Advancing Solar Modulation Ability of Near Infrared Thermochromic Cholesteric Liquid Crystal Smart Window Films Using a Waveplate

Abstract

Cholesteric liquid crystal oligomer coatings are promising smart materials for use as near infrared thermochromic reflectors to control solar radiation through windows. However, their effectiveness as solar energy transmission modulators is limited because they only reflect circularly polarized light of a specific handedness (either right- or left-handed) over a relatively narrow bandwidth (≈100 nm). In this work, these limitations are addressed using a waveplate. To enhance the reflective properties, a waveplate is placed between two baseline oligomers. By optimizing the waveplate thickness, the device switches between reflecting right-circularly polarized light at room temperature to reflecting unpolarized, ambidextrous light at 60 °C. Next, this system is combined with a second reflective cholesteric liquid crystal oligomer with the same reflection band at room temperature but with a smaller thermochromic response. This enables the device to transition from a narrow band reflection to broad band ambidextrous reflection upon heating, resulting in a solar transmission modulation of 5.2% which is four times greater than typical thermochromic cholesteric liquid crystal reflectors marking a significant step toward their application in smart solar heat-modulating windows. This work presents the use of a waveplate in stimuli-responsive optical materials to enhance its reflective changes.