Semiconductors acceptors

Polymeric solar cells with a bandgap > 2 eV are spectrally so badly mismatched to the solar spectrum that their efficiency is severely restricted. It is essential to develop polymeric semiconductors with lower bandgap. Low bandgap polymeric semiconductors behave similarly in conjunction with fullerenes as n-type semiconductors (acceptors). 

Stier W. and Prezhdo O. V. (2002), Nonadiabatic molecular dynamics simulation of light-induced electron transfer from an anchored molecular electron donor to a semiconductor acceptor , J. Phys. Ghent. B 106, 8047-8054. 

MLCT Excited States on Ti02 Experimental studies of MLCT states on Ti02 (and other semiconductors) are few mainly because of rapid interfacial charge separation that shortens their lifetimes considerably. Some aspects of MLCT excited states anchored to nanocrystalline Ti02 thin films are now becoming available through studies where the semiconductor acceptor states lie above (toward the vacuum level) the excited-state reduction potential of the sensitizer such that excited-state electron transfer from the thexi state is unfavorable. 

Figure 3.7 Schematic illustration of the relationship between structure and interfacial electron transfer properties in dye-sensitized nanostructured semiconductor materials. Control of the balance between electron injection, recombination and transport depends crucially on the physical separation and electronic coupling capabilities between the molecular donor and semiconductor acceptor states mediated by designated anchor and spacer groups.

In an extrinsic semiconductor, tlie conductivity is dominated by tlie e (or h ) in tlie CB (or VB) provided by shallow donors (or acceptors). If tlie dominant charge carriers are negative (electrons), tlie material is called n type. If tlie conduction is dominated by holes (positive charge carriers), tlie material is called p type. 

All teclmologically important properties of semiconductors are detennined by defect-associated energy levels in the gap. The conductivity of pure semiconductors varies as g expf-A CgT), where is the gap. In most semiconductors with practical applications, the size of the gap, E 1-2 eV, makes the thennal excitation of electrons across the gap a relatively unimportant process. The introduction of shallow states into the gap through doping, with either donors or acceptors, allows for large changes in conductivity (figure C2.16.1). The donor and acceptor levels are typically a few meV below the CB and a few tens of meV above the VB, respectively. The depth of these levels usually scales with the size of the gap (see below). 

In n type semiconductors, electrons are tire majority carriers. Holes will also be present tlirough accidental incoriioration of acceptor impurities or, more importantly, tlirough tlie intentional creation of electron-hole pairs. Holes in n type and electrons in p type semiconductors are minority carriers. 

Table 1. Common Donor and Acceptor Molecules Used in Organic Semiconductor and Devices...

Phosphoms pentafluoride behaves as a Lewis acid showing electron-accepting properties. It forms complexes, generally in a ratio of 1 1 with Lewis bases, with amines, ethers, nitriles, sulfoxides, and other bases. These complexes are frequently less stable than the similar BF complexes, probably owing to stearic factors. Because it is a strong acceptor, PF is an excellent catalyst especially in ionic polymeri2ations. Phosphoms pentafluoride is also used as a source of phosphoms for ion implantation (qv) in semiconductors (qv) (26). 

Fig. 1. Band-edge energy diagram where the energy of electrons is higher in the conduction band than in the valence band (a) an undoped semiconductor having a thermally excited carrier (b) n-ty e doped semiconductor having shallow donors and (c) a -type doped semiconductor having shallow acceptors.

The impurity atoms used to form the p—n junction form well-defined energy levels within the band gap. These levels are shallow in the sense that the donor levels He close to the conduction band (Fig. lb) and the acceptor levels are close to the valence band (Fig. Ic). The thermal energy at room temperature is large enough for most of the dopant atoms contributing to the impurity levels to become ionized. Thus, in the -type region, some electrons in the valence band have sufficient thermal energy to be excited into the acceptor level and leave mobile holes in the valence band. Similar excitation occurs for electrons from the donor to conduction bands of the n-ty e material. The electrons in the conduction band of the n-ty e semiconductor and the holes in the valence band of the -type semiconductor are called majority carriers. Likewise, holes in the -type, and electrons in the -type semiconductor are called minority carriers. 

Fig. 2. Representation of the band edges in a semiconductor p—n junction where shallow donor, acceptor energies, and the Fermi level are labeled Ejy E, and E respectively, (a) Without external bias is the built-in potential of the p—n junction (b) under an appHed forward voltage F. ...

Fig. 1. Photoexcitation modes iu a semiconductor having band gap energy, E, and impurity states, E. The photon energy must be sufficient to release an electron (° ) iato the conduction band (CB) or a hole (o) iato the valence band (VB) (a) an intrinsic detector (b) and (c) extrinsic donor and acceptor...

The carrier concentrations in doped or extrinsic semiconductors to which donor or acceptor atoms have been added can be deterrnined by considering the chemical kinetics or mass action of reactions between electrons and donor ions or between holes and acceptor ions. The condition for electrical neutraHty is given by equation 6. When the predominant dopants are donors, the semiconductor is... 

A hst of some impurity semiconductors is given in Table 5. Because impurity atoms introduce new localized energy levels for electrons that are intermediate between the valence and conduction bands, impurities strongly influence the properties of semiconductors. If the new energy levels are unoccupied and He close to the top of the valence band, electrons are easily excited out of the filled band into the new acceptor levels, leaving electron holes... 

Copper(I) oxide [1317-39-1] is 2lp-ty e semiconductor, Cu2 0, in which proper vacancies act as acceptors to create electron holes that conduct within a narrow band in the Cu i7-orbitals. Nickel monoxide [1313-99-17, NiO, forms a deficient semiconductor in which vacancies occur in cation sites similar to those for cuprous oxide. For each cation vacancy two electron holes must be formed, the latter assumed to be associated with regular cations ([Ni " h = Semiconduction results from the transfer of positive charges from cation to cation through the lattice. Conduction of this type is similar... 

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