Ming-Chou Chen

Asymmetric fused thiophenes for field-effect transistors: crystal structure–film microstructure–transistor performance correlations

New asymmetric phenyl and perfluorophenyl end-functionalized dithienothiophene (DTT)- and bisdithienothiophene (BDTT)-based fused-thiophene derivatives (FPP-DTT; 1 and FPP-BDTT; 3) were synthesized and characterized for organic thin-film transistor (OTFT) applications. For comparison, symmetric phenyl end-capped dithienothiophene and bisdithienothiophene derivatives DP-DTT (2) and DP-BDTT (4) were also explored in parallel. The crystal structures of all four molecules were determined via single-crystal X-ray diffraction. Asymmetric compounds 1 and 3 exhibit face-to-face π–π stacking, while symmetric 2 and 4 show herringbone stacking. Single-crystal and thin-film transistors based on these four materials were fabricated. For single-crystal transistors, asymmetric FPP-DTT and FPP-BDTT gave high p-channel mobilities of 0.74 and 0.73 cm2 V−1 s−1, respectively, as well as current on/off ratios of ∼105. Symmetric DP-DTT and DP-BDTT gave relatively lower p-channel mobilities of 0.36 and 0.41 cm2 V−1 s−1, respectively. For thin-film transistors, FPP-DTT and DP-DTT films deposited at 25 °C exhibited decent p-channel characteristics with a carrier mobility as high as 0.15 and 0.20 cm2 V−1 s−1, respectively for top-contact/bottom-gate OTFT devices. The device characteristics on various gate dielectrics have been correlated with the film morphologies and microstructures of the corresponding compounds.

Enhanced Performance of Benzothieno[3,2‐b]thiophene (BTT)‐Based Bottom‐Contact Thin‐Film Transistors

Three new benzothieno[3,2-b]thiophene (BTT; 1) derivatives, which were end-functionalized with phenyl (BTT-P; 2), benzothiophenyl (BTT-BT; 3), and benzothieno[3,2-b]thiophenyl groups (BBTT; 4; dimer of 1), were synthesized and characterized in organic thin-film transistors (OTFTs). A new and improved synthetic method for BTTs was developed, which enabled the efficient realization of new BTT-based semiconductors. The crystal structure of BBTT was determined by single-crystal X-ray diffraction. Within this family, BBTT, which had the largest conjugation of the BTT derivatives in this study, exhibited the highest p-channel characteristic, with a carrier mobility as high as 0.22 cm(2)  V(-1)  s(-1) and a current on/off ratio of 1×10(7) , as well as good ambient stability for bottom-contact/bottom-gate OTFT devices. The device characteristics were correlated with the film morphologies and microstructures of the corresponding compounds.

Multifunctional Iodide-Free Polymeric Ionic Liquid for Quasi-Solid-State Dye-Sensitized Solar Cells with a High Open-Circuit Voltage

A polymeric ionic liquid, poly(oxyethylene)-imide-imidazolium selenocyanate (POEI-IS), was newly synthesized and used for a multifunctional gel electrolyte in a quasi-solid-state dye-sensitized solar cell (QSS-DSSC). POEI-IS has several functions: (a) acts as a gelling agent for the electrolyte of the DSSC, (b) possesses a redox mediator of SeCN(-), which is aimed to form a SeCN(-)/(SeCN)3(-) redox couple with a more positive redox potential than that of traditional I(-)/I3(-), (c) chelates the potassium cations through the lone pair electrons of the oxygen atoms of its poly(oxyethylene)-imide-imidazolium (POEI-I) segments, and (d) obstructs the recombination of photoinjected electrons with (SeCN)3(-) ions in the electrolyte through its POEI-I segments. Thus, the POEI-IS renders a high open-circuit voltage (VOC) to the QSS-DSSC due to its functions of b-d and prolongs the stability of the cell due to its function of a. The QSS-DSSC with the gel electrolyte containing 30 wt % of the POEI-IS in liquid selenocyanate electrolyte exhibited a high VOC of 825.50 ± 3.51 mV and a high power conversion efficiency (η) of 8.18 ± 0.02%. The QSS-DSSC with 30 wt % POEI-IS retained up to 95% of its initial η after an at-rest stability test with the period of more than 1,000 h.

In situ STM imaging of the structures of pentacene molecules adsorbed on Au(111).

In situ scanning tunneling microscope (STM) was used to examine the spatial structures of pentacene molecules adsorbed onto a Au(111) single-crystal electrode from a benzene dosing solution containing 16-400 microM pentacene. Molecular-resolution STM imaging conducted in 0.1 M HClO(4) revealed highly ordered pentacene structures of ( radical31 x radical31)R8.9 degrees , (3 x 10), ( radical31 x 10), and ( radical7 x 2 radical7)R19.1 degrees adsorbed on the reconstructed Au(111) electrode dosed with different pentacene solutions. These pentacene structures and the reconstructed Au(111) substrate were stable between 0.2 and 0.8 V [vs reversible hydrogen electrode, RHE]. Increasing the potential to E > 0.8 V lifted the reconstructed Au(111) surface and disrupted the ordered pentacene adlattices simultaneously. Ordered pentacene structures could be restored by applying potentials negative enough to reinforce the reconstructed Au(111). At potentials negative of 0.2 V, the adsorption of protons became increasingly important to displace adsorbed pentacene admolecules. Although the reconstructed Au(111) structure was not essential to produce ordered pentacene adlayers, it seemed to help the adsorption of pentacene molecules in a long-range ordered pattern. At room temperature (25 degrees C), approximately 100 pentacene molecules seen in STM images could rotate and align themselves to a neighboring domain in 10 s, suggesting that pentacene admolecules could be mobile on Au(111) under the STM imaging conditions of -150 mV in bias voltage and 1 nA in feedback current.

Synthesis and characterization of novel symmetrical two-photon chromophores derived from bis(triphenylaminotetrathienoacenyl) and fused-thiophene units

Four new donor–π–donor (D–π1–π2–π1–D) fused-thiophene-based chromophores, end-functionalized with electron-donating triphenylamine (TPA) groups, were developed and characterized for a two-photon absorption study. Within this series, tetrathienoacene (thieno[2′,3′:4,5]thieno[3,2-b]thieno[2,3-d]thiophene; TTA) moieties were employed as side-conjugated (π1) units, and the central conjugated core (π2) units were altered with thiophene (T), bithiophene (bT), thienothiophene (TT), and dithienothiophene (DTT) for chromophores 1–4, respectively. The structural and photophysical relationships of the four compounds were compared, and all four chromophores showed strong fluorescence with good thermal stability. The energy gap compression of these chromophores was verified by electrochemistry and density functional theory (DFT) calculations. The two-photon-related properties of 1–4 were examined using femtosecond laser pulses as the probing tool. The magnitude of the two-photon absorptivity was found to be strongly dependent on the molecular conjugation length and the center fused-thiophene unit. Within the family, the most conjugated DTT-centered chromophore (4) exhibits the strongest and the most widely dispersed two-photon absorption cross-section value up to 3000 GM. To the best of our knowledge, this is the highest 2PA cross section value reported to date among the studied fused thiophene-based chromophores.

In situ STM imaging of fused thienothiopene molecules adsorbed on Au(111) electrode.

In situ scanning tunneling microscopy (STM) was used to reveal the structures of dithieno[2,3-b:3,2-d]thiophene diphenyl (DTT) molecules deposited onto Au(111) electrode from a dosing solution made of dichlorobenzene and 50 muM DTT. Potential control was proven to be of prime importance in guiding the arrangement of DTT admolecules on Au(111) in 0.1 M HClO(4), as disorder DTT adlayer seen at E > 0.3 V (vs reversible hydrogen electrode) was transformed into a highly ordered (2 x 7 square root(3))rect -2DTT structure when the potential was made to 0.05 to 0.2 V. The ordered structure was stable for hours between 0.05 and 0.2 V. However, switching the potential further negative to 0 V resulted in slow melting of the ordered structure. The (2 x 7 square root(3))rect-DTT ordered adlattices recuperated when the potential was made positive to 0.2 V. Internal molecular functionalities of the thienothiophene and benzene in DTT admolecules were clearly discerned, from which the lateral structure for the (2 x 7 square root(3))rect-2DTT structure and registries of admolecules were deduced. The dynamics of the DTT adlattices on the Au(111) electrode surface was examined by real-time STM imaging, showing reorientation of as many as 150 DTT admolecules to join a neighboring ordered array within minutes.

Enhanced performance of solution-processed TESPE-ADT thin-film transistors.

A solution-processed anthradithiophene derivative, 5,11-bis(4-triethylsilylphenylethynyl)anthradithiophene (TESPE-ADT), is studied for use as the semiconducting material in thin-film transistors (TFTs). To enhance the electrical performance of the devices, two different kinds of solution processing (spin-coating and drop-casting) on various gate dielectrics as well as additional post-treatment are employed on thin films of TESPE-ADT, and p-channel OTFT transport with hole mobilities as high as ~0.12 cm(2) V(-1) s(-1) are achieved. The film morphologies and formed microstructures of the semiconductor films are characterized in terms of film processing conditions and are correlated with variations in device performance.

High ion-conducting solid polymer electrolytes based on blending hybrids derived from monoamine and diamine polyethers for lithium solid-state batteries

In this study, polyetheramine and polyetherdiamine were cross-linked separately with (3-glycidyloxypropyl)trimethoxysilane (GLYMO) and poly(ethylene glycol) diglycidyl ether (PEGDGE) and then blended in different ratios to obtain blended hybrid solid polymer electrolytes (SPEs). The blend network architecture improved the LiClO4 salt dissociation, which led to a higher ionic conductivity of the SPEs (4.5 × 10−4 S cm−1 at 60 °C and 1.2 × 10−4 S cm−1 at 30 °C for the [O]/[Li] ratio of 16). When the salt concentration was increased, the phase of the SPEs changed from semi-crystalline to an amorphous state, as revealed by differential scanning calorimetry. Complexation with different constituents of the SPEs was evidenced from the Fourier transform infrared measurements, while 13C and 29Si solid-state NMR spectroscopy provided information about the successful formation of the blend organic–inorganic hybrid with a silicate architecture, and 7Li NMR static linewidth measurements gave an insight into the dynamic behaviour of the lithium ions inside the SPEs. The blend hybrid SPEs demonstrated an electrochemical stability window of over 5 V. The electrochemical performance of the blend hybrid SPE assembled in a battery with lithium as the anode and LiFePO4 as the cathode exhibited a good cycle life of over 100 cycles, with an initial discharge capacity of 110 mA h g−1 and a coulombic efficiency of over 99%. With these good characteristic properties, the present blend hybrid SPE has the potential to be used in lithium solid-state batteries and other electrochemical devices.

Low-voltage-driven organic phototransistors based on a solution-processed organic semiconductor channel and high k hybrid gate dielectric

Solution-processed micro-ribbon shaped organic semiconductors and high k hybrid gate dielectric were fabricated for low-voltage-driven organic phototransistors, which integrate the photodetection and memory properties within one single device.

Probing the surface viscoelasticity of polymer films

Glass transition temperature, one of the most important properties of amorphous polymers in many technological applications, exhibits deviation from bulk value for nanoconfined polymer thin films. Glass transition temperature deviation due to enhanced/restricted polymer chain dynamics at the polymer surface or substrate interface has been probed by many different experimental techniques. In this paper, we describe recent experimental techniques which could probe the surface glass transition temperature of polymer thin films, their advantages and limitations, and organic thin-film transistor as a new tool to probe the surface viscoelasticity of polymer films.