The quality of human life depends to a large degree on the availability of energy sources. The present worldwide energy consumption exceeds already the level of 6000 Gigawatt. It is expected to further increase sharply from the rising demand of energy in the developing countries. This implies enhanced depletion of fossil fuel reserves. leading to further aggravation of the environmental pollution exerting adverse effects on the well being of man kind. Adding the dangers arising from the accumulation of plutonium fission products from nuclear reactors, the quality of life on earth is threatened unless renewable energy resources can be developed in the near future. Photovoltaics is expected to make important contributions to identify environmentally friendly solutions of the energy problem. One attractive strategy discussed in this lecture is the development of systems that mimic natural photosynthesis in the conversion solar energy for the fixation of carbon dioxide. The task to be accomplished by these systems is to harvest sun light to produce electricity or drive an uphill chemical reaction, such as the cleavage of water into hydrogen and oxygen. The hydrogen can be subsequently employed to reduce carbon dioxide to produce fuels and chemical feed stocks. Learning from the concepts used by green plants we have developed a molecular photovoltaic device whose overall efficiency for solar energy conversion to electricity has already attained 10%. The system is based on the sensitization of nanocrystalline films by transition metal charge transfer sensitizers. In analogy to photosynthesis, the new chemical solar cell achieves the separation of light absorption and charge carrier transport. Extraordinary yields for the conversion of incident photons into electric current are obtained, exceeding 90% for transition metal complexes within the wavelength range of their absorption band. Conventional photovoltaic cells for solar energy conversion into electricity are solid state devices do not economically compete for base load utility electricity production. The low cost and ease of production of the new nanocrystalline cell should be benefit large scale applications in particular in underdeveloped or developing countries. These regions of the earth benefit from generous sun shine rendering the availability of a cheap solar cell technology important in view of improving the quality of life and preserving natural resources. Quite aside from its intrinsic merits as a photovoltaic device, the nanocrystalline films development opens up a whole number of additional avenues for energy storage ranging from intercalation batteries to the formation of chemical fuels. These nanocrystalline systems will undoubtedly promote the acceptance of renewable energy technologies, not least by setting new standards of convenience and economy.
