Self-supported liquid crystal film for flexible display and photonic applications

Interactive deformable electroluminescent devices enabled by an adaptable hydrogel system with optical/photothermal/mechanical tunability

Deformable electroluminescent devices (DELDs) with mechanical adaptability are promising for new applications in smart soft electronics. However, current DELDs still present some limitations, including having stimuli-insensitive electroluminescence (EL), untunable mechanical properties, and a lack of versatile stimuli response properties. Herein, a facile approach for fabricating in situ interactive and multi-stimuli responsive DELDs with optical/photothermal/mechanical tunability was proposed. A polyvinyl alcohol (PVA)/polydopamine (PDA)/graphene oxide (GO) adaptable hydrogel exhibiting optical/photothermal/mechanical tunability was used as the top ionic conductor (TIC). The TIC can transform from a viscoelastic state to an elastic state via a special freezing-salting out-rehydration (FSR) process. Meanwhile, it endows the DELDs with a photothermal response and thickness-dependent light shielding properties, allowing them to dynamically demonstrate "on" or "off" or "gradually change" EL response to various mechanical/photothermal stimuli. Thereafter, the DELDs with a viscoelastic TIC can be utilized as pressure-responsive EL devices and laser-engravable EL devices. The DELDs with an elastic TIC can withstand both linear and out-of-plane deformation, enabling the designs of various interactive EL devices/sensors to monitor linear sliders, human finger bending, and pneumatically controllable bulging. This work offers new opportunities for developing next-generation EL-responsive devices with widespread application based on adaptable hydrogel systems.

Polymer-Dispersed Liquid Crystal Films on Flexible Substrates with Excellent Bending Resistance and Spacing Stability

Polymer-dispersed liquid crystals (PDLCs) are very attractive due to their electrically switchable properties. However, current PDLC films still have problems such as high driving voltages, low contrast ratio (CR), and poor bending resistance and spacing stability. To solve these problems, a PDLC film with a system of coexisting polymer spacer columns and polymer network was proposed. First, based on the adhesive systems of IBMA and UV6301, the effects of IBMA concentration and LC content on the morphology of the polymer network and the electro-optical properties of PDLC were investigated, respectively. Then, the effects of the process conditions of mask polymerization such as temperature, time, and UV light intensity on the morphology and electro-optical properties of the polymer spacer columns were systematically investigated. It was found that PDLC films with the coexistence system exhibit both excellent electro-optical properties and outstanding bending resistance and spacing stability. Thus, it provides new practical possibilities for the preparation of high-performance PDLC films used in flexible devices.

Solution processed high performance perovskite solar cells based on a silver nanowire-titanium dioxide hybrid top electrode

Longer silver nanowires (AgNWs) > 50 μm and even 90 μm in length have been produced via a polyol method by just changing the stirring speed at a temperature of 130 °C. As-synthesized longer AgNWs are further utilized to construct transparent conductive AgNWs films by a facile drop-casting technique that attained a sheet resistance of 14.5 Ω sq-1 and transmittance over 85%, which is higher than ITO film. The use of a AgNWs/TiO2 hybrid electrode decreases the sheet resistance to 8.3 Ω sq-1, which is attributed to the enhancement of connections between AgNWs by filling the empty spaces between nanowires and TiO2 nanoparticles. Transparent perovskite solar cells (PSCs) on the basis of these AgNWs and AgNWs/TiO2 hybrid top electrodes were made and examined. Due to the light scattering nature of TiO2 nanoparticles, optical transmittance of the AgNWs/TiO2 hybrid electrode enhances to some extent after the coating of a TiO2 layer. Both cell efficiencies and stability of the PSCs are enhanced by using the AgNWs/TiO2 top electrode. A power conversion efficiency (PCE) of 10.65% was attained for perovskite devices based on only the AgNW electrode with a sheet resistance of 14.5 Ω sq-1. A PCE of 14.53% was achieved after coating with TiO2 nanoparticles, indicating the layer effect of TiO2 coating.

Quantitative Characterization of the Anisotropy of the Stress-Optical Properties of Polyethylene Terephthalate Films Based on the Photoelastic Method

Polyethylene terephthalate (PET) is one of the most commonly used substrate materials in the field of flexible electronics, and its stress-induced birefringence often has a detrimental effect on the optical properties of the device. Therefore, a deep and systematic understanding of the stress-optical properties of PET films is crucial for device design and manufacture. The photoelastic method is a direct optical measurement technique based on the stress-induced birefringence effect of materials, which has the advantages of being nondestructive and noncontact. In this work, the photoelastic method was used to quantitatively characterize the anisotropy of the stress-optical properties of PET films under the uniaxial stress state. First, a self-built reflection-transmission coaxial bidirectional photoelasticity measurement system was developed by means of a combination of transmission and reflection photoelasticity. Then, the stress-optical coefficients and isoclinic angles of PET films with different stretching angles were measured. Finally, the linear combinations of the photoelastic tensor components and refractive-index-related parameters were determined by fitting the analytical relationship between the stress-optical coefficients and isoclinic angles.

Micro-patterned photoalignment of CsPbBr3 nanowires with liquid crystal molecule composite film for polarized emission

Photoalignment technology provides high potential for the manipulation of molecular orientations and has been widely used in liquid crystal displays. In this work, we align a luminescent film composite of CsPbBr₃ nanowires (NWs) and liquid crystal molecules through photoalignment conducted on a PDMS template. We successfully define different orientations of CsPbBr₃ NWs on the same substrate and the fluorescence micrographs clearly exhibit the orthogonal polarization direction of the two regions. On the basis of this research, we develop micro-photoalignment technology, which is promising for fabricating complex and precise nanostructures for photonic applications.

Dual Network Sponge for Compressible Lithium-Ion Batteries

Compressible energy devices have received increasing attention with the rapid development of flexible electronics and wearable devices due to their size adaptability and functional stability. However, it is hard to simultaneously achieve satisfactory energy density and mechanical stability for electrodes. Here an open-porous dual network sponge (DNS) with two networks of highly conductive carbon nanotubes and Li⁺ -intercalating TiO₂ -B nanowires is synthesized and employed as compressible lithium ion battery electrodes. All 1D components inside the DNS mutually penetrate with each other to form two physically distinct but functionally coupling networks, endowing DNS excellent compressibility and stability. A prototype compressible lithium-ion battery (C-LIB) is also demonstrated, in which the DNS exhibits a specific capacity of >238 mAh g^-1 under static 50% strain, and further in situ measurements show that under 1000 times of cyclic strains, DNS can charge and discharge normally maintaining a high capacity of 240 mAh g^-1 and exhibits robustness to fast strain rates up to 500% min^-1 . The dual network structure can be extended to design high-performance compliant electrodes that are promising to serve in future compressible and deformable electronics and energy systems.

From n-layer planar ordering to the monolayer homeotropic structure of confined hard rods: The effect of shape anisotropy and wall-to-wall separation

Using the Parsons-Lee theory, we examined the effect of shape anisotropy and the wall-to-wall separation (H) on the phase behavior of the hard parallelepiped rods with dimensions L, D, and D (L>D) in such narrow slitlike pores which only one homeotropic layer can form. The phase structures, including biaxiality, planar nematic layering transition as well as planar to homeotropic, were studied for some separations in the range 2.5D≤H≤10.0D for H-D≤L<H.

Morphology-controlled silver nanowire synthesis using a cocamidopropyl betaine-based polyol process for flexible and stretchable electronics

Silver nanowire (AgNW) based transparent conductive films (TCFs) are promising building blocks for flexible and stretchable electronics to replace brittle metal oxides. Ultra-long AgNWs are preferred for enabling TCFs with excellent photoelectric properties and mechanical flexibility. Herein, a novel polyol process is proposed for the synthesis of ultra-long AgNWs, with the new finding that the addition cocamidopropyl betaine (CAB) to polyol synthesis allows the rapid production of AgNWs with an average length of ∼120 μm in a high yield of ∼90%. Also, a cocamidopropyl betaine assisted polyol method for the synthesis of ultra-long AgNWs is demonstrated with a possible mechanistic explanation. The prepared AgNWs are coated on a polyethylene glycol terephthalate (PET) substrate to fabricate a flexible transparent conductive film, which exhibits a low sheet resistance of ∼200 Ω sq⁻¹ at 88.74% transmittance with a negligible change of sheet resistance after bending. In addition, flexible TCFs based on the resulting AgNWs reveal excellent mechanical flexibility and high cyclic stability after 300 cycles of bending. The new polyol process in this work will provide a greater possibility for the practical application of long AgNWs towards flexible and wearable optoelectronic devices.

Ultra-long silver nanowires with a length of ∼120 μm were synthesised using a cocamidopropyl betaine-based polyol process.

Paper-like flexible optically isotropic liquid crystal film for tunable diffractive devices

We have demonstrated a paper-like diffractive film in which nano-structured liquid crystal droplets are embedded in elastomeric monomer incorporated polymer matrix by polymerization induced phase-separation. The film with voltage-tunable phase grating exhibits an optically isotropic phase with high transparency and an effective chromatic diffraction for an incident white light with sub-millisecond switching time. In addition, the proposed diffractive film is exhibiting excellent chemical stability against organic and inorganic solvents. In this paper, the diffraction properties of test films depending on incident polarization direction, wavelength, and spatial dispersion are characterized. Easy processing and optically isotropic nature of the film imparts potential applications to flexible electro-optic devices that can be widely implemented in wearable photonics.