Recombination centers

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. 

Recently, it is reported that Xi02 particles with metal deposition on the surface is more active than pure Ti02 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e ) in the conduction band to the external sjistem, and decrease the recombination with positive hole (h ) in the balance band of Xi02, i.e., less defects acting as the recombination center[l,2,3]. Xhe catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Xhus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. Xhe XiOa were prepared with two different methods i.e., hydrothermal method and a sol-gel method. Xhe prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. Xhey were characterized by UV-DRS, BEX surface area analyzer, and XRD[4]. 

Tenne R, Wold A (1985) Passivation of recombination centers in n-WSe2 yields high efficiency (>14%) photoelectrochemical cell. Appl Phys Lett 47 707-709 Chaparro AM, Salvador P, Peter LM (1995) The role of surface defects in the photooxidation of iodide at n-MoSe2 evidence for a local autocatalytic effect. J Phys Chem 99 6677-6683... 

Another open question is the relationship between the H-induced radiative recombination centers and the H-induced platelets. Controlled layer removal of the plasma-processed silicon surface reveals that the density of luminescence centers decays nearly exponentially with a decay length that is comparable to the depth over which the platelets form (Northrop and Oehrlein, 1986 Jeng et al., 1988 Johnson et al., 1987a). However, the defect luminescence has also been obtained from reactive-ion etched specimens in which platelets were undetectable (Wu et al., 1988). Finally, substantial changes in the luminescence spectra occur at anneal temperatures as low as 250°C (Singh et al., 1989), while higher temperatures... 

Relatively little is understood in the presence of non planar-non ideal interfaces, where electronic levels located in the band gap region act as recombination centers. Colloidal materials, low cost polycrystalline materials and films, interpenetrating networks of absorber and charge collecting phases (e.g., as in the DSSC cells), and the presence of redox active adsorbing species, all give rise to... 

A hydrogen termination of the crystallite surface is assumed in most studies. Calculations show that a small part of the hydrogen can be removed without the formation of DBs, as shown for the transformation of Si29H36 to SijjH [Hi-4-]. Further dehydrogenation, however, leads to the formation of DBs that are efficient recombination centers and thus effectively suppress the luminescence [De2, Hi4, De5]. The observed IR luminescence has been proposed to be due to recombination via DB centers [De3]. 

Contamination of silicon wafers by heavy metals is a major cause of low yields in the manufacture of electronic devices. Concentrations in the order of 1011 cm-3 [Ha2] are sufficient to affect the device performance, because impurity atoms constitute recombination centers for minority carriers and thereby reduce their lifetime [Scl7]. In addition, precipitates caused by contaminants may affect gate oxide quality. Note that a contamination of 1011 cnT3 corresponds to a pinhead of iron (1 mm3) dissolved in a swimming pool of silicon (850 m3). Such minute contamination levels are far below the detection limit of the standard analytical techniques used in chemistry. The best way to detect such traces of contaminants is to measure the induced change in electronic properties itself, such as the oxide defect density or the minority carrier lifetime, respectively diffusion length. 

It should also be briefly recalled that semiconductors can be added to nanocarbons in different ways, such as using sol-gel, hydrothermal, solvothermal and other methods (see Chapter 5). These procedures lead to different sizes and shapes in semiconductor particles resulting in different types of nanocarbon-semiconductor interactions which may significantly influence the electron-transfer charge carrier mobility, and interface states. The latter play a relevant role in introducing radiative paths (carrier-trapped-centers and electron-hole recombination centers), but also in strain-induced band gap modification [72]. These are aspects scarcely studied, particularly in relation to nanocarbon-semiconductor (Ti02) hybrids, but which are a critical element for their rational design. 

The increased crystallinity of the samples annealed at elevated temperatures reduces the number of grain boundaries, improves connectivity between grains and eliminates any amorphous regions that provide defects acting as carrier recombination centers. The width of the anatase crystallites in the walls is restricted hy the wall thickness, approximately 20 nm. The potential drop within the wall can be represented as ... 

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