Studies On The Fabrication And Physical Properties Of Electrospun Template-based Silver/copper Network Transparent Electrodes

Transparent electrodes are important components of optoelectronic devices such as solar cells,flat-panel displays and touch screens.Conventional transparent electrodes are mainly made of doped metal oxides.Particularly,tin-doped indium oxide(ITO)has dominated the transparent conducting market by virtue of its excellent optoelectronic properties.However,in the recent years,the significantly increased demand of transparent electrodes and the limited reserves of indium drive ITO’s price up.Moreover,the brittle ceramic nature of ITO severely limits its applications in the next-generation wearable devices.Therefore,searching for low-cost and non-vacuum process to produce highly flexible and stable transparent electrodes has become a research hotspot in the recent decade.Among the emerging new candidates,metal networks are the most promising materials to replace ITO as transparent electrodes.Electrospinning is a facile and low-cost method to prepare polymer nanofibers with diameter as small as tens of nanometers and length as long as several centimeters.In this work,polymer nanofibers with different geometries were prepared as templates,and both physical and chemical methods were used to deposit metals on the templates.The metal networks fabricated in this way possess advantages of large length to diameter ratio,low junction resistance and high transmittance from UV to near IR regions.The optoelectronic properties,chemical and mechanical stabilities,as well as electrical heating properties of silver/copper networks were investigated.Main results are as follows:1.Transmittance and conductivity are the key factors of transparent electrodes.Generally,high performance transparent electrodes have low sheet resistance and high transmittance.By using electrospun nanofiber networks as templates,we prepared silver networks with sheet resistance of 23 ohm/sq and specular transmittance as high as 87%.The results show that the silver networks scatter the light heavily because of their large surface roughness and the diameters which are in the range of visible wavelength.The haze of the silver networks can be controlled by adjusting the coverage of silver networks.The lower coverage is,the lower haze will be.Additionally,conductivity of silver networks can be very high by increasing the thickness of silver,for example,if we replace the 100 nm-thick silver with 400 nm-thick copper,the sheet resistance can be reduced to one-tenth of the original value at the same transmittance.2.Chemical and mechanical stabilities are the crucial factors in the practical applications of metal networks.Especially,nanostructured copper/silver,which has larger surface to volume ratio,is prone to be oxidized.To solve this problem,atomic layer deposition technique was used to prepare aluminum oxide film on the surface of Cu networks.The fabricated Cu wire/Al2O3 can withstand a temperature as high as 300 oC under atmospheric conditions without showing any increase in resistance;We found that Al2O3 film can improve the Ampacity of individual Cu wire and restrain the electromigration of copper even under large current and Joule heat.3.Transparent electrodes will generate Joule heat when electric current flows from one side to the other.This so-called “electrical heating” characteristic is widely used to defog windows or out-door displays.High performance transparent heaters require high transmittance and high saturated temperature even under low input-voltages.The electrical heating performances of random Ag networks,aligned Ag networks,and graphene-aligned Ag network composite films were compared.The results showed that the aligned Ag networks were very favorable for highly transparent and low-voltage heaters owing to their low resistance along the wire direction.Thanks to the extremely high thermal conductivity and low convection heat transfer coefficient,the graphene can reduce the heat loss of aligned Ag networks and promote heat spread simultaneously.We found that the resistance of aligned Ag network exhibited remarkably increase/remained unchanged under perpendicular/parallel to the ASNWs direction bending.Besides,the anisotropic flexibility of the aligned Ag network was improved by compositing it with monolayer graphene.The enhancement mechanism was attributed to the highly conductive graphene which restrained the stress from concentrating around the bending edge and provided extra conductive pathways at the cracked points of Ag networks.4.Using solution-processed method to deposit metal on the polymer nanofiber templates to replace the conventional physical deposition process is the key to fabricate large-area metal networks at low-cost.By using a rough copper foil as collector,we prepared large-area uniform nanofibers.The nanofibers which had low contact area with rough copper foil were easily transferred to transparent substrate without forming any fracture.We used silver mirror reaction to deposit silver on the templates.The fabricated Ag network exhibited a transmittance of 80%,at a sheet resistance of 9.5 ohm/sq.