What types of solar cells are most efficient?

The use of photovoltaics only became really interesting from a technological point of view when silicon was discovered as a material for solar cells in the middle of the 20th century. Silicon is a natural raw material, which occurs in the form of quartz sand, for example.

Silicon is used for most solar cells, and this is high-purity silicon, which does not occur in nature at all. It must, therefore, be purified from other elements, which is the most expensive part of the production process. The silicon crystals are then “doped”, which means that foreign atoms are introduced to give the silicon the semiconductor properties necessary for the photovoltaic effect.

Currently, the following types of solar cells can be distinguished:

1. crystalline silicon cells: These are the “classics” among the solar cells, which are also the most used. They are produced by sawing silicon crystal blocks into very thin slices (the “wafers”, about 0.3 mm thick).
   This produces a relatively large amount of waste. Alternative manufacturing methods for crystalline solar cells are the so-called EFG process (Edge-defined Film-fed Growth) or the String-Ribbon process, in which strings or moulded parts are drawn through a silicon melt, between which the silicon then solidifies in the desired form.
   There are monocrystalline and multi-/polycrystalline silicon cells. The monocrystalline ones are more expensive to manufacture, but also achieve higher efficiencies compared to multi-crystalline ones.
2. thin-film cells: The difference to crystalline solar cells is that here the semiconductor material is applied in one or more very thin layers directly onto a so-called “substrate” (e.g. glass, plastic or metal). This creates many new application possibilities for photovoltaics (e.g. roof elements made of metal, facade elements made of glass).
   Flexible solar cells can also be produced by using a flexible substrate (e.g. plastic or textiles for clothing or similar). The semiconductor material used is so-called amorphous silicon (a-Si) or cadmium telluride (CdTe) as well as copper indium diselenide (CIS) or copper indium gallium selenide (CIGS). Unlike the others, however, silicon is a raw material that is available in almost unlimited quantities.
   One possibility is also the combination of different semiconductors in so-called multilayer cells in order to be able to use several different wavelengths of sunlight simultaneously.
   The production process saves energy and material (thin-film cells require only about one-hundredth of the material of crystalline solar cells). Thin-film cells can also use diffuse radiation more efficiently.
3. Another technological opportunity is emerging with the development of dye cells or organic semiconductors made of hydrocarbon-based plastics. Organic semiconductors can be processed into large, thin, semi-transparent and flexible layers using simple continuous processes.
   Compared to silicon cells, production becomes more cost-effective, as it is no longer necessary to produce elaborately purified silicon crystals.