Percolation pathways

For proper operation of a bulk heterojunction photovoltaic cell, a special alignment of the HOMO and LUMO levels of the bulk heterojunction components must be accomplished, compatible with the electrodes work functions, as depicted in Scheme 5.8. If an exciton is formed in the polymer phase, then the electron is transferred to the NC phase and reaches the aluminum electrode via its percolating pathway. The remaining hole is transported to the ITO electrode through the polymer phase. In the alternative case, that is, the formation of an exciton in the NCs phase, the hole is transferred to the polymer phase and then transported to the ITO electrode, whereas the electron reaches the aluminum electrode through the NCs phase. 

The morphological problems associated with the BHJ solar cells, such as low concentration of percolating pathways which are needed in order to bring the separated charge carriers to their corresponding electrodes, have prompted the utilization of molecules in with the donor and the acceptor moieties were covalently linked. In this connection several examples of Pc-based polymers [161,162], Pc-C6o dyads [85,87,88] and triads [275] have been prepared and tested for photovoltaic applications, but the efficiencies of these systems have been proved to be still low. 

Composites with filler concentrations close to the percolation threshold exhibit conductivity which is sensitive to compressive deformation, since this brings the metal particles into contact, thereby forming percolation pathways. This sensitivity has been exploited especially in anisotropic composites. These are made by prealigning the metal particles with either electric or magnetic fields. This alignment is identical with that produced by external fields in electro- and magneto-rheological fluids where at a critical field continuous threads of... 

Changes in U and Th systematics (Figure 6) give information about the stability of the soil-rock- percolation pathway system over time. It is quite common in speleothem to find that U concentration and initial activity ratios gradually decrease with time... 

The minimally varying U and Th systematics over this record suggest stable drip routes changes in percolation pathways can thus be ruled out as cause for the shifts in isotopic values. 

Recent SAXS work by Wu [56] has demonstrated that the ionic domains in Nafion membranes are most probably spherical and exhibit a size distribution and spacing that does not vary much with equivalent weight (EW). The latter work suggested that strings of smaller spherical aggregates could be sufficiently close to coalesce upon swelling with water uptake by the membrane, and thus could provide percolation pathways for ionic transport. This picture could replace the nanoscale (1.2 nm-wide) channels suggested in earlier models for Nafion to explain transport phenomena. 

Fig. 26 Differences in the chlorobenzene (a) and toluene (b) based MDMO-PPV PCBM blend film morphologies are shown schematically. In a both the polymer nanospheres and the fullerene phase offer percolated pathways for the transport of holes and electrons, respectively. In b electrons and holes suffer recombination, as the percolation is not sufficient. (Reprinted from [61], 2005, with permission from Elsevier)...

A combination of low-pressure chemical vapour deposition (LPCVD) and plasma-enhanced chemical vapour deposition (PEC VD) was used to create a new multilayer composite SiOx/poly(paraxyly 1 ene) material for hermetic sealing of miniaturised smart micro-electromechanical systems (MEMS) implants (Hogg et al., 2014). Tailoring the thickness ratio between the layers, the percolative pathway and thereby, the permeation for direct water exposure could be considerably reduced compared to conventional parylene-C single layers with the same thickness. 

At low doping levels, percolation pathways are established, providing the means for high carrier mobility and efficient charge transfer. This has been a... 

Wetting of primary pores is essential in view of attaining high catalyst utilization and evaporation rates, while gas-filled secondary pores form the major percolation pathways for the diffusion of oxygen and water vapor. Obviously, the balance between primary and secondary pores steers the interplay of electrochemical activity, evaporation, and gaseous transport, as discussed in Section 2.6. 

Additionally, the a value of PEO declines when blended with PAc, contrary to an increase in a value when ENR is added to PEO in the absence or presence of LiClO,. The T values, as discussed earlier, show diat die salt is more soluble in PAc as compared to ENR. Therefore, with a fixed salt content, the amount of salt dissociated in the PEO amorphous phase is definitely higher for the PEO/ ENR blend compared to the PEO/PAc blend. Besides, the T values of PAc in the presence of salt is raised to a range of 29-37 °C which means the PAc is in its glassy state when ion conductivity of the blend is measured leading to restricted ion mobility in the PEO amorphous phase which forms the predominant percolating pathway of the blend electrolyte. It can be concluded that the ion conductivity of miscible or immiscible PEO-based blend electrolyte is governed by the charge... 

The effect of blend composition on the hole and electron mobilities measured using OFETs was first published in 2004 [83]. It was found that the mobilities of holes and electrons were strongly dependent upon the weight ratio of blends of poly-[2-methoxy-5-(2 -ethylhexyloxy)-l,4-(l-cyanovinylene)phenylene (MEH-PPV) and [60]PCBM. This is shown in Figure 8.10c. The mobility of electrons is not measurable until a ratio of 4 6 (MEH-PPV [60]PCBM) is reached, after which the mobility increases strongly with increasing fiillerene content. This was attributed to improvements in the percolating pathways for electrons in such systems. 

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