2.2. Materials 929 2.3. Factors Influencing Charge Mobility 931 2.3.1. Molecular Packing 931 2.3.2. Disorder 932 2.3.3. Temperature 933 2.3.4. Electric Field 934 2.3.5. Impurities 934 2.3.6. Pressure 934 2.3.7. Charge-Carrier Density 934 2.3.8. Size/molecular Weight 935 3. The Charge-Transport Parameters 935 3.1. Electronic Coupling 936 3.1.1. The Energy-Splitting-in-Dimer Method 936 3.1.2. The Orthogonality Issue 937 3.1.3. Impact of the Site Energy 937 3.1.4. Electronic Coupling in Oligoacene Derivatives 938

Charge transport in organic semiconductors.

Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

Organic semiconducting oligomers for use in thin film transistors.

Field-effect transistors based on solution-processible organic semiconductors have experienced impressive improvements in both performance and reliability in recent years, and printing-based manufacturing processes for integrated transistor circuits are being developed to realize low-cost, large-area electronic products on flexible substrates. This article reviews the materials, charge-transport, and device physics of solution-processed organic field-effect transistors, focusing in particular on the physics of the active semiconductor/dielectric interface. Issues such as the relationship between microstructure and charge transport, the critical role of the gate dielectric, the influence of polaronic relaxation and disorder effects on charge transport, charge-injection mechanisms, and the current understanding of mechanisms for charge trapping are reviewed. Many interesting questions on how the molecular and electronic structures and the presence of defects at organic/organic heterointerfaces influence the device performance and stability remain to be explored.

Device Physics of Solution‐Processed Organic Field‐Effect Transistors

Color Control in π-Conjugated Organic Polymers for Use in Electrochromic Devices

Printed electronics provides a promising potential pathway toward the realization of ultralow-cost RFID tags for item-level tracking of consumer goods. Here, we report on our progress in developing materials, processes, and devices for the realization of ultralow-cost printed RFID tags. Using printed nanoparticle patterns that are subsequently sintered at plastic-compatible temperatures, low-resistance interconnects and passive components have been realized. Simultaneously, printed transistors with mobilities >10/sup -1/ cm/sup 2//V-s have been realized using novel pentacene and oligothiophene precursors for pMOS and ZnO nanoparticles for nMOS. AC performance of these devices is adequate for 135-kHz RFID, though significant work remains to be done to achieve 13.56-MHz operation.

http://delab.csd.auth.gr/~dimitris/courses/mpc_fall05/presentation_papers/Proceedings_IEEE_July_05_rfid.pdf

Progress Toward Development of All-Printed RFID Tags: Materials, Processes, and Devices

A symmetrical α,ω-substituted sexithiophene derivative containing thermally removable branched ester solubilizing groups has been prepared. These oligomers can be solution cast into thin films and then thermolyzed to remove the solubilizing group, leaving short pendant alkene groups on the oligomer. Device testing of thin film transistors shows an increase in hole mobility from 1 × 10-5 cm2/(V s) with on/off ratios of ∼100 before thermolysis to 5 × 10-2 cm2/(V s) with on/off ratios >105 after thermolysis. This method offers an attractive route to easily processed and highly performing thiophene oligomers.

Organic thin film transistors from a soluble oligothiophene derivative containing thermally removable solubilizing groups.

The relationship between film morphology and thin film transistor (TFT) performance was investigated for two symmetrical α,ω-substituted sexithiophene derivatives containing thermally removable solubilizing groups. Solution deposition methods such as spin-coating, dip-casting, and inkjet-printing were optimized for solvent and annealing temperatures, and several substrate surface treatments were explored. The resulting thin films were characterized with AFM and the observed semiconductor performance was found to correlate with the morphology of the films, with the most crystalline films exhibiting the highest performance. Devices showed overall mobilities as high as 0.07 cm2/V s with on/off ratios > 108, which are among the highest reported values for oligothiophenes solution cast at room temperature.

Film Morphology and Thin Film Transistor Performance of Solution-Processed Oligothiophenes

Printed electronics holds promise for realizing ultra-low-cost RFID tags for item-level tracking of consumer goods. We report on our progress in developing all-printed RFID tags. We review the development of printable materials for these applications, summarize the characteristics of printed devices, and discuss the implications of these on circuit performance limits and needs. Based on this assessment, we discuss the outlook for all-printed RFID tags and identify the problems remaining to be solved and the efforts taking place in this regard.

All-printed RFID tags: materials, devices, and circuit implications

Embedded sensors based on printed organic semiconductors are attractive for use in product content monitoring due to their low cost. Arraying multiple sensor elements, in a bridge topology, yields signatures that achieve high specificity using non-specific elements. The output signal is amplified and digitized to detect numerous analytes with up to 10ppm sensitivity. A wine-spoilage application is demonstrated

J.B. Lee

Papers

Progress Toward Development of All-Printed RFID Tags: Materials, Processes, and Devices

Organic thin film transistors from a soluble oligothiophene derivative containing thermally removable solubilizing groups.

Film Morphology and Thin Film Transistor Performance of Solution-Processed Oligothiophenes

All-printed RFID tags: materials, devices, and circuit implications

Printed Electronic Nose Vapor Sensors for Consumer Product Monitoring