Study On Welding Process And Mechanism Of Silver Nanowires Based On Femtosecond Laser Irradiation

The development of flexible electronic devices has put forward higher requirements on the comprehensive performance of transparent electrode.Silver nanowire transparent electrode is regarded as a promising alternative to indium tin oxide（ITO）transparent electrode due to its excellent conductivity and ductibility,but its application is limited by the contact resistance between nanowires.Although conventional methods such as loading temperature and pressure can be used to weld silver nanowires to reduce contact resistance,these methods have poor controllability and may damage the substrate of the electrode,so it is imperative to develop new methods to weld silver nanowires.Herein,femtosecond laser irradiation is introduced to weld silver nanowires after optimizing the preparation process of silver nanowire film,and multi-physics field numerical simulation was carried out using Maxwell equations and two-temperature model based on three-dimensional finite element numerical simulation method to reveal the melting and welding rules of silver nanowire under femtosecond laser irradiation.The spin coating results showed that with the increase of the concentration of silver nanowire solution and the decrease of spin speed,the distribution of silver nanowire became denser,which provided more conductive paths but also weakened the ability of light to penetrate.When the concentration of silver nanowire solution was 2 mg/ml and the spinning speed was2000 rpm,silver nanowires were evenly distributed on the substrate with few agglomeration.The test results of the film showed that the average sheet resistance was 39.28Ω·sq^-1,the transmittance at 550 nm was 86.9%,Fo M（Figure of Merit）was 65.98 and NUF（Non-Uniformity Factor）was 0.4%.The film above possessed both high comprehensive performance and stability,which met the requirements of subsequent welding tests.The femtosecond laser welding results showed that when the laser energy density was2546 J·m^-2,the effective welding between silver nanowires can be realized without causing the fracture of conductive path.The test results of the film showed that the average sheet resistance decreased by 52%to 18.89Ω·sq^-1,the transmittance at 550 nm was 85.96%.Fo M was nearly twice of the initial value,reaching 126.99,and NUF was 1.6%.The overall performance of the film was greatly improved after femtosecond laser irradiation with appropriate intensity.Changing effective pulse number had no significant influence on the welding effect,neither the melting degree of silver nanowire nor the various indicators of the film changed drastically under different effective pulse numbers（20～2000）.The best welding effect of silver nanowires was achieved under 200 effective pulses.The numerical simulation results showed that single silver nanowire cannot achieve electric field enhancement,the increase of the number of silver nanowire at the junction can enhance the local electric field enhancement effect,and the silver nanowire aligned as parallel as possible to the polarization direction of laser was more likely to melt.At a low energy density(1273 J·m^-2),it was difficult to heat the silver nanowire network to its melting point even if the local electric field was enhanced by stacking multiple silver nanowires.While at a high energy density(5092 J·m^-2),even the single silver nanowire that cannot achieve electric field enhancement may melt.Under the irradiation of femtosecond laser with pulse width of 300 fs and frequency of 1 MHz,the time required for silver nanowire to go through a temperature cycle（1 ns）was only one thousandth of the time between two adjacent pulses（1000 ns）.Therefore,no superposition occurred in each temperature cycle of silver nanowire.The melting of silver nanowire network can be determined by studying its response to one single pulse.