Collapse of homeotropic liquid-crystal alignment by increased molecular packing on comb-like polymer surfaces.

We report an unusual alignment behavior of liquid crystals (LCs) on well-ordered comb-like poly(oxyethylene) surfaces. The homeotropic LC alignments that are observed on as-coated surfaces of the polymers are transformed to the random planar type after annealing treatment, even though the molecular structure of the polymer surface becomes more ordered and the surface energy decreases. Studies of the surface properties, such as molecular structure, morphology, and wettability, reveal that such an unexpected alteration of the LC alignment originates from the density of the alkyl side chains being enhanced by localized packing.

Thieno[3,2-b]thiophene-diketopyrrolopyrrole-containing polymers for high-performance organic field-effect transistors and organic photovoltaic devices.

We report the synthesis and polymerization of a novel thieno[3,2-b]thiophene-diketopyrrolopyrrole-based monomer. Copolymerization with thiophene afforded a polymer with a maximum hole mobility of 1.95 cm(2) V(-1) s(-1), which is the highest mobility from a polymer-based OFET reported to date. Bulk-heterojunction solar cells comprising this polymer and PC(71)BM gave a power conversion efficiency of 5.4%.

Synthesis of a novel fused thiophene-thieno[3,2-b]thiophene-thiophene donor monomer and co-polymer for use in OPV and OFETs.

The synthesis of a novel fused hexacyclic electron rich monomer incorporating thieno[3,2-b]thiophene is reported and characterized by single crystal X-ray diffraction. Suzuki co-polymerization with benzothiadiazole (BT) afforded a novel low band-gap polymer P4TBT with high molecular weights and good solution processability. Bulk heterojunction solar cell devices using the P4TBT and [70]PCBM gave power conversion efficiencies of 2.5%. Top-gate, bottom-contact field effect transistors (FETs) using P4TBT displayed high hole mobilities of 0.07 cm(2) · Vs(-1) demonstrating the suitability of the novel monomer and polymer for use in high performing organic electronic devices.

Systematic improvement in charge carrier mobility of air stable triarylamine copolymers.

The electrical performance of organic semiconducting polymers in field-effect transistor devices is now sufficient for initial low complexity circuit applications. To achieve high performance, either operation in an inert atmosphere or a hydrophobic surface treatment and annealing step is typically required. In this communication we report a strategy to prepare fully air stable, amorphous semiconducting polymers which can achieve charge carrier mobilities in the range of 0.04 cm(2)/(V s), remaining over a period of over 3 months in ambient conditions.

Photochromic optical memory based on non-destructive IR read-out.

The photochromic material 1-(2,5-dimethylthien-1,1-dioxide-3-yl)-2-(2,5-dimethylthien-3-yl)hexafluorocyclopentene (DMTFO2) undergoes reversible photocyclization and ring opening reactions upon alternate irradiation with UV and visible light, respectively. Upon photocyclization, the closed-ring isomer of DMTFO2 showed a strong IR absorption band at 1532 cm(-1) which was not observed for the open-ring isomer. To understand the IR spectra of DMTFO2, theoretical calculations were carried out using Gaussian 03 program at the B3LYP/6-31G(d) level. We also demonstrated non-destructive readout of photochromic recording by IR spectral difference between the open- and closed-ring isomers of DMTFO2 in film. We found that DMTFO2 exhibited a higher contrast ratio of IR image between open- and closed-ring isomer compared with previously reported 1,2-bis(2,5-dimethylthien-3-yl)perfluorocyclopentene (DMTF6) because of the increased photoconversion yield as well as higher vibrational transition intensity. These findings along with its high fatigue resistance indicate that DMTFO2 could be an excellent candidate for photon-mode RW (re-writable) memory devices based on photochromic recording and non-destructive IR read-out.

Kerr constant and third-order nonlinear optic susceptibility measurements in a liquid crystal composed of bent-shaped molecules.

We report the determination of the Kerr constant (B) and the real part of the third-order nonlinear optic susceptibility (chi(3)) above the nematic-isotropic phase transition temperature (TNI) of a liquid crystal composed of bent-shaped molecules. The values of B and chi(3) just above (approximately 0.3 degrees C) TNI are approximately 8x10(-12) m/V2 and 5x10(-20) m2/V2, respectively. The estimated critical temperature TC* is about 1.5 degrees C below TNI indicating that the nematic-isotropic (NI) transition is weakly first order as in the case of calamitic liquid crystals. The temperature-dependent Kerr constant is found to be in good agreement with the predictions of the Landau-de Gennes theory. The experimental results are compared with those in a calamitic liquid crystal material with negative dielectric anisotropy in the nematic phase.

Improved field-effect transistor performance of a benzotrithiophene polymer through ketal cleavage in the solid state.

A benzotrithiophene polymer with a new thermally cleavable ketal substituent is reported. It is shown how this functional group can be used to facilitate solvent processing and, subsequently, how it can be removed by a thermal annealing process to generate a structurally ordered and crystalline thin film with significantly improved field-effect transistor properties.

Molecular origin of high field-effect mobility in an indacenodithiophene-benzothiadiazole copolymer.

One of the most inspiring and puzzling developments in the organic electronics community in the last few years has been the emergence of solution-processable semiconducting polymers that lack significant long-range order but outperform the best, high-mobility, ordered semiconducting polymers to date. Here we provide new insights into the charge-transport mechanism in semiconducting polymers and offer new molecular design guidelines by examining a state-of-the-art indacenodithiophene-benzothiadiazole copolymer having field-effect mobility of up to 3.6 cm(2) V(-1) s(-1) with a combination of diffraction and polarizing spectroscopic techniques. Our results reveal that its conjugated planes exhibit a common, comprehensive orientation in both the non-crystalline regions and the ordered crystallites, which is likely to originate from its superior backbone rigidity. We argue that charge transport in high-mobility semiconducting polymers is quasi one-dimensional, that is, predominantly occurring along the backbone, and requires only occasional intermolecular hopping through short π-stacking bridges.

Surface characterization and liquid crystal alignment behavior of comb-like poly(oxyethylene)/poly(3-hexylthiophene) blend films.

The blend surfaces of poly[oxy(n-decylsulfonylmethyl)ethylene] (CH(3)-10SE) and poly (3-hexylthiophene) (P3HT) with different weight ratios were prepared by spin coating the polymer solution mixtures. In this study, their surface properties such as surface morphology, chemical composition, molecular structure, and wettability were systematically studied and correlated with liquid crystal (LC) alignment behaviors on the blend films. Therefore, we found that CH(3)-10SE part with a well-ordered side chain structure predominantly affects the both of wettability and LC alignment behavior of the blend films while there was no clear association between the wettability and the LC alignment behavior.

Random benzotrithiophene-based donor-acceptor copolymers for efficient organic photovoltaic devices.

A series of benzotrithiophene-containing random terpolymers for polymer solar cells is reported. Through variations of the two other components in the terpolymers, the absorption profile and the frontier energy levels are optimized and maximum power conversion efficiencies are nearly doubled (5.14%) relative to the parent alternating copolymer.

Surface properties and liquid crystal alignment behavior of poly(2-hydroxyethyl methacrylate) derivatives with alkyl ester side chains.

Poly(2-hydroxyethyl methacrylate) derivatives with amphiphilic side chains composed of polar ester and non-polar alkyl groups (PHEMA#C, #=9, 11, 13, 15, and 17), where # is the number of carbon atoms in the alkyl side groups, were synthesized. In this paper, the influence of ester and alkyl groups on the molecular structure and wettability of the polymers were studied through varying # in the alkyl side groups. PHEMA#Cs with relatively longer alkyl side groups (#≥15) show bilayer lamellar structures with well aligned side chains giving rise to the very low surface energies, calculated from advancing contact angles, in the range of 22.7-22.8 mN/m. In contrast, PHEMA#Cs with shorter alkyl side groups with #≤13 have disordered structures on the polymer surfaces and stick-slip behavior was observed when water was used as the test liquid for the advancing contact angle measurements. Furthermore, the alignment behavior of nematic liquid crystal, 5CB on the PHEMA#C films could be correlated with the molecular structure and wettability of the polymers.