Although there are discrepancies in estimations, it is believed that all the fossil fuels will be exhausted in not a very
far future; the estimated minable periods are 40, 60, and 250 years for oil, natural gas, and coal. These resources are decreasing because mankind is using the energy at a rate of 4.5 x 1020 J/year, and the rate is predicted to accelerate over the coming 100 years. What can we do when all these resources are exhausted, and what should we do today? The energy supply from the Sun is enormous. The total energy reaching the surface of the Earth is about 5.5 x 1024 J/year: about 10,000 times the amount we need! The solar energy has driven the photosynthesis of plants and, as a result, the fossil fuels have been accumulated in the Earth. Renewable energies such as hydroelectric, wind and wave powers are also driven by the solar energy. The exploitation of these renewable energies will become more and more important as the fossil fuels are exhausted. Their uses will also become important to prevent the greenhouse effect caused by emitting a large amount of CO2 into the atmosphere. Since solar energy supplied as photons is the largest among renewable energies, it will be the most important resource when the fossil fuels are exhausted. Its
utilization is also important to reduce the levels of greenhouse gasses. Electricity produced by solar energy has been utilized since the solar cell was developed in 1954. The solar cells were first utilized as power sources in satellites and then in remote areas. Nowadays they are placed on roofs of houses and factories and the utilization is rapidly increasing. However, the total amount of energy produced by solar cells today in the world is less than 0.1% the energy we use today. The difficulty in expanding the use of solar cells arises from the fact that the cost of electricity generated by solar cells is higher than that produced from fossil fuels. The cost of the solar electricity can be lowered by lowering the production cost of solar cells, especially silicon crystals. The cost can be lowered by increasing the energy conversion efficiency. The efficiency of a typical solar cell module is about 14%. Many approaches are being taken to lower the cost of solar electricity. They include the development of new semiconductors that can be used as thin films and improvement of the manufacturing processes. Chemist can contribute to the improvements! As a small step toward the improvement of solar cells, we as chemists succeeded in increasing the efficiency of multicrylstalline-silicon solar ells by controlling the surface morphologies using chemical
reactions. Another chemical approach to the utilization of solar energy is based on the utilization of photocatalytic reactions. Since semiconductor particles instead of crystalline semiconductor wafers can be used in these reactions, the system cost can be lowered. Another advantage of the photocatalytic reactions is that the light energy can be directly converted into chemical energy or materials, which will be very important when the fossil fuels are exhausted. The key in such photocatalytic reaction systems is the development of photocatalysts that can split water hopefully under visible light.