Dictionary of Terms in Organic Electronics. Specific Terms: E.
Mechanism of electron transport in inorganic semiconductors is different from that of organic semiconductors. Electron transport in inorganic semiconductors is due to electron movement. For organic materials, it is mostly due to delocalization and movement of various negatively charged particles, which are commonly called as negative polarons. Free electrons can form in an inorganic semiconductor (silicon on the scheme below) due-to energy fluctuation. A covalent bond between two atoms may accidentally receive an excess of energy (through thermal fluctuations), that is equal or higher of a band gap energy Eg (see energy diagram on the scheme below). In this case, breakage of the bond occurs, and one of the electrons transfers from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Remark: The use of the definition of molecular orbital for inorganic crystalline materials, such as silicon, is very approximate because the crystal lattice is not a molecule. We use it to make a 'bridge' between organic and inorganic materials in description of their electronic properties. The definition of molecular orbital is largely correct for the description of the band properties of organic compounds. If the energy of the electron is high enough, it transfers to the conducting band (CB) and roam around a positive charge (hole), thus formed. The electron may also be completely 'stripped away' if its energy exceeds ionization potential (IP).
VB: valence band Electron movement begins when a potential is applied. Electrons run through the crystal lattice until they meet a hole left from the next 'stripped' electron. They may be permanently or reversibly trapped by the hole, and the mean path of electrons between the two points is known as free mean path (see scheme below). Some physical behavior of electrons in inorganic semiconductors, such as ability to undergo 'compression' resembles that of the ideal gas, therefore a sum of these electrons is called 'Fermi gas'. 'Fermi gas' can be 'frozen' to 'Fermi glass', when a semiconductor is cooled below some critical temperature due-to trapping of all electrons by the holes. This process is known as metal-insulator transition, and it is characteristic for all semiconductors. In metals, the valence band and conducting band overlap, therefore all electrons at once start moving under the applied potential. These electrons cannot be 'compressed' or 'frozen' and they form so-called 'Fermi liquid'. The highest energy of the area of overlap is known as 'Fermi level', an important parameter that determines work function of a metal. In contrast to semiconductors, the conductivity of metals monotonically increases with decrease of the temperature down to 0 K.
Electron conductivity in organic semiconductors is not quite a correct term. Due-to high chemical activity of electrons, they react with organic compounds to form various negative particles - polarons. Thus, an idealized mechanism of the transformation of an electron to anion-radical followed by its transport along the chain of polythiophene is shown on the scheme below. Mobility of negative polarons in organic semiconductors is usually much lower than that of holes (positive polarons), apparently due to high energy necessary for the formation of negative organic polarons. Carbanions are usually much more energetic than carbocations, hence the transport of cations is much more favorable than that of anions.
Organic compounds that conduct negative polarons rather than holes are relatively sparse. Several types of organic n-conductors that are somewhat different from each other in mechanism of negative conductivity are known: 1. Electron-deficient p-conjugated systems: n-Conductors of this type possess negative conductivity due to stabilization of negative polarons. At the same time, formation and transport of carbenium ions (holes) is suppressed. Thus, reversible interaction of electrons with derivatives of perylene tertracarboxylic diimide (PDI), widely used n-conductors, shown on the scheme below. Transport occurs due to temporary reversible stabilization of electrons on molecules of the compound, moving the charges throughout the molecules (fast step), and subsequent 'hopping' from one molecule to the other (slow, speed-determining step). Other known electron-deficient organic n-trasporters are oligomers and polymers containing quinoline, oxadiazole, benzimidazole (TPBI) structural units. 2. Metal complexes containing p-conjugated ligands with conjugated negative ions bonded to metal atoms: This type of n-conductors easily form mobile conjugated negative polarons due to dissociation of a metal-heteroatom bond. The negative polarons can efficiently delocalize throughout a ligand. A static hole (metal cation) thus formed remains on a permanent position not contributing to overall conductivity of a material. Thus, dissociation of aluminum-oxygen bond in a widely used n-conductor - Alq3 affords 'mobile' conjugated negative polaron and the static hole - aluminum cation (see scheme below).
In the cases, when a metal is able to form a number of stable oxidation states, the metal atoms may participate in the n-conductivity of complexes. Electrons easily reduce metal atoms to form lower oxidation states. 'Vacant' negative polarons, thus formed, may efficiently delocalize throughout a ligand to afford n-conductivity. This process is schematically depicted below for a widely used copper phtalocyanine complex: CuPc. In contrast to electron-deficient n-conductors (above), n-conductivity of metal complexes may be altered by p-doping, oxidation of ligands that destroys conjugated negative polarons and affords conjugated ('mobile') holes, and hole conductivity.
3. 'All-organic' charge-transfer complexes containing p-conjugated electron-deficient counterparts: This type of n-conductors may transport both positive and negative polarons due to possibility of efficient charge delocalization of both charge carriers. TTF-TCNQ (see scheme below) is a widely known representative of 'all-organic' charge transfer complexes, where both n- and p-type conductivity may occur. One electron transfer from a molecule of strongly electron-donating TTF to a molecule of strongly electron withdrawing TCNQ forms an ion-radical pair. A negative polaron, thus formed, may efficiently delocalize throughout strongly electron-deficient TCNQ-counterpart to afford electron transport. 4. Fullerenes: Mechanism of electron transport in fullerenes is different from all other types of organic n-conductors described above. It is due to efficient trapping of electrons inside of carbon spheres of fullerenes.
n-Doping: Inorganic semiconductors can be easily n-doped by means of introduction of a higher valence element into a semiconductor, such as arsenic into silicon. In this case, unpaired 'extra'-electron of arsenic easily transfers to the conducting band. n-Doping of organic semiconductors (in contrast to p-doping) is rare and may be achieved by introduction of active metals (alkali metals) in a semiconductor. High, 'metallic-type' conductivity of some heavily p-doped organic materials is due-to holes rather than electrons, therefore the term 'metallic conductivity' is not quite correct. Moreover, all metallic-type organic conductors undergo metal-insulator transition at low temperatures that is characteristic of semiconductors rather than metals. However, a 'truly' metallic conductivity was recently discovered for a carefully prepared emeraldine salt.
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