Dictionary of Terms
Our dictionary of terms is created for purpose of providing simplest possible explanations of terms used in organic electronics and within our site to people with various backgrounds. However, at least basic knowledge in organic chemistry, physics, and electronics is necessary in order to follow the information provided.
Search for terms: Table of content: Next information resources have been used: Books 1. Handbook of Conducting Polymers, 2nd edn., ed. T. A. Skotheim, R. L. Elsenbaumer, and J. R. Reynolds, Marcel Dekker, New York, 1998. 2. Handbook of Liquid Crystals, ed. D. Demus, J. W. Goodby, G. W. Gray, H.-W. Spiess, and V. Vill; Wiley-VCH: Weinheim, 1998. 3. Handbook of Oligo- and Polythiophenes, ed. D. Fichou, Wiley-VCH, Weinheim, 1999. 4. Handbook of Advanced Electronic and Photonic Materials and Devices; Nalwa, H. S., Ed.; Academic: San Diego, CA, 2001. 5. Thin-Film Transistors; Kagan, C. R.; Andry P., Eds.; Marcel Dekker: New York 2003. 6. Printed Organic and Molecular Electronics; Gamota, D. R.; Brazis, P.; Kalyanasundaram, K.; Zhang, J., Eds.; Kluwer Academic Publishers: New York 2004. 7. Semiconducting Polymers: Chemistry, Physics and Engineering; Hadziioannou, G.; van Hutten, P. F., Eds.; Wiley: Weinheim, 2000. Organic semiconductors, general: 1. Roncali, J. "Conjugated Poly(thiophenes): Synthesis, Functionalization, and Applications" Chem. Rev. 1992, 92, 711. 2. Andersson, M. R.; Thomas, O.; Mammo, W.; Svensson, M.; Theander, M.; Inganas, O. "Substituted Polythiophenes Designed for Optoelectronic Devices and Conductors" J. Mater. Chem.1999, 9, 1933. 3. Fichou, D. "Structural Order in Conjugated Oligothiophenes and its imlications on optoelectronic devices" J. Mater. Chem.2000, 10, 571. 4. Forrest, S.; Burrows, P.; Thompson, M. "The Down of Organic Electronics" IEEE Spectrum 2000, 29 (popular review). 5. 1. Theme issue on organic electronics: Chem. Mater. 2004, 16, #23. 6. 2005 reviews 7. 2007 reviews Materials for organic field-effect transistors (OFETs): 1. Lodha, A., Singh, R. "Prospects of Manufacturing Organic Semiconductor-Based Integrated Circuits" IEEE Transactions on Semiconductor Manufacturing 2001, 14, 281. 2. 2007 reviews Materials for organic light-emitting diodes (OLEDs): 1. Forrest, S. R. Chem. Rev. 1997, 97, 1793. 2. 2007 reviews Materials for organic photovoltaics (OPVs): 1. Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. "Plastic Solar Cells" Adv. Funct. Mater. 2001, 11, 15. 2. Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693. 3. Reviews on hybrid solar cells. Organic magnetism: 1. Theme issue on charge transfer complexes: Chem. Rev. 2004, 104, #11 (32 reviews). 2. Sorai, M.; Nakano, M.; Miyazaki, Y. "Calorimetric investigation of phase transitions occurring in molecule-based magnets" Chem. Rev. 2006, 106, 976. 3. 2006 reviews. Liquid crystals (LCs) and related materials: 1. Binnemans, K. "Ionic Liquid Crystals" Chem. Rev. 2005, 105, 4148. Fabrication of devices in organic electronics: 1. Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. "About Supramolecular Assemblies of p-Conjugated Systems" Chem. Rev. 2005, 105, 1491. 2. Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. "New Approaches to Nanofabrication: Molding, Printing, and Other Tecnhiques" Chem. Rev. 2005, 105, 1171. 3. 2007 reviews. Molecular electronics: 1. 2005 reviews 2. 2007 reviews.
The Photonics Dictionary Semiconductor Glossary Photopolymers Dictionary Liquid Crystals Tutorial organic electronics These expressions are often confused. All of them suggest use of organic materials in electronics and microelectronics, however there is certain difference in their meaning. First, all of them imply organic compounds as functional materials that interact directly or indirectly with electric current. Insulating plastics (also organic materials) are not included unless they function in a device (as a gate dielectric insulator in transistors, for example).
Though there are no strict rules of these terms use, within our site we will follow the next relatively common definitions:
Organic electronics Organic microelectronics imply organic materials in microelectronics. Device dimensions: hundreds of nanometers to micrometers. Organic optoelectronics, and Molecular electronics is still emerging field of science/technology that implies a single molecule of an organic compound as a single electronic device, say transistor. This SciFi idea yet, seems going to be reality in the overseen future. The possible result of practical implication of this idea is hard to predict and it may be even scary. A single microchip may outperform human brain or include all libraries, all information altogether ever collected by human being. Such capabilities certainly should result in artificial intelligence. Molecular electronics use Angstroms to nanometers device dimensions.
These three commonly used terms have distinct difference. Monomers are usually very simple molecules of organic compounds that may bond to each other under the action of temperature, pressure, specific chemicals, light, irradiation, or electric current.
When only few molecules of monomer bond to each other (usually less than 50) they form heavier molecules, known as oligomers. Oligomers still behave as individual organic compounds. They may be soluble in organic solvents, may crystallize or sublimate and have a distinct melting point. However, oligomers are relatively heavy molecules that contain many atoms and bonds and they are not as volatile and soluble as monomers are.
Oligomers are often colored if contain chains of unsaturated conjugated bonds. This type of oligomers may react on the action of light or electric current. They may absorb and emit light that is known as luminescence. They may also conduct electric current. These reactions are very valuable; they form foundation for "organic electronics".
Brief overview structure, properties, preparation, and application of electrically conducting oligomers and polymers: Summary of electrically-conducting oligomers and small molecules Intrinsically conductive polymers: organic electroconductors These three terms have somewhat different meaning. They all mean organic materials that conduct electric current. Organic conductors may be subdivided in two large groups: organic conductors with metallic type conductivity and organic semiconductors. Organic materials may possess relatively high, metallic-type conductivity, usually in a doped state. Sometimes they are also called 'synthetic metals'. This term was also accepted for a title of one of the most important and information-reach periodical on organic semiconductors. "Synthetic metals" is a purely scientific journal that contain immense quantity of valuable information in the field. Organic materials in undoped state usually possess much lower conductivity and the term 'organic semiconductors' is appropriate in this case. Organic semiconductors exhibit many useful features of conventional inorganic ones, furthermore, many of them possess a variety of additional unique (optical, luminescent, magnetic) properties.
State of organic conductors in devices Crystalline: Polycrystalline: An organic compound functions in a device in a form of a film/layer/bulk of many microscopic crystals. Both small molecules and low molecular weight polymers can be used. Microcrystalline: composed of micrometers-size well-shaped crystals/lamellas. Liquid crystalline: An organic compound exhibits liquid crystalline properties, e.g. may form nematic and smectic phases. Both small molecules and low molecular weight polymers can be used. Conducting and luminescent properties of liquid crystalline semiconductors are relatively unexplored. Amorphous: Small-molecule amorphous solids: Amorphous polymers: Liquid: Commission Internationale de L'Éclairage (CIE) chromaticity diagram Commission Internationale de L'Éclairage (CIE) chromaticity diagram is a graphical representation of a set (gamut) of all colors visible by a human eye. The curved edge of the colored figure is known as spectral locus of monochromatic light, where wavelengths change from 380 (saturated violet) to 700 (saturated red) nm. The straight bottom edge of the figure is called line of purples which represents colors that have no match in the monochromatic light, but may be obtained by mixing variable amounts of blue and red. Every color distinguished by a human eye may be characterized by a point on the CIE diagram, which may be precisely located using CIE coordinates, x and y. Every color on the diagram may also be obtained by mixing two or three different colors, and the corresponding point will be located somewhere on the line or in the triangle between two or three points on the diagram. Thus, pure white color corresponds to the coordinates (x, y) = (0.33, 0.33) and may be obtained by mixing of certain amounts of either blue and yellow; green and purple; or blue, green, and red.
CIE coordinates are used as an indispensable quantitative measure of the quality of colors in the display technology. The quality of primary television colors is determined by organizations such as National Television Standards Committee (NTSC): red (x, y) = (0.67, 0.33), blue (x, y) = (0.11, 0.11), white (x, y) = (0.33, 0.33). CIE coordinates are also one of primary parameters of a device in OLED technology.
Conductivity - electrical
Conductance: S (Siemens); S = 1/ W = A/V Current density: J = A/cm2 Electric field strength: E = V/cm Conductivity: s = J/E = A · cm/V · cm2 = A/V · cm = S/cm = S · cm-1 Conductivity of metals ranges from 4 · 104 (lead) to 6 · 105 S · cm-1 (silver). Conductivity of organic semiconductors in an undoped state typically do not exceed 1 S · cm-1, whereas it may reach as high as 3000-4000 S · cm-1 in a doped state (metallic-type organic conductors). Relation between conductivity and mobility for organic semiconductors: The nature of the charge transport in organic semiconductors may be very complex and it is often unknown, which particles are responsible for the transport. It is relatively easy to determine the charge of a dominant carrier: positive or negative; however it is often impossible to determine exactly the structure of a carrier particle. For example, positive charge carriers in organic compounds may exist in a form of: The charge of the conjugated particles 1-4 can be effectively delocalized along the conjugated system on significant distances of several atoms or longer thus resulting in the positive ('hole') conductivity of an organic material. Particles 5,6 may contribute to the conductivity by temporary 'stacking' on selected atoms or bonds to form hydroxonium or carbenium ions followed by delocalization of the charge. Therefore: all positively-charged particles conjugated with a system of unsaturated (double or triple) bonds are responsible for the hole transport in a solid organic semiconductor. Metallic cations or other chemically inert cations may not delocalize or otherwise transfer their charge as well as they can not move (diffuse) efficiently in a solid, therefore they unlikely to play significant role in the 'positive' conductivity of a solid conductor. Therefore: All chemically inert positively-charged particles not conjugated with a system of unsaturated (double or triple) bonds are not or minimally responsible for the hole transport in a solid organic semiconductor. At the same time, free cations may play a critical role in the ionic conductivity of semisolid or liquid mediums which are used in electrochemical devices, such as dye sensitized solar cells. Negative charge carriers in organic compounds may exist in a form of: The charge of the conjugated particles 2-4 may effectively delocalize along the conjugated system on significant distances of several atoms or longer thus resulting in the negative ('electron') conductivity of an organic material. Electrons (1) may temporary 'stack' and delocalize on selected atoms or bonds through transformation to conjugated anion radicals (3). Therefore: All negatively-charged particles conjugated with a system of unsaturated (double or triple) bonds plus 'free' electrons are responsible for the negative ('electron') transport in a solid organic semiconductor. Free halogen anions (5) or other chemically inert anions usually may not delocalize or otherwise transfer their charge as well as they can not move (diffuse) efficiently in a solid, therefore they unlikely to play significant role in the 'negative' conductivity of a solid conductor. Therefore: All negatively-charged chemically-inert particles not conjugated with a system of unsaturated (double or triple) bonds are not or minimally responsible for the negative transport in a solid organic semiconductor. However 'free' anions may play a critical role in the ionic conductivity of semisolid or liquid mediums in electrochemical devices, such as dye sensitized solar cells. It is difficult to determine which type of charge carriers is responsible for the 'positive' or 'negative' conductivity in a given organic semiconductor, moreover the carriers my transform into each other in the majority of cases. Thus, protons may reversibly protonate double bonds to form carbocations, whereas electrons may 'stick' and delocalize in a conjugated system to form heteroatom anion-radicals etc. Therefore, a common name is accepted for positive and negative charge carriers: positive and negative polarons. Conductivity of a semiconductor is determined by two parameters: s = enp+mp+ + enp-mp- Where e = elementary charge, mp+ and mp- = mobility of positive and negative polarons respectively. Mobility of positive polarons for organic semiconductors is usually significantly higher than that of negative, therefore, the 'negative' part of the equation may be neglected for the typical organic p-conductors: s = enp+mp+ Rough relation of the electrical conductivity and mobility in organic semiconductors may be expressed also by a "simple power law": m ≈ sd, where d is about 0.7. Anisotropy of electrical conductivity of organic materials: In the cases when a highly ordered, or crystalline structure of an organic conductor can be obtained, a high degree of anisotropy of conductivity is commonly observed. Since the charge careers 'prefer' moving along the conjugated chains (see also electron transport and hole transport), the conductivity in the direction parallel to chain axis is usually orders of magnitude higher than that in the direction perpendicular to the chain axis. Thus, for the the case of iodine-doped polyacetylene at ambient conditions, the values have been measured to be 1.1 · 104 and 1 · 102 S · cm-1 respectively. Organic conductors with linear-chain structure and anisotropy of conductivity are also known as 1-D conductors. For comprehensive scientific information and theory of transport in conducting polymers, see ref. 1. pages 1-165.
k = (Q/t) · L/(A · DT) Thermal conductivity is extremely important parameter of semiconductors. Electric current always generates heat, which should be quickly transmitted out of the functional material in order to prevent it from changing of property and/or thermal degradation. In this respect, inorganic semiconductors have huge advantage over organic materials. The inorganic materials possess both much higher thermal conductivity and thermal stability than the organic ones. Poor thermal conductivity is an intrinsic property of most of organic compounds and materials, moreover they have been recognized as excellent thermal insulators thousands of years ago (first humans used wood, animal's pelts and fur to stay warm). In electronics, however, poor thermal conductivity of organic materials is a major difficulty on the way to their practical implication. Proper heat utilization is especially important in organic field effect transistors and integrated circuits. Since the heat may be conducted by charged particles (especially electrons), the thermal conductivity is correlated with electrical conductivity. Materials with higher electrical conductance are usually more efficient thermal conductors and vice versa.
Displays (main types) Cathode-Ray Tube Displays, CRDs: Working principle is based on cathodoluminescence. CRDs are composed of luminescent materials, usually inorganic, that emit light under the 'bombardment' with rays of electrons. CRDs may not be flat due to spacious vacuum bulbs necessary for the formation and distribution of the rays of electrons. CRDs flicker, e.g. change image with frequency 60-100 Hertz (times per second). CRDs also operate at high voltage, consume significant energy, and emit harmful electromagnetic fields and radiation. Plasma Displays, PDs: Working principle is based on electroluminescence of inorganic inert gases in microscopic cells arranged in 'triplets' (three major colors). They do not emit harmful radiation, however relatively high voltages are still required for their operation. Liquid Crystalline Displays, LCDs:
Working principle is based on ability of special organic compounds liquid crystals to orient their molecules variably under the action of electromagnetic field thus allowing or preventing light of passing through. Color LCDs are usually very complex multilayered systems that require back light, since an active phase, a liquid crystalline material itself do not emit, but only modify light. Color LCDs are flat, but not very thin (around 1cm) due to their complexity, and they may not be flexible or transparent. LCD operate at low voltage, do not flicker and emit harmful radiation. However, LCD production is relatively costly; major drawbacks of LCDs are insufficient brightness and relatively narrow view angle. Electroluminescent Displays, ELDs: Working principle of this type of displays is based on electroluminescence of solid inorganic materials in light emitting diode (LED); or solid organic materials in organic light emitting diodes (OLEDs) under the action of electric current. Experts believe in EL displays replacing all other types of displays in the future. EL displays possess wider view angle (close to 180o) than LCDs, flexibility (OLED ELDs), and virtually unlimited room for improvement of all characteristics such as: brightness, contrast, color quality, and potential for low cost manufacture.  |
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