Hole polarons

The combination of a hole polaron and an electron polaron, with binding energies Ep+ and Ep-, respectively, results in the formation of an exciton. Their difference corresponds to Et and is also referred to as the single particle energy gap Egsp -... 

Static solutions are found by minimizing the Hamiltonian above with respect to the nearest-neighbor distance change y =u +i-u and the electronic energies, which yields the following equations for the hole polaron ... 

Such a process can be considered to be a catalyzed recombination of the electrons and holes via a suitable solute. On the other hand when there are two solutes (X and Y) capable of reacting with the electrons as well as with the holes, Reactions 38a and 37a can effectively compete with the back Reactions 37b and 38b. From these considerations it may be inferred that in irradiated ice and frozen aqueous solutions the electron is closely coupled to the hole and may be considered to be a polaron bound to a hole polaron (36). This would also account for the first-order nature of the recombination process. 

Charge transport — When charged species move within a phase, this is called charge transport [i-viii]. Electron transport occurs in metals and -> semiconductors. (In the latter case the - charge carriers - holes, polarons, - bipolarons (- electronic defects) are also considered as moving charged species, while the superconductivity occurring at very low temperatures is explained by... 

If the lowest energy elementary excitations are bound singlet and triplet excitons, the theoretical maximum EL quantum efficiency is 25% of the photoluminescence (PL) quantum efficiency. On the contrary, if separated electron and hole polarons are the elementary excitations, the maximum theoretical EL efficiency can approach unity. Thus, a quantitative comparison of the quantum efficiencies for electroluminescence and photoluminescence can, in principle, provide fundamental information on the nature of the excited states. 

As a result of these factors, the universal paradigm for inorganic solar cells, the p-n junction, cannot be adapted for organic semiconductors. The contrast with inorganic semiconductors is shown schematically in Fig. 7.2. The alternative of a metal-semi-conductor-metal device structure, where photocurrent is directed by the difference in work function between the two metals, also cannot be used because the electric field created by available asymmetric contact materials is insufficient to separate the singlet exciton into electron and hole polarons. Therefore, alternative device architectures are needed. 

Zero effective charge defect with respect to the ideal structure Free electrons Free holes Polarons... 

However, when exceeds said critical valne a significant interface dipole can be formed. Positive charges are transferred from the metal to the semiconductor and the position of the Fermi level at the interface becomes pinned at an energy level interpreted as the hole polaron/bipolaron energy level in the polymer semiconductor. This simple picture suggests that, at least in the case of solntion-deposited polymers on common hole-injecting contacts, chemical interactions between the metal and... 

The ideas just reviewed are used below to discuss the electronic structure of specific azides. Such features of the band structure as the magnitudes of the effective masses and the positions of the band extrema in momentum space are - not sufficiently understood to warrant discussion. It should also be noted that electrons and holes tend to polarize the medium in which they move. If they are not particularly mobile, ionic nuclei have time to adjust to their presence by moving toward or away from them. An electron or hole is then dressed by this polarization and either carries it about (as an electron or hole polaron) or is trapped by it (self-trapping). Young [64] suggested that exciton models in the alkali azides should take the polaron character of electrons and holes into account. No one has yet attempted such a description. While it may be that polaron effects are necessary for a proper understanding of electronic structure, the discussion here is limited to a model involving undressed holes and electrons. 

A model for the effects of electron-lattice coupling via ring rotation on the electronic structure of leucoemeraldine base has been developed [15]. The charge defect states in leucoemeraldine and other phenyl ring-containing polymers, particularly hole polarons, are... 

It is well known that electron (or hole) excitations are unstable with respect to the formation of an electron (or hole) polaron [49]. The ground state conformation of a tPA chain with one extra electron consists of a local deformation in the middle part of the chain [41,49]. We have used this type of conformation as the initial geometry in our simulations, but with the polaron shifted away from the center of the chain. This change is introduced to achieve a longer distance for the polaron to travel before it reaches the chain end. 

Any method is not completely universal, and the approach described also has its limitations. In particular, it does not work when incorporation of the lattice relaxation qualitatively changes the electron locaUzation (e.g., for free electron and hole polarons). On the other hand, it could be very eflScient for many impurities in insulators characterized by high symmetry and when calculating forces is computationally expensive. 

Tig.1. The first singlet and triplet exciton bands of the polydiacetylene PTS shown together with the original electron and hole polaron bands. 

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