Heteroarenes as high performance organic semiconductors

In order to probe the effects of substituents (F and CN) attached to benzo[1,2-b:3,4-[Formula: see text]:5,6-[Formula: see text]]tristhianaphthene (BTTP) on their charge carrier transport properties, we investigated the characteristics of molecular structures and charge transport properties of BTTP and its derivatives (BTTP1, BTTP2, BTTP3, BTTP4, and BTTP5). Six crystal structures were predicted by the Monte Carlo-simulated annealing method with the embedded electrostatic potential charges method. Even a subtle change of geometrical structures may result in a great change of the reorganization energy. With increasing numbers of substituted fluorine atoms, the reorganization energy of the BTTP derivative increases, which is disadvantageous to the electron transport. In contrast, the attachment of the electron-withdrawing cyano groups to BTTP decreases the reorganization energy and raises the electron affinity, which is beneficial to electron injection and charge carrier stabilization. The introduction of cyano groups also results in an enhancement of [Formula: see text]–[Formula: see text] interaction and leads to an increase in the transfer integrals. Among the six compounds, the novel compound BTTP4 has the largest electron mobility (1.154[Formula: see text]cm[Formula: see text]) on account of its larger transfer integral and smaller reorganization energy, indicating that BTTP4 is a promising high-performance n-type organic semiconductor and worth to synthesize. The analysis of angular-resolution anisotropic mobilities for the BTTP and BTTP4 shows that it is helpful to control the orientations of the conducting channels for a better charge transport efficiency. This work provides a rational strategy for the design of high-performance n-type organic semiconductors from molecule to crystal structure.

Highly planar conformation is considered to be one of the most important properties for high performance organic semiconductors. Among all kinds strategies for designing highly performing materials, noncovalent conformational locks (NCLs) have been widely used to increase the planarity and rigidity for π-conjugated systems. This review summarizes π-conjugated small molecules and polymers by employing various NCLs for controlling molecular conformation in the past two years. The optoelectronic properties of the conjugated materials, together with their applications on organic field-effect transistors (OFETs) and organic photovoltaics (OPVs) are discussed. Besides, the outlook and challenges in this field are also presented. It is obvious that NCLs play an important role in the design and synthesis of high-performance organic semiconductors.

ConspectusOrganic electronics has experienced substantial advances in the past decade, driven by the development of high-performance organic semiconductors (OSCs) in combination with device engineering. While the pursuit of new aromatic building blocks has been a central topic in OSC innovation, the installation of novel side chains is also of significance for accessing high-performance solution-processable OSCs due to their great impact on (macro)molecular conformation/configuration, energy levels, intra/intermolecular interaction, and packing motifs, as well as film morphology of the materials.Compared to tuning the length, branching point, anchoring position, and terminal group of alkyl side chains, alkoxy functionalization can afford multifaceted advantages by modulating the properties of both π-conjugated main chains and side chain substituents. For instance, the oxygen atom in alkoxy chains not only greatly decreases the steric hindrance between adjacent aromatic rings due to its reduced van der Waal radius (∼1.4 Å) versus that of a CH2 moiety (∼2.0 Å) but also induces intramolecular noncovalent interaction for improving backbone coplanarity and charge transport properties. The highly electron-donating alkoxy chains can also greatly facilitate the intramolecular charge transfer (ICT), and hence the resulting semiconductors can yield absorption beyond the infrared region, which is essential for full coverage of solar absorption in photovoltaic devices. Additionally, the high polarity of oligo(ethylene glycol)-type alkoxy side chains can improve the miscibility of the OSCs with molecular dopants and ions, thus enabling them as a key part of the OSCs for emerging applications such as organic thermoelectrics and organic electrochemical transistors.In this Account, we summarize our pioneering and systematic efforts on the rational design, synthesis, and applications of novel alkoxy-functionalized head-to-head (HH)-linked bithiophene/thiazole-based building blocks and related organic/polymeric semiconductors. First, starting with a brief retrospective to the long pursuit of regioregular polythiophenes to avoid HH linkages for accessing highly planar polymers with high mobility, we introduce the basic design guidelines for developing alkoxy-functionalized bithiophene/thiazole-based building blocks via various molecular engineering strategies, including noncovalent interaction incorporation, symmetry-breaking, fluorination, cyanation, esterification, etc. In this part, the merits of HH-linked alkoxy-functionalized bithiophene/thiazoles in constructing high-performance OSCs will be elaborated. Then the principles of designing organic and polymeric semiconductors based on these building blocks toward applications in different optoelectronic devices will be further discussed. Afterward, we present recent examples of alkoxy-functionalized bithiophene/thiazoles-based materials which have delivered state-of-the-art performance in various optoelectronic devices, showing how the judicious structure tailoring of these building blocks can optimize the materials properties and device performance. Finally, we offer our insights into the further development of alkoxy-functionalized bithiophene/thiazoles-based derivatives and semiconductors for high-performance optoelectronic devices.

Computational screening and molecular design of anthracene-based semiconductors

The purpose of this feature article is to give an overview of recent advances in development of high performance organic semiconductors for field-effect transistors, especially those with mobility of/over amorphous silicon, since they are believed to be promising candidates with practical applications in the near future's organic electronic industry. We hope this comprehensive summary of high performance organic semiconductors will provide guidelines for the design and synthesis of novel, high performance organic field-effect semiconductors.

The primary goal of the field concerned with organic semiconductors is to produce devices with performance approaching that of silicon electronics, but with the deformability—flexibility and stretchability—of conventional plastics. However, an inherent competition between deformability and charge transport has long been observed in these materials, and achieving the extreme (or even moderate) deformability implied by the word “plastic” concurrently with high charge transport may be elusive. This competition arises because the properties needed for high carrier mobilities—e.g., rigid chains in π-conjugated polymers and high degrees of crystallinity in the solid state—are antithetical to deformability. On the device scale, this competition can lead to low-performance yet mechanically robust devices, or high-performance devices that fail catastrophically (e.g., cracking, cohesive failure, and delamination) under strain. There are, however, some observations that contradict the notion of the mutual exclusivity of electronic and mechanical performances. These observations suggest that this problem may not be a fundamental trade-off, but rather an inconvenience that may be negotiated by a logical selection of materials and processing conditions. For example, the selection of the poly(3-alkylthiophene) with a critical side-chain length—poly(3-heptylthiophene) (n = 7)—marries the high deformability of poly(3-octylthiophene) (n = 8) with the high electronic performance (as manifested in photovoltaic efficiency) of poly(3-hexylthiophene) (n = 6). This review explores the relationship between deformability and charge transport in organic semiconductors. The principal conclusions are that reducing the competition between these two parameters is in fact possible, with two demonstrated routes being: (1) incorporation of softer, insulating material into a stiffer, semiconducting material and (2) increasing disorder in a highly ordered film, but not enough to disrupt charge transport pathways. The aim of this review is to provide a bridge between the fields interested in electronic properties and mechanical properties of conjugated polymers. We provide a high-level introduction to some of the important electronic and mechanical properties and measurement techniques for organic electronic devices, demonstrate an apparent competition between good electronic performance and mechanical deformability, and highlight potential strategies for overcoming this undesirable competition. A marriage of these two fields would allow for rational design of materials for applications requiring large-area, low-cost, printable devices that are ultra-flexible or stretchable, such as organic photovoltaic devices and wearable, conformable, or implantable sensors.

Organic Single Crystals: An Essential Step to New Physics and Higher Performances of Optoelectronic Devices

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Significant progress has been made in both molecular design and fundamental scientific understanding of organic semiconductors (OSCs) in recent years. Suitable charge-carrier mobilities (μ) have been obtained by many high-performance OSCs (μ > 10 cm2 V-1 s-1), but drawbacks remain, including low solution processability and poor thermal durability. In addition, since aggregation of OSCs involves weak intermolecular interactions, the molecules are perpetually in thermal motion, even in the solid state, which disrupts charge-carrier transport. These issues limit potential applications of OSCs. The present work examines a molecular design for hole-transporting (p-type) OSCs based on the "bent-shaped" geometry with specific molecular orbital configurations, which aims to enhance effective intermolecular orbital overlaps, stabilize crystal phases, suppress detrimental molecular motions in the solid state, and improve solution processability. The results indicated that such OSCs have high μ and suitable solution processability, and are resistant to ambient and thermal conditions, making them suitable for practical applications.

We have succeeded in developing high-performance p-type of organic semiconductors with phenylethynyl groups, which have high filed-effect mobilities (> 3cm2V-1s-1) by improving molecular planarity. A single crystal of the organic semiconductors has a herringbone structure. It plays an important role for carrier transport. In addition, we found that they had lower contact resistances to Au electrodes as well. Then, we used the materials for the carrier injection layer deposited onto another organic semiconductor we developed recently, which achieved a high field-effect mobility, and a low threshold voltage (Vth).

Tailoring π-Conjugated Systems: From π-π Stacking to High-Rate-Performance Organic Cathodes

Here we discuss requirements for high performance and solution processable organic semiconductors, by presenting a systematic investigation of 7-alkyl-2-phenyl[1]benzothieno[3,2-b][1]benzothiophenes (Ph-BTBT-C n ’s). We found that the solubility and thermal properties of Ph-BTBT-C n ’s depend systematically on the substituted alkyl-chain length n. The observed features are well understood in terms of the change of molecular packing motif with n: The compounds with n ≤ 4 do not form independent alkyl chain layers, whereas those with n ≥ 5 form isolated alkyl chain layers. The latter compounds afford a series of isomorphous bilayer-type crystal structures that form two-dimensional carrier transport layers within the crystals. We also show that the Ph-BTBT-C10 afford high performance single-crystalline field-effect transistors the mobility of which reaches as high as 15.9 cm2/Vs. These results demonstrate a crucial role of the substituted alkyl chain length for obtaining high performance organic semiconductors and field-effect transistors.

Organic electronics devices based on high-performance small-molecule organic semiconductors have gained substantial attention because of their unique advantages such as excellent charge transport, solution processability, and environmental stability. However, the intrinsic crystallization of small-molecule organic semiconductors is anisotropic, resulting in significant device performance variations of organic electronics devices. In this article, the authors review the various approaches and techniques developed to control and align the crystallization of some benchmark solution-processable, high-performance, small-molecule organic semiconductors, such as 6,13-bis(triisopropylsilylethynyl) pentacene, N,N′-1H,1H-perfluorobutyl dicyanoperylenecarboxydiimide, and 5,11-bis(triethylgermylethynyl)anthradithiophene. These alignment approaches are studied in the context of capillary force-based techniques, patterning-based techniques, solution-shearing-based techniques, and other miscellaneous techniques, including zone-casting, vertical flowing, air flow navigation, temperature gradient alignment, etc. The organic semiconductors and crystal alignment techniques reviewed in this article shed light on important relationship among crystallization, charge transport, and device performance and can be applied to various high-performance organic electronics devices, such as organic thin film transistors and solar cells.

Theoretical investigations on electronic and charge transport properties of novel organic semiconductors – Triisopropylsilylethynyl(TIPS)-functionalized anthradifuran and anthradithiophene derivatives

Molecular crystals and molecules: A. I. Kitaigorodsky. Volume 29 of Physical Chemistry, Edited by Ernest M. Loebl. Pp. 553. Academic Press, New York, 1973. Price: $44.00