Molecular Photovoltaics

A primitive photovoltaic cell may be obtained by shining light on the surface of one of the electrodes in an electrochemical cell. Electrons are lifted to the excited state, thereby creating a voltage between the two electrodes. 

Nevertheless, there is a problem with the mobilization of the excited electron. It is reasonable to assume that the 02p hole left behind is attractive, making it hard for 

FIG U RE 13.10 (See color insert) Examples of photosensitizers in Gratzel cells (a) indoline-like molecule and (b) PCBM. 

FIGURE 13.11 Orbital energies for the system Ti02 and sensitizer. 

The rate of ET from the sensitizer to Ti02 has been measured in several cases using transient spectroscopy. Very high rates on the femtosecond scale have been found. The donation of electrons to Ti02 is one of the fastest ET reactions known. 

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The successful design of both natural and artificial molecular photovoltaic devices rests on meeting three fundamental requirements, namely 121 (1) The quantum yield for the charge separation process should be as high as possible. That is, kcs > kd (Figure 1). (2) The lifetime, tcr (= 1 lkcr), of the CS state must be sufficiently long to enable it to carry out... 

These significant findings form the basis of a set of design principles for the construction of molecular photovoltaic cells and other nanoscale electronic devices in which the control of both the rate and directionality of ET processes is an essential requirement. The successful construction of an artificial light-driven proton pump, based on principles of long-range ET processes illustrates the promise of this approach.1501... 

There are, however, several fields of current research in which a corresponding level of understanding would be of interest also for large molecular adsorbates. For example, adsorbate-substrate interactions are relevant in the general areas of biocompatibility [51] and chemical sensors [52]. The requirement of dye-sensitization of metal oxide semiconductors also makes this an important aspect of many molecular photovoltaic devices. In fact, a good interfacial contact between dye and substrate, characterized by long-term stability and intimate electric contact, is vital for the efficiency of e.g. the dye-sensitized solar cells which have been at the center of our attention for the last five years. 

Thus, the ideas behind the bottom-up approach are as simple as powerful. The general aim lies in the design of novel materials employable for molecular-scale electronics, such as molecular transistors, molecular photovoltaic applications, molecular display technology, etc. Hereby, the functions are carefully adjusted by synthetic tools to build up a certain chemical structure. In this context, we need to address the key steps/challenges in such a work-flow. With this in hands the impetus of this thesis should be illustrated. 

Graetzel, M. The artificial leaf, molecular photovoltaics achieve efficient generation of electricity from sunlight, Coord. Chem. Rev. 1991, 111, 167. 

Moser JE, Bonnote P, Gratzel M. Molecular photovoltaics. Coord Chem Rev 1998 171 245-50. 

The photochemical properties of bipyridine mthenium compounds are at the origin of impressive results in the field of molecular photovoltaics. In 1991, Gratzel et al. prepared a molecular photovoltaic system by coating Ti02 with a bipyridine complex linked to the surface... 

The successful design of both natural and artificial molecular photovoltaic devices rests on meeting three fundamental requirements, namely [1] ... 

In summary, the comparatively small internal reorganization energy of C o, together with its excellent redox properties, should make the Cgo chromophore a valuable electron acceptor component in molecular photovoltaic devices. 

Molecular Photovoltaics 603 modify Ti-isopropoxyde with acetic acid... 

Figure 9. Heterogeneous charge transfer sensitizer for molecular photovoltaic cells.

Recognition is due to the members of the Laboratory of Photonics Interfaces of the Swiss Federal Institute of Technology (EPFL), some of whose work is referenced below to those industrial organizations whose interest in the molecular photovoltaic system has induced them to license the concept and thereby support our research to EPFL and FNRS (Swiss National Science Foundation) for financial support and to OFEN (Swiss Federal Office of Energy) for past encouragement and support. Thanks are also due to Dr. Pierre Bonhote for valuable help in the writing of Section 1.4.5. 

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