Tunable Approach to Structural Color Powers Displays, Cuts Energy

Researchers at Linköping University have discovered a way to generate structural color that could potentially be applied to the manufacture of low-cost, energy-efficient displays and electronic labels. The method uses monochromic conducting polymer films, which the researchers prepared on metallic surfaces (that is, mirrors) using vapor phase polymerization (VPP) and a single ultraviolet (UV) light patterning step.

“We have developed a simple method to produce structural color images with electrically conducting plastics, or conducting polymers,” researcher Shangzhi Chen said. “The polymer is applied at nanoscale thicknesses onto a mirror by a technique known as vapor phase polymerization, after the substrate has been illuminated with UV light. The stronger the UV illumination, the thicker the polymer film, and this allows us to control the structural colors that appear at different locations on the substrate.”

The researchers found that structural coloration was modulated jointly by the thickness of the film and the permittivity of the conducting polymer. By controlling the amount of UV light that was shone on the film, the researchers could control the film’s thickness and generate structural colors from violet to red. Using grayscale photomasks, they could fabricate high-resolution structural color images.

The researchers tuned colored surfaces and images through the electrochemical modulation of the polymer’s redox state. Due to the tunable properties of the conducting polymer, they achieved reversibly switchable color images.

Though the approach used by the researchers has been applied to monochrome reflective displays, the Linköping team showed that the same materials could provide dynamic images in color, using optical interference effects combined with spatial control of nanoscale thicknesses.

Most reflective displays are primarily monochrome, team members said; existing methods to create color versions are complex and only moderately successful. The approach they developed can be used to make and tune structural colors in the entire visible spectrum, and with low power consumption.

In contrast, LED displays are expensive to make and their global use consumes a lot of energy.

The developed approach could aid the development of next-generation electronic color labels and backlight-free, energy-efficient displays, the researchers said. Future efforts could focus on optimizing UV patterning parameters and exploring other polymers that can be prepared by VPP, as well as different mirrors and nanocavity structures, to further improve color purity and electrochemical switching properties.

Researchers at Linköping University have developed a method that may lead to new types of displays based on structural colors. Courtesy of Thor Balkhed.

Professor Magnus Jonsson said that the method has great potential, for example, for applications such as electronic labels in color.

“We receive increasing amounts of information via digital displays, and if we can contribute to more people gaining access to information through cheap and energy-efficient displays, that would be a major benefit. But much research remains to be done, and new projects are already underway,” he said.

The research was published in Advanced Materials (www.doi.org/10.1002/adma.202102451).