Sean Parkin

XABS2: an empirical absorption correction program

The XABS2 Fortran program calculates an empirical absorption correction based on minimization of the 2 differences between F2obs and F2calc. The basic algorithm has been used in the crystallography laboratory at the University of California, Davis for over a decade, in the form of the program XABS. XABS relied upon an approximately linear relationship for spherical crystals between the transmission factor and sin2 θ for θ ≃ 30° when μR ≃ 5. In XABS2, deviations from linearity are accounted for by a cubic equation in sin2 θ, which makes it applicable over the whole range of diffraction angle. The program needs no data in addition to the unique set but assumes a bisecting (symmetric) data-collection mode on a four-circle diffractometer. Since it does not require knowledge of the diffractometer setting angles, it can be applied even in cases where details of the original data collection are unknown.

Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits

The use of organic materials presents a tremendous opportunity to significantly impact the functionality and pervasiveness of large-area electronics. Commercialization of this technology requires reduction in manufacturing costs by exploiting inexpensive low-temperature deposition and patterning techniques, which typically lead to lower device performance. We report a low-cost approach to control the microstructure of solution-cast acene-based organic thin films through modification of interfacial chemistry. Chemically and selectively tailoring the source/drain contact interface is a novel route to initiating the crystallization of soluble organic semiconductors, leading to the growth on opposing contacts of crystalline films that extend into the transistor channel. This selective crystallization enables us to fabricate high-performance organic thin-film transistors and circuits, and to deterministically study the influence of the microstructure on the device characteristics. By connecting device fabrication to molecular design, we demonstrate that rapid film processing under ambient room conditions and high performance are not mutually exclusive.

Chromophore Fluorination Enhances Crystallization and Stability of Soluble Anthradithiophene Semiconductors

We report dramatic improvements in the stability and crystallinity arising from partial fluorination of soluble anthradithiophene derivatives. These fluorinated materials still behave as p-type semiconductors but with dramatic increases in thermal and photostability compared to the non-fluorinated derivatives. The triethylsilyl-substituted material forms highly crystalline films even from spin-cast solutions, leading to devices with maximum hole mobility greater than 1.0 cm(2)/V s. In contrast, the triisopropylsilyl derivative forms large, high-quality crystals that could serve as the substrate for transistor fabrication. For this compound, mobility as high as 0.1 cm(2)/V s was measured on the free-standing crystal.

Synthesis and Structural Characterization of Crystalline Nonacenes

these linearly fused hydro-carbons have made significant contributions to the under-standing of electronic processes in organic semiconductors.Despite the utility of these compounds, the selection ofmaterials suitable for exploration essentially stops at penta-cene. Although numerous studies have predicted enticingelectronic properties for larger acenes,

Alkaloids from the antarctic sponge Kirkpatrickia varialosa. : Part 1: Variolin b, a new antitumour and antiviral compound

Abstract Variolin B (1), a new type of pyridopyrrolopyrimidine alkaloid with antitumour and antiviral properties, has been isolated from the Antarctic sponge Kirkpatrickia varialosa, and its structure determined by X-ray crystallography. A degradation product, variolin D (2), has been identified from its spectroscopic data.

A survey of electron-deficient pentacenes as acceptors in polymer bulk heterojunction solar cells

We have prepared, characterized and surveyed device performance for a series of electron deficient pentacenes for use as acceptors in polymer bulk heterojunction solar cells, using P3HT as the donor material. All of the materials reported here behaved as acceptors, and variations in the position and nature of the electron-withdrawing group on the pentacene core allowed tuning of device open-circuit voltage. Photocurrent was strongly correlated with the pentacene crystal packing motif; materials with 2D π-stacking interactions performed poorly compared with materials exhibiting 1D π-stacking interactions. The best pentacene acceptors gave repeatable device efficiency in excess of 1.2%, compared with 3.5% exhibited for PCBM-based devices.

Highly Fluorinated Benzobisbenzothiophenes

Expedient, facile syntheses of highly fluorinated benzobisbenzothiophenes (BBBT) are reported. Defined peripheral arrangements of sulfur and fluorine atoms lead to extensive crystalline networks of edge-to-edge S-F close contacts. The effects of various substitution patterns on self-assembly and electronic properties are described.

Optical and transient photoconductive properties of pentacene and functionalized pentacene thin films: Dependence on film morphology

We present a comprehensive study of the optical and transient photoconductive properties of pentacene and functionalized pentacene thin films grown by evaporation or from solution onto a variety of substrates. The transient photoconductivity was studied over picosecond time scales using time-resolved terahertz pulse spectroscopy. The structure and morphology of the films were assessed using x-ray diffraction, atomic force microscopy, and scanning electron microscopy. Regular pentacene films grown by evaporation under similar conditions but on different substrates yielded polycrystalline films with similar morphology and similar optical and transient photoconductive properties. Single exponential or biexponential decay dynamics was observed in all of the regular pentacene films studied. Functionalized pentacene films grown by evaporation at two different substrate temperatures (as well as from solution) yielded significant variations in morphology, resulting in different optical-absorption spectra and tran...

High Mobility Field‐Effect Transistors with Versatile Processing from a Small‐Molecule Organic Semiconductor

Trialkylgermyl functionalization allows the development of high-performance soluble small-molecule organic semiconductors with mobilities greater than 5 cm(2) V(-1) s(-1) . Spray-deposited organic thin-film transistors show a record mobility of 2.2 cm(2) V(-1) s(-1) and demonstrate the potential for incorporation in large-area, low-cost electronic applications.

Soluble n-type pentacene derivatives as novel acceptors for organic solar cells

6,13-Bis(triisopropylsilylethynyl) (TIPS)-pentacene has proven to be a promising soluble p-type material for organic thin film transistors as well as for photovoltaics. In this work, we show that adding electron-withdrawing nitrile functional groups to TIPS-pentacene turns it into an n-type material, which can be used as an acceptor for organic solar cells. Several new cyanopentacenes with different trialkylsilyl functional groups have been synthesized. The HOMO–LUMO energy levels can be tuned by varying the number of nitrile groups, while the trialkylsilyl groups control crystal packing and film morphology. Solar cells were fabricated from a blend of poly(3-hexylthiophene) (P3HT) as the donor and the cyanopentacenes as acceptors, and we found that the acceptors that stack in a 1D “sandwich-herringbone” exhibited the best performance of derivatives in this study. A solar cell fabricated from a blend of P3HT and 2,3-dicyano-6,13-bis-(tricyclopentylsilylethynyl)pentacene (2,3-CN2-TCPS-Pn) exhibited a power conversion efficiency of 0.43% under 100 mW cm−2AM 1.5 illumination.