Solar energy conversion systems

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

We have seen that, in photosynthetic bacteria, visible light is harvested by the antenna complexes, from which the collected energy is funnelled into the special pair in the reaction centre. A series of electron-transfer steps occurs, producing a charge-separated state across the photosynthetic membrane with a quantum efficiency approaching 100%. The nano-sized structure of this solar energy-conversion system has led researchers over the past two decades to try to imitate the effects that occur in nature. 

Figure 12.11 Component building blocks used in triads for solar energy-conversion systems...

Some future directions in inorganic photochemistry have been outlined by Adamson (56). A pessimistic picture of the practical uses of solar energy conversion systems is painted, but a rosy view of the academic future of the subject is held. It is anticipated that there will be further examination of thermally equilibrated excited (thexi) states—their lifetimes, and spectroscopic and structural properties—and an extension of present efforts to organometallics and metalloproteins is also envisaged (56). The interpretation of spectroscopic data from excited states will continue to be controversial and require future experimentation (57). 

Until a recent x-ray diffraction study (17) provided direct evidence of the arrangement of the pigment species in the reaction center of the photosynthetic bacterium Rhodopseudomonas Viridis, a considerable amount of all evidence pertaining to the internal molecular architecture of plant or bacterial reaction centers was inferred from the results of in vitro spectroscopic experiments and from work on model systems (5, 18, 19). Aside from their use as indirect probes of the structure and function of plant and bacterial reaction centers, model studies have also provided insights into the development of potential biomimetic solar energy conversion systems. In this regard, the work of Netzel and co-workers (20-22) is particularly noteworthy, and in addition, is quite relevant to the material discussed at this conference. 

Biomimetic Solar Energy Conversion Systems Design Issues... 

Three key issues must be addressed in the development of effective biomimetic solar energy conversion systems. First, the molecular system should possess a large optical absorption cross-section in the desired spectral region. Second, the system should possess appropriate characteristics to insure formation of a sufficiently long-lived, low-lying state which can initiate the primary ET efficiently. And third, the system should be able to effect the ET process irreversibility, that is electron-hole recombination should be substantially inhibited. 

Molecular assemblies such as micells or liposomes, and polymers are useful to construct solar energy conversion systems. Their effects are summarized as follows. 

These molecular assemblies are unfortunately not stable enough to construct practical solar energy conversion systems. Vesicles composed of polymerizable monomers (e.g., 4, 5) were polymerized to give polymeric vesicles having enhanced stability 25 26). 

The photoreduction of polymer pendant viologen by 2-propanol was reported to proceed by the successive two-electron transfer processes between the adjacent viologen units and the propanol which is a two-electron reducing agent44). Preferential formation of a dimeric cation radical of viologen observed was ascribed to the polymeric structure and the two-electron process. These fundamental studies on polymeric electron mediators contribute to the construction of solar energy conversion systems. 

As described above, polymeric materials provide specific microenvironment in solution which contributes much to construct solar energy conversion systems. Macrohetero-geneous systems constructed from polymers are of great value especially from the practical point of view. 

Polymers are attracting much attention as functional materials to construct photochemical solar energy conversion systems. Polymers and molecular assemblies are of great value for a conversion system to realize the necessary one-directional electron flow. Colloids of polymer supported metal and polynuclear metal complex are especially effective as catalysts for water photolysis. Fixation and reduction of N2 or C02 are also attractive in solar energy utilization, although they were not described in this article. If the reduction products such as alcohols, hydrocarbons, and ammonia are to be used as fuels, water should be the electron source for the economical reduction. This is why water photolysis has to be studied first. 

The construction of solar energy conversion systems requires the combination of molecularly designed functional materials. Functional polymers play deciding roles for this purpose. 

In a regenerative solar energy conversion system, the device efficiency (r ) is simply given by the ratio of the power delivered by the photovoltaic converter and the incident solar power (Ps in W/m2 or mW/cm2). However, we are concerned here with devices producing a fuel (H2) and several expressions exist for the device efficiency. Thus, this efficiency can be expressed in kinetic terms 70192... 

This type of device has been contrasted489 with a series connection of a photovoltaic p-n junction solar cell and a water electrolyzer. Unlike the latter which is a majority carrier system (i.e., the n-side of the junction is the cathode and the p-side becomes the anode), in a photochemical diode, minority carriers (holes for the n-type and electrons for the p-type) are injected into the electrolyte. This distinction translates to certain advantages in terms of the overall energetics of the solar energy conversion system (see Ref. 489). 

The emission spectrum of the sun consists approximately of 9 % UV light, 40 % visible, and 51 % IR [1]. Only UV, visible, and a small fraction of the near-IR can cause electronic excitation. Furthermore, we have to take into account that the excitation energy collected should be enough to drive the chemical reactions we would like to use to store the solar energy. Therefore, any practical photochemical solar energy conversion system has to be based on visible light absorption. 

C.G. Granqvist, Materials Science for Solar Energy Conversion Systems, C.G. Granqvist (Ed.), Pergamon, Oxford, 1991, p. 106. 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

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