Photocurrent generation mechanism

MFEs on the photocurrents of the modified electrode with nanoclusters of the mixture of C qN and MePH were examined to verify the photocurrent generation mechanism. In the presence of a magnetic field, the photocurrents clearly increased. 

Figure 13.7 Schematic diagram of the photocurrent generation mechanism in buik heterojunction hybrid soiarceiis (a) schematics of the different energy ieveis of the donor and acceptor materiais and the individuai processes taking piace exciton generation (1), exciton diffusion (2), charge transfer (3), charge carrier...

Takahashi and co-workers (69,70,71) reported both cathodic and anodic photocurrents in addition to corresponding positive and negative photovoltages at solvent-evaporated films of a Chl-oxidant mixture and a Chl-reductant mixture, respectively, on platinum electrodes. Various redox species were examined, respectively, as a donor or acceptor added in an aqueous electrolyte (69). In a typical experiment (71), NAD and Fe(CN)g, each dissolved in a neutral electrolyte solution, were employed as an acceptor for a photocathode and a donor for a photoanode, respectively, and the photoreduction of NAD at a Chl-naphthoquinone-coated cathode and the photooxidation of Fe(CN)J at a Chl-anthrahydroquinone-coated anode were performed under either short circuit conditions or potentiostatic conditions. The reduction of NAD at the photocathode was demonstrated as a model for the photosynthetic system I. In their studies, the photoactive species was attributed to the composite of Chl-oxidant or -reductant (70). A p-type semiconductor model was proposed as the mechanism for photocurrent generation at the Chi photocathode (71). 

The short carrier lifetimes together with time, field, intensity and temperature dependences of the photocurrent indicate an intrinsic generation mechanism with the carrier yield limited by geminate recombination. 

Scheme 13.9 Mechanism of the photocurrent generation in 0TE/Sn02/Mes—Acr+—COOH + (C60) electrode [77].

Figure 5.52 General molecular structure of the surface active porphyrin-ferrocene-thiol supra-molecular complexes reported by Uosaki and co-workers [82]. (b) Energy diagram illustrating the mechanism behind photocurrent generation in SAMs of such complexes at cathodic electrode potentials of the Fc/Fc+ couple P, porphyrin Fc, ferrocene MV2+, methyl viologen...

In general, organic photovoltaic devices are constructed in a sandwich structure where the organic layer(s) are found between two highly conducting electrodes. One of them is transparent to let the light in, normally indium tin oxide (ITO), and the other mirror reflective, usually aluminum. The mechanism of photocurrent generation includes four basic processes ... 

The cathode photocurrent is in proportion to the number of TCPP layers at least up to 10 cycles. The efficiency is greatly dependent on the kinds of oxide gel. These experimental observations suggest that electron transfer from the electrode to the porphyrin via the oxide gel layer is an essential mechanism of the photocurrent generation. Oxygen molecules as an electron acceptor readily diffuse in the oxide gel films of about 20 nm thickness. The electron transfer from the electrode to the porphyrin is assisted by satisfactory conductivity of the gel layer. The overall photocurrent value is considerably smaller than the conventional wet solar cell [11]. However, modification of the electrode surface by ultrathin oxide gel films will facilitate the design of novel light harvesting devices. 

In the case of porphyrin systems, there is an agreement that electron injection occurs from the lowest singlet excited state (438, 439). In the tetraruthenated porphyrins, the mechanisms involved in ET are more complex, since both components are responsible for photocurrent generation. As already discussed, from the HOMO and LUMO compositions, the peripheral ruthenium complexes can effectively transfer electronic charge to the porphyrin center via Ru( Jtt) porphyrin MLCT transitions. In addition, the direct interaction between the ZnTPyP core and Ti02 plays an important role in the photoresponse efficiency. 

Figure 67. Scheme showing the mechanism of photocurrent generation in the Ti02/[Ru30 (Ac)6(py)2(pzC02H)]PF6 photoelectrochemical cell. 

The carrier generation mechanism in PPV has been addressed by studying the transient photoconductivity and the photoluminescence as a function of the external electric field, E, in samples oriented by tensile drawing [166]. The transient photocurrent is proportional to E at low fields, but increases nonlinearly for E> 10 V/cm. The field at which the photoconductivity becomes nonlinear (the onset field, EP ) depends on the degree of alignment the higher the draw... 

Quantitative measurements are best carried out at much lower light intensities than those responsible for the large effects illustrated in Fig. 5. It is desirable to avoid, as far as possible, the photoelectrochemical oxidation or reduction of the film on the time scale of the measurements and this generally restricts incident power densities to less than 10-4 Wcm"2. Since the photocurrents generated by such low levels of illumination are too small to be measured directly, it is necessary to use a lock-in amplifier in conjunction with a mechanical chopper in the experimental arrangement shown in Fig. 6. The sensitivity of the photocurrent spectrometer is usually determined by the noise current arising from the electrode capacitance and the noise voltage in the system. Under favourable conditions, it is possible to measure photocurrents as small as 10 10 A. For typical illumination inten-... 

In this paper, we have developed new multi-l er film electrodes for solar cell application, composed of PANI and PANI/Ti02 films deposited on Au/p-ATP substrates. Fu er investigations involved photoelectrochemistry and mechanism of photocurrent generation in these electrodes. 

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