Controlling ink's spreadability to reduce its disorderly flow for promoting the uniformity of silver nanowire transparent electrode

Abstract

In the wet-process film preparation, much surfactant is conventionally incorporated into the ink because traditional viewpoint believes more surfactant can bring about higher film uniformity. In this work, we discover that this popular viewpoint is not completely correct and may hinder the preparation of a uniform film instead. In fact, with the increase of the surfactant dosage, the silver-nanowire-film uniformity first increases at a low surfactant concentration but then decreases at other wide concentrations. Further studies reveal the reason: (1) an appropriate ink spreading distance is critical for obtaining a uniform film, and both insufficient and excess spreading distances will bring about disorderly ink flow to lower the film uniformity; (2) at low concentrations, the increase in the surfactant dosage promotes the ink's wettability and spreadability simultaneously from an insufficient level to a suitable level, and hence, the film uniformity increases; (3) at other wide concentrations, the increase in the surfactant dosage does not further improve the ink's wettability obviously but instead brings about excess spreadability that results in disorderly ink flow, so the film uniformity decreases. Based on the above theoretical results, we reduce the FS-3100 surfactant concentration from 0.10% to 0.022%. The sheet-resistance variation coefficient of the film decreases from 47.56% to 9.80%, indicating that the film uniformity is greatly improved. Meanwhile, no ink overflow from the substrate is observed at 0.022% FS-3100, and the ink-waste problem in the industry can be solved. The above theoretical results correct the traditional viewpoint on surfactants, and the above film-uniformity promotion method can be widely used by scientists and in industrial applications.