DYE Sensitisation Processes

The mechanism of spectral sensitisation has been shown to involve the direct transfer of an electron from the excited state of the dye into the AgX conduction band (see Fig. 11.8). In confirmation of this mechanism, excellent correlation has been demonstrated between the sensitisation capability of dyes and their electrochemical reduction potential. For the most efficient dyes, the quantum yield of the electron transfer step is close to 1.0. Using picosecond laser pulse excitation to measure the fluorescence decay rates of dyes adsorbed to AgX crystals, it has been shown that the electron transfer is very fast, with rate constants in the region of 10 s having been reported [24]. Once an electron has been transferred into the AgX conduction band, the latent image formation process ensues more or less as it does following direct absorption by AgX itself. The difference in the case of the dye-sensitised process is that the positive holes are trapped at the dye molecules. 

A host of extremely ingenious and informative experiments has been conducted on multilayer L-B films, including studies of fluorescence energy transfer, dye sensitisation processes, electron tunnelling and many others(2). 

Dye-sensitisation of ZnO Electrodes Compared with Ti02, the dye-sensitisation process of ZnO is more complicated due to reduced stability towards acidic media. Carboxyl groups are commonly used as anchoring groups for chemisorption of dye molecules onto metal oxide surfaces. Protons from this anchoring group cause dissolution of Zn surface atoms and the formation of complexes in the pores of the... 

Figure 3.46 A schematic drawing of the problems associated with the dye-sensitisation process of protonated anchoring groups on ZnO surfaces. Reprinted with permission from Boschloo et al., Copyright (2006) with permission from Elsevier...

The DnPont photopolymeric system consists of polymeric binder resins, e.g. PVA, PMMA, cellnlose acetates and styrene-acrylates, reactive acrylic monomers, e.g. aryloxy or alkoxy acrylates, a dye sensitiser and a radical or charge transfer photoinitiator, e.g. DEAW and HABI respectively (see Chapter 4, section 4.5.2), and plasticisers. The process for producing the refractive index structures is as follows ... 

In order to transfer the results achieved for small laboratory cells to a full production line for dye-sensitised solar modules to be used for indoor and outdoor applications, all process steps and technological parameters relevant for industrial production have to be investigated. Topics that are essential for reliable and cheap production technology are listed below ... 

The main conceptual advance made in the last few years is the acceptance that electron-transfer process at dye-sensitised systems under barrierless conditions can be purely electronic. A measurement of the nonradiative decay channel due to electron transfer under these conditions gives a direct determination of the electronic coupling. Subsequent to the initial work pointing this out, there have been a number of determinations of extremely fast electron-transfer times at dye-sensitised surfaces. For dye-derivatised TiOi electron-transfer times from 10 fs to 100 fs have been reported by a number of groups (Rehm et al, 1996 Tachibana et al, 1996 Hannappel et al,... 

When considering organic dyes for use in DSSCs, porphyrins and phthalocyanines have attracted particular attention, the former because of the analogy with natural photosynthetic processes, the latter because of their photochemical and phototherapeutic applications. However, porphyrins cannot compete with the N3 or black dye sensitiser due to their lack of red light and near-lR absorption. Phthalocyanines, on the other hand, show intense absorption bands in this spectral region. However, problems with aggregation and the unsuitable energetic position of the LUMO level, which is too low for electron transfer to the TiOa conduction band, have turned out to be intractable for the moment. 

It is important to recognise that a sub-band gap optical transition leads to a delocalised carrier of one type and a localised carrier of opposite type. Steady-state photocurrent flow requires that the localised carrier is excited subsequently to the valence or conduction band, either by absorption of a second photon (process (b) in Fig. 3) or by thermal excitation (processes (c, d)). Bandgap states localised at the semiconductor surface may be of special importance for sub-band gap photocurrent flow. In process (e), an electron (majority carrier) is optically excited into the conduction band, and the resulting empty surface state is refilled by an interfacial electron transfer process. The latter process is similar to the process of dye sensitised electron injection in the nanocrystalline Ti02 solar cell [20-26, 129). 

Durrant, J.R., Haque, S.A., and Palomares, E. (2004) Towards optimisation of electron transfer processes in dye sensitised solar cells. Coordin. Chem. Rev., 248,1247-1257. 

Developments in DSSCs depend on our understanding and control of the fundamental kinetic and thermochemical nanoscale phenomena that govern the conversion of photon energy into electron energy , namely on the interfacial electron transfer and charge transport dynamics. Efficient dye-sensitised solar cells depend on the fine-tuning of the energies of the states implicated and of the rates of the processes involved. 

The introduction of the electrolyte and the sealing of the cell are critical steps to assemble a durable DSSC. Although highly efficient and durable dye-sensitised solar cells need pure materials, complex and controlled procedures [23], reasonable cells can be constructed using commercial available lower cost materials and following simplified processing steps [24]. 

Another important example of interfacial PCET can occur between semiconductor surfaces and adsorbate molecules, and is particularly relevant for some of the current energy conversion strategies, such as dye-sensitised solar cells (DSSC) or photoelectrochemical (PEC) water splitting cells. A simple proof of the involvement of PCET in interfacial redox processes is the dependence of the conduction and valence band potentials of semiconducting metal oxides, such as Ti02, with pH. The nature of the surface terminal groups (typically O or OH in metal oxides) will have a strong influence in the thermodynamics and kinetics of the system. 

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