John E. Anthony

n‐Type Organic Semiconductors in Organic Electronics

Organic semiconductors have been the subject of intensive academic and commercial interest over the past two decades, and successful commercial devices incorporating them are slowly beginning to enter the market. Much of the focus has been on the development of hole transporting, or p-type, semiconductors that have seen a dramatic rise in performance over the last decade. Much less attention has been devoted to electron transporting, or so called n-type, materials, and in this paper we focus upon recent developments in several classes of n-type materials and the design guidelines used to develop them.

High mobility solution processed 6,13-bis(triisopropyl-silylethynyl) pentacene organic thin film transistors

Using the small molecule organic semiconductor 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene), the authors have fabricated the solution-processed organic thin film transistors (OTFTs) with carrier mobility >1cm2∕Vs, current on/off ratio greater than 107, and subthreshold slope <0.3V/decade. The high mobility TIPS-pentacene solution-processed films are deposited from high boiling point solvents and show strong molecular ordering including molecular terracing. Film ordering varies substantially for different solvents and film deposition techniques and OTFT mobility correlates well with film ordering.

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.

Photovoltaics from soluble small molecules

Solution-processable small molecules have attractive features for application in photovoltaic cells. They offer the facile processing associated with polymers, yet are easier to synthesize and purify, are monodisperse, and typically show higher charge carrier mobilities. Recent progress in solution-processable small molecule blends has yielded photovoltaic cells with efficiencies exceeding 1%. This article reviews progress in this nascent field and discusses the requirements imposed by the need for charge separation within an interpenetrating network, energy level tuning for light absorption and voltage output, and processing techniques to achieve phase separation on excitonic length scales. Design criteria for next-generation materials are provided.

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.

Ultra-flexible solution-processed organic field-effect transistors

Organic semiconductors might enable new applications in low-cost, light-weight, flexible electronics. To build a solid foundation for these technologies, more fundamental studies of electro-mechanical properties of various types of organic semiconductors are necessary. Here we perform basic studies of charge transport in highly crystalline solution-processed organic semiconductors as a function of applied mechanical strain. As a test bed, we use small molecules crystallized on thin plastic sheets, resulting in high-performance flexible field-effect transistors. These devices can be bent multiple times without degradation to a radius as small as ~200 μm, demonstrating that crystalline solution-processed organic semiconductors are intrinsically highly flexible. This study of electro-mechanical properties suggests that solution-processable organic semiconductors are suitable for applications in flexible electronics, provided that integration with other important technological advances, such as device scalability and low-voltage operation, is achieved in the future.

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,

Competition between Singlet Fission and Charge Separation in Solution-Processed Blend Films of 6,13-Bis(triisopropylsilylethynyl)pentacene with Sterically-Encumbered Perylene-3,4:9,10-bis(dicarboximide)s

The photophysics and morphology of thin films of N,N-bis(2,6-diisopropylphenyl)perylene-3,4:9,10-bis(dicarboximide) (1) and the 1,7-diphenyl (2) and 1,7-bis(3,5-di-tert-butylphenyl) (3) derivatives blended with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) were studied for their potential use as photoactive layers in organic photovoltaic (OPV) devices. Increasing the steric bulk of the 1,7-substituents of the perylene-3,4:9,10-bis(dicarboximide) (PDI) impedes aggregation in the solid state. Film characterization data using both atomic force microscopy and X-ray diffraction showed that decreasing the PDI aggregation by increasing the steric bulk in the order 1 < 2 < 3 correlates with a decrease in the density/size of crystalline TIPS-Pn domains. Transient absorption spectroscopy was performed on ~100 nm solution-processed TIPS-Pn:PDI blend films to characterize the charge separation dynamics. These results showed that selective excitation of the TIPS-Pn results in competition between ultrafast singlet fission ((1*)TIPS-Pn + TIPS-Pn → 2 (3*)TIPS-Pn) and charge transfer from (1*)TIPS-Pn to PDIs 1-3. As the blend films become more homogeneous across the series TIPS-Pn:PDI 1 → 2 → 3, charge separation becomes competitive with singlet fission. Ultrafast charge separation forms the geminate radical ion pair state (1)(TIPS-Pn(+•)-PDI(-•)) that undergoes radical pair intersystem crossing to form (3)(TIPS-Pn(+•)-PDI(-•)), which then undergoes charge recombination to yield either (3*)PDI or (3*)TIPS-Pn. Energy transfer from (3*)PDI to TIPS-Pn also yields (3*)TIPS-Pn. These results show that multiple pathways produce the (3*)TIPS-Pn state, so that OPV design strategies based on this system must utilize this triplet state for charge separation.

Anisotropic mobility in large grain size solution processed organic semiconductor thin films

The hollow pen method for writing thin films of materials from solution is utilized to deposit films of 6,13-bis(tri-isopropylsilylethynyl) pentacene (TIPS pentacene) onto SiO2 surfaces with pre-patterned source/drain gold contacts. We demonstrate that large domains are obtained for TIPS pentacene films deposited from 0.5–4.0wt% solutions with toluene. Crystalline grains with (001) orientation are observed to grow with sizes that can exceed 1mm along the writing direction. A preferred azimuthal orientation is also selected by the process, resulting in anisotropic field effect transistor mobility in the films.

Controlled Deposition of Highly Ordered Soluble Acene Thin Films: Effect of Morphology and Crystal Orientation on Transistor Performance

Controlling the morphology of soluble small molecule organic semiconductors is crucial for the application of such materials in electronic devices. Using a simple dip-coating process we systematically vary the film drying speed to produce a range of morphologies, including oriented needle-like crystals. Structural characterization as well as electrical transistor measurements show that intermediate drying velocities produce the most uniformly aligned films.