Donor-acceptor bilayers

Figure 7.4 (a) A donor-acceptor bilayer heterojunction. Donor regions are shaded blue and acceptor... 

Originally, a donor-acceptor bilayer device of two films was used as an n-p junction in solar cells. Thus, they were fabricated as sandwich structures. An example would be one where a transparent substrate is first coated with a conductor, like indium-tin oxide. A conducting polymer like, poly (ethylene dioxythiphene), doped with polystyrene-sulfonic acid, would then be applied from and aqueous solution. The indium-tin oxide acts as an electrode for hole injection or extraction. The polymer is then covered with a conductor, an aluminum foil. The doped polymer can be illustrated as follows ... 

Comparison of the spectral response and of the power efficiency of these first conjugated polymer/fullerene bilayer devices with single layer pure conjugated polymer devices showed that the large potential of the photoinduced charge transfer of a donor-acceptor system was not fully exploited in the bilayers. The devices still suffer from antibatic behavior as well as from a low power conversion efficiency. However, the diode behavior, i.e. the rectification of these devices, was excellent. 

The photoinduction ion flux derives from the similarity of vesicle systems to the proton flux in halobacterium halobium cell envelopes in the bacteriorhodopsin photocycle [126]. Liposome permeability to glucose can similarly be induced by photoexdtation in vesicles containing polyacetylene or thiophene as ion mediators [127]. As in planar bilayers, the surface charge [128] of the vesicle and the chain length of the component surfactant [129] influence assodation between the donor-acceptor pairs, and hence the distance of separation of components inside and outside the vesicle walls. 

The interpenetrating network in bulk hetero junction solar cells [9] helps to overcome the limitations of bilayer systems [25,95] with low mobility materials. However, less is known about the nanometer morphology of an interpenetrating network or the optimum density of donor/acceptor interfacial... 

It is seen from Table 5.1 that the values of the conversion efficiency in bilayer solar cells also is quite low. As mentioned in the introduction it is difficult to dissociate excitons in the conducting polymers. The Donor/Acceptor (D/A) junction between the polymer and the fullerene is rectifying and can be used for designing photovoltaic cells or photodetectors. In this bilayer cell also the conversion efficiency is low. The cause of the low efficiency is that the charge separation occurs only at the D/A interface that results low collection efficiency. The diffusion length of the exciton is a factor 10, lower than the typical penetration depth of the photon. 

Bilayer devices [27,65] apply the donor-acceptor concept introduced above here the exciton is dissociated at their interface, leading to holes on the donor and electrons on the acceptor. Thus, the different types of charge carriers may travel independently within separate materials and bimolecu-lar recombination is largely suppressed. Therefore fight intensity-dependent... 

Transfer of calcium cations (Ca2 + ) across membranes and against a thermodynamic gradient is important to biological processes, such as muscle contraction, release of neurotransmitters or biological signal transduction and immune response. The active transport can be artificially driven (switched) by photoinduced electron transfer processes (Section 6.4.4) between a photoactivatable molecule and a hydroquinone Ca2 + chelator (405) (Scheme 6.194).1210 In this example, oxidation of hydroquinone generates a quinone to release Ca2+ to the aqueous phase inside the bilayer of a liposome, followed by reduction of the quinone back to hydroquinone to complete the redox loop, which results in cyclic transport of Ca2 +. The electron donor/acceptor moiety is a carotenoid porphyrin naphthoquinone molecular triad (see Special Topic 6.26). 

The majority of studies on the distance dependence in electron transfer systems have involved modified enzymatic substrates/" " potential models of chlorophyll photosystems/ and donor-acceptor pairs separated by phospholipid bilayers/ There has been considerable progress in these areas despite the formidable synthetic problems, the difficulty in systematically varying only one parameter in a series of related studies, and the difficulty in evaluating k u in such complicated systems. Some reviews of this work have appeared. Electrochemical studies, using organic bridging groups attached to metal surfaces, should also be noted. ... 

O. Thiebaut, H. Bock, E. Grelet, Eace-on oriented bilayer of two discotic columnar liquid crystals for tn-ganic donor-acceptor heterojunctiotL J. Am. Chem. Soc. 132, 6886-6887... 

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