绿色柔性喷墨打印银纳米墨水研究综述

A Review on Green and Flexible Silver Nano-ink for Inkjet Printing

传统导电薄膜的图形化需使用光刻工艺,涉及显影、刻蚀等多个工艺步骤,存在耗时长、器件各功能层构建复杂、成本高昂等不足。喷墨打印作为一种非真空、非接触、无掩膜、低成本、高通量、精准、便捷的桌面型沉积技术,可用于金属导线直接图形化制备,能够节约材料与时间,在柔性大面积电子器件中的应用潜力日益凸显。高性能环保导电墨水的开发是喷墨打印柔性电极制备技术最主要的瓶颈之一,其应满足无毒安全、低温烧结、高导电等需求。早期有机材料由于具有易溶液加工特性被选作可打印导体材料,如聚苯胺和PEDOT/PSS,但它们的导电性低,且化学、热、电学性质不稳定。而金墨水价格昂贵,铜墨水易氧化。相比较而言,银墨水具有高导电性、抗氧化性、稳定性以及相对合理的成本等优势,在打印电子器件领域引起了更加广泛的关注,目前已经实现商业化生产。随着电子器件在低成本、低能量消耗、高效率生产与柔性方面的需求日益增长,导电纳米银墨水的配制不仅需要考虑打印电子器件的高性能,更要满足绿色环保与低温烧结的要求。近年来,针对环保低温的高导电喷墨打印墨水制备技术,诸多研究者不仅致力于改善墨水体系,开发环保的水性墨水,还关注新型的低温烧结技术。在传统热烧结条件下,以月桂酸为分散剂、硼酸钠为还原剂的银纳米颗粒型墨水在烧结温度为125℃时电阻率为6.6μΩ·cm,而以氨水为络合剂、甲酸为还原剂的无颗粒纯溶液墨水在90℃烧结时电阻率为1.6μΩ·cm。新型烧结技术在一定程度上突破了高性能墨水喷墨打印工艺与低温烧结的限制,目前已使银电极烧结温度降低到100℃以下,电阻率低于6μΩ·cm。水性墨水的开发可减少毒性废料的产生,同时若采用多聚电解质作为分散剂,则稳定性更优于有机墨水,利于大规模生产。本文介绍了水性银纳米颗粒(SNPs)悬浮液和有机银复合物(SOC)溶液这两种水性银纳米墨水,比较了不同分散剂、络合剂、还原剂、颗粒聚集方式的墨水体系,总结了适用于柔性印刷的化学烧结、蒸气还原烧结、热水辅助烧结、光还原烧结等新型低温烧结技术。这些新型墨水体系与烧结技术有助于实现导电墨水高性能、高稳定性与柔性的统一,它们的开发和完善将极大促进柔性低温墨水和柔性打印电子器件的发展。

The patterning of conductive films in a traditional way involves photolithography process that consists of multiple steps like developing,etching,etc.,and thus is time consuming,complex in constructing functional layers of devices,and expensive.Inkjet printing,a non-vacuum,non-contact,mask-free,low-cost,high-flux,precise and convenient desktop deposition technology,can be adopted to directly metallize conductive lines with relatively less materials and time,and shows increasing application potential in the fabrication of flexible and large scale display electronics.The development of high-performance and environmental-friendly conductive inks,which require non-toxicity,low-temperature sintering and high conductivity,is one of the main obstacles to fabricating flexible electrodes by inkjet printing.Organic materials,such as polyaniline and PEDOT/PSS,are previously chosen as printable conductive precursors due to its favorable solution processability,but they are poor in conductivity and chemical,thermal and electrical instability.Compared with other conductive materials,silver nanoparticles which are gifted with high conductivity,anti-oxidation,stability and rational price have provoked considerable research interest in the field of printable electrical devices,and have already achieved commercialization.On the other hand,the conti-nuously increasing demands of electronic devices for low cost,low energy consumption,high yield and flexibility exaggerate the significance of both environmental friendliness and low-temperature sintering in the developing of nano-inks besides the high product performance.Recently,apart from the optimization of ink system,e.g.developing green and water-soluble inks,researchers also showed passion for novel low-temperature sintering techniques in order to achieve green,low-temperature and high conductivity printable inks.In a traditional thermal sintering condition,the resistivity of 6.6μΩ·cm can be obtained by sintering a silver nanoparticles(SNPs)ink with Lauric acid as dispersant and NaBH 4 as reducing agent at 125℃,while the silver-organic complex(SOC)ink with ammonia as complexing agent and HCOOH as reducing agent can achieve the resistivity of 1.6μΩ·cm by adopting 90℃sintering temperature.The novel sintering techniques are conducive to overcoming the limitations of the inkjet printing process and low temperature sintering(<100℃)of high performance(<6μΩ·cm)inks.Aqueous silver ink contributes to the attenuation of toxic waste emissions,and possesses higher stability than organic ink when using polyelectrolyte as dispersion agent,and thus is applicable for mass production.This paper offers an introduction of aqueous SNPs suspension and SOC solution,both of which can serve as aqueous silver ink,gives comparative analysis for the ink systems with different dispersants,complexing agents,reducing agents and aggregation modes,and summarizes the representative novel low-temperature sintering techniques including chemical sintering,vapor reduction sintering,hot-water-assisted sintering and photo-induced reduction sintering.These optimized ink systems and sintering methods can facilitate to impart high performance,high stability and flexibility to conductive inks,and will promote to a large extent the innovation with respect to flexible low-temperature ink pro-ducts and flexible printing electronics.