Dichroic-dye-doped short pitch cholesteric liquid crystals for the application of electrically switchable smart windows

A short pitch cholesteric liquid crystal (CLC) acts as one-dimensional photonic crystals. It was widely studied for electro-optical switching devices. Although it was proposed for a switchable smart window, its spectral characteristics have critical limitations for window applications. In this report, we present dichroic-dye-doped short pitch CLC films, suitable for switchable window applications. The spectral limitations of a conventional CLC film in the voltage-off screening states were resolved by combining two different approaches of doping dichroic dye and increasing film thickness. Although each adjustment showed insufficient improvements, the implementation of a combined approach resulted in a drastically enhanced shielding performance in the voltage-off state, desirable for privacy windows. The increased film thickness promoted both the scattering focal conic and reflective multi-domain planar configurations for two different voltage-off states. The modified planar configuration caused a specific colored reflection with some turbidities. Furthermore, the dichroic dyes in both states effectively absorbed transmitting and scattered lights, resulting in the dark gray focal conic state and opaque colored reflection in the multi-domain planar state. Such combined efforts brought a great synergetic effect for enhancing a shielding efficiency. As a result, spectral properties of the voltage-off screening states were greatly different from the conventional CLC films, which show a milky white and tinted color, respectively, in a bright environment. The spectral performances of three switchable states were dramatically improved for the application of electrically switchable smart windows. The dichroic-dye-doped CLC films may be usefully adopted for switchable ultraviolet and heat windows if the cholesteric pitch and dichroic dyes are optimized for the corresponding wavelengths of electromagnetic radiation.