Electrothermally Driven Paintable Photonic Devices for Large-Area Flexible Optoelectronic Applications

Optoelectronic devices have found use in numerous applications such as displays, sensors and smart windows. The development of flexible and large-area optoelectronic devices that can respond both autonomously by environmental changes and via an electric stimulus might lead to new applications. Our research led to the development of such a flexible (electro)thermally responsive device by using easy scalable industrial techniques such as gravure printing and bar coating that can lead to the manufacturing of large-area systems. Color tuning over the entire visible spectrum with high optical quality was demonstrated for a flexible, transparent cholesteric liquid crystal-based device by applying an electric field or via changes in the surrounding temperature.<br/>The multilayered device consisted of a flexible, transparent heater, which acted as a heating source that could tune the reflective color of a robust temperature-responsive coating. In absence of an electric field, the device was colorless at room temperature while a reflective color was displayed upon applying a DC electric field. Rapid color tuning was demonstrated by tuning the electric field. No influence on the electrical/optical properties was observed upon bending the foil to a large degree thereby demonstrating simultaneously the high flexibility and transparency of the device.<br/>The transparent heater was developed via gravure printing, allowing the fabrication of large-area conductive substrates. As conductive material, a silver nanowire-based ink was deposited in a fast and controlled way over a transparent PET substrate and subsequently cured to form a thin conductive layer. Joule heating was induced by applying a DC electric field to the system, causing homogeneous heating over the entire surface of the conductive AgNW/PET foil. Additionally, the heating capability of such heaters could easily be tuned by depositing multiple printing layers over the PET foil thereby changing the silver nanowire density on top of the surface.<br/><br/>An optical response was provided by a temperature-responsive photonic coating based on cholesteric liquid crystals. This coating was formed out of a homogeneous mixture, containing acrylic monomers and non-polymerizable liquid crystal mesogens, and was bar coated over the AgNW/PET heater. During photo-polymerization, phase separation occurred within the mixture since the reactive monomers started to diffuse towards the light source. This led to the formation of a robust polymeric topcoat that protected the non-polymerized thermosensitive cholesteric liquid crystal fraction underneath. The photonic coating was colorless in the rest state, but upon temperature elevation, a reflective color was observed that could be rapidly shifted towards lower wavelengths.<br/><br/>Thus, applying an electric field to the AgNW/PET foil resulted in a similar change of the reflective color in the photonic coating, caused by Joule heating. Applying a DC voltage in the range of 0-7 V allows the reflection band to be shifted over the entire visible spectrum. Due to the homogeneous electrothermal response, a uniform reflective color is displayed in the final device while retaining high optical quality. Additionally, bendability studies demonstrated the high flexibility of the device without altering the electrical/optical properties. Scaling up of the above-mentioned method could lead to large-area, flexible optoelectronic devices that generate a uniform response, showing promise for implemention of such systems in the field of flexible displays, smart windows, and sensors.