A solar cell converts light into electrical energy and always generates DC voltage or current. It works according to the photoelectric effect, which is based on the space charge zone(RLZ) formed between a positively and negatively doped semiconductor.
The photoelectric effect as the principle of the solar cell
Solar cells work according to the photoelectric effect. They absorb photons, which causes charge carriers to be pulled out of the conduction band and jump the energy gap between it and the valence band, exciting the conduction band.
The charge carriers from the valence band, which are the holes, and from the conduction band, the electrons, can be separated by the space charge region. The outflow of electrons takes place via an electrode on the positively doped semiconductor, the inflow of light via a translucent electrode on the negatively doped semiconductor.
Solar cells are square and have an edge length of 10 cm, 12.5 cm or 15 cm. They are coated with an anti-reflective coating( ARC). This reduces the reflection of sunlight and thus improves the absorption and thus the efficiency of the solar cell, which is specified as Power Conversion Efficiency( PCE). The solar cells themselves are made of monocrystalline, polycrystalline and amorphous silicon. Whereby the efficiency of monocrystalline silicon is the highest at 20 % to 22 %. The theoretically achievable efficiency of monocrystalline silicon is just under 30 %. Polycrystalline silicon achieves efficiencies of 15 % to 20 % and amorphous silicon an efficiency of about 8 %.
The direct current generated in solar cells is dissipated via front or rear contacts. These contacts are called contact fingers and are usually in the form of a grid. The current is passed through them to the busbar. The contact fingers have a width of between 50 µm and 100 µm. Several solar cells are joined together to form a larger solar module.
The silicon of the solar cells
Silicon solar cells: monocrystalline, black (left) and blue (center), and polycrystalline, blue, Photo: solarnova.de
Monocrystalline silicon consists of a crystal whose atoms are regularly arranged. During the production of the solar cell, the atoms of the molten silicon are aligned in one direction. Therefore, the production is very complex and expensive, but monocrystalline silicon also has the highest efficiency with 14% to 18%, its surface is flat and graphite-colored.
Polycrystalline silicon consists of several crystals, each of which has a regular atomic structure. Polycrystalline solar cells are about 200 µm to 300 µm thin. They can be produced more cheaply than monocrystalline solar cells, but have a slightly lower efficiency. This is between 10 % and 15 %. The surface of polycrystalline silicon is blue.
Amorphous silicon has no ordered atomic structures, they are irregular. It can be produced relatively cheaply, the efficiency is only 6 % to 10 %. On the other hand, amorphous silicon has a high absorption capacity.
Characteristic values of solar cells
The open circuit voltage (Vco) of solar cells depends on the semiconductor material. For silicon, the source voltage is about 0.5 V and is independent of solar irradiance. The current, on the other hand, increases with higher illuminance and can generate a current of up to 2 A at a solar irradiance of 1,000 W/m2. The relationship between the current and the voltage corresponds to the characteristic behavior of the solar cell and is shown in the I-V characteristic curve. From this characteristic curve, the short-circuit current (Isc), the open-circuit voltage, the fill factor, the maximum power point( MPP) and the efficiency can be obtained.
In addition to silicuim-based base materials, there are already solar cells made of other semiconductor compounds such as cadmium telluride( CdTe), copper indium selenide( CIS) and gallium arsenide( GaAs), as well as those based on organic photovoltaics (OPV) or dye cells. The aforementioned materials are used in thin-film solar cells, but their efficiency is much lower than that of silicon solar cells, ranging from about 6% to 10%.
The development of solar cells is aimed at improving efficiency. Corresponding designs work with anti-reflective layers, surface structures with roughened surfaces, with a back side field as in the PERC solar cell as well as the CPV solar cell in which the light is bundled via lenses.
In addition, there is a decisive factor with which the efficiency of solar cells can be improved. This is the dependence of the semiconductor material on the wavelength for the light yield. For crystalline silicon, the wavelengths with the highest light yield are between 600 nm and 900 nm. That is, from red light to the infrared range. This means that short-wave light and UV light do not contribute to the light yield of silicon and thus to electricity generation. This disadvantage can be overcome by solar cells made of two or more different semiconductor materials. These include heterojunction solar cells, tandem solar cells, and perovskite solar cells. The latter offer a cubic crystal structure and production advantages. Perovskite is a naturally occurring mineral of calcium titanium oxide. Efficiencies of over 25 % have been achieved in the laboratory with perovskite solar cells.