Doping electrodes

As aforementioned, the introduction of carbon nanomaterials is an effective strategy to take on some of the contemporary challenges in the field of DSSCs. In particular, enhanced charge injection and charge transport processes in carbon nanomaterial-doped electrodes, efficient carbon nanomaterial-based, iodine-free, quasi-solid state electrolytes, and the use of novel nanographene hybrids as dyes are some of the most stunning milestones. All of these milestones are considered as solid proof for the excellent prospect of carbon nanomaterials in DSSCs. The major goal of this chapter is to... 

Similar to what has been seen with CNT-doped electrodes, the presence of graphene resulted in enhanced charge transport and inhibited charge recombination. Additionally, as a consequence of doping rougher surfaces evolved, which, in turn, increased light scattering and amount of dye adsorbed onto the surface (Fig. 18.2). 

By contrast, the cobalt and iron doped electrodes form a perfectly reversible redox system showing voltammograms which are independent of film thickness. It was suggested that differences arose from more facile electron hopping in the case of four coordinate copper(II) than for five or six coordinate M11/111 (M = Co, Fe). Electrodes doped with manganese(II) or zinc(II) gave no response. 

Samples of the ceramic polycrystalline Ti02 (rutile) doped electrodes of the VxTi . x02 composition were studied at different vanadium content (0.001 < x < 0,05) in [128, 129]. It was shown that at x < 0,003 the EPR spectra perform a well resolved hyperfine structure (hfs) typical of V4+-doped rutile (Fig. 8.10), in which V4+ ions substituted Ti4+ ions in the crystal lattice. At 0.003 < x < 0.01, the dipolar broadening of the individual lines 8H occurred. At x > 0.01, in parallel with continuing broadening of the hfs lines, a broad single line appears (Fig. 8.11). Its part in the spectrum increased with the increase of vanadium content. 

In the liquid environment the situation is different from that described above. Highly doped electrodes are often avoided because they present an enhanced reactivity with the solution. The sample and tip biases are set independently with respect to levels in solution and the position of the band edges of the semiconductor is fixed with respect to the tip Fermi level iU is indeed generally fixed within a narrow potential window). It follows that the situation is often the one shown in Fig. 6, with the tip Fermi level located in the band gap. In this situation, maintaining the tip at a constant height above the surface requires that the n-type electrode be cathodically biased so as to provide a sufficiently large density of electrons (Fig. 6 c). The stability of this situation is governed by the sample bias and not by the tip bias since the position of the tip Fermi level is not critical here, unless it is outside the band gap. 

It should be mentioned that direct recombination from the excited molecule to the ground state, and also the ground state recovery due to recombination and transfer of electrons via surface states (compare with Fig. 10.21), have been neglected in this derivation. According to Eq. (10.17), the exponential decay is determined by k. Since in heavily doped electrodes, ki includes the tunneling rate constant kj, the decay should... 

Still the electrochemical behavior of diamond electrodes is influenced by more parameters than the surface termination alone. Most of all it is a doping of the diamond phase that provides the required electric conductivity. Usually boron-doped electrodes are employed. In this case, the electric conductivity increases with the content of boron-the resulting properties range from an isolator at low... 

At the present state of the art, however, the entire held of nanochemistry is still at a level of basic research. Despite the relatively high photocurrent quantum yields, solar applications did not arise until now, since the Q-particle doped electrodes decompose under illumination within several days or weeks. Keeping in mind, however, the complicated manner in which the photochemistry and photophysics of Q-particles is determined by the interplay of particle size and surface chemistry, it appears reasonable that a lot of detailed investigations have to be carried out to reliably judge the potential of Q-particle materials. 

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