Dictionary of Terms in Organic Electronics. Specific Terms from N to O.
Search for terms: Oligomers and small molecules - conducting Summary of the most important classes of conductive oligomers and small molecules. Conducting oligomers are used in small molecule organic electronic devices, such as smOLEDs, smOFETs, and smPVs. Vacuum deposition method is the main method for the fabrication of small molecule devices. Oligomers: p-conductors. General: Oligoacenes have been used for the fabrication of the very first organic electronic devices (electroluminescence of anthracene, Pope and coworkers, 1963). Pentacene (6 on the scheme below) possesses highest carrier mobility achieved to date for organic materials. Preparation: Many oligoacenes are extracted from fossil oil and coal. Complex molecules are usually synthesized by cyclocondensation of lighter precursors containing 1-2 benzene rings followed by reductive aromatization or dehydroaromatization on the final step. Structure and properties: Derivatives of 3-membered oligoacene, anthracene (1 on the scheme below), such as DPA (2) possess relatively wide band gap due to relatively short conjugation. They found application as blue emitters in OLED. Rubrene (3) and perylene (4) are known p-transporters and fluorescent dopants. Pentacene 5 is a p-conductor with high carrier mobility. Some of it's derivatives are also used as red emitters in OLED.
Application: Historically, oligoacenes have been most commonly used in small molecule devices. Primary device fabrication method: vacuum deposition. Oligoacenes possess outstanding luminescent properies, they are especially promising for use in smOLED technology as p-transporting and light-emitting layers as well as fluorescent dopants. Pentacene (6) is a primary p-conductor for small molecule organic transistor (smOFET and smOTFT) applications. General: Oligothiophenes are important small molecule p-conductors for smOTFT applications. Some of oligothiophenes are solution processible that may afford certain advantage in device fabrication over oligoacenes. Polythiophenes are polymeric ('extended') analogs of oligothiophenes. Preparation: a-Oligothiophenes (1-5 on the scheme below) are prepared via transition-metal (usually Cu2+ salts) induced oxidative coupling of shorter oligomers (bithiophenes, terthiophene 1). Transition-metal (usually nickel, iron or palladium)-catalyzed reductive coupling of various selectively brominated thiophenes affords higher purity products. Structure and properties: Derivatives of terthiophene (1) and quaterthiophene a-4T (2) are soluble in organic solvents, whereas a-6T (3) is insoluble and infusible. In order to increase solubility, alkyl chains may be introduced in the various positions of oligomers such as in DH-a-6T (4) and DH-PTTP (5). Pure a-8T (not shown on the scheme) is hardly available synthetically. This product of good quality can be supplied from our company.
Application: Oligothiophenes are mostly used for small molecule transistor applications (smOFET and smOTFT). a-6T (3) can be processed only utilizing vacuum deposition, whereas the other oligomers are also suitable for the solution processing. General: Triarylamines are important class of small molecule p-conductors used in every type of organic electronic devices, especially popular in OLED technology. They differ from the other types of conductive oligomers in ability to form stabilized bication-radicals, so-called bipolarons, efficient positive charge-transport particles. Polyanilines are polymeric analogs of triarylamines. Preparation: Triarylamines are prepared from simpler derivatives of aniline or benzidine by menas of reactions of arylation, such as copper-catalyzed Ullmann arylation. Structure and properties: Derivatives of benzidine, such as TPD (1), and NPB (3) are good conductors due to high degree of the bipolaron stabilization. Carbazole derivatives: CBP (2) and mCP (4) possess higher rigidness, ionization potential and band gap.
Application: Triarylamines are most widely used as p-transporters for small molecule OLED applications. They can be processed utilizing vacuum deposition methods. General: Unsubstituted oligo-para-phenylenes are of less utility than the other classes of conducting oligomers, described above due to lower synthetic availability, solubility, and conductivity determined by relatively wide band gap. They are also less frequently used than their polymeric analogs: PPPs. One of the most valuable properties of oligophenylenes is in ability to absorb and emit light in the blue region of visible light spectra. Preparation: Oligo-para-phenylenes are usually prepared by transition metal-catalyzed reductive coupling of selectively halogenated lighter monomers and oligomers, such as brominated and iodinated derivatives of benzene and biphenyl. Structure and properties: para-Hexaphenyl (P6) (2 on the scheme below) is intractable and insoluble compound. It exhibits high quantum efficiency blue photoluminescence (PL) and electroluminescence (EL). Highly luminescent DPVBI (2) contains structural units of both para-phenylene and para-phenyl-vinylene.
Application: Oligo-para-phenylenes are used in small molecule OLED technology as p-transporting and light-emitting components. High quantum efficiency emission in the blue region of visible light spectrum, high ionization potential (IP), and environmental stability are characteristic of oligophenylenes. They can be processed only by vacuum-deposition methods.
Oligomers and small molecules: n-conductors. General: n-Conducting oligomers and small molecules are relatively sparse. Molecules of an organic n-conductor should easily accept electrons and stabilize them in a form of negative polarons. At the same time, formation and/or stabilization of positive polarons (holes) should be hampered. The other important condition of the successful functioning of organic n-conductors: they should not easily undergo irreversible chemical reduction under the action of electrons. Structure and properties: In order to exhibit the aforementioned properties, the structure of an n-conductor should include electron-deficient conjugated system. At the same time, electron-withdrawing functional groups conjugated with an unsaturated system should not undergo irreversible chemical reduction under the action of electrons. Therefore, many easily reducible electron-withdrawing groups, such as nitro-, nitroso-groups, or aldehydes are not applicable. Among electron-deficient conjugated systems, derivatives of pyridine and quinoline are efficiently employed. Bathocuproine (BCP) (1 on the scheme below) is one of the most popular conducting conjugated quinolines. Polyquinolines (PQs) are polymeric analogs of bathocuproine. Other frequently used electron-deficient n-transporters are derivatives of benzimidazole, such as TPBI (2), and oxadiazole, such as PBD (4) and OXD-7 (5). The latter compound is actually p-extended electron-deficient system enforced with a pyridine ring in addition to two oxadiazole rings. Derivatives of perylene tetracarboxylic diimide PTCDI (or PDI, 5) contain electron-deficient carbonyl groups at the perylene core. Variation of substituents at the nitrogen positions, or at the perylene core of PDIs may alter strongly film-forming and/or electronic properties. This flexible functionality and cost efficiency of PDIs made them the most widely used n-transporting oligomers.
Mechanism of electron transport in fullerenes is somewhat different and is considered to be due-to 'trapping' of an electron inside of the carbon 'ball', the unique structural feature of fullerenes. Unsubstituted fullerene C60 (6 on the scheme below) is not convenient for use in devices due to tendency of phase separation (precipitation) from an active layer followed by deterioration of the properties of a material. Therefore a variety of modified derivatives of fullerene, such as PCBM (7) are most commonly used.
Application: Oligo-n-conductors are most widely used in electron transport (ETL) and luminescent layers of small molecule OLED and photovoltaic devices. Insoluble compounds are processed by vacuum-deposition methods. Oligomers, containing long or 'swallow tailed' alkyl chains (PDIs), and fullerenes are soluble in organic solvents and may be processed from a solution.
Oligomers and small molecules: ambipolar. General: Ambipolar oligomers and small molecules may conduct both electrons and holes. From this respect, organic semiconductors are advantageous over the inorganic ones, because the former ones are intrinsically capable of conducting both types of charge carriers. Even typical organic p-conductors, such as polythiophenes may conduct electrons (negative polarons) with comparable efficiency (see hole transport and electron transport). However, it is difficult to realize on practice reliable electron-transporting properties in typical p-conductors due-to instability of intermediate carbanion-radicals, which prone to oxidize rapidly when exposed to air. Nevertheless, ambipolar transport in some common organic conductors, such as fullerenes has been demonstrated. The other important condition they should satisfy: reasonably efficient injection of both types of carriers from the same electrode whose work function is located on a middle between HOMO and LUMO levels of an organic semiconductor. This is a serious technical challenge, nevertheless it is possible to achieve sometimes even with common (gold) electrodes. A potential organic ambipolar transporter should not contain also very strong electron-withdrawing functional groups which may exhibit hole-blocking properties. Therefore, many electron-withdrawing moieties commonly used in organic n-transporters are not applicable for ambipolar transporters. Structure and properties: The molecule of an ambipolar conductor typically includes both hole-transporting electron-rich moieties and electron-transporting moieties containing mild electron-withdrawing functions. Oligomers for ambipolar transistor applications should possess highest possible mobility of both charge carriers. Few compounds were found to be suitable for ambipolar OFETs, such as 18T-C80 (1 on the scheme below), which contains p-conducting oligothiophene moiety and n-conducting fullerene moiety in the same molecule. In DCMT (2), electron-withdrawing counterparts are located on the sides of a strongly electron-donating terthiophene system. High carrier mobilities of ambipolar conductors for LED applications are not very important. More important, they should be equal for both charge carriers, and materials should possess amorphous properties with a glass-transition temperatures (Tg) as high as possible. Some of suitable compounds were found to be triarylamine derivatives containing various mild electron-withdrawing moieties: AODF (3) with oxadiazole counterparts; pyridine and carbazole counterparts (4); cyano-groups (NPAFN, 5); or imides NPAMLMe 6).
Application: Ambipolar molecular conductors have been tested for single component organic ambipolar transistor (OFET) and light-emitting ambipolar transistor (OLET) applications. They play an important role in monolayered OLEDs for the delivery of both electron and holes to luminophores.
General: Metal complexes are used mainly in small-molecule organic electronic devices. Sometimes, organic 'shell' of metal complexes that is called 'ligands' can be chemically bound, or otherwise incorporated into polymers. Many metal complexes are widely used as n- or p-conductors, or luminescent components. Structure and properties: Metal complexes often possess n-type conductivity in undoped state. Mechanism of electron (more strictly - negative polaron) transport in metal complexes differs from the one of either electron-deficient oligomers or fullerens. In metal complexes, it is mostly due to ease with which a metal atom surrenders its electron to a conjugated ligand. A conjugated, negatively charged anion, thus formed, can delocalize throughout a ligand to afford n-conductivity. The negative charge roams freely around of the static hole (the metal cation), and may be transferred to the next molecule if electrical field is applied. n-Conductance of some metal complexes may also be enhanced by doping with alkali metals. One of the unique features of metal complexes is in facile switching of n-type conductivity to p-type conductivity by doping with oxidizing agents. The doping gives rise to oxidation of ligands and formation of so-called 'open shell' holes to afford hole-type conductivity. The most widely known Alq3 (1 on the scheme below) and copper phtalocyanine CuPc (2) are inexpensive and stable n-conductors. Alq3 is also often used as luminescent 'host' material that emits light in the presence of 'guests', efficient fluorescent or phosphorescent dopants (for characteristic example see). Metal complexes themselves are also extensively used as luminescent dopants, and among them ruthenium (Ru(bpy)3-salts 3 and similar structures) and iridium complexes (Ir(ppz)3 5 and similar structures) are the most popular. Ruthenium complex N3 (4) was found to be the most efficient sensitizing dye in dye sensitized solar cells to afford power conversion efficiency of up to 12%, the highest achieved to date for organic devices.
Application: Metal complexes are extensively used in small molecule OLED, and PV technologies as n-, or p-transporting and light-emitting materials. They often play the role of either luminescent 'hosts' or 'guests'. Metal complexes are often well soluble in organic solvents and suitable for both solution and vacuum deposition processing.
Tetrathiafulvalenes and charge-transfer complexes. General: Tetrathiafulvalenes (TTFs) and TTF-based charge-transfer complexes are widely used in small molecule organic electronic devices. Extensive research that incorporates TTF-derivatives in polymers and polymer-like self-assembled systems is also underway. TTFs were one of the first types of conducting organic materials discovered about 30 years ago along with polyacetylenes, polyanilines, and oligoacenes. In addition to electroconductivity, TTF-based complexes posses some unique, such as ferromagnetic properties. Structure and properties: Tetrathiafulvalenes (1-3 on the scheme below) are strong electron-donating compounds due to four sulfur atoms conjugated with 'central' and 'side' double bonds, and they are typical p-conductors. Electron-donating properties of the 'parent' TTF, the simplest member of the family (1) may be even more enhanced by introduction of additional sulfur atoms conjugated to 'side' double bonds. One such derivative is well-known BEDT-TTF (2). Introduction of fused phenyl rings (DN-TTF (3), strongly improves p-stacking of conjugation planes and hence carrier mobility. Conductivity of TTFs may be strongly increased up to metallic-type if they subject to the action of strong electron-acceptors. This process is similar to p-doping of other types of organic conductors, but difference for the case of TTFs is in the formation of stable stoichiometric complexes with salt-like properties. The TTF salts are known as charge-transfer complexes. Thus, TTF forms salt with one molecule of iodine (4), whereas BEDT-TTF (2) may coordinate as many as three molecules of iodine. TTFs can be reversibly oxidized also by many oxidizing metal salts to form complexes similar to (5). Charge-transfer complexes of TTFs with strongly electron-deficient organic counterparts such as tetracyanoquinodimethane TCNQ (5) are widely known good conductors that possess essential n-conductivity in addition to p-conductivity (ambipolar conductors). n-Conductivity occurs due to delocalization and mobility of negative polarons in the conjugated electron-deficient counterpart (TCNQ).
Application: TTFs and their complexes are extensively studied for use in |
