Alejandro L. Briseno

Patterning organic single-crystal transistor arrays

Field-effect transistors made of organic single crystals are ideal for studying the charge transport characteristics of organic semiconductor materials. Their outstanding device performance, relative to that of transistors made of organic thin films, makes them also attractive candidates for electronic applications such as active matrix displays and sensor arrays. These applications require minimal cross-talk between neighbouring devices. In the case of thin film systems, simple patterning of the active semiconductor layer minimizes cross-talk. But when using organic single crystals, the only approach currently available for creating arrays of separate devices is manual selection and placing of individual crystals—a process prohibitive for producing devices at high density and with reasonable throughput. In contrast, inorganic crystals have been grown in extended arrays, and efficient and large-area fabrication of silicon crystalline islands with high mobilities for electronic applications has been reported. Here we describe a method for effectively fabricating large arrays of single crystals of a wide range of organic semiconductor materials directly onto transistor source–drain electrodes. We find that film domains of octadecyltriethoxysilane microcontact-printed onto either clean Si/SiO2 surfaces or flexible plastic provide control over the nucleation of vapour-grown organic single crystals. This allows us to fabricate large arrays of high-performance organic single-crystal field-effect transistors with mobilities as high as 2.4 cm2 V-1 s-1 and on/off ratios greater than 107, and devices on flexible substrates that retain their performance after significant bending. These results suggest that our fabrication approach constitutes a promising step that might ultimately allow us to utilize high-performance organic single-crystal field-effect transistors for large-area electronics applications.

Oligo- and Polythiophene/ZnO Hybrid Nanowire Solar Cells

We demonstrate the basic operation of an organic/inorganic hybrid single nanowire solar cell. End-functionalized oligo- and polythiophenes were grafted onto ZnO nanowires to produce p-n heterojunction nanowires. The hybrid nanostructures were characterized via absorption and electron microscopy to determine the optoelectronic properties and to probe the morphology at the organic/inorganic interface. Individual nanowire solar cell devices exhibited well-resolved characteristics with efficiencies as high as 0.036%, J(sc) = 0.32 mA/cm(2), V(oc) = 0.4 V, and a FF = 0.28 under AM 1.5 illumination with 100 mW/cm(2) light intensity. These individual test structures will enable detailed analysis to be carried out in areas that have been difficult to study in bulk heterojunction devices.

Introducing organic nanowire transistors

Organic nanowires self-assembled from small-molecule semiconductors and conducting polymers have attracted an enormous amount of interest for use in organic field-effect transistors. This new class of materials offers solution processability, the potential for elucidating transport mechanisms and structure-property relationships, and the realization of high-performance transistors that rival the performance of amorphous Si. We discuss the self-assembly of one-dimensional, single-crystalline organic nanowires, show the structures of commonly employed organic semiconductors, and review some of the advances in this field.

Efficient Polymer Solar Cells Based on a Low Bandgap Semi‐crystalline DPP Polymer‐PCBM Blends

Solar cell performance and morphology characterization of a diketopyrrolopyrrole-based low bandgap polymer is reported. The polymer adopts an H-type aggregation and solvent mixture processing gives a better morphology. The morphology evolution is characterized by combined GIXD and GISAXS experiments and a four step morphology development mechanism is proposed.

Polymer semiconductor crystals

One of the long-standing challenges in the field of polymer semiconductors is to figure out how long interpenetrating and entangled polymer chains self-assemble into single crystals from the solution phase or melt. The ability to produce these crystalline solids has fascinated scientists from a broad range of backgrounds including physicists, chemists, and engineers. Scientists are still on the hunt for determining the mechanism of crystallization in these information-rich materials. Understanding the theory and concept of crystallization of polymer semiconductors will undoubtedly transform this area from an art to an area that will host a bandwagon of scientists and engineers. In this article we describe the basic concept of crystallization and highlight some of the advances in polymer crystallization from crystals to nanocrystalline fibers.

Organic Single-Crystalline p−n Junction Nanoribbons

This article focuses on the growth and transport properties of organic single-crystalline p-n junction nanoribbons. The development of organic nanoelectronics requires the fabrication of organic nanometer-sized p-n junctions for high-performance devices and integrated circuits. Here we demonstrate the formation of single-crystalline p-n junction nanoribbons of organic semiconductors by selective crystallization of copper hexadecafluorophthalocyanine (F(16)CuPc, n-type) on copper phthalocyanine (CuPc, p-type) single-crystalline nanoribbons. The crystallization of F(16)CuPc onto CuPc requires several parameters, including similar molecular structures, similar lattice constants, and pi-stacking along the nanoribbon axis. Ambipolar transport of the p-n junction nanoribbons was observed in field-effect transistors with balanced carrier mobilities of 0.05 and 0.07 cm(2) V(-1) s(-1) for F(16)CuPc and CuPc, respectively. A basic p-n junction nanoribbon photovoltaic device showed current rectification under AM 1.5 simulated light. The discrete p-n junction nanoribbons may serve as ideal systems for understanding basic charge-transport and photovoltaic behaviors at organic-organic interfaces.

Oligothiophene semiconductors: synthesis, characterization, and applications for organic devices.

Oligothiophenes provide a highly controlled and adaptable platform to explore various synthetic, morphologic, and electronic relationships in organic semiconductor systems. These short-chain systems serve as models for establishing valuable structure-property relationships to their polymer analogs. In contrast to their polymer counterparts, oligothiophenes afford high-purity and well-defined materials that can be easily modified with a variety of functional groups. Recent work by a number of research groups has revealed functionalized oligothiophenes to be the up-and-coming generation of advanced materials for organic electronic devices. In this review, we discuss the synthesis and characterization of linear oligothiophenes with a focus on applications in organic field effect transistors and organic photovoltaics. We will highlight key structural parameters, such as crystal packing, intermolecular interactions, polymorphism, and energy levels, which in turn define the device performance.

Additive-driven assembly of block copolymer-nanoparticle hybrid materials for solution processable floating gate memory.

Floating gate memory devices were fabricated using well-ordered gold nanoparticle/block copolymer hybrid films as the charge trapping layers, SiO(2) as the dielectric layer, and poly(3-hexylthiophene) as the semiconductor layer. The charge trapping layer was prepared via self-assembly. The addition of Au nanoparticles that selectively hydrogen bond with pyridine in a poly(styrene-b-2-vinyl pyridine) block copolymer yields well-ordered hybrid materials at Au nanoparticle loadings up to 40 wt %. The characteristics of the memory window were tuned by simple control of the Au nanoparticle concentration. This approach enables the fabrication of well-ordered charge storage layers by solution processing, which is extendable for the fabrications of large area and high density devices via roll-to-roll processing.

High Mobility Single-Crystal Field-Effect Transistors from Bisindoloquinoline Semiconductors

A novel heptacyclic bisindoloquinoline-based organic semiconductor has been synthesized, characterized, and used to fabricate single-crystal field-effect transistors. A synthetic route was developed for the synthesis of heptacyclic bis(indolo{1,2-a})quinoline via an intramolecular cyclization of anthrazoline derivatives. Single-crystal X-ray structure revealed that the seven fused rings of bis(indolo{1,2-a})quinoline are relatively coplanar and lead to a slipped face-to-face π-stacking with the shortest intermolecular spacing of 3.3 A. Single-crystal field-effect transistors based on the bis(indolo{1,2-a})quinoline had carrier mobility as high as 1.0 cm2/V·s with on/off ratios greater than 104.

Unconventional, Chemically Stable, and Soluble Two-Dimensional Angular Polycyclic Aromatic Hydrocarbons: From Molecular Design to Device Applications

Polycyclic aromatic hydrocarbons (PAHs), consisting of laterally fused benzene rings, are among the most widely studied small-molecule organic semiconductors, with potential applications in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs). Linear acenes, including tetracene, pentacene, and their derivatives, have received particular attention due to the synthetic flexibility in tuning their chemical structure and properties and to their high device performance. Unfortunately, longer acenes, which could exhibit even better performance, are susceptible to oxidation, photodegradation, and, in solar cells which contain fullerenes, Diels-Alder reactions. This Account highlights recent advances in the molecular design of two-dimensional (2-D) PAHs that combine device performance with environmental stability. New synthetic techniques have been developed to create stable PAHs that extend conjugation in two dimensions. The stability of these novel compounds is consistent with Clars sextet rule as the 2-D PAHs have greater numbers of sextets in their ground-state configuration than their linear analogues. The ionization potentials (IPs) of nonlinear acenes decrease more slowly with annellation in comparison to their linear counterparts. As a result, 2-D bistetracene derivatives that are composed of eight fused benzene rings are measured to be about 200 times more stable in chlorinated organic solvents than pentacene derivatives with only five fused rings. Single crystals of the bistetracene derivatives have hole mobilities, measured in OFET configuration, up to 6.1 cm(2) V(-1) s(-1), with remarkable Ion/Ioff ratios of 10(7). The density functional theory (DFT) calculations can provide insight into the electronic structures at both molecular and material levels and to evaluate the main charge-transport parameters. The 2-D acenes with large aspect ratios and appropriate substituents have the potential to provide favorable interstack electronic interactions, and correspondingly high carrier mobilities. In stark contrast to the 1-D acenes that form mono- and bis-adducts with fullerenes, 2-D PAHs show less reactivity with fullerenes. The geometry of 2-D PAHs plays a crucial role in determining both the barrier and the adduct stability. The reactivity and stability of the 2-D PAHs with regard to Diels-Alder reactions at different reactive sites were explained via DFT calculations of the reaction kinetics and of thermodynamics of reactions and simple Hückel molecular orbital considerations. Also, because of their increased stability in the presence of fullerenes, these compounds have been successfully used in OPVs. The small-molecule semiconductors highlighted in this Account exhibit good charge-transport properties, comparable to those of traditional linear acenes, while being much more environmentally stable. These features have made these 2-D PAHs excellent molecules for fundamental research and device applications.