Fabrication of high-quality silver nanowire conductive film and its application for transparent film heaters

Abstract

Here, we report a facile method to produce pure silver nanowires (AgNWs) with high yield. A highly conductive dispersant was used to ensure uniform dispersion of the AgNWs. Without any posttreatment, the AgNW networks, deposited on flexible substrates, showed excellent optoelectrical performance owing to minimal junction resistance between the AgNWs. To explore their potential in flexible optoelectronic devices, a transparent film heater was constructed based on the present AgNW networks. The heater could achieve rapid response at low input voltage and reach a relatively high temperature in a short response time. Since this high-quality AgNW film exhibits relatively low production costs and fast production time, it may have value for future electronic industry applications.

Introduction

Recently, tin-doped indium oxide (ITO), a widely used optoelectronic material, has been severely restricted in flexible transparent conductive films (TCFs) owing to the increasing cost of indium, its high deposition temperature and especially its intrinsic brittleness [[1], [2], [3], [4]]. Therefore, various potential candidates have been developed, including carbon nanotubes [5,6], graphene [7,8], metallic nanowires [[9], [10], [11], [12]], and conducting polymers [13,14]. Among these materials, silver nanowires (AgNWs) exhibit outstanding optoelectrical properties that are superior to those of other materials and are regarded as the most promising alternative [15,16]. The desirable characteristics of AgNW conductive film mainly depend on the length of the AgNWs and the purity of the surface.

Currently, polyol synthesis, developed by Xia and coworkers, has been the most versatile approach used for the large-scale production of high-quality AgNWs [17,18]. In a typical polyol method, polyvinylpyrrolidone (PVP) plays a crucial role as a capping agent in controlling the morphology and size of AgNWs [19,20]. In contrast, traditional PVP-free methods are rarely utilized because this method can only synthesize low-yield AgNWs with different sizes. Unfortunately, for AgNW conductive films, the PVP adsorbed onto the AgNWs during polyol synthesis will create a large junction resistance and decrease the optoelectrical performance [21,22]. Although many posttreatments have been suggested to overcome this problem, the PVP on the AgNWs cannot be completely removed. In addition, this postprocessing will increase the production costs and time, which inhibits the commercialization of AgNW-based TCFs. Therefore, it is necessary to create a novel PVP-free process for the large-scale production of AgNWs with pure surfaces to avoid the introduction of a postprocessing step.

It is worth noting that uniform AgNW dispersion is a prerequisite for producing high-performance AgNW networks. However, pristine AgNWs will agglomerate together without PVP. Thus, it is imperative to add a dispersant (insulating ligand) to ensure the uniformity of AgNW dispersion, which also increases the junction resistance to some extent. Therefore, a new dispersant with high conductivity should be created to disperse pure AgNWs. Poly(3,4-ethylenedioxythiopene) (PEDOT), possessing excellent electrical performance, good stability in the oxidized state and unique spectrochemical properties, is one of the most intensively investigated conducting polymers [[23], [24], [25]]. However, its inherent insolubility in most solvents restricts its practical application. Incorporating poly(styrene sulfonic acid) (PSS) as a charge compensator and oxidant during EDOT polymerization is an effective means to address the insolubility problem [[26], [27], [28], [29]]. PEDOT:PSS is believed to be a colloidal suspension in which the PEDOT chain decorates segments of higher-molecular-weight PSS chains, and the electrostatic effect increases the water solubility of PEDOT:PSS. Since the ligand of carbonyl in PVP can contribute electrons to the sp orbital of silver ions to form a coordinative bond, PVP can adsorb onto the surface of AgNWs to prevent AgNWs from agglomerating. Because there is a similar electron-donating group in PEDOT:PSS, we believe this may also coordinate with the AgNWs or physically adsorb onto the AgNWs due to the high surface energy of the AgNWs. Then, the AgNWs can uniformly disperse in the solution owing to the electrostatic repulsion between the PEDOT:PSS colloid. Therefore, it may be feasible to disperse AgNWs with PEDOT:PSS instead of traditional dispersants.

In this paper, a novel polyol process without PVP was proposed to produce long-length and high-yield AgNWs. Then, the traditional dispersant was replaced with PEDOT:PSS to disperse the as-prepared pure AgNWs. In the absence of any postprocessing, an outstanding optoelectrical performance AgNW network could be prepared and deposited on a flexible substrate. Furthermore, we explored the usage of the AgNW conductive film in a flexible optoelectronic device. Thermal response time, operation voltage, and steady-state temperature are essential issues used to assess the quality of flexible film heaters [30,31]. A film heater based on the present AgNW conductive film exhibited excellent performance, achieving rapid heating at a low input voltage and reaching a high steady-state temperature in a short response time.