Charge transfer interface states

This exciton diffuses to the donor/acceptor interface via an energy-transfer mechanism (i.e., no net transport of mass or charge occurs). (3) Charge-transfer quenching of the exciton at the D/A interface produces a charge- transfer (CT) state, in the form of a coulombically interacting donor/acceptor complex (D A ). The nomenclature used to describe this species has been relatively imprecise, and has... 

Fig. 4 Schematic illustration of the processes leading to photocurrent generation in organic solar cells, (a) Photon absorption in Step 1 leads to excitons that may diffuse in Step 2 to the donor/ acceptor (D/A) interface. Quenching of the exciton at the D/A interface in Step 3 leads to formation of the charge-transfer (CT) state. Note that processes analogous to Steps 1-3 may also occur in the acceptor material, (b) Charge separation in Step 4 leads to free polarons that are transported through the organic layers and collected at the electrodes in Steps 5 and 6, respectively, (c) The equilibria involved in Steps 1-4- strongly influence device efficiency...

Fig. 6 Schematic illustration of the phonon-assisted exciton dissociation process. Due to the electronic state couplings, the photogenerated exciton (XT) wavepacket undergoes transitions to an interfacial charge transfer (CT) state, along with indirect XT — IS — CT transitions via an intermediate (IS) state (see panel (b)). In Ref. [52], the diabatic Hamiltonian of Eqs. (19)-(20) was parametrized for two relevant interface configurations (eclipsed (E) vs. staggered (S) as shown in panel (a)) which correspond to the configurations of Fig. 3.

For insulating materials, such as ceramics, this normally does not pose problems, because the conduction band is empty and the band gap error is not reflected energetically. However, for M/C interfaces there may be some reason to worry, since mixing between oxide conduction bands and metal states will influence the magnitude of adhesion and charge transfer. These states may become filled, depending on the Fermi level on the metal side. Further research on this issue is warranted. 

Transport of a species in solution to and from an electrode/solution interface may occur by migration, diffusion and convection although in any specific system they will not necessarily be of equal importance. However, at the steady state all steps involved in the electrode reaction must proceed at the same rate, irrespective of whether the rate is controlled by a slow step in the charge transfer process or by the rate of transport to or from the electrode surface. It follows that the rate of transport must equal the rate of charge transfer ... 

An ideally polarized electrode is rigorously defined as the electrode at which no charge transfer across the metal/solution interface can occur, regardless of the potential externally imposed on the electrode. At any fixed potential, such an electrode system attains a true state of equilibrium. 

Figure 42. Scheme comparing expected potential-independent charge-transfer rates from Marcus-Gerischer theory of interfacia) electron transfer (left) with possible mechanisms for explaining the experimental observation of potential-dependent electron-transfer rates (right) a potential-dependent concentration of surface states, or a charge-transfer rate that depends on the thermodynamic force (electric potential difference) in the interface. 

Insulators lack free charges (mobile electrons or ions). At interfaces with electrolyte solutions, steady-state electrochemical reactions involving charge transfer across the interface cannot occur. It would seem, for this reason, that there is no basis at this interface for the development of interfacial potentials. 

Phospholipid monolayers at liquid-liquid interfaces influence the charge transfer processes in two ways. On the one hand, the phospholipids constitute a barrier that blocks the process by impeding the transferring species to reach the interface [1,15,48]. On the other hand, the phospholipids modify the electrical potential difference governing the process [60]. While the first influence invariably leads to a decreased rate, the second one might result in either a decreased or an increased rate of charge transfer. The net effect of the phospholipids on the charge transfer process depends on the state of the monolayer, and therefore studies with simultaneous electrochemical and surface pressure control are preferable [10,41,45]. 

The intercept should reflect the unchanging activation polarization at the two interfaces, as well as some other effects (presence of a film before anodization, time lag in attainment of the steady state, etc.). Nevertheless, the fact that it is small or negligible indicates that charge transfer processes at the interfaces are fast and that the kinetics of the growth are entirely transport controlled. 

The sensitizers display a crucial role in harvesting of sunlight. To trap solar radiation efficiently in the visible and the near IR region of the solar spectrum requires engineering of sensitizers at a molecular level (see Section 9.16.3).26 The electrochemical and photophysical properties of the ground and the excited states of the sensitizer have a significant influence on the charge transfer (CT) dynamics at the semiconductor interface (see Section 9.16.4). The open-circuit potential of the cell depends on the redox couple, which shuttles between the sensitizer and the counter electrode (for details see Section 9.16.5). 

Both ion and electron transfer reactions entail the transfer of charge through the interface, which can be measured as the electric current. If only one charge transfer reaction takes place in the system, its rate is directly proportional to the current density, i.e. the current per unit area. This makes it possible to measure the rates of electrochemical reactions with greater ease and precision than the rates of chemical reactions occurring in the bulk of a phase. On the other hand, electrochemical reactions are usually quite sensitive to the state of the electrode surface. Impurities have an unfortunate tendency to aggregate at the interface. Therefore electrochemical studies require extremely pure system components. 

For the familiar dropping mercury electrode, the electrical potential 1J1 at the metal surface relative to the bulk region of the electrolyte is controlled by an external potential source - a constant voltage source. In this case, can be set to any value (within reasonable physical limits) as the mercury/electrolyte interface does not allow charge transfer or chemical reactions to occur (at least to a good approximation for the case of NaF). Therefore, we can say that the equation of state of the mercury surface is... 

A Schottky diode is always operated under depletion conditions flat-band condition would involve giant currents. A Schottky diode, therefore, models the silicon electrolyte interface only accurately as long as the charge transfer is limited by the electrode. If the charge transfer becomes reaction-limited or diffusion-limited, the electrode may as well be under accumulation or inversion. The solid-state equivalent would now be a metal-insulator-semiconductor (MIS) structure. However, the I-V characteristic of a real silicon-electrolyte interface may exhibit features unlike any solid-state device, as... 

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