Christopher Johannes1
University of Kassel1
The integration/processing of flexible electrochromic multilayer films in common plastic injection molding processes could be advantageous with regard to the possible range of applications and mass production of electrochromic devices (ECDs). Especially light weight, slightly three dimensional formed and optically modulating windows are of interest in the transportation sector. Most optical applications based on amorph plastics, such as plastic windows or headlight coverings, are made of polycarbonate or polymethylmethacrylate (e.g. the well known brands Makrolon® and Plexiglas®). Especially polycarbonate as substrate material for ECDs could be advantageous regarding the optical properties compared to the often used PET substrate, particularly regarding the haze.<br/>Therefore, a battery-type ECD multilayer structure based on polycarbonate substrate was developed aiming at good optical properties, especially a neutral colored and highly transmissive bleached state. For this reason, a neutral counter electrode of TiO2 and a highly transmissive polyurethane based gel electrolyte [1] was chosen. ITO was applied as current collector layer and PEDOT:PSS as working electrode.<br/>Within a funded project an injection mold was developed and manufactured that allows to back mold the multilayer films (active area: 4 cm x 4 cm) and introduces a bending radius (200 mm). The integrated embossing unit leads to drastically lower injection pressure the cavity compared to standard processing. This results in lower shear and compressive stress on the multilayer film as well as reduced orientation and residual stresses in the injected component.<br/>First back injection molding experiments with polycarbonate showed severel problems, especially considering the brittle metal oxide layers. After processing micro cracks all over the surface and macroscopic cracks at the kinks were detectable (beginning and end of radius). Due to different shrinkage of film and injected melt, the half cells partially detached from each other and deformed in most cases. In case of lamination defects at the multilayer films edges melt penetrated between the half cells. Most ECDs showed no function afterwards. Few surviving ECDs showed a non homogeneous coloring, defects, lower current responses and slower switching speed compared to the function before. Due to these findings the above described ECD configuration was adjusted and the brittle metal oxide layers were replaced. Instead of ITO a very conductive PDEOT:PSS formulation was used as current collector layer and TiO2 was replaced by Prussian Blue. The first ECD showed comparable transmittance changes, but considerable slower coloring/decoloring, probably due to lower conductivity of the current collector layer. The bright state was slightly blue colored and the total transmittance in the bright state was pretty low with 49 % compared to the original configuration with almost 70 %. Very promising on the other hand are the results with back injection molding since the major problems did not occur. Neither cracks were visible nor the cell detached. An accompanying peel study revealed that the new configuration had a 2.5 times better adhesion between the half cells (over the electrolyte). The total visible transmittance in the oxidized state at +1.5 V was 49 % before and 50 % after the process. The transmittance in the reduced state at -2.5 V after 60 s was 37 % before and 41 % after the process. So, the transmittance change was 12 % before and 9 % after back injection molding. Increasing the cycle time to 120 s led to a transmittance change of 14 % before and 13 % after the process. Accordingly, the switching speed decreased, but the total transmittance change only decreased little.<br/><br/>[1] Johannes, C., Hartung, M., & Heim, H.-P. (2022). Polyurethane-Based Gel Electrolyte for Application in Flexible Electrochromic Devices. <i>Polymers</i>, <i>14</i>(13).