Conducting Polymers for Tuneable Structural Colours

Conductive polymers have been having a fundamental role in the development of organic electronics and show great potential for tuneable photonic systems. The fact that organic materials are soft and flexible and are often compatible with high throughput fabrication methods gives them great technological potential in developing flexible, wearable, and low-cost devices. Besides the extensive use for emissive devices in optoelectronics, there has been a large interest for electrochromic displays and smart windows, where their tuneable absorption properties are used. However, instead of having just a few electrochromic states available, a novel compelling application is in reflective displays in colour. While those displays have the advantage of a high eye comfort and visibility against the sun, coupled with very low power consumption, the bottleneck is the low brightness, having no other light sources than ambient light. This can be addressed by having an electroactive tuneable structural colour that can cover the entire visible spectrum, with no need for a traditional subpixel division. Plasmonics, optical nanocavities, and photonic crystals have been proposed as electroactive structural colors in the visible, but only a few were shown to be able to cover the entire visible spectrum and achieve a high reflectance. Most of the organic-based devices used redox electroactive polymeric or gel-like structures with redox sites, but conductive polymers have only been recently considered.<br/>Electrochromism has been the most exploited optical property of conductive polymers, but their redox tunability makes them a viable choice for tuneable optics. That is, because along with the absorption also the permittivity and the thickness or volume can be electrochemically changed. We have demonstrated that conductive polymers based on poly(3,4-ethylenedioxythiophene) (PEDOT) exhibit a negative permittivity in certain spectral regions and can be used as plasmonic materials [1]. Moreover, we also showed that PEDOT can be structured by photolithography in combination with vapour phase polymerization, making possible to directly obtain images by structural coloration, with a certain degree of tunability [2]. We focus here on our recent efforts in studying and exploiting conductive polymers for electroactive photonic devices. We show that some novel polymers, such as thienothiophenes and suitably engineered polythiophenes, exhibit a relatively low absorption in the visible for all the redox states, which make them good candidates for electroactive tuneable reflective structural colors. To test their optical performances as actuating materials, we successfully integrated those polymers in optical nanocavities inside an electrochemical cell [3]. We achieved complete coverage of the visible spectrum, maintaining a high reflectance for all the colors while using a low voltage range. This is made possible by a synergistic combination of the variation of the optical properties and thickness change of the polymeric spacer. We offer here an overview and spectroelectrochemical characterization of the properties of conductive polymers in respect to the change of their redox state and the use in organic photonics. Besides electroactive structural coloration, those materials might find applications in other types of optical metasurfaces, with potential also in the infrared region, and could also be used in the field of electrochemical actuators, due to their thickness changes.<br/>[1] <i>S. Chen et al.</i>, Nature Nanotechnology 2020, <b>15</b>, 35-40<br/>[2] <i>S. Chen et al.</i>, Advanced Materials 2021, <b>33</b> 2102451<br/>[3] <i>S. Rossi et al.</i>, Advanced Materials 2021, 2105004