Xianghua Wang

A bis(2-oxoindolin-3-ylidene)-benzodifuran-dione containing copolymer for high-mobility ambipolar transistors

A bis(2-oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF)-based low band gap polymer (PBIBDF-BT), containing a solubilizing alkyl chain bithiophene unit as a donor, has been synthesized. The polymer with a low-lying LUMO/HOMO energy level (-4.03/-5.55 eV) exhibits efficient ambipolar charge transport. The electron and hole mobilities are as high as 1.08 and 0.30 cm(2) V(-1) s(-1), respectively.

Electrically switchable photoluminescence of fluorescent-molecule-dispersed liquid crystals prepared via photoisomerization-induced phase separation

A novel α-cyanostilbenic derivative, (Z)-2-(4-aminophenyl)-3-(4-(dodecyloxy)phenyl) acrylonitrile, is reported, wherein a Z–E isomerization process is accompanied by a remarkable decrease in compatibility with liquid crystals. Accordingly, fluorescent-molecule-dispersed liquid crystals have been prepared through a photoisomerization-induced phase separation method. The photoluminescence can be repeatedly switched by an electric field. Modulating the excitation light offers a new technique to explore electrically controllable photoluminescence.

An ABA triblock copolymer strategy for intrinsically stretchable semiconductors

A novel semiconductor–rubber–semiconductor (P3HT–PMA–P3HT) triblock copolymer has been designed and prepared according to the principle of thermoplastic elastomers. It behaves as a thermoplastic elastomer with a Youngs modulus (E) of 6 MPa for an elongation at break of 140% and exhibits good electrical properties with a carrier mobility of 9 × 10−4 cm2 V−1 s−1. This novel semiconductor may play an important role in low-cost and large-area stretchable electronics.

A luminescent liquid crystal with multistimuli tunable emission colors based on different molecular packing structures

A novel luminescent liquid crystal, (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(4-(dodecyloxy)phenyl)acrylonitrile) (PDPA), is reported, which shows green, yellow and orange colors depending on the self-assembled structure. The luminescence of PDPA can vary between those three different colors invoked by mechanical shearing and thermal annealing. Moreover, an excellent liquid crystal phase transition of PDPA is observed using polarized optical microscopy (POM) and differential scanning calorimetry (DSC) analysis. Finally, a prototype device is fabricated that demonstrates rewritable fluorescence optical recording with multistimuli luminescence tuning.

Thickness dependence of the electro-optical properties of reverse-mode polymer-stabilised cholesteric texture

The cell gap dependence of the electro-optical properties of reverse-mode polymer-stabilised cholesteric texture has been investigated. In this study, the morphology and electro-optical properties were analysed as a function of the thickness of the cells. The scanning electron microscopy indicates that the polymer networks become tighter with the decrease of cell gap. Increasing cell gap can improve the contrast ratio exponentially, while the threshold voltage increases linearly. The field-off response time increases, which is ascribed to the decreasing elastic force between the polymer network and the LC molecules.

Electrically switchable multi-stable cholesteric liquid crystal based on chiral ionic liquid

A multi-stable and electrically switchable cholesteric liquid crystal based on chiral ionic liquid is demonstrated. The cholesteric liquid crystal can be switched among the planar texture, focal conic texture, wide-band reflected state, and fingerprint texture by applying specific electric fields. Each of these four states exists stably for several hours without any obvious change observed at room temperature. The electro-optical properties and driving scheme of the cholesteric liquid crystal are also reported.

A phthalimide- and diketopyrrolopyrrole-based A1–π–A2 conjugated polymer for high-performance organic thin-film transistors

An acceptor–π–acceptor (A1–π–A2) type polymer based on phthalimide and diketopyrrolopyrrole units has been designed and synthesized by Stille cross-coupling reaction. The photo-physical, electrochemical and transistor properties have been also investigated. The polymer exhibited broad absorption in the near infrared region and low LUMO/HOMO energy levels. Organic thin-film transistors (OTFTs) devices with common architectures were fabricated to evaluate the device properties. Polymer-based OTFTs devices exhibited n-type charge transfer under vacuum and yielded an electron mobility as high as 0.52 cm2 V−1 s−1. This is a record value for n-type polymers based on phthalimide structure. The OTFTs devices were also investigated in air and displayed p-type transporting behavior with a hole mobility of up to 0.13 cm2 V−1 s−1. The corresponding thin film morphology and polymer packing were also investigated in detail by atomic force microscopy (AFM) and X-ray diffraction (XRD) to correlate with the mobility. The results showed that the polymer film possessed highly uniform polymer nanofibers, a long-range-ordered lamellar structure and a small π–π stacking distance.

Phototransistors based on a donor–acceptor conjugated polymer with a high response speed

A photoresponsive polymer thin-film transistor based on a donor–acceptor conjugated polymer ambipolar semiconductor (PBIBDF-BT) is proposed in this report. The device exhibited both hole- and electron-carrier transport response to incident light with photoswitching speeds below 14 ms. In addition, the photocurrent/dark-current ratio and the photoresponsivity were 4552 and 108.43 mA W−1 for the p-type channel, and 1044 and 38.72 mA W−1 for the n-type channel, respectively. The PBIBDF-BT films exhibit more pronounced sensitivity to red light than previous polymer semiconductors, and the drain current gradually increased with an increase in the illumination intensity, resulting in typical output field-effect transistor characteristics. The influence of mobility of PBIBDF-BT thin films, which were formed via spin coating at different spinning speeds on the OTS/SiO2/Si substrate, on the phototransistor response to light illumination was further investigated.

Interfacial nucleation behavior of inkjet-printed 6,13 bis(tri-isopropylsilylethynyl) pentacene on dielectric surfaces

The performance of organic thin film transistors (OTFTs) is heavily dependent on the interface property between the organic semiconductor and the dielectric substrate. Device fabrication with bottom-gate architecture by depositing the semiconductors with a solution method is highly recommended for cost-effectiveness. Surface modification of the dielectric layer is employed as an effective approach to control film growth. Here, we perform surface modification via a self-assembled monolayer of silanes, a spin-coated polymer layer or UV-ozone cleaning, to prepare surfaces with different surface polarities and morphologies. The semiconductor is inkjet-printed on the surface-treated substrates as single-line films with overlapping drop assignment. Surface morphologies of the dielectric before film deposition and film morphologies of the inkjet-printed semiconductor are characterized with polarized microscopy and AFM. Electrical properties of the films are studied through organic thin-film transistors with bottom-gate/bottom-contact structure. With reduced surface polarity and nanoscale aggregation of silane molecules on the substrates, semiconductor nucleates from the interior interface between the ink solution and the substrate, which contributes to film growth with higher crystal coverage and better film quality at the interface. Surface treatment with hydrophobic silanes is a promising approach to fabrication of high performance OTFTs with nonpolar conjugated molecules via solution methods.

Cholesteric liquid crystals with an electrically controllable reflection bandwidth based on ionic polymer networks and chiral ions

The reflection bandwidth of cholesteric liquid crystals is typically on the order of 50–100 nm in the visible region. Static bandwidths greater than 100 nm can be observed in polymer stabilized cholesteric liquid crystals (PSCLCs) that possess a pitch gradient throughout the thickness of the cell. This work presents PSCLCs based on ionic polymer networks and chiral ions, which exhibit large reflection bandwidth tunability under a small applied direct current (DC) electric field. The reflection colors dynamically switch between white, orange, red and transparent. The underlying mechanism of the electrically controllable bandwidth in PSCLCs is the redistribution of the chiral ions under DC electric fields resulting in a pitch gradient variation along the optical axis. Removal of the electric field returns the PSCLC to its original optical properties due to the strong electronic attraction between the ionic polymer network and chiral ions. Moreover, the reflection bandwidth is also sensitive to the temperature.