graphene transfer,which is complicated,and unsuitable for the non-destructive transfer process.Meanwhile,it easily causes the damage and pollution of the sample.Alternativley,the copper nanowire film with good transmittance and conductivity can realize the preparation of a high-performance transparent electrode.However,its easy oxidiation and high surface roughness have restricted the industrial application of copper nanowires in the field of optoelectronic devices.Herein,a single-crystal graphene-encapsulated copper nanowire network was prepared,considering the advantages and disadvantages of the above two materials,and UV curable Resin was used as a flexible substrate to embedded nanowires.This strategy not only ensures the high conductivity and transmission of the graphene/copper nanowire composite structure,but also improves the stability and the smoothness of the copper nanowire.The surface morphology and microstructure of the composite film were characterized,and their optical and electrical properties,mechanical flexibility,as well as substrate adhesion were investigated.The interaction mechanism of graphene and copper nanowires was explored.The stabilities of graphene-coated copper nanowires in water and oxygen atmospheres were also analyzed.Finally,the flexible graphene/copper nanowire composite film exhibited the excellent optical and electronic properties(R_sh=19±1.5Ωsq^-1,T_{550 nm}=84.3%),high quality factor（F.O.M=121.18）and ultra-low roughness（RMS=1.85 nm）.In addition,the resistance of the flexible graphene/copper nanowire composite film changed to be only 1.06 times than the initial value after 1000 bending cycles at a small bend radius of 2.5 mm.Meanwhile,the resistance changed to be only 1times than the initial value after the film was placed in the atmospheric environment at room temperature for 30 days.3.The application of flexible single-crystal graphene/copper nanowire composite electrode in quantum dot light emitting diode（QLED）.A flexible green quantum dot light emitting diode（QLED）device was fabricated with the patterned graphene/copper nanowire composite films as a positive electrode,PEDOT:PSS as a hole injection layer,TFB as a hole transport layer,ZnCdSeS/ZnS green light quantum dots as a light emitting layer,ZnO as an electron transport layer and Al as a negative electrode.The Gr/CuNWs/R-based QLEDs have excellent chromatographic purity.The maximum luminance of the Gr/CuNWs/R-based QLED was up to 9000 cd/m².What’s more,the current efficiency of the Gr/CuNWs/R-based QLED was 4.25 cd/A.