What is meant by polycrystalline silicon

How a PV system works

Photovoltaics refers to the direct conversion of sunlight into electrical energy using solar cells.

Solar cells consist of various semiconductor materials. Semiconductors are substances that become electrically conductive when exposed to light or heat, while they have an insulating effect at low temperatures. Over 95% of all solar cells produced in the world are made of the semiconductor material silicon (Si). Silicon has the advantage that it is the second most common element in the earth's crust and that it is available in sufficient quantities and the processing of the material is environmentally friendly.
To manufacture a solar cell, the semiconductor material is “doped”. This means the defined introduction of chemical elements with which either a positive charge carrier excess (p-conducting semiconductor layer) or a negative charge carrier excess (n-conducting semiconductor layer) can be achieved in the semiconductor material. If two differently doped semiconductor layers are formed, a so-called p-n junction is created at the boundary layer.

An internal electric field builds up at this transition, which leads to a charge separation of the charge carriers released upon incidence of light. An electrical voltage can be tapped via metal contacts. If the outer circuit is closed, i.e. an electrical consumer (inverter / rectifier) ​​is connected, a direct current flows.
Silicon cells are about 10 cm ´ 10 cm in size (recently also 15 cm ´ 15 cm). A transparent anti-reflective layer serves to protect the cell and to reduce reflection losses on the cell surface.

The voltage that can be tapped at solar cells depends on the semiconductor material. In the case of silicon, it is around 0.5 V. The terminal voltage is only slightly dependent on the light irradiation, while the current intensity increases with higher illuminance levels.

The output (product of current and voltage) of a solar cell is temperature-dependent. Higher cell temperatures lead to lower performance and thus to poorer efficiency. The efficiency indicates how much of the incident light is converted into usable electrical energy.

Depending on the type of crystal, a distinction is made between three cell types: monocrystalline, polycrystalline and amorphous.
High-purity semiconductor material is required to manufacture monocrystalline silicon cells. Monocrystalline rods are drawn from a silicon melt and then sawn into thin slices. This manufacturing process guarantees relatively high levels of efficiency.
The production of polycrystalline cells is more economical. Liquid silicon is poured into blocks, which are then sawn into slices. When the material solidifies, crystal structures of different sizes form, at the limits of which defects occur. These crystal defects result in a lower efficiency of the solar cell.
If a silicon layer is deposited on glass or other substrate material, one speaks of amorphous or thin-film cells. The layer thicknesses are less than 1 µm (thickness of a human hair: 50-100 µm), so that the production costs are lower simply because of the lower material costs. The efficiencies of amorphous cells are, however, still far below those of the other two cell types. They are mainly used in the low-power sector (clocks, pocket calculators) or as facade elements.

In order to be able to provide suitable voltages or outputs for the different areas of application, individual solar cells are interconnected to form larger units (module). A series connection of the cells results in a higher voltage, a parallel connection a higher current. The interconnected solar cells are usually embedded in transparent ethylene vinyl acetate (EVA film), provided with a frame made of aluminum or stainless steel and covered with transparent glass on the front.
The typical rated outputs of such solar modules are between 170 Wpeak and 270 Wpeak. The characteristics of the solar modules relate to the standard test conditions of 1000 W / m² solar radiation at a cell temperature of 25 ° C. The warranty periods given by the manufacturers are usually 2 to 10 years and testify to the high quality standard and the high life expectancy of today's products.

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