Introduction

The only sustainable energy source on earth is sunlight. Within one hour the current annual energy demand of the world reaches the surface of earth as sunlight. Many concepts to harvest, use and store this energy directly or from secondary processes, such as wind or waves, are now under development to finally reduce our dependency on fossil energy sources. While the conversion of light into electrical energy has reached a high level of efficiency and is already at the stage of commercial applications, the use of visible light to drive chemical reactions is much less developed. This is surprising looking at the omnipresent biological photosynthesis, which harvests and stores energy from sunlight in chemical products as the basis for life on earth.