Recombination and Efficiency

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. 

The design of the organic LEDs should be optimized so that the double charge injection factor and the probability for singlet exciton formation is near to 

Bassler et al. [110-113] treated charge recombination in organic LEDs in terms of chemical kinetics. The probability of recombination depends on the ratio of recombination rate yn Mp- (where y represents a bimolecular rate constant) and the transition time t,r=dliJ,E) of the charge carriers through the device. 

The recombination rate Prec, which is defined as the number of generated excitons in At divided by the number of generated excitons in At plus the number of minority carriers passing through the device in At, for an unbalanced LED (where j x k, jp x) x) and jXp is given by the following equation  

Obviously, the recombination rate increases with decreasing transition time of the minority charge carriers. 

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In a subsequent work [182], it was shown that the photoelectrochemical performance of InSe can be considerably improved by means of selective (photo)electrochemical etching. Interestingly, whereas the cleavage vdW plane showed little improvement, the photocurrent in the face parallel to the c-axis was doubled. Note that, in contrast to InSe crystals cleaved in the plane perpendicular to the c-axis that are almost defect free, the crystals cut in the plane parallel to the c-axis contain a high density of defects on their surface which leads to a high rate of electron-hole recombinations and inferior quantum efficiency. The asymmetry in the role of electrons and holes, as manifested, e.g., in the fact that surface holes carry out the selective corrosion of the semiconductor surface in both cleavage orientations, was discussed. 

Ema, T., Okita, N., Ide, S. and Sakai, T. (2007) Highly enantioselective and efficient synthesis of methyl (R)-o-chloromandelate with recombinant E. coli toward practical and green access to clopidogrel. Organic and Biomolecular Chemistry, 5 (8), 1175-1176. 

The addition of a second species can cause a decrease in charge recombination and an increase in the TiOz photocatalytic efficiency. Such behavior was examined by loading a series of species on the surface or into the crystal lattice of photocatalysts inorganic ions [148-152], noble metals [153,154], and other semiconductor metal oxides [155], It was thus proven that modifications produced by these species can change semiconductor surface properties by altering interfacial electron-transfer events and thus the photocatalytic efficiency. 

The overall efficiency of LED emission depends on three factors which vary between the different types of LEDs. These are the efficiency of electron-hole production, the radiative efficiency of recombination, and the efficiency of extraction of the optical signal from the junction. 

Highly Enantioselective and Efficient Synthesis of Methyl (R)-o-Chloromandelate, Key Intermediate for Clopidogrel Synthesis, with Recombinant Escherichia coli... 

The biocatalytic reduction of carboxylic acids to their respective aldehydes or alcohols is a relatively new biocatalytic process with the potential to replace conventional chemical processes that use toxic metal catalysts and noxious reagents. An enzyme known as carboxylic acid reductase (Car) from Nocardia sp. NRRL 5646 was cloned into Escherichia coli BL21(DE3). This E. coli based biocatalyst grows faster, expresses Car, and produces fewer side products than Nocardia. Although the enzyme itself can be used in small-scale reactions, whole E. coli cells containing Car and the natural cofactors ATP and NADPH, are easily used to reduce a wide range of carboxylic acids, conceivably at any scale. The biocatalytic reduction of vanillic acid to the commercially valuable product vanillin is used to illustrate the ease and efficiency of the recombinant Car E. coli reduction system." A comprehensive overview is given in Reference 6, and experimental details below are taken primarily from Reference 7. 

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