Yaowu He

Side-chain engineering of green color electrochromic polymer materials: toward adaptive camouflage application

The syntheses of adaptive camouflage devices based on novel side-chain engineered organic electrochromic materials have been demonstrated. Herein we report a molecule engineering approach for the tuning and syntheses of green-brown switchable electrochromic materials and also demonstrate their applications in chameleonic fabric devices. We have also successfully demonstrated the fabrication of chameleonic fabric devices.

Influence of heteroatoms on the charge mobility of anthracene derivatives

The introduction of polarizable heteroatom, such as O, and S, attached peripheral side chains of conjugated moieties such as polyacenes has not been systematically investigated. To study such effects, and to explore semiconductors with both high charge mobility and luminescence properties, we present a comparative systematic study of heteroatom effects on the conduction of organic semiconductors in a representative series of new organic semiconductors based on the blue phenyl-anthracene molecule core. Elucidated by the single-crystal X-ray analysis, thin film XRD and AFM measurements, a correlation between the molecular structure variation, film ordering, and charge mobility has been established. Quantum chemistry calculations combined with the Marcus–Hush electron transfer theory interpret the transport parameters. The anisotropic transport properties of these compounds were suggested by the DFT predictions and the high hole mobility in BEPAnt and BOPAnt is contributed mainly by the parallel packing of these compounds with the highest ∥μh; these results are in good agreement with the experimental observations. Heteroatoms are demonstrated to influence the charge mobility dramatically. Our systematic investigation will provide valuable guidance for a judicious material design of semiconductors for OTFT applications.

Molecular phase engineering of organic semiconductors based on a [1]benzothieno[3,2-b][1]benzothiophene core

Two environmentally and thermally stable [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives, 2-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT) and 2-(4-hexylphenyl)[1]benzothieno[3,2-b][1]benzothiophene (C6-Ph-BTBT), are prepared by Suzuki coupling. Organic thin-film transistors with a top-contact and bottom-gate based on BTBT, Ph-BTBT and C6-Ph-BTBT are fabricated by vacuum-deposition on octyltrichlorosilane treated Si/SiO2 substrates. Experimental results show that the thin-film based on BTBT sublimes instantly after the deposition of electrodes, and no semiconductor signal is detected. Ph-BTBT shows a mobility of 0.034 cm2 V−1 s−1. Furthermore, C6-Ph-BTBT exhibits three liquid crystal phases (Sm A, Sm E and Sm K or H) and achieves the highest hole mobility of 4.6 cm2 V−1 s−1 with an on/off ratio of 2.2 × 107 for polycrystalline organic thin-film transistors in ambient air. The present study exemplifies that the introduction of mesoscopic order in molecular design provides a general approach for the high electronic performance of organic semiconductors.

Highly transmissive blue electrochromic polymers based on thieno[3,2-b]thiophene

A series of three polymers based on a thieno[3,2-b]thiophene core is synthesized and polymerized via electrochemical polymerization. The addition of benzene and thiophene rings as two different types of substituents on the 3,6-position of the thieno[3,2-b]thiophene core brings about a variance in color changing, optical contrast and morphological demeanor. Electrochromical studies demonstrate that P1 and P2 switch between deep blue neutral and colorless transparent oxidized states, while P3 switches between violet and light green transmissive states. Amid the three polymers, P1 shows the highest optical contrast (71%) in the visible region with complete coloring and bleaching in just 1.10 s and 1.80 s, respectively. In addition, all three polymers reveal about 60% of the transmittance change in the near-IR region, which endows them with commendable applications in NIR electrochromic devices. AFM images depict an augmented surface roughness due to the introduction of alkyl chains in the thieno[3,2-b]thiophene core, which gives rise to a better stability of the polymer thin film.

Highly responsive phototransistors based on 2,6-bis(4-methoxyphenyl)anthracene single crystal

Herein, thin film and single crystal phototransistors based on 2,6-bis(4-methoxyphenyl)anthracene (BOPAnt) are systematically studied. High quality BOPAnt single crystals are grown via the PVT (physical vapor transport) method to fabricate bottom gate top contact phototransistors. The thin film phototransistor shows a photoresponsivity of 9.75 A W−1 under 1 mW cm−2 blue LED illumination, whereas under the same conditions the single crystal based phototransistor displays a photoresponsivity of 414 A W−1. Furthermore, using low power monochromatic light with the wavelength of 350 nm, the photoresponsivity of 3100 A W−1 under 0.11 mW cm−2 illumination and EQE of 9.5 × 105% are obtained for the BOPAnt single crystal phototransistor with the channel length of 65 μm. The much higher photoresponsivity of the single crystal based phototransistor is due to its much higher exciton diffusion length compared to that of the thin film device, which is also evident by the large Von shift towards the positive voltage direction. The photoswitching behaviors of the single crystal based phototransistor are also studied. It is observed that after a short warming up period, the single crystal based phototransistor shows stable switching behavior.

Thermal and Optical Modulation of the Carrier Mobility in OTFTs Based on an Azo-anthracene Liquid Crystal Organic Semiconductor

One of the most striking features of organic semiconductors compared with their corresponding inorganic counterparts is their molecular diversity. The major challenge in organic semiconductor material technology is creating molecular structural motifs to develop multifunctional materials in order to achieve the desired functionalities yet to optimize the specific device performance. Azo-compounds, because of their special photoresponsive property, have attracted extensive interest in photonic and optoelectronic applications; if incorporated wisely in the organic semiconductor groups, they can be innovatively utilized in advanced smart electronic applications, where thermal and photo modulation is applied to tune the electronic properties. On the basis of this aspiration, a novel azo-functionalized liquid crystal semiconductor material, (E)-1-(4-(anthracen-2-yl)phenyl)-2-(4-(decyloxy)phenyl)diazene (APDPD), is designed and synthesized for application in organic thin-film transistors (OTFTs). The UV-vis spectra of APDPD exhibit reversible photoisomerizaton upon photoexcitation, and the thin films of APDPD show a long-range orientational order based on its liquid crystal phase. The performance of OTFTs based on this material as well as the effects of thermal treatment and UV-irradiation on mobility are investigated. The molecular structure, stability of the material, and morphology of the thin films are characterized by thermal gravimetric analysis (TGA), polarizing optical microscopy (POM), (differential scanning calorimetry (DSC), UV-vis spectroscopy, atomic force microscopy (AFM), and scanning tunneling microscopy (STM). This study reveals that our new material has the potential to be applied in optical sensors, memories, logic circuits, and functional switches.

Exploring the electrochromic properties of poly(thieno[3,2-b]thiophene)s decorated with electron-deficient side groups

Two novel thieno[3,2-b]thiophene (TT)/3,4-ethylenedioxythiophene (EDOT)-based compounds of 2,5-(EDOT–TT–EDOT) type bearing electron-withdrawing side groups (4-cyanophenyl or 4-pyridyl) at 3,6-positions of the TT moiety have been synthesized. Their electropolymerization leads to electroactive conjugated polymers, P(CNPh-ETTE) and P(Py-ETTE), which possess electrochromic properties changing the color from purple to pale grey-blue or from sand brown to pale grey-green, respectively. Cyclic voltammetry and spectroelectrochemical experiments reveal that functionalization with electron-withdrawing side groups decreases the HOMO and LUMO energy levels and contracts the band gap of materials. Both new polymers demonstrated extremely short response times of 0.9–1.1 s for bleaching and 0.34–0.35 s for coloring. P(CNPh-ETTE) and P(Py-ETTE) polymers showed reasonably good contrast (16–23%) and coloration efficiency (120–190 cm2 C−1) in the visible region (at the maxima of their π–π* transitions, 540/570 nm), and high contrast and coloration efficiency in the near-infrared region (50–62% and 324–440 cm2 C−1 at 1500 nm, respectively). While the stability of the pyridine-functionalized polymer, P(Py-ETTE), was shown to be low (with unstable charge–discharge characteristics, presumably due to the protonation of the pyridine ring during the redox process), P(CNPh-ETTE) demonstrated superior electrochromic performance retaining 91–96% of its electroactivity after 2000 cycles between −0.5 and +1.0 V. DFT calculations on these and related EDOT–TT–EDOT polymers reported by us early have been presented and analyzed to understand the structure–property relationships in this class of electrochromic polymers.

Molecular engineering tuning optoelectronic properties of thieno[3,2-b]thiophenes-based electrochromic polymers

Thieno[3,2-b]thiophene (TT) monomers end-capped with 3,4-ethylenedioxythiophene (EDOT) moieties are electropolymerized to form π-conjugated polymers with distinct electrochromic (EC) properties. Steric and electronic factors (electron donor and acceptor substituents) in the side groups of the TT core, as well as the structure of the polymer backbone strongly affect the electrochemical and optical properties of the polymers and their electrochromic characteristics. The studied polymers show low oxidation potentials, tunable from–0.78 to +0.30 V (vs. Fc/Fc+) and the band gaps from 1.46 to 1.92 eV and demonstrate wide variety of color palettes in polymer films in different states, finely tunable by structural variations in the polymer backbone and the side chains. EC materials of different colors in their doped/dedoped states have been developed (violet, deep blue, light blue, green, brown, purple-red, pinkish-red, orange-red, light gray, cyan and colorless transparent). High optical contrast (up to 79%), short response time (0.57–0.80 s), good cycling stability (up to 91% at 2000 cycles) and high coloration efficiency (up to 234.6 cm2 C–1) have been demonstrated and the influence of different factors on the above parameters of EC polymers have been discussed.

Design and characterization of methoxy modified organic semiconductors based on phenyl[1]benzothieno[3,2-b][1]benzothiophene

Two environmentally and thermally stable [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives, BOP-BTBT and DBOP-BTBT are successfully synthesized and analyzed as active layers in organic thin film transistors. The effects of methoxy on BTBT based OTFT materials are reported for the first time. The experimental results show an excellent optimization influence of methoxy group on the OTFT performance with an improved hole transport mobility up to 0.63 cm2 V−1 s−1 (BOP-BTBT) and 3.57 cm2 V−1 s−1 (DBOP-BTBT). Meantime the mono- and bis-substituted derivatives are compared in terms of their physical properties and device performance. We find that the threshold voltage decreases when more methoxy groups are introduced.

Blending crystalline/liquid crystalline small molecule semiconductors: A strategy towards high performance organic thin film transistors

Solution processed small molecule polycrystalline thin films often suffer from the problems of inhomogeneity and discontinuity. Here, we describe a strategy to solve these problems through deposition of the active layer from a blended solution of crystalline (2-phenyl[1]benzothieno[3,2-b][1]benzothiophene, Ph-BTBT) and liquid crystalline (2-(4-dodecylphenyl) [1]benzothieno[3,2-b]benzothiophene, C12-Ph-BTBT) small molecule semiconductors with the hot spin-coating method. Organic thin film transistors with average hole mobility approaching 1 cm2/V s, much higher than that of single component devices, have been demonstrated, mainly due to the improved uniformity, continuity, crystallinity, and stronger intermolecular π-π stacking in blend thin films. Our results indicate that the crystalline/liquid crystalline semiconductor blend method is an effective way to enhance the performance of organic transistors.