Electron - affinity injection

The photoanodic dissolution of n-silicon in acidic fluoride media provides an example of the complexity of multistep photoelectrochemical reactions [33, 34]. The reaction requires the transfer of four electrons, but it is clear that not all of the steps involve photogenerated holes because the photocurrent quantum efficiency is between 2 and 4. The explanation of the high quantum efficiencies is that the initial hole capture step can be followed by a series of steps in which intermediates with low electron affinity inject electrons into the conduction band. These intermediates can be assigned nominal oxidation states as shown in the following scheme. 

The metallic electrode materials are characterized by their Fermi levels. The position of the Fermi level relative to the eneigetic levels of the organic layer determines the potential barrier for charge carrier injection. The workfunction of most metal electrodes relative to vacuum are tabulated [103]. However, this nominal value will usually strongly differ from the effective workfunction in the device due to interactions of the metallic- with the organic material, which can be of physical or chemical nature [104-106]. Therefore, to calculate the potential barrier height at the interface, the effective work function of the metal and the effective ionization potential and electron affinity of the organic material at the interface have to be measured [55, 107],... 

Bunz et al. pointed out that it would be of interest to develop materials that combine the stability, electron affinity, and high emissive quantum yield of PPEs with the excellent hole injection capabilities of poly(p-phenylene vinylene)s (PPVs) [48]. In line with this notion,recent synthetic activities have focused on the engineering of the band gap, conduction band, and valence band of PAEs with the objective to render these materials more useful for practical applications that exploit their electrically (semi)conducting nature. Examples of materials that emerged from these efforts are discussed in detail in other portions of this volume (in particular the chapters by Bunz, Klemm, and Yamamoto). They include, among others, poly(heteroarylene ethynylenes) such... 

DNA, laced with an intercalator characterized by a high electron affinity, is y-ir-radiated and observed by EPR. The one-electron reduced intercalator presents an EPR spectrum that is readily distinguishable from that of the DNA-trapped radicals. A key example is mitoxantrone (MX), with an electron affinity of 6.25 eV and a radical anion spectrum that is a sharp singlet. Charges are injected into the DNA by y-irradiation at a preselected temperature (4 130 K). Holding the temperature constant, the EPR spectrum changes as a function of time (0.5-30 h). Thereby, a direct measure of the rate of electron transfer from one-electron reduced pyrimidines (Pryre) to the intercalator, e.g., MX, is measured. The turmeling rate is observed to depend on the electron affinity (EA) of the... 

For gas chromatography volatile derivatives such as the methyl ester of ABA (27, 29, 30) or trimethylsilylated ABA (20, 28) must be prepared. ABA is a molecule with a high electron affinity so that the methyl ester can be measured with a gas chromatograph equipped with an electron capture detector. Injections of as little as 5 pg of Me-ABA cause a detector response (27). Metabolites of ABA such as phaseic and dihydro-phaseic acid can also be measured by this method (33). 

The degree of ionicity in the bond between a metal atom and a polymer, or molecule, is related to the ionization potential and electron affinities of the substituents. The metals we have studied are of interest as electron injecting contacts in electronic devices. These metals must have a low ionization potential (or work function), of the same order as the electron affinity of the polymer, in order for the charge transfer process to occur. If the ionization potential of the metal is lower than the polymery-electron affinity, spontaneous charge transfer occurs which is the signature of an ionic bond. Thus, the character of the charge distribution in the metal-polymer complexes we are studying is related to the situation in the electronic device. 

As stated above, the notions of n and p-type do not have the same meaning as for inorganic semiconductors. At the current state of the art, an organic n-type material is one in which electrons are more easily injected than holes. This is therefore more a matter of HOMO and LUMO energy level rather than possibility of doping. In other words, an n-type organic semiconductor is characterized by high electron affinity. 

Here 4> is the work function of the metal and x is the electron affinity of the insulator. Table 3.3 shows that N0 is very sensitive to the values of 0 - x - In the organic polymer devices the injecting contact is made as nearly ohmic as possible and — x is small. In computation of the I-V relations 0 — X is assumed to be zero. In this case the value of No is very large and can be taken as infinity [37,38],... 

Another important property is the position of the lowest unoccupied levels, the electron affinity A. Symmetrically to Ip, it determines the efficiency of potential n-dopants and the energy-band positions at an electron injecting contact, which is very important in LEDs (Section V.C). Moreover, the quantity (A - Ip) is the bandgap Eg, which should be accessible to experiment. The two VEH calculations that have been mentioned [186,194] yield more different values for Eg than for Ip, as shown in Table 3. 

It is perhaps important to note here that PE exhibits a negative electron affinity (Bloor, 1976), a feature shared with only a very restricted set of materials. This means that excess electrons prefer energetically to reside outside PE rather than be in any way bound to the PE molecular structure. Nevertheless, recent calculations show that there are electronic surface states lying below the vacuum level in the forbidden band gap of PE (Righi et al., 2001). These surface states will certainly be expected to act as traps for transferred or injected electrons, and they will therefore be involved in contact charging. Their resonance interaction with negative ion states of typical PE dopants (02 and H20) may be very important too. 

Electrical conduction will occur by the hopping of either electrons or holes within these distributions of energy levels. Charge transport can be either of holes by transfer between the LUMO states or of electrons between the HOMO states. These correspond to the formation of either a radical cation by the removal of an electron to an adjacent electrode or an anion by the injection of an electron. The nature of the majority carriers will, therefore, be determined by the ionisation potentials and electron affinities of the conjugated moieties. A low ionisation potential will favour hole transport while a high electron affinity will favour electron transport. Most of the conductive polymers reported in the literature have low ionisation potentials and are hole, conductors. ... 

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