Transport recombination

Frank A. J., Kopidakis N. and van de Lagemaat J. (2004), Electrons in nano-structured TiOa solar cells transport, recombination and photovoltaic properties . Coordination Chem. Rev. 248,1165-1179. 

A.J. Frank, N. Kopidakis, J. van de Lagemaat, Electrons in nanostructured Ti02 solar cells transport, recombination and photovoltaic properties (review) . Coordination Chemistry Reviews, 248, 1165-1179, (2004). 

All the important processes of light absorption, charge transport, recombination and contact properties discussed previously can be incorporated into a device model. Such a model is highly desired as the... 

The first broad topic considers the formation, transport, recombination, and transfer of charge carriers, including the dynamics aspects. These phenomena are evidently crucial because they govern the occurrence and rates of photocatalytic reactions. Among the reactants to which the charges can be transferred, water and... 

In solid state materials, single-step electron transport between dopant species is well known. For example, electron-hole recombination accounts for luminescence in some materials [H]. Multistep hopping is also well known. Models for single and multistep transport are enjoying renewed interest in tlie context of DNA electron transfer [12, 13, 14 and 15]. Indeed, tliere are strong links between tire ET literature and tire literature of hopping conductivity in polymers [16]. 

When electrons are injected as minority carriers into a -type semiconductor they may diffuse, drift, or disappear. That is, their electrical behavior is determined by diffusion in concentration gradients, drift in electric fields (potential gradients), or disappearance through recombination with majority carrier holes. Thus, the transport behavior of minority carriers can be described by a continuity equation. To derive the p—n junction equation, steady-state is assumed, so that = 0, and a neutral region outside the depletion region is assumed, so that the electric field is zero. Under these circumstances,... 

Some battery designs have a one-way valve for pressure rehef and operate on an oxygen cycle. In these systems the oxygen gas formed at the positive electrode is transported to the negative electrode where it reacts to reform water. Hydrogen evolution at the negative electrode is normally suppressed by this reaction. The extent to which this process occurs in these valve regulated lead —acid batteries is called the recombination-efficiency. These processes are reviewed in the Hterature (50—52). 

Thus, in order to reproduce the effect of an experimentally existing activation barrier for the scission/recombination process, one may introduce into the MC simulation the notion of frequency , lo, with which, every so many MC steps, an attempt for scission and/or recombination is undertaken. Clearly, as uj is reduced to zero, the average lifetime of the chains, which is proportional by detailed balance to Tbreak) will grow to infinity until the limit of conventional dead polymers is reached. In a computer experiment Lo can be easily controlled and various transport properties such as mean-square displacements (MSQ) and diffusion constants, which essentially depend on Tbreak) can be studied. 

Several authors " have suggested that in some systems voids, far from acting as diffusion barriers, may actually assist transport by permitting a dissociation-recombination mechanism. The presence of elements which could give rise to carrier molecules, e.g. carbon or hydrogen , and thus to the behaviour illustrated in Fig. 1.87, would particularly favour this mechanism. The oxidant side of the pore functions as a sink for vacancies diffusing from the oxide/gas interface by a reaction which yields gas of sufficiently high chemical potential to oxidise the metal side of the pore. The vacancies created by this reaction then travel to the metal/oxide interface where they are accommodated by plastic flow, or they may form additional voids by the mechanisms already discussed. The reaction sequence at the various interfaces (Fig. 1.87b) for the oxidation of iron (prior to the formation of Fe Oj) would be... 

Clearly additional layers may be used to accomplish other benefits, tailoring the energy profiles and mobilities across the entire organic stack. Splitting the transport layer(s) into two separate layers permits the optimization of injection into the layer nearest the electrode (sometimes called the injection layer), and transport in the farther layer [101]. Layers of insulator (charge confinement layers) have also been used in an attempt to control the motion of the charges and ensure recombination in the desired region [102]. 

Studies of double carrier injection and transport in insulators and semiconductors (the so called bipolar current problem) date all the way back to the 1950s. A solution that relates to the operation of OLEDs was provided recently by Scott et al. [142], who extended the work of Parmenter and Ruppel [143] to include Lange-vin recombination. In order to obtain an analytic solution, diffusion was ignored and the electron and hole mobilities were taken to be electric field-independent. The current-voltage relation was derived and expressed in terms of two independent boundary conditions, the relative electron contributions to the current at the anode, jJfVj, and at the cathode, JKplJ. 

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

The efficient formation of singlet excitons from the positive and negative charge carriers, which are injected via the metallic contacts and transported as positive and negative polarons (P+ and P ) in the layer, and the efficient radiative recombination of these singlet excitons formed are crucial processes for the function of efficient electroluminescence devices. 

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