Enhanced Bandwidth Broadening of Infrared Reflector Based on Polymer Stabilized Cholesteric Liquid Crystals with Poly(N-vinylcarbazole) Used as Alignment Layer

Abstract

Alignment layer plays a critical role on liquid crystal (LC) conformation for most LC devices. Normally, polyimide (PI) or polyvinyl alcohol (PVA), characterized by their outstanding thermal and electrical properties, have been widely applied as the alignment layer to align LC molecules. Here, we used a semi-conductive material poly(N-vinylcarbazole) (PVK) as the alignment layer to fabricate the cholesteric liquid crystal (CLC) device and the polymer-stabilized cholesteric liquid crystals (PSCLC)-based infrared (IR) reflectors. In the presence of ultraviolet (UV) irradiation, there are hole–electron pairs generated in the PVK layer, which neutralizes the impurity electrons in the LC–PVK junction, resulting in the reduction in the built-in electric field in the LC device. Therefore, the operational voltage of the CLC device switching from cholesteric texture to focal conic texture decreases from 45 V to 30 V. For the PSCLC-based IR reflectors with the PVK alignment layer, at the same applied electric field, the reflection bandwidth is enhanced from 647 to 821 nm, ranging from 685 to 1506 nm in the IR region, which makes it attractive for saving energy as a smart window.

Photothermal Dual Passively Driven Liquid Crystal Smart Window

“…Therefore, smart windows are required to achieve intelligent indoor light and thermal management by modulating the visible and infrared light range of sunlight . Up to now, plenty of research about smart windows has been developed, which can adjust transmittance through electro-, − thermal, − photo-, − and mechanoresponses. , Changing the transmittance of smart windows artificially and actively by electrical stimulation requires an external circuit, additional energy consumption, and safety hazards . Photochromic and thermochromic windows are considered cost-effective, stimuli-rational, and energy-efficient for their simple structure, passive light modulation, and zero-energy input characteristics …”

Recent Advances in Electro-Optic Response of Polymer-Stabilized Cholesteric Liquid Crystals

“…Bandwidth broadening in PSCLCs has been an intensive research topic over the last decade from both an academic and practical application perspective. While a number of approaches exist to induce bandwidth broadening, electrically induced responses in PSCLCs are of great interest [ 42 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ]. PSCLCs employed for such studies are typically comprised of CLCs with negative Δε and an anisotropic polymer network with a strong alignment effect on the LC [ 58 , 61 ].…”

Research Progress of Cholesteric Liquid Crystals with Broadband Reflection

“…Under UV irradiation, the PVK layer generated hole-electron pairs that neutralized the impurity electrons in the LC-PVK junction, resulting in a lower built-in electric field in the LC device, which was more favorable for the polymer network to move under the electric field. Compared with the PVA orientation layer, the bandwidth broadening of the PVK orientation layer was better at the same electric field (up to 821 nm at 60 V) [ 81 ].…”

“…Compared with the PVA orientation layer, the bandwidth broadening of the PVK orientation layer was better at the same electric field (up to 821 nm at 60 V) [81]. the PVA orientation layer, the bandwidth broadening of the PVK orientation layer was better at the same electric field (up to 821 nm at 60 V) [81]. Based on the ion capture of oligomers, Hu developed an in-situ DC curing strategy, which involves UV irradiation while loading DC voltage (as shown in Figure 14).…”