The first solar cells were developed at the beginning of the 1950s by US scientists to supply power to satellites and space craft. They had a conversion efficiency of around 6 %. By the start of the 1970s, the efficiency had been increased to around 14 %. As interest grew in renewable energies, around 100 semiconductor materials were tested for their practicality for photovoltaic power generation. Silicon initially proved to be the best suited material thanks to its physical properties, worldwide availability and easy disposal. Silicon is the second most common element on the earth’s crust. However, it can only be produced in a complex process from compounds such as sand, quartz, quartz sand or rock crystal. Depending on how the silicon crystallises in this process, a differentiation is made between mono-crystalline, poly-crystalline and amorphous silicon.

The most efficient but also most cost-intensive with regard to the production process is the highly pure mono-crystalline silicon which is also used as a basic material in the microelectronics industry. Mono-crystalline silicon solar cells can be recognised by their even, smooth surface and the broken corners that are the result of the production process. They are sawn out of the originally round disks, called wafers, with special saws. These wafers used to be around 0.45 mm thick. As the production costs are directly correlated to the layer thickness, there are ongoing attempts to reduce the thickness of the layer. Today, thicknesses of 0.2 millimetres and less can be controlled in mass production. Further cost reduction potential is however limited. The reduction of the layer thickness will reach the technological limits at some point. Also the price for mono-crystalline silicium is affected by the high demand from the microelectronics industry and the development of the energy prices.

Less abundant as a raw material for solar cells but much more cost-effective to manufacture is poly-crystalline silicon. Polycrystalline solar cells are as square as the cast blocks from which they are sawn. The various courses taken by the crystallites can be clearly seen on their irregular surface; these range from a few millimetres to a few centimetres in diameter.

The lowest costs per solar-generated energy unit are achieved by panels made of amorphous silicon. Here, the silicon atoms are steamed onto a substrate in no particular arrangement. Due to its high absorption, amorphous silicon can be processed to thin-film-panels with a layer thickness of only 0.5 μm. The level of efficiency of solar cells made of amorphous silicon is however only a quarter of the level of those made of mono-crystalline silicon. We often see this material on a daily basis in solar cells for calculators or watches.