A solar cell (photovoltaic cell) is an electrical component that converts radiant energy, usually sunlight, directly into electrical energy. The application of the solar cell is photovoltaics, where it serves as a power source. The physical basis of the conversion is the photovoltaic effect, which is a special case of the internal photoelectric effect.
There are many different cell types, which can be differentiated according to the semiconductor material used as well as according to the cell technology (wafer-based or thin film). The most important semiconductor material is silicon, from which about 90% of all solar cells produced worldwide were manufactured in 2013; the market share of thin-film cells was around 10%. The solar modules used to generate energy are created by connecting individual solar cells in series and finally encapsulating them. In the case of thin-film modules, series connection is integrated into the cell production process, while in the case of widely used crystalline modules, it is implemented by soldering connectors onto finished solar cells. Sometimes, elements of a solar panel are also colloquially mistakenly referred to as solar cells. However, they do not generate electricity, but process heat and store their energy in a hot water tank (boiler).
Solar cells can be classified according to various criteria. The most common criterion is the thickness of the material. A distinction is made between thick-film and thin-film cells. Another criterion is the semiconductor material used. Silicon is the most commonly used. In addition, other semiconductors such as cadmium telluride and gallium arsenide are also used. In so-called tandem solar cells, layers of different semiconductors are used, for example indium gallium arsenide in combination with indium gallium phosphide.
The crystal structure can be crystalline (mono-/polycrystalline) or amorphous. In addition to inorganic semiconductor materials, there are also organic solar cells and dye-sensitized solar cells, as well as inorganic-organic hybrids. The development is by no means complete.
Monocrystalline Silicon cells (c-Si) have an efficiency of over 25 % and a power density of 20 to 50 W/kg in large-scale applications. The technique is considered to be well mastered.
Polycrystalline Cells, also called multicrystalline cells (poly-Si or mc-Si), have relatively short energy return times and have become the most common cells. They achieve efficiencies of almost 18% in large-scale use. The renunciation of the energy-consuming and time-consuming recrystallization of a single crystal comes at the price of somewhat lower performances. Experimental cells achieve efficiencies of over 20%.
From the 1980s onwards, amorphous silicon (a-Si) achieved the largest market share among thin-film cells. They are known from small applications such as calculators. The module efficiencies are between 5 and 7 % and have a power density of up to approx. 2000 W/kg. There are no material bottlenecks here, even with production on a terawatt scale. Tandem and triple cells, some of which have different spectral sensitivities, have increased efficiency by 10 to 20% to reduce degradation problems.
Crystalline silicon, e.g. microcrystalline silicon (μc-Si), is also used in combination with amorphous silicon as tandem cells, achieving higher efficiencies of up to an expected 15%. Similar to solar cells, they are made of amorphous silicon. By combining two solar cells with different spectral sensitivity (band gap), whereby the front one must of course be semi-transparent, a higher overall efficiency is achievable. However, with an easy-to-implement series connection, the required correspondence of the currents can only be achieved very imperfectly. Solar cell duos in a more promising parallel connection under practical conditions or with matching electronics are so far only known as laboratory experiments.
Si Wire Array (laboratory stage): By equipping a surface with the thinnest wires, this new solar cell is flexible and requires only 1% of the amount of silicon compared to conventional solar cells.
III-V Semiconductor Solar Cells
Gallium arsenide (GaAs) cells are characterized by high efficiencies (up to 41.1% experimentally), very good temperature resistance, lower performance loss when heated than crystalline silicon cells, and robustness against UV radiation. However, they are very expensive to produce. They are often used in space travel (gallium indium phosphide, (Ga,In)P/gallium arsenide, GaAs/germanium, Ge). Triple cells (tandem solar cells with three monolithically stacked p-n junctions) have the highest commercially available efficiency of almost 30%.
II-VI Semiconductor Solar Cells
CdTe cells can be produced very cheaply on an industrial scale by chemical bath deposition (CBD) or chemical vapor deposition (CVD) and are used in thin-film solar cells; for a laboratory solar cell have already been achieved, module efficiencies are now at 10%, long-term behavior is not yet known.
I-III-VI Semiconductor Solar Cells
CIS, CIGS (chalcopyrite) solar cells consist of copper-indium-gallium-selenide or copper-indium disulfide. This material is used in thin-film solar cells, where CIGS is the most powerful material with laboratory efficiencies of 22.6%. The module efficiency is currently 17.4% (as of February 2012). In 1999, Siemens Solar was able to show the first modules. A wide variety of manufacturers have developed different manufacturing processes. So far, despite the excellent design, none has achieved any significant market share. Indium is expensive and limited as a resource.
Organic solar cells (OPV)
Organic chemistry provides materials that may allow the cost-effective production of solar cells. The disadvantage so far is their currently still poor efficiency of a maximum of 17.3 % and the rather short service life (max. 5000 h) of the cells.
DSC or DSSC (dye-sensitized (solar) cell) – use organic dyes to convert light into electrical energy; a process that is based on photosynthesis. They are mostly purple. These cells, with a conductive polymer such as polypyrrole at the cathode, provide the best efficiency of any organic solar cell of over 10%, but have a limited lifespan due to aggressive electrolytes.
Semiconductor electrolyte cells
Copper oxide/NaCl solution. Very easy to manufacture, but limited in performance and reliability.