Charge injection, sensitized

In principle, such devices should take advantage of efficient intercomponent energy transfer from a number of "antenna chromophoric units to a specific chromophoric unit which behaves at the same time as an energy collector and as a charge injection sensitizer. In this way, the light... 

Geiischer H, Michel-Beyerle ME, Rebentrost F, Tributsch H (1968) Sensitization of charge injection into semiconductors with large band gap. Electrochim Acta 13 1509-1515 Geiischer H (1972) Electrochemical techniques for the study of photosensitization. Photochem Photobiol 16 243-260... 

The charge injection from the sensitizer, S, dissolved in the electrolyte solution, might formally be considered as a photogalvanic process. The S molecules must be able to diffuse to the semiconductor surface during their lifetime, t0. Assuming the usual lifetime, r0—10 6 to 10 4s and the diffusion coefficient, D of 10 5cm2/s, the effective diffusion distance of S, <5d (see Eq. 2.5.9) is very small ... 

For a typical monomolecular coverage, T — 10 10 mol/cm2, an electrode roughness factor r = 1000 and an extinction coefficient ads = 107 cm2/mol, the light-harvesting efficiency is, in comparison to the preceding case, very high, intimate contact with the semiconductor surface, hence the conditions for charge injection from S into the semiconductor are almost ideal (q9j—>100 per cent). 

Cyclic voltammetry is an excellent tool to explore electrochemical reactions and to extract thermodynamic as well as kinetic information. Cyclic voltammetric data of complexes in solution show waves corresponding to successive oxidation and reduction processes. In the localized orbital approximation of ruthenium(II) polypyridyl complexes, these processes are viewed as MC and LC, respectively. Electrochemical and luminescence data are useful for calculating excited state redox potentials of sensitizers, an important piece of information from the point of view of determining whether charge injection into Ti02 is favorable. 

Lee, J.-K. Yang, M., Progress in light harvesting and charge injection of dye-sensitized solar cells. Materials Science Eng., B Adv. Funct. Solid-State Mat. 2011,176 1142-1160. 

It is unlikely in such cases where different species exist on the surface that the photocurrent spectrum coincides with the absorption spectrum, since the efficiency of charge transfer will be different for different species. Memming 52> has concluded that for a cyanine dye adsorbed on a SnO 2-electrode the monomer seems to be more effective for charge injection than the dimer or higher aggregates. Hauffe and co-worker S3> have found that chelating dyes are especially efficient for sensitized electron injection into ZnO-electrodes which is seen in the variation of the photocurrent spectrum. 

Fig. 23. Experimental set-up for the measurement of dye sensitized charge injection in organic crystals and of dye sensitized delayed fluorescence of the crystal...

Bach et al. have successfully introduced the concept of a solid p-type semiconductor (heterojunction), with the amorphous organic hole-transport material 2,2, 7,7 -tetrakis (, V, V-di-/ -methoxyphcnyl-aminc)9,9 -spirobifluorenc [96]. This hole-conducting material allows the regeneration of the sensitizers after electron injection due to its hole-transport properties. Nevertheless, the incident photon-to-current conversion efficiencies using complex 22 as a charge-transfer sensitizer... 

In Dr. M. Gratzel s plenary lecture at IPS-2000,103 he presented the following research topics to improve DSC. 1) Mastering the interfaces, electron transfer dynamics, control of dark current. 2) Charge transport in nanocrystalline films. 3) Panchromatic sensitizers, dye cocktail, quantum dot charge injection. 4) Light management, mixed metal oxide films, core-shell metal oxide films. 5) New... 

Over the past 15 years there has been a wealth of research on development and application of transition metal complex sensitizers to the development of dye sensitized photoelectrochemical (solar) cells (DSSCs) [113]. Charge injection from the excited state of many sensitizers has been found to be on the subpicosecond timescale, and a key objective has been to identify chromophores that absorb throughout the visible spectrum. For this reason, Os(II) complexes appear attractive and a variety of attempts were made to make use of these complexes in DSSCs in the 1990s [114-116]. Work has continued in this area in recent years and representative examples are given below. 

Vos and Bignozzi reported another Ru(II)/Os(II) donor/acceptor complex, [(dcb)2Ru(dpt)Os(bpy)2]4+, as a sensitizer for TiC>2 photoelectrodes. Their work indicates that charge injection occurs from both Ru(II) and Os(II) centered excited states, but that the resulting transient species is the thermodynamic Ru(II)/Os(III) species [118]. 

Figure 2.23 Schematic illustrating the dye sensitization of a semiconductor electrode via electron transfer straight lines indicate radiative transitions, curved lines electron transfer, and wavy lines non-radiative (nr) transitions. Photoexcitation into the Si state of the dye may result in charge injection into the conduction band of the semiconductor or fluorescence and inter-system crossing, from where charge injection may occur from the triplet state or phosphorescence...

Photoinduced electron injection is by no means a new development. This process has already been applied in areas such as silver halide photography. In this discussion, only sensitized TiC>2 surfaces will be considered. Many experiments have shown that the charge injection into the semiconductor surface is very fast. In order to study these processes, fast spectroscopic techniques are preferred. Whether or not charge injection takes place can be studied conveniently on the nanosecond time-scale by using transient absorption spectroscopy. However, to address the injection process directly, experiments are carried out on the femtosecond time-scale, while recombination and charge separation require the nanosecond to microsecond range. 

A number of different approaches can be taken to investigate the charge-injection process. The first one, outlined in the last section, is based on the absorption rise observed at about 1200 nm which is associated with the presence of electrons in TiC>2. A second method is based on the measurement of the IPCE values for the assemblies in the presence of iodide, while the third approach is based on the intrinsic spectroscopic features of the sensitizer. In this present section, the focus is on the latter two approaches since the absolute rate for charge injection is not of direct interest but simply whether or not injection is taking place. To estimate the injection and charge-separation process, the transient absorption spectra of the sensitizer in solution are compared with those obtained in the interfacial supramolecular assembly. A typical example of this approach is shown in Figure 6.17 for the compound [Ru(dcbpy)2(bpzt)] [8], (see Figure 6.7 above for the structure). 

Figure 1 Photoinduced charge transfer processes in semiconductor nanoclusters, (a) Under bandgap excitation and (b) sensitized charge injection by exciting adsorbed sensitizer (S). CB and VB refer to conduction and valence bands of the semiconductor and et and ht refer to trapped electrons and holes, respectively.

The sensitizer molecules adsorbed on Ti02 surface have a significantly shorter fluorescence lifetime than in the homogeneous solution and this decrease in lifetime has been attributed to the charge injection process [83,181-183,186-188, 197,218,225,236-239]. Heterogeneous electron transfer rate constants in the range of 107—1011 have been reported in these studies. 

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