Abstract
The quality of human life depends to a large degree on the availability of energy sources. The present annual worldwide energy consumption has already attained a level of over 400 exajoules and is expected to further augment steeply from the increase in world population and 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. Quality of life on earth is threatened unless renewable energy resources can be developed in the near future. Chemistry is expected to make important contributions to identify environmentally friendly solutions of the energy problem. One attractive strategy discussed in this article is the development of systems that mimic natural photosynthesis in the conversion solar energy. The task to be accomplished by these systems is to harvest sunlight and convert it to electricity or drive an uphill chemical reaction. 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 11%. The system is based on the sensitization of nanocrystalline oxide films by charge-transfer sensitizers. The underlying fundamental processes of light trapping by the sensitizer, heterogeneous electron transfer from the electronically excited chromophore into the conduction band of the semiconductor oxide and the percolative migration of the injected electrons through the mesoporous film to the collector electrode, will be described below in detail. The low cost and ease of production of the new cell should benefit large scale applications. These systems will undoubtedly promote the acceptance of renewable energy technologies, not least by setting new standards of convenience and economy. The nanocrystalline semiconductor junctions developed first for dye sensitized solar cells have meanwhile found a realm of other applications ranging from electrochromic and electroluminescent displays to high power lithium insertion batteries. They are also used in tandem cells for the cleavage of water into hydrogen and oxygen by sunlight.