Research On Laser-assisted Fabrication Of Silver Nanowire Flexible Electrodes

With the development of flexible electronic technology,flexible sensing,flexible display,flexible battery,electronic skin,etc.are gradually changing people’s lives.At the same time,higher requirements are put forward for the miniaturization,multi-function,wearable and other methods of flexible devices.To prepare flexible devices with high performance,flexible electrodes are the key components that determine the devices performance.Silver nanowires,which can be used for the preparation of flexible electrodes,have gradually shown broad application prospects in flexible electronics and other fields due to their good flexibility,electrical conductivity and oxidation resistance.However,silver nanowires will face many problems in the process of using,such as high contact resistance between wires,easy thermal damage of substrate,difficult patterning and so on.In order to solve the above problems,femtosecond laser technology with flexible processing and high processing precision is introduced into the welding,fusing and self-assembly processes of nanowires to improve the performance and preparation efficiency of silver nanowire electrodes.This paper analyzes the mechanism of laser welding and silver nanowires cutting.The conductivity of silver nanowire flexible films is improved by laser welding.The high-precision patterned preparation for silver nanowire electrodes is achieved by laser ablation and wetting-assisted method,and the tensile properties of the electrodes in the interconnected structures of silver nanowires is improved through optimization of design analysis and preparation method.The main contents of this article are as follows:(1)A simulation analysis model of the electric field intensity distribution of the Gaussian laser beam irradiated silver nanowires is established.The influence of laser wavelength,nanowire spacing,and nanowire diameter on the local field enhancement at the nodes of two perpendicularly intersecting nanowires during laser welding is analyzed.The laser in the 350-1050 nm band can excite the local field enhancement at the nanowire gap,and local high temperature will be generated at the local field enhancement,so that the nanowire gap will form a solder joint.Meanwhile,the local field enhancement also shows its self-limiting characteristic.When the nanowires overlap and fuse during the welding process,the local electric field enhancement effect is weakened rapidly,and the melting welding generated by the corresponding local high temperature will also be weakened,which can avoid the fuse of silver nanowires at the joint by the excessive welding.Compared to the change of wavelength,the size of the nanowire diameter has a greater influence on the local field enhancement at the nanowire gap.In the process of laser cutting a single nanowire,only the laser with polarization direction paralleled to the long axis of the nanowire,can generate local field enhancement at the end of the nanowire.The local field enhancement effect at the end of a single nanowire is affected by the laser wavelength,while the effect of nanowire diameter is weaker.(2)The sheet resistance and the resistance uniformity of silver nanowire transparent conductive films are improved by using femtosecond laser irradiation welding method.When the nanowires are welded by laser irradiation,the field enhancement occurs at the nodes where the two nanowires intersect.As a result,the welding is achieved by the generating local heat,reducing the contact resistance between the nanowires.The resistance of nanowire conductive film is reduced by more than 75% to16.1 Ω/sq,and the light transmittance of 91% is maintained.Moreover,the damage to the heat-sensitive flexible substrate is avoided by the femtosecond laser irradiation welding method.In addition,the Gaussian beam is shaped into a linear beam by spatial light modulator,which improves the laser welding speed dozens of times higher than that of the traditional spherical lens focused irradiation welding.Stability test shows the soldered nanowires still have good oxidation resistance.(3)The ablation threshold of the nanowire network and the damage threshold of the substrate are analyzed.It shows that the femtosecond laser can accurately remove silver nanowires without damaging the thermal sensitive PET.Furthermore,the effect of pulse number on the ablation threshold of silver nanowires and the effect of nanowires on the substrate ablation threshold are further analyzed.The results show that a wider process window is obtained with less pulse number and relatively higher energy during the ablation or welding of silver nanowires in case of the damage of substrate.To remove the silver nanowires,a reciprocating scanning method is exploited with spot repetition rate about 70%,which can form the patterns with clear and neat boundaries.In addition,by using the laser patterning technique of silver nanowire flexible conductive films,the fabrication of stretchable electrodes combing with holes array and silver nanowire mesh is realized.(4)A method combined with laser cutting and wetting is proposed for the fabrication of patterned silver nanowire electrodes with high resolution.The method mainly includes the following three key steps: precision cutting of thin masks by laser;selective treatment of oxygen plasma for the exposed substrate to induce hydrophilicity;after removing the mask,the designed area on the substrate is hydrophilic,while the area without treatment by the plasma shows hydrophobic property.As a result,the PDMS substrate exhibits hydrophilic/hydrophobic property in different areas.When the nanowire suspension is coated on the substrate,the nanowires will aggregate into the hydrophilic area.After the suspension is dried,the conductive structure will be formed in the hydrophilic area.By optimizing the femtosecond laser cutting process and the ratio of the suspension,a silver nanowire electrode with a width of 50 μm is selfassembled by such method.And the high-precision patterned nanowire electrode is used as a bottom electrode to prepare a flexible electroluminescent device.The luminescent patterns with different sizes and complex shapes show uniform brightness and clear boundaries.(5)A stretchable interconnect structure electrode of silver nanowires is designed and fabricated.Through the finite element analysis,the strain distribution of serpentine and toothed electrodes in the tensile state is compared,and it is concluded that the toothed convex electrode has better stretchable performance.After PDMS is poured on the groove electrode prepared by the wetting assisted method,the convex electrode is formed by reverse demoulding,and the silver nanowire grid originally deposited in the groove is fixed to the convex structure to form a stable composite structure.Compared with the flat substrate,the convex structure can better resist the tensile deformation;Compared with the serpentine structure,the tooth structure will have fewer stress concentration areas under the same tensile deformation,which is conducive to reducing the number of nanowire fractures.After the cyclic tensile test with 40% strain,the electrode with toothed convex structure can still maintain its initial stable conductivity.The research on laser-assisted preparation of high-performance silver nanowire electrodes in this paper can effectively reduce the sheet resistance of silver nanowire flexible transparent conductive films without thermal damage to the substrate;Both laser ablation patterning technology and selfassembly patterning technology have the advantages of flexible and high precise processing,which is beneficial to broaden the application range of silver nanowire flexible electrodes and improve the performance of flexible electronic devices.